Differential requirements for proliferation of CD4+ and γδ+ T cells to spirochetal antigens

αβ+ and γδ+ T cells have different mechanisms of epitope recognition and are stimulated by antigens of different chemical nature. An immunization model with antigens from the spirochete Brachyspira hyodysenteriae was used to examine the requirements for proliferation of circulating porcine CD4+ and γδ+ T cells in mixed lymphocyte cultures. CD4+ T cells only responded to stimulation with B. hyodysenteriae antigens, whereas γδ+ T cells proliferated when cultures were stimulated with either spirochetal antigens or interleukin-2 (IL-2). T cells that had proliferated expressed high levels of IL-2-receptor-α (IL-2Rα). Furthermore, neutralization of IL-2 at the beginning of the culture period was more efficient in blocking γδ+ than CD4+ T cell proliferation. Immunization induced interferon-γ (IFN-γ) production by CD4+ T cells, whereas only a small fraction of the antigen-stimulated γδ+ T cells produced this cytokine. Our results indicate that, under the same environmental conditions, CD4+ T cell functions are more tightly regulated when compared to γδ+ T cells. We conclude that these differences are due, in part, to the enhanced γδ+ T cell responsiveness to IL-2.