Factor(s) elaborated by bone marrow that promote persistent engraftment of xenogeneic and semiallogeneic marrow.

Physiological media in which bone marrow suspensions are prepared (original supernatants) contain biologically active substances which affect the engraftment of semiallogeneic or xenogeneic bone marrow in lethally irradiated recipient mice. The activity of these factors is non species-specific. When F1 hybrid mice were transplanted with parental bone marrow, a high survival rate and enduring chimerism were achieved if these marrow-derived factors were administered to the recipients. Thus, the bone marrow seems to possess an autonomous self-regulatory system represented by products of its microenvironment. Apparently, the histocompatibility barrier can be overcome when the semiallogeneic marrow is transferred to the irradiated host together with components of its own microenvironmental regulatory system.