CD8⁺ but Not CD4⁺ T Cells Require Cognate Interactions with Target Tissues To Mediate GVHD across Only Minor H Antigens but CD4⁺ and CD8⁺ T Cells Both Require Direct Leukemic Contact for GVL.

There has been debate as to whether CD4⁺ or CD8⁺ T cells require direct cognate interactions with their tissue targets to mediate GVHD. In one report, GVHD mediated by CD4 or CD8 cells did not require target tissue MHCI or MHCII expression in MHC-disparate models (Teshima, et al 2002). However, GVHD lethality may have been due to cytokines produced by high-frequency alloreactive T cells primed by MHC-disparate host APCs. In contrast, MHC-matched, multiple minor H antigen (miHA) disparate GVHD mediated by CD4 or CD8 cells has been reported to be reduced or absent in host → donor chimeric recipients of a second transplant with donor BM and T cells. In this case, host antigen was restricted to hematopoietic cells and the authors concluded that tissue miHA expression must be required (Korngold and Sprent, 1982; Jones et al, 2003). However these experiments did not address whether T cells directly interacted with MHC on target tissues. Rather they demonstrated that a continuing source of host antigen was essential. To resolve this we created bone marrow chimeras in which hematopoiesis was wild type (wt) while the parenchyma was either MHC I^- (B6→B6 beta-2-microglobulin^−/− (β2M^−/−)) or MHC II⁻ (B6→B6 IA^b−/−) and used these chimeras as recipients in GVHD-inducing second transplants. We found that B6→B6β2M^−/− chimeras were completely resistant clinically and pathologically to CD8-mediated GVHD induced by a second transplant with C3H.SW (H-2b) BM and CD8⁺ T cells whereas control B6→B6 chimeras developed severe disease. Thus, CD8 cells require direct cognate interactions with target host tissues to induce MHC-matched, miHA disparate GVHD. In contrast, B6→B6 IA^b−/− chimeras developed similar clinical and histologic GVHD (liver, ear, skin and bowel) as did control B6→B6 chimeras when retransplanted with 129/J (H-2b) bone marrow and purified CD4⁺ T cells. Notably we observed lymphocytic infiltrates in involved organs. Therefore, CD4 cells can mediate tissue damage without directly recognizing alloantigen presented by MHCII on target epithelial cells. This suggests an indirect mechanism, perhaps mediated by T cell release of factors after stimulation in tissues by donor-derived APCs presenting host antigens. Alternatively, donor CD4 cells may activate miHA-bearing macrophages to release inflammatory mediators. To investigate the requirement for cognate recognition in GVL, we created murine CML via retroviral-mediated bcr-abl (p210) transduction of bone marrow from wt B6, B6 IA^b−/− and B6 β2M^−/− mice. Using the C3H.SW→B6 and 129→B6 GVHD models we found that both CD8-and CD4-mediated GVL requires leukemic expression of MHCI and MHCII, respectively. Thus both CD8-mediated GVHD and GVL required cognate T cell:target interactions. However, for CD4 cells only GVL, but not GVHD required target cell MHCII expression. This indicates that CD4-mediated GVL and GVHD have distinct mechanisms of action. Further understanding of these may provide insight in how to deliver GVL with less GVHD.