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. MSP1 is a heteromeric complex of a single MSP1a protein covalently associated with MSP1b polypeptides, of which at least two (designated MSP1F1 and MSP1F3) in the Florida strain are expressed. Immunization with recombinant MSP1a and MSP1b alone or in combination fails to provide protection. The protective immunity in calves immunized with native MSP1 is associated with the development of opsonizing and neutralizing antibodies, but CD4(+) T-lymphocyte responses have not been evaluated. CD4(+) T lymphocytes participate in protective immunity to ehrlichial pathogens through 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 related organisms should contain both T- and B-lymphocyte epitopes that induce a strong memory response that can be recalled upon challenge with homologous and heterologous strains. This study was designed to determine the relative contributions of MSP1a and MSP1b proteins, which contain both variant and conserved amino acid sequences, in stimulating memory CD4(+) T-lymphocyte responses in calves immunized with native MSP1. Peripheral blood mononuclear cells and CD4(+) T-cell lines from MSP1-immunized calves proliferated vigorously in response to the immunizing strain (Florida) and heterologous strains of A. marginale. The conserved MSP1-specific response was preferentially directed to the carboxyl-terminal region of MSP1a, which stimulated high 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 polypeptides. These findings suggest that in calves immunized with MSP1 heteromeric complex, MSP1a-specific T lymphocytes may provide help to MSP1b-specific B lymphocytes. The data provide a basis for determining whether selected MSP1a CD4(+) T-lymphocyte epitopes and selected MSP1a and MSP1b B-lymphocyte epitopes presented on the same molecule can stimulate a protective immune response.