CD4(+) T lymphocytes from calves immunized with Anaplasma marginale major surface protein 1 (MSP1), a heteromeric complex of MSP1a and MSP1b, preferentially recognize the MSP1a carboxyl terminus that is conserved among strains

Native major surface protein 1 (MSP1) of the ehrlichial pathogen Anaplasma marginale induces protective immunity in calves challenged with homologous and heterologous strains. MSPI is a heteromeric complex of a single MSP1a p rotein covalently associated with MSP1b polypeptides, of which at least two (designated MSP1F1 and MSP1F3) in the Florida strain are expressed. Immuni zation with recombinant MSP1a and MSP1b alone or in combination fails to pr ovide protection. The protective immunity in calves immunized with native M SP1 is associated with the development of opsonizing and neutralizing antib odies, but CD4(+) T-lymphocyte responses have not been evaluated. CD4(+) T lymphocytes participate in protective immunity to ehrlichial pathogens thro ugh production of gamma interferon (IFN-gamma), which promotes switching to high-affinity immunoglobulin G (IgG) and activation of phagocytic cells to produce nitric oxide. Thus, an effective vaccine for A. marginale and rela ted organisms should contain both T- and B-lymphocyte epitopes that induce a strong memory response that can be recalled upon challenge with homologou s and heterologous strains. This study was designed to determine the relati ve contributions of MSP1a and MSP1b proteins, which contain both variant an d conserved amino acid sequences, in stimulating memory CD4(+) T-lymphocyte responses in calves immunized with native MSP1. Peripheral blood mononucle ar cells and CD4(+) T-cell lines from MSP1-immunized calves proliferated vi gorously in response to the immunizing strain (Florida) and heterologous st rains of A. marginale. The conserved MSPI-specific response was preferentia lly directed to the carboxyl-terminal region of MSP1a, which stimulated hig h levels of IFN-gamma production by CD4(+) T cells. In contrast, there was either weak or no recognition of MSP1b proteins. Paradoxically, all calves developed high titers of IgG antibodies to both MSP1a and MSP1b polypeptide s. These findings suggest that in calves immunized with MSP1 heteromeric co mplex, MSP1a-specific T lymphocytes may provide help to MSP1b-specific B ly mphocytes. The data provide a basis for determining whether selected MSP1a CD4(+) T-lymphocyte epitopes and selected MSP1a and MSP1b B-lymphocyte epit opes presented on the same molecule can stimulate a protective immune respo nse.