| Organisms in the genus Anaplasma express an immunodominant major surface protein 2 (MSP2), composed of a hypervariable region (HVR) flanked by conserved regions. MSP2 undergoes antigenic variation by gene conversion of msp2 pseudogenes and pseudogene segments into a single expression site. Throughout A. marginale infection, recombination results in the sequential appearance of novel MSP2 variants and subsequent control of rickettsemia by the immune response. We hypothesized that T- and B-cell responses would be directed predominantly against epitopes within the HVR, which could facilitate immune evasion and persistent infection. Similar numbers of MSP2-specific T-cell epitopes were in conserved and hypervariable regions, and responses were of comparable magnitude. Epitope clusters recognized by the majority of animals were identified in the HVR (aa 171--229), and conserved regions (aa 101--170 and as 272--361). Linear B-cell epitopes were concentrated in the HVR, residing within hydrophilic sequences. This pattern of T-lymphocyte recognition of specific epitope clusters and dominant B-lymphocyte recognition of HVR epitopes was observed regardless of the overall magnitude of the individual MSP2-specific responses, consistent with protein structure influencing the patterns of epitope recognition.; Furthermore, we hypothesized that challenge of cattle with A. marginale expressing MSP2 variants to which the animals had been immunized, would stimulate variant epitope-specific recall CD4+ T-cell and IgG responses and variant-specific organism clearance. IFN-gamma secreting cells, proliferation, and IFN-gamma production were measured in response to 27 overlapping peptides spanning MSP2. Immunized calves were challenged with organisms expressing the same major MSP2 variants. Although immunization elicited robust MSP2-specific CD4+ T-cell and IgG responses, animals were not protected. Acute rickettsemia was not due to the emergence of organisms expressing minor or novel MSP2 variants. MSP2-specific T-lymphocyte responses were strong initially, but surprisingly, were insignificant at the peak of rickettsemia, and thereafter. The suppressed T-lymphocyte response was specific for A. marginale, as responses to Clostridium vaccine antigen were observed at all times. Neither an increase in CD4+ CD25+ T cells or suppressive cytokines IL-10 and TGF-beta1, nor a suppressive effect of non-responding cells was observed. Thus, we conclude that A. marginale infection may have caused peripheral deletion of MSP2-specific T cells, suggesting a novel mechanism for A. marginale persistence. |
