Age-related factors in cyclosporine-induced syngeneic graft-versus-host disease: regulatory role of marrow-derived T lymphocytes

Role of the Host Resistance Mechanism on the Induction of SGVHD A series of experiments was undertaken to identify the role of the host resistance mechanism in preventing the induction of SGVHD. The results from the studies described above indicate that addition of mature T lymphocytes from animals 6 mo of age to the marrow inoculum at Graft composition the time of transplant prevented the development of SGVHD. One hypothesis to account for the inhibition of the development of SGVHD is that the mature T lymphocytes (contained in marrow or in spleen) expand during CsA therapy to levels capable of preventing the expression ofthe autoreactive cells generated during therapy. Recent data suggest that mature T lymphocytes can expand in a thymec-and antigenindependent environment (18). To assess ifthe mature T lymphocytes including the cells responsible for host resistance clonally expand, thymectomized Lewis rats were grafted with marrow depleted ofT lymphocytes by elutriation . 2 wk after transplantation, the recipients were infused with 5 x 106 splenic T lymphocytes (nylon wool-nonadherent cells) and treated with CsA (10 mg/kg/d) or the control diluent for 30 d. After therapy, the recipients were infused with 3 x 101 effector T lymphocytes from animals with active SGVHD. The results in Table 6 demonstrate that SGVHD could not be transferred into the bone marrow-reconstituted, thymectomized recipients grafted with 5 x 106 T lymphocytes. However, SGVHD could be transferred if the recipients were treated with CsA . This was not due to the primary induction of SGVHD, since control thymectomized recipients treated with CsA did not develop disease, thus confirming the data of Sorokin et al. (7) . These data suggest that CsA interferes with the expansion of a T lymphocyte-dependent host resistance mechanism carried over with mature T lymphocytes and could not prevent the adoptive transfer of disease with 3 x 107 effector cells . Graded doses of T lymphocytes in fraction 25 of elutriated marrow and in nylon wool-nonadherent spleen cells derived from Lewis rats of 1 and 6 mo of age were added to marrow of 1-mo-old syngeneic animals. The number of T cells was estimated by expression of OX19 with immunofluorescent staining . The mixture of cells was infused into irradiated syngeneic Lewis recipients (6-8 wk of age), and the rats were treated with the standard course of CsA therapy (15 mg/kg/d, for 30 d) . Animals were observed for the onset of SGVHD after discontinuation of CsA treatment. We also compared the efficacy of infusing mature splenic T lymphocytes from animals 6 mo of age, either on the day of transplantation or the day of withdrawal of CsA therapy (day 30), and the results are summarized in Table 7 . SGVHD could not be induced if the T lymphocytes were infused on the day oftransplant . However, development ofSGVHD was observed despite the infusion of 3 x 107 normal splenic T lymphocytes (from animals 6 mo of age) on the day of C&A withdrawal. These data suggested that the frequency of autoreactive lymphocytes exceeded the minimum number of cells that can be regulated by the host resistance mechanism transferred with 3 x 10' normal T lymphocytes. To assess if autoreactive cells could be detected in animals receiving marrow plus mature T lymphocytes at the time of transplant, a series ofadoptive transfer studies was performed harvesting cells on the day of CsA withdrawal. Since CsA therapy prevented the clonal expansion of the host resistanc