Blastogenic response of lymphocytes separated from bone marrow to allogeneic lymphoid cells in vitro.

Fractions of cells were separated from the bone marrow and spleen of Lewis rats by brief centrifugation in linear sucrose-serum density gradients and were cultured either alone or mixed with F (Lewis × Brown Norway) hybrid rat lymphoid cells. After 4 days the incorporation of [H]thymidine into DNA was measured by scintillation counting, and the morphology and incidence of [H]thymidine-labelled cells were determined in radioautographs. Lymphocyte-rich, slowly-sedimenting fractions of Lewis rat bone marrow cells showed increased incorporation of [H]thymidine and the development of many large proliferating blast-like cells when cultured with F hybrid cells. This blastogenic response was greater than could be ascribed to contaminating intravascular blood lymphocytes. Slowly-sedimenting fractions of spleen cells, consisting mainly of small lymphocytes, showed a greater blastogenic responsiveness to F hybrid cells than that of whole spleen, while rapidly-sedimenting spleen cell fractions, containing many large cells, showed a reduced responsiveness. Paradoxically, rapidly-sedimenting marrow cell fractions containing few small lymphocytes, showed an increment in [H]thymidine incorporation when cultured with F cells but this was due to active macrophage proliferation as well as to blastogenesis. The results demonstrate that some parenchymal bone marrow lymphocytes undergo blastogenic transformation in response to allogeneic lymphocytes .