Regulation of bone marrow cell survival in short-term cultures: A new macrophage function

Abstract

The involvement of macrophages (Mφ) in the regulation of bone marrow (BM) cell survival in short-term cultures was studied. We developed a system to measure the survival of fresh BM cells in vitro, by evaluating 111indium ( 111In) release from prelabeled BM cells. 111In release was proportional to cell death and inversely related to the number of trypan blue excluding cells. Upon 24 hr of culture in conventional medium, more than 50% of BM cells died. In order to investigate whether BM cell death could be reduced by coculture with other cell types, 111In-labeled BM cells were incubated for 24 hr with peritoneal Mφ, thymocytes (THY), or polymorphonuclear cells (PMN) and then assayed for their survival. We found that coculture of BM cells with Mφ dramatically increased BM survival, whereas THY or PMN consistently failed to enhance BM survival. The ability to promote BM cell survival, here designated nurse activity, represented a novel function of Mφ and was further characterized. The stage of activation of Mφ did not influence their nurse activity, since Mφ elicited in vivo by proteose-peptone, thioglycollate, or Bacillus Calmette-Guérin, as well as resident Mφ unstimulated or activated in vitro with lipopolysaccharide, equally sustained survival of BM cells. BM-derived Mø (adherent cells from BM cultures maintained in 20% L-cell-conditioned medium for 14 days) were equally effective in exerting nurse activity. Moreover, nurse activity was also exerted across the histocompatibility barriers. Supernatants from Mφ cultures or killed Mφ were ineffective. We propose that the nurse effect of Mφ on BM is a primitive function that may play an important role in the development of the hemopoietic system.