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ACTIVE ENHANCEMENT IN THE PRIMATE BY BONE MARROW: I. IA‐LIKE DETERMINANTS ON NUCLEATED BONE MARROW CELLS

SUMMARY This study investigates the possibility that the active enhancing properties of bone marrow (BM) cells may be related to la-like determinants on their surface membranes. Less chromium was released on a cell per cell basis from nucleated baboon BM cells used as targets in complement-dependent cytotoxicity than from labelled lymph node lymphocytes. On the other hand, BM cells stimulated allogeneic lymphocytes in culture more vigorously than lymph node lymphocytes. BM cells with lymphocyte-activating properties could be enriched by fractionation on a discontinuous bovine serum albumin gradient. BM cells responded poorly to allogeneic cell stimulation in mixed culture. Platelet absorption studies of an alloan-tiserum supported the conclusion that nucleated BM cells, and particularly BM fractions collected from the less dense interfaces of a bovine serum albumin gradient, express relatively more lymphocyte-activating or la-like determinants than “serologically defined” determinants. We have previously reported highly significant prolongation of liver allograft survival in chacma baboons following the administration of a polyspecific alloantiserum (PSS) and a single injection of 25 × 106 donor bone marrow (BM) cells at the time of transplantation (7). Graft survival was also significantly longer than that seen in a group of animals passively enhanced with PSS alone. All eight of the animals tested in the PSS + BM group formed alloantibodies detected by mixed cell agglutination against the lymphocytes of a random panel of 10 baboons. The effect of the BM was abrogated (7) or lessened (8) by coincident administration of cyclophosphamide or anti-lymphocyte serum and azathioprine and prednisolone, respectively. The studies reported here investigate the possibility that the active enhancing properties of BM cells may be related to la-like determinants on their surface membranes, and show that nucleated BM cells are rich in lymphocyte-activating (Ia-like) determinants, and relatively poor in serologically defined (SD) determinants. The accompanying paper gives the results of active immunization with whole suspensions and fractions of nucleated BM cells.