EXOGENOUS CYTOKINES ENHANCE SURVIVAL OF MACROPHAGES FROM ORGAN-CULTURED EMBRYONIC RAT-TISSUES

Background: Macrophage precursors are present in embryonic rats shortl y after the onset of hematopoiesis. During organogenesis they soon est ablish residency in many parts of the body and become convertible into phagocytes, at first gaining morphological characteristics of macroph ages and later a range of surface antigens used to characterize subpop ulations in adults. Nonetheless, it is uncertain whether representativ es of this fetal lineage continue to exist past birth. We investigated the question indirectly by seeing if such cells can be made to surviv e in vitro to an age equivalent to adulthood and by examining underlyi ng conditions that favor this outcome. Methods: Fourteen-day embryonic lungs, hearts, and limb buds were organ cultured on a firm serum-cont aining medium. Fetal macrophages developed within all explants and the n migrated out to form a corona of cells surrounding each explant. The lung cultures were selected for subsequent work which mainly used cor onal area as the measure of macrophage population size in experimental and control groups. Baseline growth and survival of macrophages were established for cultures grown on standard medium, then effects of the following were examined: indomethacin (10(-6) M) as it influences ini tial production of macrophages from precursors and later survival of d ifferentiated cells; and macrophage colony-stimulating factor (M-CSF), used alone at moderate dosage (50-100 U), and combined with granulocy te-macrophage CSF (both 200 U), for its importance to long-term surviv al of the population. Mitogenic influence of M-CSF on differentiated m acrophages was demonstrated by uptake of 5-bromo-2'-deoxyuridine. Resu lts: Indomethacin inhibited the formation of macrophages from precurso rs but enhanced the survival of differentiated cells. M-CSF increased BrdU uptake of differentiated macrophages and permitted coronal growth to continue long past the similar to 30 day limit of controls. Beyond this interval, M-CSF was essential for macrophage survival, since cor onas quickly shrank after the cytokine was withdrawn. Administration o f the M-CSF/GM-CSF mixture to the 2 oldest M-CSF-exposed cultures betw een 98 and 127 days in vitro resulted in an increase in the number of coronal macrophages (P < 0.001); withdrawal between 129 and 140 days l ed to a decrease (P < 0.005). Ultimately a few cells were still surviv ing at 183 days. Conclusions: Intrinsic factors promote early formatio n of macrophages within the explants, but the availability of factors is lessened by the antiinflammatory action of indomethacin. Its later promotion of macrophage survival may be based on suppression of autoge nous prostaglandin (PGE(2)) synthesis. M-CSF greatly promotes macropha ge survival; in context this is sufficient to show that the fetal macr ophage line has a clear potential to survive well into adulthood. (C) 1994 Wiley-Liss, Inc.