Horse immunized with Th1 and CTL lipopeptides experience reduced clinical disease following lentivirus challenge

Equine infectious anemia virus (EIAV) is a lentivirus with a genome organization and characteristic persistent infection similar to other lentiviruses including HIV-1, although EIAV does not infect T lymphocytes. Signs of clinical EIAV disease include fever, thrombocytopenia and anemia, accompanied by detectable plasma viremia. This viremia drops significantly 3-4 weeks after infection, when virus-specific CTL responses appear, but before neutralizing antibodies are present. Recurrent episodes of acute disease, associated with antigenic variants, often occur the first year, but most horses enter a life-long inapparent carrier state, resembling HIV-1-infected longterm non-progressors. Cellular immune responses are critical to control lentiviral infections, and successful defense usually depends on coordinated CTL and helper T (Th) lymphocyte responses, particularly Th1 responses. Thus, inducing Th1 responses is part of current vaccine designs where CTL responses are desired. Nonetheless, it is difficult to define immunologically relevant epitopes broadly-recognized in the affected population, and that are highly conserved within and across virus strains. This difficulty is compounded in horses by our limited ability to type equine MHC alleles. Still, broadly recognized epitopes apparently exist, particularly among class II restricted epitopes, and several studies demonstrated that immunizing with Th1 epitopes alone enhanced both Th1 and CTL responses after infection. We identified three highly conserved EIAV peptides (2Gag, 1Pol) that promoted Th1 responses ex vivo in PBMC from class II-disparate, EIAV carrier horses. An ELA-A1 class I-restricted CTL epitope in the envelope surface unit protein (EIAV-SU) was also previously identified. The current studies sought to determine if immunization with Th1 epitopes or a CTL epitope would afford protection against EIAV challenge. To that end, three class II-disparate horses were immunized with lipidated versions of the Th1 peptides described above. Lipopeptides were weakly immunostimulatory, however two of three immunized horses had transient responses. Concurrently, three other ELA-A1 horses were immunized with a lipidated version of the ELA-A1 EIAV-SU CTL epitope. Peptide-specific ELA-A1-restricted CTL were induced in all three horses, but again the responses were transient. These six immunized plus three unimmunized horses were challenged intravenously with 300 50% tissue culture infective doses of EIAV (PV strain). After challenge, all horses developed plasma viremia, fever and thrombocytopenia. Comparing Th1 lipopeptide-immunized with unimmunized horses, only the two horses responding to immunization had early, sustained T lymphocyte proliferative responses. Furthermore, these proliferative responses were significantly, positively correlated with reduced EIAV disease (Spearman's rank order correlation coefficient, rs=0.917, p<0.05). Comparing CTL lipopeptide-immunized with unimmunized horses, statistically lower fever and thrombocytopenia severity scores occurred in immunized horses (Mann-Whitney one-tailed U test, p=0.05). Thus, our results indicated that both Th1 and CTL lipopeptide immunization alone moderated clinical disease following virus challenge. While lipopeptide immunization of horses appeared only weakly immunostimulatory, the specific immunization regimen chosen may be inadequate to elicit strong immune responses. Moreover, although immunizing with CTL or Th1 epitopes alone may be beneficial in moderating early stages of EIAV disease, co-immunization with Th1 and CTL epitopes will likely be more effective. Our ongoing studies address these issues. (Texte intégral)