The direct pathway of human T-Cell allorecognition is not tolerized by stimulation with allogeneic peripheral blood mononuclear cells irradiated with high-dose ultraviolet B

Transfusions of high-dose (> or =10,000 Joule/m(2)) ultraviolet-B (UVB)-irradiated allogeneic leukocytes in rodent models have been shown to induce immunologic tolerance that is mediated by allospecific regulatory CD4(+) T cells. Whether these regulatory T cells recognize alloantigens through the direct or indirect pathway of allorecognition is controversial. Here, we demonstrate that the proliferative response obtained in standard primary mixed leukocyte reactions (MLRs) with human peripheral blood mononuclear cells (PBMCs) reflected a CD4(+) T-cell-dependent direct pathway of allorecognition and that high-dose UVB irradiation of PBMCs totally inhibited their capacity to induce a proliferative alloresponse. Re-stimulation with gamma-irradiated PBMCs from the same allogeneic donor (secondary MLR) elicited a proliferative and Th1-deviated response that was similar to the response induced in unprimed PBMCs. Finally, high-dose UVB was found to induce a rapid and massive apoptosis of irradiated PBMCs. Collectively, these data indicate that leukocytes irradiated with high-dose UVB are unable to prime for unresponsiveness or immune deviation in T cells directly recognizing allogeneic major histocompatibility complex molecules. Because it is well-established that antigens within transfused apoptotic cells are captured by resident tolerogenic spleen dendritic cells, we propose that tolerance induced by transfusions of high-dose UVB-irradiated leukocytes primarily involve T cells indirectly recognizing alloantigens.