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 in vitro either alone or mixed with F₁ (Lewis × Brown Norway) hybrid rat lymphoid cells. After 4 days the incorporation of ³H]thymidine into DNA was measured by scintillation counting, and the morphology and incidence of ³H]thymidine-labelled cells were determined in radioautographs.

Lymphocyte-rich, slowly-sedimenting fractions of Lewis rat bone marrow cells showed increased incorporation of ³H]thymidine and the development of many large proliferating blast-like cells when cultured with F₁ 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 F₁ 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 ³H]thymidine incorporation when cultured with F₁ 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 in vitro.