CD4-positive helper T lymphocytes mediate mouse cardiac allograft rejection independent of donor alloantigen specific cytotoxic T lymphocytes.

Mouse heterotopic cardiac allograft recipients were depleted of CD4+ or CD8+ T lymphocytes in vivo to assess cellular requirements for graft infiltration, tissue damage, and acute allograft rejection. Modified limiting dilution analysis was employed to quantitate IL-2-producing Th lymphocytes (HTL) and CTL infiltrating the graft. Results were correlated with graft function and histologic evidence of tissue damage. In unmodified recipients, large numbers of donor alloantigen-specific CTL infiltrated the graft, overshadowing a modest number of HTL. CTL infiltration coincided with tissue damage and loss of graft function, suggesting a key role for CTL in rejection. In vivo treatment with anti-CD4 mAb inhibited both HTL and CTL infiltration, and no histologic evidence of tissue damage was observed. This observation suggested that HTL, although few in number, regulated the development of effector CTL and/or entry of these CTL into the graft. Reconstitution of HTL-depleted recipients with IL-2 resulted in graft infiltration by stimulated CTL, as assessed by modified limiting dilution analysis. However, these stimulated CTL failed to mediate tissue damage, and graft survival was prolonged. Unlike CTL obtained from unmodified recipients, graft-infiltrating CTL of IL-2-reconstituted mice were incapable of directly lysing donor cells in a 51Cr release assay. Hence, while IL-2 facilitated partial CTL differentiation and mobilization to the graft, additional signals appear necessary for maturation into lytic CTL. Furthermore, in recipients depleted of CTL by treatment with anti-CD8 mAb, HTL infiltrating the allograft, though few in number, were associated with extensive tissue damage and loss of graft function. These data suggest a less important role for CTL in the rejection process, and indicate that graft-infiltrating CTL are insufficient as sole mediators of cardiac allograft rejection. Potential mechanisms by which CD4+ HTL mediate cardiac allograft rejection independent of CTL are discussed.