Acute rejection in the absence of cognate recognition of allograft by T cells.

We studied the effects of the indirect pathway of allograft recognition using T cells from TCR transgenic Marilyn mice, which recognize the male Ag H-Y in an I-A(b)-restricted fashion. The T cells are not alloreactive to the H-2(k) haplotype, because they are not activated when adoptively transferred into recombinase-activating gene-2(-/-) common gamma-chain(-/-) double-mutant H-2(k) male or female mice. However, skin from H-2(k) males, but not from H-2(k) females, is acutely rejected by recombinase-activating gene-2(-/-) transgenic female recipients. In vitro, Marylin spleen cells primed by H-2(k) skin grafting proliferated and secreted both IL-4 and IFN-gamma in response to H-2(k) male stimulators. However, the removal of H-2(b) APC from the responding population abolished the response. Taken together, these results show that the indirect recognition that triggers rejection in this model is due to the recognition of H-Y Ag shed from H-2(k) male allograft and presented by the recipient's own I-A(b) APC to transgenic T cells. This study demonstrates unequivocally the capacity of naive CD4(+) T cells to promote the rejection of allografts through mechanisms that involve indirect destruction of grafted tissues.