Identification of a bovine surface antigen uniquely expressed on CD4⁻CD8⁻ T cell receptor γ/δ⁺ T lymphocytes

In this study, two monoclonal antibodies, IL‐A29 and CC15, are described that identify a novel bovine cell surface marker of 215/300 kDa. The antibodies reacted with a discrete population of resting lymphocytes in peripheral blood which, in young animals, constituted about 25% of the mononuclear cells. Thymus, lymph nodes and spleen contained < 5% positive cells. These cells were negative for surface Ig, a monocyte/granulocyte marker, and the T lymphocyte antigens CD2, CD6, CD4 and CD8. Immunohistological analyses revealed the presence of IL‐A29/CC15‐positive lymphocytes in the thymic medulla, in the outer cortex of lymph nodes, in the marginal zones of the spleen, in the dermal and epidermal layers of the skin and in the lamina propria of the gut. The IL‐A29/CC15⁺ cells in unfractionated blood mononuclear cells responded in autologous and allogeneic mixed lymphocyte cultures, and when purified they responded to concanavalin A in the presence of recombinant interleukin 2. These observations suggested this population of cells belonged to the T cell lineage. In order to unambiguously define their lineage, cDNA clones encoding bovine T cell receptor (TcR) and CD3 proteins were isolated. Northern blot analyses of IL‐A29/CC15⁺ cell populations and of established cell lines of various lineages demonstrated that they expressed TcR δ and CD3γ, δ and ϵ mRNA; TcR α was not expressed, whereas only a truncated form of TcR β mRNA was present. These results indicate that the IL‐A29 and CC15 antibodies define a unique population of CD4⁻CD8⁻, γ/δ T cells.