Lymphocyte proliferation responses induced to broadly reactive Th peptides did not protect against equine infectious anemia virus challenge

Abstract

The effect of immunization with five lipopeptides, three containing T-helper (Th) epitopes and two with both Th and cytotoxic T-lymphocyte (CTL) epitopes, on equine infectious anemia virus (EIAV) challenge was evaluated. Peripheral blood mononuclear cells from EIAV lipopeptide-immunized horses had significant proliferative responses to Th peptides compared with those preimmunization, and the responses were attributed to significant responses to peptides Gag from positions 221 to 245 (Gag 221-245), Gag 250-269, and Pol 326-347; however, there were no consistent CTL responses. The significant proliferative responses in the EIAV lipopeptide-immunized horses allowed testing of the hypothesis that Th responses to immunization would enhance Th and CTL responses following EIAV challenge and lessen the viral load and the severity of clinical disease. The EIAV lipopeptide-immunized group did have a significant increase in proliferative responses to Th peptides 1 week after virus challenge, whereas the control group did not. Two weeks after challenge, a significant CTL response to virus-infected cell targets occurred in the EIAV lipopeptide-immunized group compared to that in the control group. These Th and CTL responses did not significantly alter either the number of viral RNA copies/ml or disease severity. Thus, lipopeptide-induced proliferative responses and enhanced Th and CTL responses early after virus challenge were unable to control challenge virus load and clinical disease.

FIG. 1.

CTL activities of PBMCs from horses before immunization (prepeptide), after five lipopeptide immunizations (previrus), and following EIAV_PV challenge (weeks 1 to 5). The first five horses listed in each legend (H637, H640, H641, H646, and H651) were immunized with EIAV lipopeptides, and the next four horses (H643, H648, H649, and H652) were immunized with an Anaplasma control lipopeptide. (a) EK cell targets were EIAV_PV infected, and prepeptide and previrus PBMC effectors were stimulated with EIAV_PV, whereas PBMCs obtained after challenge (weeks 1 to 5) were stimulated with EIAV_WSU5. (b) The Gag 13-32 peptide was used to stimulate PBMCs and pulse EK cell targets. (c) The Env_PV 195-206 peptide was used to stimulate PBMCs and pulse EK cell targets. In panel a, the percent specific lysis of noninfected targets was subtracted from the percent specific lysis of infected targets; in panels b and c, the percent specific lysis of non-peptide-pulsed targets was subtracted from that of peptide-pulsed targets. The corrected specific lysis in this figure of ≥10% was considered a significant positive assay result, as these assay results were also ≥3 SEs above those for the noninfected or nonpulsed targets. Peptide targets for H637 previrus had levels of spontaneous lysis that were too high and were not used. Neither H637 PBMCs nor H652 PBMCs were available for testing at week 5 (a and b), and no assay was performed in week 5 with EnvPV 195-206 (c). The effector cell-to-target cell ratio was 20:1.

FIG. 2.

Disease parameters for EIAV lipopeptide-immunized (a) and control lipopeptide-immunized (b) horses following EIAV challenge. Morning rectal temperatures (in °F) are on the left scale (closed squares) and periodic platelet counts × 1,000 per μl blood are on the right scale (open circles). Rectal temperatures above the dark solid horizontal line were considered fevers, and platelet counts below the lighter dotted horizontal line were considered thrombocytopenic.