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    CTLEquine Infectious Anemia Virus-Specific Recognition of Gag and Rev Epitopes byEquine MHC Class I Molecules Alters the -2 Domain of Two Naturally Occurring

    A Single Amino Acid Difference within the

    Littke and Travis C. McGuireRobert H. Mealey, Jae-Hyung Lee, Steven R. Leib, Matt H.

    http://www.jimmunol.org/content/177/10/7377doi: 10.4049/jimmunol.177.10.7377

    2006; 177:7377-7390; ;J Immunol

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  • A Single Amino Acid Difference within the -2 Domain of TwoNaturally Occurring Equine MHC Class I Molecules Alters theRecognition of Gag and Rev Epitopes by Equine InfectiousAnemia Virus-Specific CTL1

    Robert H. Mealey,2* Jae-Hyung Lee, Steven R. Leib,* Matt H. Littke,* and Travis C. McGuire*

    Although CTL are critical for control of lentiviruses, including equine infectious anemia virus, relatively little is known regardingthe MHC class I molecules that present important epitopes to equine infectious anemia virus-specific CTL. The equine class Imolecule 7-6 is associated with the equine leukocyte Ag (ELA)-A1 haplotype and presents the Env-RW12 and Gag-GW12 CTLepitopes. Some ELA-A1 target cells present both epitopes, whereas others are not recognized by Gag-GW12-specific CTL, sug-gesting that the ELA-A1 haplotype comprises functionally distinct alleles. The Rev-QW11 CTL epitope is also ELA-A1-restricted,but the molecule that presents Rev-QW11 is unknown. To determine whether functionally distinct class I molecules presentELA-A1-restricted CTL epitopes, we sequenced and expressed MHC class I genes from three ELA-A1 horses. Two horses had the7-6 allele, which when expressed, presented Env-RW12, Gag-GW12, and Rev-QW11 to CTL. The other horse had a distinct allele,designated 141, encoding a molecule that differed from 7-6 by a single amino acid within the -2 domain. This substitution did notaffect recognition of Env-RW12, but resulted in more efficient recognition of Rev-QW11. Significantly, CTL recognition of Gag-GW12 was abrogated, despite Gag-GW12 binding to 141. Molecular modeling suggested that conformational changes in the141/Gag-GW12 complex led to a loss of TCR recognition. These results confirmed that the ELA-A1 haplotype is comprised offunctionally distinct alleles, and demonstrated for the first time that naturally occurring MHC class I molecules that vary by onlya single amino acid can result in significantly different patterns of epitope recognition by lentivirus-specific CTL. The Journal ofImmunology, 2006, 177: 73777390.

    I nfections by lentiviruses induce virus-specific CTL responsesthat are critical for control of viral load and clinical disease.Specifically, Env, Gag, and Rev proteins are important tar-gets for CTL in HIV-1-infected individuals (14). SustainedHIV-1 Gag-specific CTL responses are associated with very lownumbers of infected CD4 T cells and stable CD4 T cell countsin long-term nonprogressors, whereas loss of Gag-specific CTL isassociated with clinical progression to AIDS (5). Moreover, HIV-1Gag-specific CTL responses and frequency are inversely corre-lated with viral load (68), and high frequencies of Gag-specificCTL are significantly associated with slower disease progression(9). In addition, high levels of HIV-1 Env- and Gag-specific mem-ory CTL are strongly associated with low viral load and lack ofdisease in long-term nonprogressors (10), and CTL responses di-rected against the early expressed viral regulatory protein Rev in-versely correlate with rapid HIV-1 disease progression (11, 12).

    Equine infectious anemia virus (EIAV)3 is a macrophage-tropiclentivirus that causes persistent infections in horses worldwide. Incontrast to HIV-1 infection however, most EIAV-infected horseseventually control plasma viremia and clinical disease and remainlife-long inapparently infected carriers (1315). The initial as wellas the long-term control of viremia and clinical disease is the resultof adaptive immune responses, including neutralizing Ab and im-portantly, CTL (1622). Due to these robust viral-specific immuneresponses that contain viral replication, EIAV infection in horses isa unique and useful model system for the study of lentiviral im-mune control.

    As in HIV-1, Env, Gag, and Rev proteins are important CTLtargets in EIAV-infected horses. EIAV Env- and Gag-specific CTLare detected during acute and inapparent infection (17, 23, 24),with Gag-specific CTL responses occurring in the majority ofhorses tested (25, 26). Although EIAV Env-specific CTL can beimmunodominant and may be important in early viral control, viralescape can limit the effectiveness of CTL directed against variableEnv epitopes, and Gag-specific CTL frequency can correlate withthe ability to control viral load and clinical disease (27, 28).Epitope clusters occur within EIAV Gag proteins that are recog-nized by CTL (including high-avidity CTL) in horses with dispar-ate MHC class I (MHC I) haplotypes and are likely important incontrol of clinical disease (29, 30). Additionally, moderate andhigh-avidity Rev-specific CTL are associated with control of vire-mia and disease in EIAV-infected nonprogressor horses (27). Sig-nificantly, no viral escape from high-avidity Rev-specific CTL hasbeen observed. The Rev epitope recognized by these CTL is highly

    *Department of Veterinary Microbiology and Pathology, Washington State Univer-sity, Pullman, WA 99164; and Bioinformatics and Computational Biology Program,Iowa State University, Ames, IA 50011

    Received for publication January 17, 2006. Accepted for publication August 30, 2006.

    The costs of publication of this article were defrayed in part by the payment of pagecharges. This article must therefore be hereby marked advertisement in accordancewith 18 U.S.C. Section 1734 solely to indicate this fact.1 This work was supported in part by U.S. Public Health Service, National Institutesof Health Grants AI058787 (to R.H.M. and T.C.M.), AI067125 (to R.H.M. andT.C.M.), AI060395 (to T.C.M. and R.H.M.), CA97936 (to J.L.), U.S. Department ofAgriculture National Research Initiative Grant 2002-35204-12699 (to J.L.), and theCenter for Integrated Animal Genomics at Iowa State University (to J.L.).2 Address correspondence and reprint requests to Dr. Robert H. Mealey, Departmentof Veterinary Microbiology and Pathology, Washington State University, Pullman,WA 99164-7040. E-mail address: rhm@vetmed.wsu.edu

    3 Abbreviations used in this paper: EIAV, equine infectious anemia virus; MHC I,MHC class I; 2m, 2-microglobulin; ELA, equine leukocyte Ag; EK, equine kidney.

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  • conserved among other strains of EIAV, and independent studiesby other investigators show that this sequence does not changeduring long-term EIAV infection (31, 32).

    Given that CTL epitopes in Env, Gag, and Rev are critical forlentiviral immune control, factors that affect the presentation and rec-ognition of these peptides by CTL in a population of infected indi-viduals are important considerations for vaccine development. One ofthe most important factors is MHC I polymorphism, because CTLrecognize peptide epitopes only in the context of allelic forms ofMHC I molecules. The CTL TCR binds to a cell surface MHC Icomplex consisting of the MHC I molecule, a peptide processed froma viral protein, and 2-microglobulin (2m) (33). The HLA classicalMHC I genes are the products of three loci, and as of July 2006, therewere 478 HLA-A, 805 HLA-B, and 256 HLA-C named alleles (www.ebi.ac.uk/imgt/hla/intro.html) (34). MHC I polymorphism occursprimarily in the residues that form the peptide-binding domain, there-fore influencing the types of peptides that are selectively bound. Sim-ilarities in peptide-binding specificity between human MHC I mole-cules have been identified based on the structure of peptide-bindingpockets, peptide-binding assays, and analysis of motifs, and