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Results Cell Surface Phenotype of Syngeneic Bone Marrow Cell Responsible for Alloresistance to Engraftment in Mixed Allogeneic Reconstitution

We recently reported a murine model for the study of specific transplantation tolerance induced by total body irradiation and reconstitution with T cell depleted syngeneic (B10) plus T cell-depleted allogeneic (B10.D2/nSn) bone marrow (B10 + B10.D2 ~ B10) (1). Such recipients repopulated as mixed lymphopoietic chimeras and were specifically tolerant to the B10.D2 allogeneic donor in vivo and in vitro, yet retained immunocompetence at a level similar to that of syngeneically reconstituted (B10 ~ B10) recipients (2). T cell depletion of the syngeneic component of the mixed allogeneic bone marrow inoculum was essential for engraftment of the allogeneic donor and for induction of this specific transplantation tolerance. If the syngeneic donor marrow was not T celldepleted, animals repopulated as fully syngeneic chimeras were not tolerant to BI0.D2 allogeneic donor skin grafts, and were reactive in vitro to B10.D2 lymphoid elements in MLR and cell mediated lympholysis (CML) assays (3). To the extent that failure ofallogeneic engraftment in this model may correlate with failure of allogeneic bone marrow engraftment in general, these results provide a means for identifying the critical syngeneic cell phenotype responsible for the alloresistance to engraftment observed. We report here the results of selective depletions of various T cell subsets (L3T4 +, Lyt-2 ÷, Thy-1 +) from the syngeneic component of the mixed bone marrow inoculum by the use of mAbs plus complement. Our results indicate that the critical syngeneic cell phenotype that prevents engraftment of allogeneic donor bone marrow and tolerance to the donor alloantigens in mixed allogeneic reconstitution resides among the Lyt2 + subset of T lymphocytes.