Thymus-dependent lymphocytes in human bone marrow.

Human bone marrow is known to contain significant numbers of bursa-dependent lymphocytes. The presence of thymus-dependent (T) cells is controversial. Bone marrow cells obtained from healthy volunteers was fractionated by density centrifugation. A lymphocte-enriched subpopulation was shown to be reactive to alloantigens in mixed lymphocyte culture and to contain substantial numbers of T lymphocytes. The T lymphocytes were detected by cell surface markers (rosette formation with sheep RBC) and by response to the mitogens phytohemagglutinin and concanavalin A. Bone marrow T cells exhibited functional characteristics quantitatively different from peripheral blood T cells, suggesting that they may represent a particular subpopulation of T cells. The lymphocyte-enriched fraction additionally contained committed granulopoietic stem cells capable of colony formation in semisolid gel. The presence of T cells in human bone marrow is consistent with findings in other mammals and may explain the high incidence of graft versus host disease in bone marrow transplant recipients.