The Progression of Graft Versus Host Disease Is Characterized by the Breaking of Tolerance to Self Antigens and Is Dependent upon Donor Antigen Presenting Cells.

The progression of acute to chronic graft versus host disease (GVHD) is characterized by the development of clinical manifestations that bear close resemblance to what is observed in autoimmune disorders. A longstanding unresolved issue is how acute GVHD which is initiated by alloreactive donor T cells evolves into chronic GVHD where autoreactive donor T cells appear to play a major role in the pathophysiology of this syndrome. We hypothesized that during the course of GVHD, there is breaking of tolerance such that donor T cells acquire the ability to recognize self antigens. To test this hypthesis, we examined the ability of donor T cells obtained from mice undergoing GVHD to respond to both donor and recipient antigens using an adoptive transfer model. Lethally irradiated Balb/c [H-2^d] mice were transplanted with BM and spleen cells from MHC-mismatched C57BL/6 (B6)[H-2^b] mice to induce severe GVHD. Spleen cells from fully donor T cell engrafted GVHD mice were then adoptively transferred into unirradiated B6 or Balb Rag animals. Adoptive transfer of GVHD spleen cells into B6-Rag animals resulted in either colitis or pulmonary inflammation in a >90% of recipients. Colitis was characterized by infiltration of mononuclear and granulocytic cells into the lamina propria with transmural extension and mucosal sloughing, while inflammation in the lung was characterized by pronounced perivascular and peribronchial mononuclear cell cuffing accompanied by areas of extensive pneumonitis.Immunohistochemical staining demonstrated significant T cell infiltration into damaged tissue in both the colon and lungs of affected animals. Neither Balb Rag nor third party C3H-Rag mice, however, exhibited pathological damage after adoptive transfer of the same spleen cell population. Administration of asialo GM1 to deplete host NK cells did not effect any histological abnormalities in Balb Rag mice, indicating that failure to observe pathology in these mice was not due to NK-mediated rejection of donor T cells. Autoreactivity could also be serially passaged through B6 Rag but not Balb Rag mice demonstrating that tolerance could not be reestablished by exposure to donor antigens in a different environmental milieu. To exclude the possibility that the observed pathological lesions were model dependent, similar transplant studies were performed in B6[H-2^b]→B10.BR[H-2^k] chimeras. Transfer of GVHD spleen cells into B6 Rag mice recapitulated the pathology observed in recipients of B6→Balb spleen cells. Independent experiments showed that transplantation of GVHD spleen cells into irradiated B6 as opposed to B6 Rag mice failed to induce any pathology, suggesting that a radiosensitive cell in host animals was necessary for propagation of autoreactivity. To determine if this cell was an antigen presenting cell (APC), Balb Rag mice were transplanted with B6 Rag BM to create chimeric animals where APCs were of donor origin. GVHD spleen cells were then transferred into chimeric mice where the same pathological lesions were observed as in B6 Rags. We conclude that GVHD results in the breaking of tolerance against self such that donor T cells acquire the ability to respond to both self and alloantigens but only within the context of donor APCs. These data suggest a new paradigm to GVHD pathophysiology whereby progression of GVHD is dependent upon donor APCs and provides an explanation for why chronic GVHD is characterized by autoimmune manifestations in man.