Different target antigens for T-cell subsets acting synergistically in vivo

After allogenic transplantation of lymphoid cells into immunologically incompetent recipients, a graft-versus-host (GvH) reaction can occur. The original observation that the GvH reaction is mediated by two interacting types of T cell has led to the proposal that T cells must be subdivided into two subpopulations according to life span and migratory properties. These consist of T1 cells, which are short-lived, sessile cells sensitive to adult thymectomy (ATx), and T2 cells, which are long-lived, recirculating cells sensitive to anti-thymocyte serum (ATS). T1-T2 cooperation has also been demonstrated in vitro in murine and rat mixed lymphocyte culture. The question arises as to whether these T-cell subpopulations are activated by different or identical parts of the major histocompatibility complex (MHC). We report here that for optimal development of the anti-host immune response in a murine GvH reaction, T2 cells have to be amplified by T1 cells. The former cells are activated by a set of MHC gene products that are expressed mainly on immunological cells (H-2I-coded antigens), whereas the latter recognize a different set of MHC gene products that are expressed on almost all cells of the mouse (H-2K/D-coded antigens).