The CD4 + T cell immunodominant Anaplasma marginale major surface protein 2 stimulates γδ T cell clones that express unique T cell receptors

Major surface protein 2 (MSP2) of the bovine rickettsial pathogen Anaplasma marginale is an abundant, serologically immunodominant outer membrane protein. Immunodominance partially results from numerous CD4 + T cell epitopes in highly conserved amino and carboxy regions and the central hypervariable region of MSP2. However, in long‐term cultures of lymphocytes stimulated with A. marginale , workshop cluster 1 (WC1) + γδ T cells and CD4 + αβ T cells proliferated, leading to a predominance of γδ T cells. As γδ T cells proliferate in A. marginale‐ stimulated lymphocyte cultures, this study hypothesized that γδ T cells respond to the abundant, immunodominant MSP2. To test this hypothesis, γδ T cell clones were isolated from MSP2 vaccinates and assessed for antigen‐specific proliferation and interferon‐γ secretion. Seven WC1 + γδ T cell clones responded to A. marginale and MSP2, and three of these proliferated to overlapping peptides from the conserved carboxy region. The γδ T cell response was not major histocompatibility complex‐restricted, although it required antigen‐presenting cells and was blocked by addition of antibody specific for the T cell receptor (TCR). Sequence analysis of TCR‐γ and ‐δ chains of peripheral blood lymphocytes identified two novel TCR‐γ chain constant (C γ ) regions. It is important that all seven MSP2‐specific γδ T cell clones used the same one of these novel C γ regions. The TCR complementarity‐determining region 3 was less conserved than those of MSP2‐specific CD4 + αβ T cell clones. Together, these data indicate that WC1 + γδ T cells recognize A. marginale MSP2 through the TCR and contribute to the immunodominant response to this protein.