The Hypervariable Region of Anaplasma marginale Major Surface Protein 2 (MSP2) Contains Multiple Immunodominant CD4+ T Lymphocyte Epitopes That Elicit Variant-Specific Proliferative and IFN-γ Responses in MSP2 Vaccinates

Major surface protein 2 (MSP2) is an immunodominant outer membrane protein of Anaplasma marginale and Anaplasma phagocytophilum pathogens that cause bovine anaplasmosis and human granulocytic ehrlichiosis, respectively. MSP2 has a central hypervariable region (HVR) flanked by highly conserved amino and carboxyl termini. During A. marginale infection, dynamic and extensive amino acid sequence variation in MSP2 occurs through recombination of msp2 pseudogenes into the msp2 expression site, followed by sequential segmental gene conversions to generate additional variants. We hypothesized that MSP2 variation leads to significant changes in Th cell recognition of epitopes in the HVR. T cell epitopes were mapped using T cells from native MSP2-immunized cattle and overlapping peptides spanning the most abundant of five different MSP2 HVRs in the immunogen. Several epitopes elicited potent effector/memory Th cell proliferative and IFN-gamma responses, including those in three discreet blocks of sequence that undergo segmental gene conversion. Th cell clones specific for an epitope in the block 1 region of the predominant MSP2 variant type failed to respond to naturally occurring variants. However, some of these variants were recognized by oligoclonal T cell lines from MSP2 vaccinates, indicating that the variant sequences contain immunogenic CD4(+) T cell epitopes. In competition/antagonism assays, the nonstimulatory variants were not inhibitory for CD4(+) T cells specific for the agonist peptide. Dynamic amino acid sequence variation in MSP2 results in escape from recognition by some effector/memory MSP2-specific Th cells. Antigenic variation in MSP2 Th cell and B cell epitopes may contribute to immune evasion that allows long-term persistence of A. marginale in the mammalian reservoir.