Short survey
Role of neutralizing antibodies in PRRSV protective immunity
O.J.Lopez 1,F.A.Osorio *
Department of Veterinary and Biomedical Sciences,University of Nebraska–Lincoln,141VBS,
East Campus,UNL,Lincoln,NE 68583-0905,USA
Abstract
Little has been known about the components of the immune system that are effective in the protection of a pig against PRRSV infection.Although antibodies were initially perceived as a deleterious,ineffective component of the PRRSV-speci?c immune response,neutralizing antibodies (NA)are now considered to be an important correlate of protective immunity against PRRSV.This paper reviews the current knowledge on arterivirus-speci?c NA,the role that NA have in protection against infection with PRRSV ,as well as the viral molecular structures that are responsible for the production of this type of antibodies by the pig.This information should prove central to the design of new generation vaccines against PRRSV.#2004Elsevier B.V .All rights reserved.
Fig.1shows the temporary sequence of events during the process of infection of an immune-competent pig with porcine reproductive and respira-tory syndrome virus (PRRSV).The early post-infection (PI)period which follows initial exposure to PRRSV ,is characterized,in many cases,by the presence of clinical symptoms and always by abundant replication in target cells:alveolar and other tissue macrophages (Pol et al.,1991).This acute PI period is then characterized by a high viral load in tissues and by the presence of cell-free (serum-associated)viremia,which may last up to 1month.
Beyond the acute stage of PRRSV infection,a persistent phase of infection ensues (Zimmerman et
al.,1992).In this late infectious phase,the PRRSV persists at lower load levels in selected sites of the body,primarily lymphoid tissue (Ben?eld et al.,1998).The persistence of PRRSV involves a continuous low level of viral replication but not as a true steady-state persistent infection (Allende et al.,2000).The virus eventually clears from the body and seems to do so in most of the animals after several months PI.This process has been estimated to involve at least 150days PI or more in order to clear the infection in 100%of the animals (Allende et al.,2000).Regarding the shedding to other contact animals,most of the laboratories which worked with experimentally infected young pigs,cite a period of around two months PI for effective contagion to others animals.However,?eld experiments conducted by Bierk et al.(2001)indicate that the shedding of older
animals (sows)may infect na?
¨ve contact controls up to 45–86days post-infection.During the persistent phase of infection,high genetic stability is seen in several
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Veterinary Immunology and Immunopathology 102(2004)155–163
*Corresponding author.Tel.:+14024727809;fax:+14024723094.
E-mail address:fosorio@https://www.doczj.com/doc/fe18850276.html, (F.A.Osorio).1
Present address:Department of Biology,Northern Michigan University,1401Presque Isle Ave.,Marquette,MI 49855,USA.0165-2427/$–see front matter #2004Elsevier B.V .All rights reserved.doi:10.1016/j.vetimm.2004.09.005
regions of the genome of the persistent PRRSV ,although some consistent mutations in the genes of envelope glycoproteins and M protein were also observed (Allende et al.,2000).While the actual mechanisms of persistence of PRRSV are not known yet,it seems logical that one major contributing factor to PRRSV persistence in convalescent animals would be the inability of the host ’s immune system to mount an effective protective response against this virus.Little is known about the components of the immune system that are effective in the protection of a pig against PRRSV infection.It has been known for some time that some attenuated vaccines are protective (Gorcyca et al.,1995;Osorio et al.,1998;Plana-Duran et al.,1997;van Woensel et al.,1998)but the basis of this protection was not studied.In addition,convalescent animals show speci ?c protective immunity (Gorcyca et al.,1993;Lager et al.,1997,1999;Mengeling et al.,1999).This protection seems to be preferentially strain-speci ?c,although a certain level of heterologous protection to other strains of PRRSV appears to exist (Lager et al.,1999).Infected pigs develop a T cell mediated response,beginning at 4weeks PI.This response can be detected from day 9(Bautista and Molitor,1997)to 14weeks PI (Lopez Fuertes et al.,1999).The pigs infected with PRRSV also develop a retarded hypersensitivity against the virus (Bautista and Molitor,1997)indicating a response with CD4T helper cells.The matrix protein M and glycoprotein GP5are targets for these CD4T cell response
(Bautista et al.,1999).Speci ?c g -IFN production by T cells,which is a measure of T cell cytotoxicity,show a delay in the response similar to the one observed with neutralizing antibodies (Meier et al.,2000,2003;Xiao et al.,2004).
1.PRRSV-speci?c antibody response during infection
Early after exposure a vigorous anti-PRRSV antibody response can be detected by ELISA (Fig.1)starting at 7–9days PI.Such seroconversion can also be detected by indirect ?uorescent antibody assay or by immunoperoxidase monolayer assay.However,there is no evidence that this early antibody response would play a role in the protection against the infection with PRRSV (Labarque et al.,2000;Yoon et al.,1994).The antibodies that appear during the early PI period do not neutralize PRRSV in vitro (Yoon et al.,1994)and when used in passive protection experiments these early PI antibodies (i.e.antibodies collected at 21days PI)do not mediate passive protection against challenge with virulent PRRSV (Lopez and Osorio,University of Nebraska –Lincoln,unpublished data,2004).Antibodies with PRRSV-neutralizing activity appear only at later PI times (sp.at periods equal or higher than 4weeks PI)(Meier et al.,2000,2003;Yoon et al.,1994).Simultaneously with the appearance of an early,non-protective but vigorous homologous antibody response,a polyclonal
O.J.Lopez,F .A.Osorio /Veterinary Immunology and Immunopathology 102(2004)155–163
156Fig.1.Temporal sequence of events after infection of a pig with porcine reproductive and respiratory syndrome virus.
activation of B cells appears to take place(Drew, 2000;Lamontagne et al.,2001)inducing auto-antibodies early after exposure to PRRSV(Lemke et al.,2004).
The apparently simultaneous presence of PRRSV and PRRSV-speci?c antibodies in serum during the early PI viremic phase of PRRSV infection has been interpreted as an indicator of the inability of antibodies to play a role in protection against PRRSV infection (Albina,1997;Collins,1998;Loemba et al.,1996; Molitor,1993;Molitor et al.,1997;Snijder and Meulenberg,2001).The notion that serums containing PRRSV antibodies beyond the endpoint of neutraliza-tion(that is,at sub-neutralizing concentrations of anti-PRRSV antibodies)could exacerbate PRRSV replica-tion in macrophages in vitro and in vivo(Yoon et al., 1996),?nally led to the general conclusion that anti-PRRSV antibodies constitute a non-protective but deleterious response(reviewed by Snijder and Meulenberg,2001).Due to the slow and irregular appearance of PRRSV-neutralizing antibodies(NA) after PRRSV exposure,not much credit was given to the possible role of this type of antibody in PRRSV protection.In addition,literature related to the role of NA against PRRSV infection has been confusing.For example,a report describing persistence of infectious PRRSV in tissues(tonsils)at long term PI periods in animals that exhibited high titers of circulating homologous PRRSV NA was considered consistent with the idea that PRRSV NA would be incapable of clearing off the virus from the circulation(Murtaugh et al.,2002;Wills et al.,1997).In contrast to this assumption,several reports coincide in that NA prevent the appearance of or blocks viremia.Preven-tion of viremia by NA has been reported by Yoon et al. (1996)and Osorio et al.(2002).Likewise,a recent report(Murtaugh et al.,2003)clearly correlates clearance of viremia with the concomitant appearance of antibodies against the major NA determinant:GP5. Likewise,immunization with proteins GP5and M of PRRSV(Bastos et al.,2004;Pirzadeh and Dea,1997, 1998)confer certain degree of protection,and this protection can be correlated to the appearance of NA. This is consistent with the overall observation that the most successful viral vaccines protect through NA (Zinkernagel et al.,2001).
A standard experimental approach to assess protection mediated by NA consists of passive transfer of monoclonal or polyclonal antibodies followed by viral challenge of the recipient animal.Such experi-ments have been effective for demonstrating that,for many viruses affecting different species and involving infections by different routes,a good correlation exists between virus-neutralizing activity in vitro and protection in vivo(Burton,2002).In some cases the appearance of NA provides sterilizing immunity against certain viruses such is the case for respiratory syncytial virus(Prince et al.,1985)and simian/human immunode?ciency virus(Baba et al.,2000).In other situations,solid protection against clinical disease is evident,even if some level of viral replication is still detectable in the passively transferred animals,such is the case,for instance,of lymphocytic-choriomenin-gitis virus(Wright and Buchmeier,1991).In addition, a commercially available,‘‘humanized’’anti-viral monoclonal antibody speci?c against human respira-tory syncytial virus has been produced and used successfully in those patients considered to be at high risk of infection(Subramanian et al.,1998).The successful use of antibodies in mice,in combination with anti-in?ammatory antibodies,also opens the possibility of their use as a therapeutical approach of an already established infection(Haynes et al.,2002).
The importance of NA in protection has been demonstrated with other arteriviruses.For example, protection against equine arteritis virus(EAV),the prototype for the arterivirus group,is mediated mainly by NA.The appearance of NA in serum coincides with the elimination of virus from circulation(Hammond et al.,1997)and likewise,passive transfer of antibodies to foals lowers or prevents infection with EAV (McCollum,1976).NA is directed mainly against glycoprotein GL(the EA V homologue for PRRSVs GP5).GL has an ectodomain of around100amino acids.The main neutralizing epitope(NE)in this glycoprotein is located within the second half of the N-terminal hydrophilic ectodomain of the GL protein between amino acids99–106(Balasuriya et al.,1997). Immunization with a recombinant vaccine comprising amino acids18–122of GL expressed in E.coli induced NA in serum.After challenge,a correlation between the level of NA and the degree of protection was found(Castillo-Olivares et al.,2001)con?rming the importance of NA in protection against EAV. Lactate dehydrogenase-elevating virus(LDV)is another well-studied arterivirus and the causative
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agent of an asymptomatic infection in mice.Transfer of NA into mice protects them from infection with LDV-C(Chen et al.,1999).
Several observations point to an important role of NA in protection against PRRSV infection.In our experiments,PRRSV inactivated autogenous vac-cines,which failed to induce NA were not protective, and conversely,attenuated vaccines that induced some levels of NA were protective(Osorio et al.,1998).The onset of NA after experimental infection is known to be accompanied by clearance of the virus from circulation and from tissues(Labarque et al.,2000). Although,both mechanisms,cellular immunity and neutralizing antibodies,may be involved in clearance of the virus after infection,there is so far only experimental evidence about the importance of the latter.In addition,vaccination of pigs with a DNA vaccine encoding PRRSV GP5induced moderate levels of NA.Nevertheless,the pigs were protected against challenge with the homologous virulent strain. They presented a mild fever and virus could be recovered after second passage in MARC-145cells only from lungs and mediastinal lymph nodes.Two weeks after challenge,the titer of NA increased up to 128(Pirzadeh and Dea,1998).Thus,the NA response after vaccination with the DNA vaccine resembles the immune response to an attenuated strain of PRRSV observed in our experiments.After vaccination with a modi?ed live vaccine,there is a low or non-detectable NA response,which rapidly increases in titer upon challenge with virulent PRRSV(Osorio et al.,1998).
Recently,a serum transfer experiment conducted in our laboratory(Osorio et al.,2002)provided unequi-vocal evidence that PRRSV NA alone can fully prevent transplacental infection by PRRSV and completely extinguish or prevent PRRSV infection in the pregnant females.Moreover,the transfer of these NA provided sterilizing immunity since no PRRSV could be detected in lymphoid organs by viral isolation,RT-PCR,or swine bioassay in either the dams or the offspring.Along the same line,we have recently observed that transfer of NA to piglets also appears to protect this type of animals against PRRSV infection.The minimal endpoint neutralizing titer in recipient piglets that would protect 100%of the animals against PRRSV viremia upon challenge appears to be at1:8(Lopez and Osorio, University of Nebraska,2004,unpublished results). Thus,neutralizing antibody titer in serum constitutes a correlate of protective immunity against PRRSV,for sure in pregnant sows and possibly in young pigs (Osorio et al.,2002).This data supports the interesting possibility that NA could be used to provide instant protection of animals at high risk of infection with PRRSV,by means of passive transfer of neutralizing immunoglobulins.In addition,NA could be used as a therapeutic approach,although the ability of NA to clear or curtail a previously established infection by PRRSV remains to be tested.We anticipate that a large concentration of immunoglobulins would be required for either therapeutic or preventive use of NA for control of PRRSV infections.Such concentrations of NA could be attained by cloning NA through recombinant DNA technology(Jar et al.,2003)or,as recently reported(Morales et al.,2003)by means of the use of immunization of hens with PRRSV and puri?cation of high titers of PRRSV-speci?c NA directly from egg-yolk.
It is known that the major neutralizing epitopes are located on GP5of PRRSV(Pirzadeh and Dea,1997; Pirzadeh et al.,1998;Weiland et al.,1999;Yang et al., 2000).Besides the major neutralizing epitope on GP5,at least another neutralizing epitope is recognized by a monoclonal antibody on the envelope’s GP4(Yang et al.,2000)and another one on the envelope protein M (codi?ed by the ORF6)(Yang et al.,2000).GP5is heavily glycosylated in the amino-terminal portion ofits ectodomain and forms a hetero-dimer with protein M.
Using reverse vaccinology(Ostrowski et al.,2002) as well as through other studies involving overlapped peptides(Plagemann et al.,2002)the main neutraliz-ing epitope(NE)of PRRSVs GP5responsible for the NA activity has been identi?ed.The minimum antigenic area of this epitope comprising amino acids 37–44as the core of the main NE has been identi?ed (Ostrowski et al.,2002).This antigenic NE is referred to as epitope B.This neutralizing epitope B is also recognized by a neutralizing monoclonal antibody (Yang et al.,2000)and by the highly protective stock solution of PRRSV of antibodies that we used in the passive transfer experiment described above(Osorio et al.,2002).In addition,Wissink et al.(2003)showed that neutralizing monoclonal antibodies recognize their epitope on the N-terminal ectodomain for the European type of PRRSV.In contrast,Plagemann (2004),using serum from convalescent animals,found that the NE is both in the European and American
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strains,in both cases mapping to the area coinciding with our description of epitope B.In all these results suggest an important role for the ectodomain of GP5in the overall biology of PRRSV.
An immunodominant region in the ectodomain of PRRSV GP5named epitope A was also identi?ed (Ostrowski et al.,2002).Epitope A induces a strong non-neutralizing antibody response rapidly after infection.The core of epitope A(between amino acid27and31)is located seven amino acid residues ahead of the neutralizing epitope B towards the amino-terminal of the protein.These features are consistent with the function of a decoy epitope.Decoy epitopes are those that may cause the diminishment of the immune responsiveness against an adjacent NE as it is the case with protein GP41of HIV(Cleveland et al., 2000).The NE(ERDRD)on gp41of HIV-1was demonstrated to be immunogenic in rabbits.However, this response is completely abrogated when epitope ERDRD is presented on a peptide juxtaposed to the immunodominant non-neutralizing epitope IEEE, located at a two-amino acid distance in the native sequence of gp41.Thus,the decoy epitope IEEE decreases the NA response to neutralizing epitope ERDRD.Serum obtained by immunization of mice with a peptide containing motif ERDRD and devoid of IEEE induced polyclonal antibodies capable of neutralization at a concentration of0.1–0.2m g/ml.
NA against the well-studied arterivirus lactate dehydrogenase-elevating virus(LDV)are directed to a region of the LDV VP-3P comprising amino acids37–44,which is the only NE of VP-3P in LDV ((Plagemann,2001).The neurotropic variant of this virus has lost two of the three glycosylation sites in VP-3P making it more susceptible to neutralization in vivo. Moreover,synthetic peptides representing this region of VP-3P were recognized by neutralizing monoclonal antibodies allowing for the development of an ELISA test that detects NA in serum(Plagemann et al.,2001). However,antibodies against these peptides did not neutralize LDV,indicating that the conformation of these peptides was not optimal or that some other residues outside this core region are necessary to ef?ciently induce NA.Peptides resembling PRRSV GP5epitope B coupled to a KLH through a cysteine located in the carboxy-terminal or presented as multiple arti?cial peptides(MAP)were used to immunize mice and pigs.Although the animals produced antibodies against the peptides,these antibodies did not neutralize PRRSV(Lopez et al.,University of Nebraska–Lincoln, 2004,unpublished results).Thus,it seems that the described NE of PRRSV has similar characteristics to that described for LDV:(1)just like the NE of LDV,the PRRSV LDV seems to be antigenic,as indicated by the fact the NE is recognized by NA,but(2)not immunogenic,because it does not induce NA, suggesting then that other additional residues might be critical for the induction of NA.
In summary,these observations made on the PRRSV model,would con?rm,in light of what is known for other arteriviruses,two main concepts:(1) NA play an essential role in prevention of infection with PRRSV(as it is the case for EAV);(2)NA are mainly directed against a main epitope in the ectodomain of PRRSVs GP5(as it is also the case for GL of EAV and VP-3P of LDV).
The mutation of NEs,a process called antigenic drift,is a typical mechanism for immune escape employed by RNAviruses.However,in some cases NEs are located in a region of the glycoprotein that has an important pathogenic function(i.e.interaction with the cellular receptors,a fundamental step in the process of multiplication and perpetuation in the host).In this case, mutations in this particular epitope would represent a disadvantage for the virus,thus other alternative mechanisms would become important to be displayed by PRRSV in order for this virus to escape immune surveillance and persist in the host.Antibodies against decoy epitope A are elicited in the early phase of the infection,while no antibodies against epitope B become detectable before30days PI.Thus,the presence of epitope A and the sugars surrounding epitope B might hinder the response against neutralizing epitope B.The delay in the production of neutralizing,anti-epitope B antibodies(Ostrowski et al.,2002)could then be the basis for explaining the absence of a PRRSV NA described by the earlier authors(Nelson et al.,1994). Thus,this delay would be a chief mechanism for the escape from immune surveillance,which seems to be a feature of PRRSV infection.
1.1.A working model of the protective immune response against PRRSV in swine
Based on the data described above,which have been contributed by several laboratories,we elabo-
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rated a working model for a PRRSV-driven modula-tion of the virus-speci ?c immune response.After exposure to PRRSV ,a prolonged,acute productive infection takes place,characterized by viremia that may last up to 30dpi.Virus is eliminated when cellular immunity,identi ?ed by PRRSV-speci ?c g -IFN pro-ducing cells (Meier et al.,2003)and/or non-speci ?c cells (Xiao et al.,2004)as well as NA appear.This incipient anti-PRRSV protective immunity would begin not sooner than 4weeks post-infection and build up during the following weeks.PRRSV would then appear to have developed alternative mechanisms to escape immune surveillance.For example,induction of low levels of a -IFN that are induced after infection (Albina et al.,1998;Van Reeth et al.,1999)decreasing the levels of danger signals (Matzinger,1994),which normally induce a strong speci ?c cellular response.Regarding antibodies,a polyclonal expansion is induced distracting the speci ?c antibody response (Lamontagne et al.,2001;Lemke et al.,2004).In addition,during the ?rst few weeks post-infection the antibody response against the ectodomain of GP5is mostly centered on the immunodominant and non-neutralizing epitope A.One possibility is that epitope A together with sugars in the ectodomain hinder epitope B from the recognition by membrane IgM on the surface of epitope B-speci ?c B https://www.doczj.com/doc/fe18850276.html,ing different prediction methods,some researchers postulated that the site of cleavage of the leader of GP5is located between amino acids 31–32(Andreyev et al.,1997),while others suggested that such site is located between amino acids 25and 26(Dea et al.,2000).We postulate that it is possible that late in the infection process,variants of PRRSV arise which are deprived of sugars and/or that suffer alternative cleavage of GP5,producing some form of GP5without epitope A by cleavage between amino acids 31and 32.PRRSV containing this type of molecular form of gP5would induce a weak but protective immune response against epitope B.For instance,it has been reported the occurrence of a neurotropic strain of PRRSV with an N !S mutation in residue 44of GP5,which would delete glycosylation at this position of GP5(Rossow et al.,1999).This mutation would be similar to another N !S mutation in LDV ,which confers neurotropism to this isolate and at the same time renders LDV more sensitive to neutraliza-tion (Plagemann,2001).
The delay in the induction of protective cellular and humoral immunity provides a window of time throughout which PRRSV replicates profusely in the host,sheds and infects other contact na ?¨ve animals.At a later time in this process,the induction of neutralizing antibodies (which clear viremia)together with the cellular immune response which would terminate virus infected cells,would eventually eliminate the virus from the organism.Therefore,during this persistent phase of infection,when antibodies are ineffective in clearing the virus,mutations in epitope B would not longer represent an evolutionary advantage to the virus.The level of neutralizing antibodies in serum is related to the increase in frequency of epitope B-speci ?c lymphocyte B clones.A re-infection with PRRSV will activate these clones and will induce a faster and stronger response of NA as it has been demonstrated after virulent challenge of immunized animals (Osorio et al.,1998).Thus,the attenuated vaccine and other immunogens such as DNA vaccine (Pirzadeh and Dea,1998)would reproduce the same mechanism of evasion developed by wild type PRRSV (Fig.2A).An ideal vaccine against PRRSV should then induce,within a short period of time,a high level of NA and speci ?c cellular response against the virus (Fig.2B).
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160Fig.2.The ideal response to a vaccine against porcine reproductive and respiratory syndrome virus.
References
Albina,E.,1997.Porcine reproductive and respiratory syndrome: ten years of experience(1986–1996)with this undesirable viral infection.Vet.Res.28,305–352.
Albina,E.,Carrat,C.,Charley,B.,1998.Interferon-alpha response to swine arterivirus(PoA V),the porcine reproductive and respiratory syndrome virus.J.Interferon Cytokine Res.18, 485–490.
Allende,R.,Laegreid,W.W.,Kutish,G.F.,Galeota,J.A.,Wills, R.W.,Osorio,F.A.,2000.Porcine reproductive and respiratory syndrome virus:description of persistence in individual pigs upon experimental infection.J.Virol.74,10834–10837. Andreyev,V.G.,Wesley,R.D.,Mengeling,W.L.,V orwald,A.C., Lager,K.M.,1997.Genetic variation and phylogenetic relation-ships of22porcine reproductive and respiratory syndrome virus (PRRSV)?eld strains based on sequence analysis of open reading frame5.Arch.Virol.142,993–1001.
Baba,T.W.,Liska,V.,Hofmann-Lehmann,R.,Vlasak,J.,Xu,W., Ayehunie,S.,Cavacini,L.A.,Posner,M.R.,Katinger,H.,Stieg-ler,G.,Bernacky,B.J.,Rizvi,T.A.,Schmidt,R.,Hill,L.R., Keeling,M.E.,Lu,Y.,Wright,J.E.,Chou,T.C.,Ruprecht,R.M., 2000.Human neutralizing monoclonal antibodies of the IgG1 subtype protect against mucosal simian-human immunode?-ciency virus infection.Nat.Med.6,200–206.
Balasuriya,U.B.,Patton,J.F.,Rossitto,P.V.,Timoney,P.J.,McCol-lum,W.H.,MacLachlan,N.J.,1997.Neutralization determinants of laboratory strains and?eld isolates of equine arteritis virus: identi?cation of four neutralization sites in the amino-terminal ectodomain of the G(L)envelope glycoprotein.Virology232, 114–128.
Bastos,R.G.,Dellagostin,O.A.,Barletta,R.G.,Doster,A.R.,Nel-son,E.,Zuckermann,F.,Osorio,F.A.,2004.Immune response of pigs inoculated with Mycobacterium bovis BCG expressing a truncated form of GP5and M protein of porcine reproductive and respiratory syndrome virus.Vaccine22,467–474. Bautista,E.M.,Molitor,T.W.,1997.Cell-mediated immunity to porcine reproductive and respiratory syndrome virus in swine.
Viral.Immunol.10,83–94.
Bautista,E.M.,Suarez,P.,Molitor,T.W.,1999.T cell responses to the structural polypeptides of porcine reproductive and respira-tory syndrome virus.Arch.Virol.144,117–134.
Ben?eld,D.,Nelson,J.K.,Rossow,K.,Rowland,R.R.,Lawson, S.R.,Steffen,M.,Collins,J.E.,1998.Pathogenesis and persis-tence of PRRS.In:Proceedings of the Allen D.Leman Swine Conference,vol.25.pp.169–171.
Bierk,M.D.,Dee,S.A.,Rossow,K.D.,Collins,J.E.,Guedes,M.I., Pijoan,C.,Molitor,T.W.,2001.Diagnostic investigation of chronic porcine reproductive and respiratory syndrome virus in a breeding herd of pigs.Vet.Rec.148,687–690. Burton,D.R.,2002.Antibodies,viruses and vaccines.Nat.Rev.
Immunol.2(9),706–713.
Castillo-Olivares,J.,de Vries,A.A.,Raamsman,M.J.,Rottier,P.J., Lakhani,K.,Westcott,D.,Tearle,J.P.,Wood,J.L.,Mumford, J.A.,Hannant,D.,Davis-Poynter,N.J.,2001.Evaluation of a prototype sub-unit vaccine against equine arteritis virus com-prising the entire ectodomain of the virus large envelope gly-
coprotein(G(L)):induction of virus-neutralizing antibody and assessment of protection in ponies.J.Gen.Virol.82,2425–2435. Chen,Z.,Li,K.,Rowland,R.R.,Plagemann,P.G.,1999.Selective antibody neutralization prevents neuropathogenic lactate dehy-drogenase-elevating virus from causing paralytic disease in immunocompetent mice.J.Neurovirol.5,200–208. Cleveland,S.M.,Buratti,E.,Jones,T.D.,North,P.,Baralle,F., McLain,L.,McInerney,T.,Durrani,Z.,Dimmock,N.J.,2000.
Immunogenic and antigenic dominance of a non-neutralizing epitope over a highly conserved neutralizing epitope in the gp41 envelope glycoprotein of human immunode?ciency virus type1: its deletion leads to a strong neutralizing response.Virology266, 66–78.
Collins,J.E.,1998.Porcine reproductive and respiratory syndrome: the disease.In:Proceedings of the15th IPVS Congress, Birmingham,England,5–9July,pp.149–156.
Dea,S.,Gagnon,C.A.,Mardassi,H.,Pirzadeh,B.,Rogan,D.,2000.
Current knowledge on the structural proteins of porcine repro-ductive and respiratory syndrome(PRRS)virus:comparison of the North American and European isolates.Arch.Virol.145, 659–688.
Drew,T.W.,2000.A review of evidence for immunosuppression due to porcine reproductive and respiratory syndrome virus.Vet.
Res.31,27–39.
Gorcyca,D.,Spiess,D.,Harris,L.,Schlesinger,K.,Chladek,D., Morrison,R.,Schultz,R.,1993.The effect of porcine repro-ductive and respiratory syndrome(PRRS)virus challenge in pregnant sows after subsequent exposure to virulent virus.In: Proceedings of the24th Annual Meeting of American Associa-tion of Swine Practitioners,Missouri,pp.51–52. Gorcyca,D.E.,Schlesinger,K.,Chladek,D.,Morrison,R.,Wens-voort,G.,Dee,S.,Polson,D.,1995.A summary of experimental and?eld studies evaluating the safety and ef?cacy of RespPRRS/Repro for the control of PRRS-induced reproductive disease.NOBL Laboratories,Sioux Center,IA. Hammond,S.A.,Cook,S.J.,Lichtenstein,D.L.,Issel,C.J.,Mon-telaro,R.C.,1997.Maturation of the cellular and humoral immune responses to persistent infection in horses by equine infectious anemia virus is a complex and lengthy process.J.
Virol.71,3840–3852.
Haynes,L.M.,Tonkin,J.,Anderson,L.J.,Tripp,R.A.,2002.Neu-tralizing anti-F glycoprotein and anti-substance P antibody treatment effectively reduces infection and in?ammation asso-ciated with respiratory syncytial virus infection.J.Virol.76, 6873–6881.
Jar,A.M.,Platt,K.B.,Osorio,F.A.,Lopez,O.J.,2003.Production of chimeric(mouse?porcine)anti-porcine reproductive and respiratory syndrome virus antibodies.In:Proceedings of the 84th Annual Meeting of Research Workers in Animal Diseases.
Chicago,IL,November2003(Abstract143).
Labarque,G.G.,Nauwynck,H.J.,Van Reeth,K.,Pensaert,M.B., 2000.Effect of cellular changes and onset of humoral immunity on the replication of porcine reproductive and respiratory syn-drome virus in the lungs of pigs.J.Gen.Virol.81,1327–1334. Lager,K.M.,Mengeling,W.L.,Brockmeier,S.L.,1997.Duration of homologous porcine reproductive and respiratory syndrome virus immunity in pregnant swine.Vet.Microbiol.58,127–133.
O.J.Lopez,F.A.Osorio/Veterinary Immunology and Immunopathology102(2004)155–163161
Lager,K.M.,Mengeling,W.L.,Brockmeier,S.L.,1999.Evaluation of protective immunity in gilts inoculated with the NADC-8 isolate of porcine reproductive and respiratory syndrome virus (PRRSV)and challenge-exposed with an antigenically distinct PRRSV isolate.Am.J.Vet.Res.60,1022–1027. Lamontagne,L.,Page,C.,Larochelle,R.,Longtin,D.,Magar,R., 2001.Polyclonal activation of B cells occurs in lymphoid organs from porcine reproductive and respiratory syndrome virus (PRRSV)-infected pigs.Vet.Immunol.Immunopathol.82, 165–182.
Lemke,C.D.,Haynes,J.S.,Spaete,R.,Adolphson,D.,V orwald,A., Lager,K.,Butler,J.E.,2004.Lymphoid hyperplasia resulting in immune dysregulation is caused by porcine reproductive and respiratory syndrome virus infection in neonatal pigs.J.Immu-nol.172,1916–1925.
Loemba,H.D.,Mounir,S.,Mardassi,H.,Archambault,D.,Dea,S., 1996.Kinetics of humoral immune response to the major structural proteins of the porcine reproductive and respiratory syndrome virus.Arch.Virol.141,751–761.
Lopez Fuertes,L.,Domenech,N.,Alvarez, B.,Ezquerra, A., Dominguez,J.,Castro,J.M.,Alonso,F.,1999.Analysis of cellular immune response in pigs recovered from porcine respiratory and reproductive syndrome infection.Virus Res.
64,33–42.
Matzinger,P.,1994.Tolerance,danger,and the extended family.
Annu.Rev.Immunol.12,991–1045.
McCollum,W.H.,1976.Studies of passive immunity in foals to equine viral arteritis.Vet.Microbiol.1,45–64.
Meier,W.,Wheeler,J.G.,Husmann,R.J.,Osorio,F.A.,Zuckermann,
F.,2000.Characteristics of the immune response of pigs to
PRRS virus.Vet.Res.31,41.
Meier,W.A.,Galeota,J.,Osorio,F.A.,Husmann,R.J.,Schnitzlein, W.M.,Zuckermann,F.A.,2003.Gradual development of the interferon-gamma response of swine to porcine reproductive and respiratory syndrome virus infection or vaccination.Virology 309,18–31.
Mengeling,W.L.,Lager,K.M.,V orwald,A.C.,1999.Safety and ef?cacy of vaccination of pregnant gilts against porcine repro-ductive and respiratory syndrome.Am.J.Vet.Res.60,796–801. Molitor,T.,1993.Immune response to PRRS virus.In:Proceedings of the Allen D.Leman Conference,St.Paul,p.20. Molitor,T.W.,Bautista, E.M.,Choi, C.S.,1997.Immunity to PRRSV:double-edged sword.Vet.Microbiol.55,265–276. Morales,G.A.,Lucio,D.E.,Coeto,G.A.,Chapa,B.J.,Santarrosa,
F.E.,2003.PRRSV-speci?c neutralization assay in MARC
cell cultures using anti-PRRSV immunoglobulins extracted from egg-yolk.In:Proceedings of the36th AMVEC Meeting on Asociacion Mexicana de Veterinarios Especialistas eb Cerdos.Guadalajara,Mexico,July2003,(in Spanish),pp.
281–282.
Murtaugh,M.P.,Xiao,Z.,Johnson,C.R.,Dee,S.A.,Batista,L., 2003.Porcine immunity to porcine reproductive and respiratory syndrome virus(PRRSV):systemic and local response in acute and persistent infection.In:Proceedings of the IX International Symposium on Nidoviruses(Arteriviruses and Coronaviruses).
May2003,(Abstract5.7),p.61.Murtaugh,M.P.,Xiao,Z.,Zuckermann,F.,2002.Immunological responses of swine to porcine reproductive and respiratory syndrome virus infection.Viral.Immunol.15,533–547. Nelson, E.A.,Christopher-Hennings,J.,Ben?eld, D.A.,1994.
Serum immune responses to the proteins of porcine reproductive and respiratory syndrome(PRRS)virus.J.Vet.Diagn.Invest.6, 410–415.
Osorio,F.A.,Galeota,J.A.,Nelson,E.,Brodersen,B.,Doster,A., Wills,R.,Zuckermann, F.,Laegreid,W.W.,2002.Passive transfer of virus-speci?c antibodies confers protection against reproductive failure induced by a virulent strain of porcine reproductive and respiratory syndrome virus and establishes sterilizing immunity.Virology302,9–20.
Osorio,F.A.,Zuckermann,F.,Wills,R.,Meier,W.,Christian,S., Galeota,J.,Doster,A.,1998.PRRSV:comparison of commer-cial vaccines in their ability to induce protection against current PRRSV strains of high virulence.Allen D.Leman Swine Conf.
25,176–182.
Ostrowski,M.,Galeota,J.A.,Jar,A.M.,Platt,K.B.,Osorio,F.A., Lopez,O.J.,2002.Identi?cation of neutralizing and non-neu-tralizing epitopes in the porcine reproductive and respiratory syndrome virus GP5ectodomain.J.Virol.76,4241–4250. Pirzadeh,B.,Dea,S.,1997.Monoclonal antibodies to the ORF5 product of porcine reproductive and respiratory syndrome virus de?ne linear neutralizing determinants.J.Gen.Virol.78(Pt8), 1867–1873.
Pirzadeh,B.,Dea,S.,1998.Immune response in pigs vaccinated with plasmid DNA encoding ORF5of porcine reproductive and respiratory syndrome virus.J.Gen.Virol.79(Pt5),989–999. Pirzadeh,B.,Gagnon,C.A.,Dea,S.,1998.Genomic and antigenic variations of porcine reproductive and respiratory syndrome virus major envelope GP5glycoprotein.Can.J.Vet.Res.62, 170–177.
Plagemann,P.G.,https://www.doczj.com/doc/fe18850276.html,plexity of the single linear neutraliza-tion epitope of the mouse arterivirus lactate dehydrogenase-elevating virus.Virology290,11–20.
Plagemann,P.G.,2004.GP5ectodomain epitope of porcine repro-ductive and respiratory syndrome virus,strain Lelystad virus.
Virus Res.102,225–230.
Plagemann,P.G.,Chen,Z.,Li,K.,2001.Replication competition between lactate dehydrogenase-elevating virus quasispecies in mice.Implications for quasispecies selection and evolution.
Arch.Virol.146,1283–1296.
Plagemann,P.G.,Rowland,R.R.,Faaberg,K.S.,2002.The primary neutralization epitope of porcine respiratory and reproductive syndrome virus strain VR-2332is located in the middle of the GP5ectodomain.Arch.Virol.147,2327–2347.
Plana-Duran,J.,Bastons,M.,Urniza,A.,Vayreda,M.,Vila,X., Mane,H.,1997.Ef?cacy of an inactivated vaccine for preven-tion of reproductive failure induced by porcine reproductive and respiratory syndrome virus.Vet.Microbiol.55,361–370. Pol,J.M.,van Dijk,J.E.,Wensvoort,G.,Terpstra,C.,1991.Patho-logical,ultrastructural,and immunohistochemical changes caused by Lelystad virus in experimentally induced infections of mystery swine disease(synonym:porcine epidemic abortion and respiratory syndrome(PEARS)).Vet.Q.13,137–143.
O.J.Lopez,F.A.Osorio/Veterinary Immunology and Immunopathology102(2004)155–163 162
Prince,G.A.,Hemming,V.G.,Horswood,R.L.,Chanock,R.M., 1985.Immunoprophylaxis and immunotherapy of respiratory syncytial virus infection in the cotton rat.Virus Res.3,193–206. Rossow,K.D.,Shivers,J.L.,Yeske,P.E.,Polson,D.D.,Rowland, R.R.,Lawson,S.R.,Murtaugh,M.P.,Nelson,E.A.,Collins,J.E., 1999.Porcine reproductive and respiratory syndrome virus infection in neonatal pigs characterised by marked neuroviru-lence.Vet.Rec.144,444–448.
Snijder,E.J.,Meulenberg,J.M.,2001.In:Kniper,D.,et al.(Eds.), Arterivirusesin Fields Virology,vol.1,4th ed.Lippincott, Williams&Wilkins,Philadelphia,pp.1205–1220. Subramanian,K.N.,Weisman,L.E.,Rhodes,T.,Ariagno,R.,San-chez,P.J.,Steichen,J.,Givner,L.B.,Jennings,T.L.,Top,F.H.J., Carlin,D.,Connor,E.,1998.Safety,tolerance and pharmaco-kinetics of a humanized monoclonal antibody to respiratory syncytial virus in premature infants and infants with broncho-pulmonary dysplasia.MEDI-493Study Group.Pediatr.Infect.
Dis.J.17,110–115.
Van Reeth,K.,Labarque,G.,Nauwynck,H.,Pensaert,M.,1999.
Differential production of proin?ammatory cytokines in the pig lung during different respiratory virus infections:correlations with pathogenicity.Res.Vet.Sci.67,47–52.
van Woensel,P.A.,Liefkens,K.,Demaret,S.,1998.Effect on viraemia of an American and a European serotype PRRSV vaccine after challenge with European wild-type strains of the virus.Vet.Rec.142,510–512.
Weiland,E.,Wieczorek-Krohmer,M.,Kohl,D.,Conzelmann,K.K., Weiland,F.,1999.Monoclonal antibodies to the GP5of porcine reproductive and respiratory syndrome virus are more effective in virus neutralization than monoclonal antibodies to the GP4.
Vet.Microbiol.66,171–186.
Wills,R.W.,Zimmerman,J.J.,Yoon,K.J.,Swenson,S.L.,McGin-ley,M.J.,Hill,H.T.,Platt,K.B.,Christopher-Hennings,J., Nelson,E.A.,1997.Porcine reproductive and respiratory syn-drome virus:a persistent infection.Vet.Microbiol.55,231–240.Wissink, E.H.,van Wijk,H.A.,Kroese,M.V.,Weiland, E., Meulenberg,J.J.,Rottier,P.J.,van Rijn,P.A.,2003.The major envelope protein,GP5,of a European porcine reproductive and respiratory syndrome virus contains a neutralization epitope in its N-terminal ectodomain.J.Gen.Virol.84, 1535–1543.
Wright,K.E.,Buchmeier,M.J.,1991.Antiviral antibodies attenuate T-cell-mediated immunopathology following acute lymphocytic choriomeningitis virus infection.J.Virol.65, 3001–3006.
Xiao,Z.,Batista,L.,Dee,S.,Halbur,P.,Murtaugh,M.P.,2004.The level of virus-speci?c T-cell and macrophage recruitment in porcine reproductive and respiratory syndrome virus infection in pigs is independent of virus load.J.Virol.78,5923–5933. Yang,L.,Frey,M.L.,Yoon,K.J.,Zimmerman,J.J.,Platt,K.B.,2000.
Categorization of North American porcine reproductive and respiratory syndrome viruses:epitopic pro?les of the N,M, GP5and GP3proteins and susceptibility to neutralization.Arch.
Virol.145,1599–1619.
Yoon,I.J.,Joo,H.S.,Goyal,S.M.,Molitor,T.W.,1994.A modi?ed serum neutralization test for the detection of antibody to porcine reproductive and respiratory syndrome virus in swine sera.J.
Vet.Diagn.Invest.6,289–292.
Yoon,K.J.,Wu,L.L.,Zimmerman,J.J.,Hill,H.T.,Platt,K.B.,1996.
Antibody-dependent enhancement(ADE)of porcine reproduc-tive and respiratory syndrome virus(PRRSV)infection in pigs.
Viral.Immunol.9,51–63.
Zimmerman,J.,Sanderson,T.,Eernisse,K.,Hill,H.,Frey,M.,1992.
Transmission of SIRS virus in convalescent animals to com-mingled penmates under experimental conditions.Am.Assoc.
Swine Pract.Newslett.4,25.
Zinkernagel,R.M.,LaMarre,A.,Ciurea,A.,Hunziker,L.,Ochsen-bein,A.F.,McCoy,K.D.,Fehr,T.,Bachmann,M.F.,Kalinke,U., Hengartner,H.,2001.Neutralizing antiviral antibody responses.
Adv.Immunol.79,1–53.
O.J.Lopez,F.A.Osorio/Veterinary Immunology and Immunopathology102(2004)155–163163
比较级与最高级 1.as...as 与(not) as(so)...as as...as...句型中，as的词性 第一个as是副词，用在形容词和副词的原级前，常译为“同样地”。第二个as是连词，连接与前面句子结构相同的一个句子(相同部分常省略)，可译为“同..... He is as tall as his brother is (tall) . (后面的as 为连词) 只有在否定句中，第一个as才可换为so 改错: He is so tall as his brother.（X） 2.在比较状语从句中，主句和从句的句式结构一般是相同的 与as...as 句式中第二个as一样，than 也是连词。as和than这两个连词后面的从句的结构与前面的句子大部分情况下结构是相同的，相同部分可以省略。 He picked more apples than she did. 完整的表达为: He picked more apples than she picked apples. 后而的picked apples和前面相同，用did 替代。 He walked as slowly as she did.完整表达为: He walked as slowly as she walked slowly. she后面walked slowly与前面相同，用did替代。
3.谓语的替代 在as和than 引导的比较状语从句中，由于句式同前面 主句相同，为避免重复，常把主句中出现而从句中又出现的动词用do的适当形式来代替。 John speaks German as fluently as Mary does. 4.前后的比较对象应一致 不管后面连词是than 还是as,前后的比较对象应一致。The weather of Beijing is colder than Guangzhou. x than前面比较对象是“天气”，than 后面比较对象是“广州”，不能相比较。应改为: The weather of Bejing is colder than that of Guangzhou. 再如: His handwriting is as good as me. 应改为: His handwriting is as good as mine. 5.可以修饰比较级的词 常用来修饰比较级的词或短语有: Much,even,far,a little,a lot,a bit,by far,rather,any,still,a great deal等。 by far的用法: 用于强调，意为“...得多”“最最...”“显然”等，可修饰形容词或副词的比较级和最高级，通常置于其后，但是若比较级或最高级前有冠词，则可置于其前或其后。
人教版（新目标）初中英语形容词与副词的比较级与最高级 （一）规则变化： 1．绝大多数的单音节和少数双音节词，加词尾-er ，-est tall—taller—tallest 2．以不发音的e结尾的单音节词和少数以-le结尾的双音节词只加-r，-st nice—nicer—nicest , able—abler—ablest 3．以一个辅音字母结尾的重读闭音节词或少数双音节词，双写结尾的辅音字母，再加-er，-est big—bigger—biggest 4．以辅音字母加y结尾的双音节词，改y为i再加-er，-est easy—easier—easiest 5．少数以-er，-ow结尾的双音节词末尾加-er，-est clever—cleverer—cleverest, narrow—narrower—narrowest 6．其他双音节词和多音节词，在前面加more，most来构成比较级和最高级 easily—more easily—most easily （二）不规则变化 常见的有： good / well—better—best ; bad (ly)/ ill—worse—worst ; old—older/elder—oldest/eldest many / much—more—most ; little—less—least ; far—farther/further—farthest/furthest
用法： 1．原级比较：as + adj./adv. +as（否定为not so/as + adj./adv. +as）当as… as中间有名字时，采用as + adj. + a + n.或as + many / much + n. This is as good an example as the other is . I can carry as much paper as you can. 表示倍数的词或其他程度副词做修饰语时放在as的前面 This room is twice as big as that one. 倍数+as+adj.+as = 倍数+the +n.+of Your room is twice as larger as mine. = Your room is twice the size of mine. 2．比较级+ than 比较级前可加程度状语much, still, even, far, a lot, a little, three years. five times,20%等 He is three years older than I (am). 表示“（两个中）较……的那个”时，比较级前常加the（后面有名字时前面才能加冠词） He is the taller of the two brothers. / He is taller than his two brothers. Which is larger, Canada or Australia? / Which is the larger country, Canada or Australia? 可用比较级形式表示最高级概念，关键是要用或或否定词等把一事物（或人）与其他同类事物（或人）相分离 He is taller than any other boy / anybody else.
有时造句大全 [标签:栏目] ，有时造句大全 1、人们有时候是在做正确的决定，有时候是在证明决定正确。 2、家不只是房子，更重要的是人。有时候你会想方设法的离开他，有时候你却要不顾一切地回归他的怀抱。 3、再高的人有时也需踮足，再矮的人有时也需屈身。 4、人生啊，是这样不可预测，没有永恒的痛苦，也没有永恒的幸福，生活象流水一般，有时是那么平展，有时又是那么曲折。 5、使你惨败的，有时是你的朋友；使你成功的，有时是你的敌手。我们应学会感恩；更应该学会感怨。 6、这雨让我懂得了，父母是爱我么的，虽然有时很罗嗦，虽然有时脾气会很大，但是他们是爱我们的。 7、爱情是缘，就像瞌睡碰到了枕头，久旱遇到了雨天；爱情是思念，有时想着会失眠，有时梦里笑得甜。愿你爱情甜蜜！ 8、没有比时间更容易浪费的，同时没有比时间更珍贵的了，因为没有时间我们几乎无法做任何事。 9、一阵春风吹来，小草跳起欢快的舞蹈，有时舒展双臂，有时弯腰触地，有时左右摇晃，有时拥抱春风，真是姿态万千。 10、天鹅们同临一乱湖水，有时心怀幽情，咯守规行，有时也会意会神，雌雄彼此呵护。 11、月亮有时候象圆盘，有时候像西瓜，有时候象镰刀，有时候又像个光环。 12、炒股和赛车一样，有时候要停，有时候要冲，不要永远想着冲那样会撞死的。 13、最高明的骗子，可能在某个时刻欺骗所有人，也可能在所有时刻欺骗某些人，但不可能在所有时刻欺骗所有的人。 14、天空由于多变化，又是晴朗的天空是蓝蓝的，有时天空是黑沉沉的，有时天空像披上了雪白的衣服。 15、世界上没有原则，只有世故，没有法律，只有时势，高明的人同世故跟
时事打成一片，任意支配。 16、身体偶尔的背叛可以原谅，心灵的长期背叛不能容忍。有时要难得糊涂，有时要当机立断，这是婚姻的大智慧。 17、天空中的白云，有时像飞腾的巨龙，有时如威武的雄狮，有时又似奔腾的骏马。 18、小明正在做回家作业，只见他有时抓耳挠腮，有时写写画画，正急得满头大汗。 19、含羞草的**力是不可抵。有时，它的魅力甚至比牡丹还强，它的风格是典雅的，它的性格是独一无二的！含羞草，是真正值得我去爱的。 20、天上的云彩变化多端，有时候像一匹骏马在奔驰，有时候像一条狗在摆头摇尾，有时候像一只鸡在找食，有时候又像一头大象在喝水。 21、**常常走向自己的反面：吝啬有时导致挥霍，挥霍有时导致吝啬；我们常常经由软弱而达到坚强，经由怯懦而达到勇敢。 22、春雨，有时像飘渺的云烟，有时像如丝的针线，有时像密织的罗莎。 23、我的爸爸长着乌黑的头发，浓浓的眉毛下长着一双大大的眼睛，唱闪烁着智慧和机敏的神采，有时还使人感到几分诙谐和幽默。 24、对小钱不要过分去计较。金钱是生着羽翼的东西，有时它会自行飞去，有时必须将它放出去，才能带更多回来。 25、鸟们有时在天空中展翅高飞，有时停在树枝上婉转啼叫，有时在林间欢蹦乱跳。 26、我课余生活很丰富，有时看书，有时跳绳，有的画画。 27、春雨有时像牛毛，有时像花针，有时像细丝，像细小的珍珠，有时像细小的尘埃。 28、我很好奇，有时候我仿佛尝到了爱的味道，有时候爱很甜。 29、伤心有时是一种动力，失望有时是一种解脱，执迷不悟有时是一种磨练。 30、有时候相爱是一种无奈，有时候离开是另一种安排。为了爱你和你爱的人，请不要吸烟。 31、有时可能别人不在乎你，但你不能不在乎自己。 32、金翅雀唱着、跳跃着，有时也扑打着，像一群不知疲倦的孩子，给这幽
大多数形容词有三种形式,原级,比较级和最高级, 以表示形容词说明的性质在程度上的不同。 形容词的原级: 形容词的原级形式就是词典中出现的形容词的原形。例如: poor tall great glad bad 形容词的比较级和最高级: 形容词的比较级和最高级形式是在形容词的原级形式的基础上变化的。分为规则变化和不规则变化。 规则变化如下: 1) 单音节形容词的比较级和最高级形式是在词尾加 -er 和 -est 构成。 great (原级) (比较级) (最高级) 2) 以 -e 结尾的单音节形容词的比较级和最高级是在词尾加 -r 和 -st 构成。wide (原级) (比较级) (最高级) 3)少数以-y, -er, -ow, -ble结尾的双音节形容词的比较级和最高级是在词尾加 -er 和 -est 构成。 clever(原级) (比较级) (最高级) 4) 以 -y 结尾,但 -y 前是辅音字母的形容词的比较级和最高级是把 -y 去掉,加上 -ier 和-est 构成. happy (原形) (比较级) (最高级) 5) 以一个辅音字母结尾其前面的元音字母发短元音的形容词的比较级和最高级是双写该辅音字母然后再加 -er和-est。 big (原级) (比较级) (最高级) 6) 双音节和多音节形容词的比较级和最高级需用more 和 most 加在形容词前面来构成。 beautiful (原级) (比较级) (比较级) difficult (原级) (最高级) (最高级) 常用的不规则变化的形容词的比较级和最高级: 原级------比较级------最高级 good------better------best many------more------most much------more------most bad------worse------worst far------farther, further------farthest, furthest 形容词前如加 less 和 least 则表示"较不"和"最不 形容词比较级的用法: 形容词的比较级用于两个人或事物的比较,其结构形式如下: 主语+谓语(系动词)+ 形容词比较级+than+ 对比成分。也就是, 含有形容词比较级的主句+than+从句。注意从句常常省去意义上和主句相同的部分, 而只剩下对比的成分。
但是造句大全 1、你可以选择坚持，也可以选择放弃。没有对错。我是说对爱情来说，但是重要的是坚持你的选择。 2、我站在窗前，静静地欣赏着，天空中下着一群可爱的小精灵，虽然都是雪花，但是，你只要仔细观察，就会发现有许多形状、图案不同的雪花。 3、以和蔼亲切的态度说话并不会伤到舌头，但是，以愤怒和不悦的态度说话，则可能招致不幸。 4、有自己的人生观和价值观。出现问题可以忍让并寻求解决，但是触及原则，要保持自己原则。丧失原则会让你失去生活的目的。 5、秋天，柳树的叶儿慢慢变黄，继而掉落，让人感到无限凄凉。这时的风也变得恶了，是不是柳树姑娘和风姐姐吵架了呢？我不知道。但是，我见到，秋天到处是落叶缤纷，仿佛一场自然界的葬礼。 6、虽然他不聪明，但是他学习十分的用功。 7、想要获得成功，不像数学题一样，没有一个绝对的公式。但是如果知道一些原则的话，可以离成功更近一步。 8、生活过，而不会宽容别人的人，是不配受到别人的宽容的。
但是谁能说是不需要宽容的呢？ 9、虽然这是你亲手做的巧克力蛋糕，但是我不喜欢。 10、虽然万物好像逝去了，但是，你瞧！那火红的枫叶在树枝上摇摆着，就像是一大群顽皮的孩子在手拉手一起跳着欢快的舞蹈呢。有些枫叶因跳舞不慎，从树枝上跌了下来，瞬间化作了一只只飞舞的蝴蝶，在空中飞来飞去。 11、对于18岁的我们来说，有些事情的确会影响我们的一生，但是没有一件事能决定我们的一生！高考试卷是一把刻度不均匀的尺子：对于你自己来说，难题的分值不一定高。 12、她虽然态度冷淡，但是还算客气。 13、是人都有惰性，这是与生俱来的，但是我们后天可以改变这种惰性，谁改变的越多，谁就越成功。 14、我们前进的脚步虽然会让挫折绊住，但是我们要做生活的主人，不要坐在绊脚石的面前唉声叹气而耗尽了自己。让我们学会微笑着用有限的生命来超越无限的自己！我坚信，挫折让我更自信，更成功！ 15、夏天是个多姿多彩的世界。在这个季节，是最充满快乐的。也许，尽管很枯燥；也许，令人热得无法忍受；也许，植物们懒洋洋
英语比较级和最高级的用法归纳 在学习英语过程中，会遇到很多的语法问题，比如比较级和最高级的用法，对于 这些语法你能够掌握吗？下面是小编整理的英语比较级和最高级的用法，欢迎阅读！ 英语比较级和最高级的用法 一、形容词、副词的比较级和最高级的构成规则 1.一般单音节词和少数以-er，-ow结尾的双音节词，比较级在后面加-er，最高级 在后面加-est; (1)单音节词 如：small→smaller→smallest short→shorter→shortest tall→taller→tallest great→greater→greatest (2)双音节词 如：clever→cleverer→cleverest narrow→narrower→narrowest 2.以不发音e结尾的单音节词，比较在原级后加-r，最高级在原级后加-st; 如：large→larger→largest nice→nicer→nicest able→abler→ablest 3.在重读闭音节(即：辅音+元音+辅音)中，先双写末尾的辅音字母，比较级加-er，最高级加-est; 如：big→bigger→biggest hot→hotter→hottest fat→fatter→fattest 4.以“辅音字母+y”结尾的双音节词，把y改为i，比较级加-er，最高级加-est; 如：easy→easier→easiest heavy→heavier→heaviest busy→busier→busiest happy→happier→happiest 5.其他双音节词和多音节词，比较级在前面加more，最高级在前面加most; 如：bea utiful→more beautiful→most beautiful different→more different→most different easily→more easily→most easily 注意：(1)形容词最高级前通常必须用定冠词 the，副词最高级前可不用。 例句： The Sahara is the biggest desert in the world. (2) 形容词most前面没有the，不表示最高级的含义，只表示"非常"。 It is a most important problem. =It is a very important problem.
一会一会造句大全 导读：本文是关于一会一会造句大全，如果觉得很不错，欢迎点评和分享！ 1、天上的云，颜色一会儿红通通的，一会儿半紫半黄的，一会儿金灿灿的。 2、科学家们好勤奋，一会儿观察自然现象，一会儿观察天气，一会儿做实验，一会儿相互提问，忙得不可开交。 3、小明一会儿打扫房间一会儿拖地，忙着手忙脚乱。 4、灯下的影子一会儿变长，一会儿缩短，可真有趣。 5、她起身站着想，神经质地一会儿把两手绞在一起，一会儿又松开。 6、不知在远处的什么地方，一辆汽车一会儿起动，一会儿熄火，一会儿起动，一会儿熄火，终于无声无息了。 7、火烧云的变化形态万千，一会儿就变成了一只可爱的小狗，一会儿又变成了一匹飞奔的骏马。 8、他一边走，头一边不停地来回转动，一会儿看看桥，一会儿看看桥后面的树。 9、健身房里，人们一会儿跑步一会儿做仰卧起坐，快乐的挥洒着汗水。 10、妈妈一会儿在家织毛衣，一会儿在厨房里做饭，我回到家很快就可以吃饭了。
11、白云飘飘一会儿东一会儿西一会儿左一会儿右。 12、秋高气爽的时节，一群大雁正往南飞，一会儿排成一字形，一会儿排成人字形。 13、他像只调皮的猴子，一会儿挠腮一会儿跳上跳下。 14、我做作业老是不专心，一会儿做作业，一会儿又吃东西，一会儿又看电视。 15、上课时他一会儿东张西望，一会儿交头接耳，结果什么也没有学到。 16、美丽的小燕子一会儿蜻蜓点水般掠过湖面，一会儿像离弦的弓箭飞向远方。 17、小蜜蜂一会我采集花粉回来，一会儿采信蜂蜜回来。 18、我也不明白是怎么回事，一会儿觉得丧气，一会儿又觉得轻快。 19、动物园里的猴子一会儿爬上，一会儿窜下，可调皮了。 20、数学课上，小明一会儿跟同桌小刚聊天，一会儿和邻桌小红聊天，最后被老师批评了。 21、小猫钓鱼很不专心，一会儿捉蜻蜓，一会儿捉蝴蝶。 22、野鸭悠闲自在地浮着，一会儿跌入水底，一会儿又立在浪尖上，像孩子在打秋千。 23、小花狗跑到楼顶上，一会儿大声叫，一会儿趴在那不动。 24、我好喜欢我的语文老师，读课文的时候一会儿大声，一会儿小声，一会儿神色飞扬。
More than的用法 A. “More than+名词”表示“不仅仅是” 1)Modern science is more than a large amount of information. 2)Jason is more than a lecturer; he is a writer, too. 3) We need more than material wealth to build our country.建设我们国家，不仅仅需要物质财富. B. “More than+数词”含“以上”或“不止”之意，如： 4)I have known David for more than 20 years. 5)Let's carry out the test with more than the sample copy. 6) More than one person has made this suggestion. 不止一人提过这个建议. C. “More than+形容词”等于“很”或“非常”的意思，如： 7)In doing scientific experiments, one must be more than careful with the instruments. 8)I assure you I am more than glad to help you. D. more than + (that)从句，其基本意义是“超过（=over）”，但可译成“简直不”“远非”.难以，完全不能（其后通常连用情态动词can） 9) That is more than I can understand . 那非我所能懂的. 10) That is more than I can tell. 那事我实在不明白。 11) The heat there was more than he could stand. 那儿的炎热程度是他所不能忍受的 此外，“more than”也在一些惯用语中出现，如： more...than 的用法 1. 比……多，比……更 He has more books than me. 他的书比我多。 He is more careful than the others. 他比其他人更仔细。 2. 与其……不如 He is more lucky than clever. 与其说他聪明，不如说他幸运。 He is more （a）scholar than （a）teacher. 与其说他是位教师，不如说他是位学者。 注：该句型主要用于同一个人或物在两个不同性质或特征等方面的比较，其中的比较级必须用加more 的形式，不能用加词尾-er 的形式。 No more than/not more than 1. no more than 的意思是“仅仅”“只有”“最多不超过”，强调少。如： --This test takes no more than thirty minutes. 这个测验只要30分钟。 --The pub was no more than half full. 该酒吧的上座率最多不超过五成。-For thirty years，he had done no more than he （had）needed to. 30年来，他只干了他需要干的工作。 2. not more than 为more than （多于）的否定式，其意为“不多于”“不超过”。如：Not more than 10 guests came to her birthday party. 来参加她的生日宴会的客人不超过十人。 比较： She has no more than three hats. 她只有3顶帽子。（太少了） She has not more than three hats. 她至多有3顶帽子。（也许不到3顶帽子） I have no more than five yuan in my pocket. 我口袋里的钱最多不过5元。（言其少） I have not more than five yuan in my pocket. 我口袋里的钱不多于5元。（也许不到5元） more than, less than 的用法 1. （指数量）不到，不足 It’s less than half an hour’s drive from here. 开车到那里不到半个钟头。 In less than an hour he finished the work. 没要上一个小时，他就完成了工作。 2. 比……（小）少 She eats less than she should. 她吃得比她应该吃的少。 Half the group felt they spent less than average. 半数人觉得他们的花费低于平均水平。 more…than,/no more than/not more than (1)Mr．Li is ________ a professor; he is also a famous scientist． (2)As I had ________ five dollars with me, I couldn’t afford the new jacket then． (3)He had to work at the age of ________ twelve． (4)There were ________ ten chairs in the room．However, the number of the children is twelve． (5)If you tel l your father what you’ve done, he’ll be ________ angry． (6)－What did you think of this novel? －I was disappointed to find it ________ interesting ________ that one． 倍数表达法 1. “倍数+形容词（或副词）的比较级+than+从句”表示“A比B大（长、高、宽等）多少倍” This rope is twice longer than that one.这根绳是那根绳的三倍（比那根绳长两倍）。The car runs twice faster than that truck.这辆小车的速度比那辆卡车快两倍（是那辆卡车的三倍）。 2. “倍数+as+形容词或副词的原级+as+从句”表示“A正好是B的多少倍”。
初中英语比较级和最高级讲解与练习 形容词比较级和最高级 一．绝大多数形容词有三种形式,原级,比较级和最高级, 以表示形容词说明的性质在程度上的不同。 1. 形容词的原级: 形容词的原级形式就是词典中出现的形容词的原形。例如: poor tall great glad bad 2. 形容词的比较级和最高级: 形容词的比较级和最高级形式是在形容词的原级形式的基 础上变化的。分为规则变化和不规则变化。 二．形容词比较级和最高级规则变化如下: 1) 单音节形容词的比较级和最高级形式是在词尾加-er 和-est 构成。 great (原级) greater(比较级) greatest(最高级) 2) 以-e 结尾的单音节形容词的比较级和最高级是在词尾加-r 和-st 构成。 wide (原级) wider (比较级) widest (最高级) 3) 少数以-y, -er, -ow, -ble结尾的双音节形容词的比较级和最高级是在词尾加 -er 和-est构成。 clever(原级) cleverer(比较级) cleverest(最高级)， slow(原级) slower(比较级) slowest (最高级) 4) 以-y 结尾,但-y 前是辅音字母的形容词的比较级和最高级是把-y 去掉,加上-ier 和-est 构成. happy (原形) happier (比较级) happiest (最高级) 5) 以一个辅音字母结尾其前面的元音字母发短元音的形容词的比较级和最高级是双写该 辅音字母然后再加-er和-est。 原形比较级最高级原形比较级最高级 big bigger biggest hot hotter hottest red redder reddest thin thinner thinnest 6) 双音节和多音节形容词的比较级和最高级需用more 和most 加在形容词前面来构 成。 原形比较级最高级 careful careful more careful most careful difficult more difficult most difficult delicious more delicious most delicious 7)常用的不规则变化的形容词的比较级和最高级: 原级比较级最高级 good better best 好的 well better best 身体好的 bad worse worst 坏的 ill worse worst 病的 many more most 许多 much more most 许多 few less least 少数几个 little less least 少数一点儿 （little littler littlest 小的） far further furthest 远（指更进一步，深度。亦可指更远） far farther farthest 远（指更远，路程）
用2个有时有时造句大全 用有时有时造句 1、天上的云彩变化多端，有时像一匹骏马在奔驰，有时像一条狗在摆头摇尾。 2、大白菜的价格有时高有时低。 3、小狗豆豆有时很温驯，有时又很顽皮。 4、在这个竞争激烈的社会里，我们有时会感到压力重重，而有时会感到生活很充实。 5、乐曲有时高昂，有时低沉。 6、鸟们有时在天空中展翅高飞，有时停在树枝上婉转啼叫，有时在林间欢蹦乱跳。 7、一分钟有时很长，有时又很短。 8、放学后，我有时直接回家，有时留在学校做作业，有时和同
学去打球。 9、课间休息时间，我有时去操场散步，有时和同学跳皮筋。 10、天上的月亮有时像一把镰刀，静静地挂在天空，懒得动一下；有时像一的大银盆，又圆又大的。 11、人生总是包含着五味杂陈，有时你会感到快乐，有时你会遭遇困难，有时你会期待梦想，有时也会充满感伤，必须每样都经历才算完整。 12、观看比赛的观众有时鼓掌为运动员加油，有时呐喊为选手鼓劲。 13、喷泉有时像百花怒放，有时像凤凰一飞冲天。 14、我的爸爸有时候严厉，有时候慈祥。 15、小猫咪有时淘气，有时可爱。 16、我有时可笑，有时悲伤。 17、在周末，小明有时在家里看电视，有时去爷爷奶奶家玩。 18、一群大雁往南飞，它们有时排成一字，有时排成人字。
19、天气千变万化，有时晴天，有时阴天。 20、人生际遇总是变幻莫测，有时如顺风行船，喜笑颜开；有时如雪中登山，凶险万分。 21、月亮有时像圆盘，有时像镰刀。 22、亮有时象圆盘，有是象镰刀，有时又象个光环。 23、我有时步行去学校，有时骑车去。 24、妈妈去外地出差了，我有时给妈妈打电话，有时我还给妈妈发短信。 25、这些天，她有时高兴，有时悲伤，有时又整天沉默无语，一定是发生了什么事。 26、生命如水，有时潺潺的流动，有时静静的盘桓。 27、仙女们在这片神奇的绿野上游戏，有时跳舞，有时歌唱，有时漫步，有时飞翔。 28、小鸟们有时在天空中展翅高飞，有时停在树枝上婉转啼叫。 29、他的课余生活丰富。有时打篮球，有时上网。
形容词比较级和最高级的形式 一、形容词比较级和最高级的构成 形容词的比较级和最高级变化形式规则如下 构成法原级比较级最高级 ①一般单音节词末尾加 er 和 est strong stronger strongest ②单音节词如果以 e结尾，只加 r 和 st strange stranger strangest ③闭音节单音节词如末尾只有一个辅音字母， 须先双写这个辅音字母，再加 er和 est sad big hot sadder bigger hotter saddest biggest hottest ④少数以 y, er(或 ure), ow, ble结尾的双音节词， 末尾加 er和 est(以 y结尾的词，如 y前是辅音字母， 把y变成i，再加 er和 est，以 e结尾的词仍 只加 r和 st) angry Clever Narrow Noble angrier Cleverer narrower nobler angriest cleverest narrowest noblest ⑤其他双音节和多音节词都在前面加单词more和most different more different most different 1) The most high 〔A〕mountain in 〔B〕the world is Mount Everest，which is situated 〔C〕in Nepal and is twenty nine thousand one hundred and fourty one feet high 〔D〕 . 2) This house is spaciouser 〔A〕than that 〔B〕white 〔C〕one I bought in Rapid City，South Dakota 〔D〕last year. 3) Research in the social 〔A〕sciences often proves difficulter 〔B〕than similar 〔C〕work in the physical 〔D〕sciences. 二、形容词比较级或最高级的特殊形式：
出现造句大全 导读：本文是关于出现造句大全，如果觉得很不错，欢迎点评和分享！ 1、你们同学白天上课时的音量比晚自习时小多了，这是极不正常的，如果这种情况出现在野生动物身上，那就意味着大的自然灾害即将到来。 2、我不是带着将失败的情绪走进赛场的。我回来的时候满怀信心，我相信仍然能够有所作为。如果我只是坐在这里听别人告诉我不能复出，那我肯定不会出现在这里。 3、小贝，我去向琛哥要我的明天，全身上下最值钱的东西，我都留给你，谢谢你给了我一个值得活下来的理由，为了这个理由，我愿意用生命去赌我们的未来，如果没回来，不要来找我，就当我从来没出现在你的生命里。 4、夏天的晚上，太阳太劳累了，又钻回了云朵被窝去睡觉了，星星和月亮，养足了精神，出现在了天空，发出动人的光芒，蟋蟀和没有睡觉的青蛙知了，在草丛里池塘里大树上唱着歌。 5、曾经也有一个笑容出现在我的生命里，可是最后还是如雾般消散，而那个笑容，就成为我中心深深埋藏的一条湍急河流，无法泅渡，那河流的声音，就成为我每日每夜绝望的歌唱。 6、知道自己出现在别人的梦里是很让人开心的事，这能证明你的存在，而且在某种程度上，还可以证明你在别的地方也具有实体和
价值。 7、正当我高兴的流下泪水时，闹钟忽然响了，它把我从梦中拉出来，才知道原来是场梦，但我坚信，在不久的将来，我一定会出现在奥运会上，为中国队努力拼搏。 8、世界上没有谁选择谁，只有谁遇到谁，所以我不选ABCD，只看着时间刚刚好的时候，出现在你面前，然后，牵着手，去哪里那里，然后，就是一辈子。 9、只见月亮像一个害羞的姑娘，羞答答地从一片乌云背后伸出半个脑袋，偷偷地向下窥探，发现没有什么动静，一扭身，出现在天空中，天空就好像出现了一盏明亮的灯，周围的乌云被白色的月光照着。 10、情痴先生，多谢你的痴情。对于我在你的梦中出现这件事，令我十分震惊。你没有征求我的同意，便让我出现在你的梦里，侵犯了我的自由，我保留所有法律上追究的权利跟赔偿。还有，你没有告诉我在梦里你对我做了什么？ 11、到19世纪70年代，那些实际生活中的作奸犯科者、道德沦丧者，竟然已经堂而皇之地出现在歌舞表演中，这一幕与如今八卦电视节目何其相似，文化垃圾商品的真人版而已！ 12、以前只是一种经历与感觉，而不是证据，不需要为以前的喜欢付出现在或以后的责任。不要揪住以前的事情不放。现在的事实比以前的回忆更有实效性与说服力。 13、法律不等于正义，这是一种非常不完美的机制，如果你按
比较级和最高级 1.在形容词词尾加上―er‖ ―est‖ 构成比较级、最高级： bright（明亮的）—brighter—brightest broad（广阔的）—broader—broadest cheap（便宜的）—cheaper—cheapest clean（干净的）—cleaner—cleanest clever（聪明的）—cleverer—cleverest cold（寒冷的）—colder—coldest cool（凉的）—cooler—coolest dark（黑暗的）—darker—darkest dear（贵的）—dearer—dearest deep（深的）—deeper—deepest fast（迅速的）—faster—fastest few（少的）—fewer—fewest great（伟大的）—greater—greatest hard（困难的,硬的）—harder—hardest high（高的）—higher—highest kind（善良的）—kinder—kindest light（轻的）—lighter—lightest long（长的）—longer—longest loud（响亮的）—louder—loudest low（低的）—lower—lowest near（近的）—nearer—nearest new（新的）—newer—newest poor（穷的）—poorer—poorest quick（快的）—quicker—quickest quiet（安静的）—quieter—quietest rich（富裕的）—richer—richest short（短的）—shorter—shortest slow（慢的）—slower—slowest small（小的）—smaller—smallest smart（聪明的）—smarter—smartest soft（柔软的）—softer—softest strong（强壮的）—stronger—strongest sweet（甜的）—sweeter—sweetest tall（高的）-taller-tallest thick（厚的）—thicker—thickest warm（温暖的）—warmer—warmest weak（弱的）—weaker—weakest young（年轻的）—younger—youngest 2.双写最后一个字母,再加上―er‖ ―est‖构成比较级、最高级： big（大的）—bigger—biggest fat（胖的）—fatter—fattest hot（热的）—hotter—hottest red（红的）—redder—reddest sad（伤心的）—sadder—saddest thin（瘦的）—thinner—thinnest wet（湿的）—wetter—wettest mad（疯的）—madder—maddest 3.以不发音的字母e结尾的形容词,加上―r‖ ―st‖ 构成比较级、最高级：able（能干的）—abler—ablest brave（勇敢的）—braver—bravest close（接近的）—closer—closest fine（好的,完美的）—finer—finest large（巨大的）—larger—largest late（迟的）—later—latest nice（好的）—nicer—nicest ripe（成熟的）—riper—ripest
一下子造句大全 导读：本文是关于一下子造句大全，如果觉得很不错，欢迎点评和分享！ 1、幼稚的眼睛常常看不清楚。小鸟怀着热烈的希望展翅向天空飞去，但一下子就碰着铁丝网落了下来。 2、瞧！当我给小金鱼喂鱼食时，有的鱼迫不及待地跳上来；好像鲤鱼跃龙门；有的鱼摇着头，翘着尾巴慢慢地吃鱼食；还有的鱼很胆小，专门吃别人剩下的。整个鱼缸一下子因为我这几个鱼食而沸腾了。 3、忽然，一颗豆大的雨点恰好重重地打在了这嫩生生的细条上。它猛地一颤，弯下了腰。我正担心它被打折了，不想它马上像摆脱了重担似的，一下子直起了腰，晃了两晃，又像先前一样，直立在树的断口处。 4、俗话说“六月的天，娃娃的脸说变就变。”刚刚还是晴空万里，一下子就变得阴沉沉的，乌云布满了整个天空，风也吹起来了，吹着树叶沙沙作响。不远处传来几声雷响，仿佛正告诉人们要下雷雨啦！ 5、小偷认为自己的谎言天衣无缝，一点破绽都没有，可**一下子就看出了他是坏人。 6、花瓣一片一片地绽放，花朵也不是一下子全开的，我明白昨天是与今天紧密相连的，好高兴。
7、过了一会儿，雨变小了，微风吹来，雨帘斜了，像一根根针线似的刺向草木、墙壁。雨落在小草上，看，草儿轻轻地在微风中摇动，雨珠顺着它那翠绿的茎滚下来，有一滴一下子钻到土里，又一滴钻到小草的嘴里，找不着了。 8、妈妈的手也是温柔的手。每当我生病了，妈妈便用她的手无微不至地照料着我，我感觉病一下子好了许多，心中无比温暖。每当我受到挫折感到委屈时，妈妈也会用她那双手轻轻拍拍我的肩膀，鼓励我，给我勇气和力量。 9、只见他嘴张得像箱子口那么大，一下子就愣住了，接着他就咽了两三口唾沫，好像是嗓子里发干似的。 10、突然门外传来嘈杂声，接着是敲门声，一下子又给大叫声所覆盖。我连忙去外面看看发生什麽事，当我看到我家门外的几个人时，我呆在那儿了，那陌生又熟悉的脸庞，都带着亲切的微笑，我不禁也露出微笑，请他们进家中。 11、早上起雾了，小溪里最浓，什么也看不见，我便高兴了，走下去，哪知它们一下子便将我拥围起来。慢慢地太阳升起，穿过树林，溪边的小草显得更加鲜嫩油滑了。我坐在木屋窗口下读书，让这一切陪我畅想。 12、蒋百里在宜山的南边路上，汽车坏了，他受了一下子凉，就有一点半身不遂的样子。 13、在座的人全都受到他影响，一下子也都变得严肃起来。就像钢水突然落到冷水里，一下子整个脸都凝固了。
英语语法---比较级和最高级的用法 在英语中通常用下列方式表示的词:在形容词或副词前加more(如 more natural,more clearly )或加后缀 -er(newer,sooner )。典型的是指形容词或副词所表示的质、量或关系的增加。英语句子中，将比较两个主体的方法叫做“比较句型”。其中，像“A比B更……”的表达方式称为比较级；而“A最……”的表达方式则称为最高级。组成句子的方式是将形容词或副词变化成比较级或最高级的形态。 一、形容词、副词的比较级和最高级的构成规则 1．一般单音节词和少数以-er，-ow结尾的双音节词，比较级在后面加-er，最高级在后面加-est； （1）单音节词 如：small→smaller→smallest short→shorter→shortest tall→taller→tallest great→greater→greatest （2）双音节词 如：clever→cleverer→cleverest narrow→narrower→narrowest 2．以不发音e结尾的单音节词，比较在原级后加-r，最高级在原级后加-st； 如：large→larger→largest nice→nicer→nicest able→abler→ablest 3．在重读闭音节（即：辅音＋元音＋辅音）中，先双写末尾的辅音字母，比较级加-er，最高级加-est； 如：big→bigger→biggest hot→hotter→hottest fat→fatter→fattest 4．以“辅音字母＋y”结尾的双音节词，把y改为i，比较级加-er，最高级加-est； 如：easy→easier→easiest heavy→heavier→heaviest busy→busier→busiest happy→happier→happiest 5．其他双音节词和多音节词，比较级在前面加more，最高级在前面加most； 如：beautiful→more beautiful→most beautiful different→more different→most different easily→more easily→most easily