Effect of strain and age on in vitro proliferation of murine thymus-derived macrophages

In our laboratory we developed a technique which enabled us to carry out long term cultures of pure murine thymus-derived macrophage (TDM phi) populations having morphology, phagocytic capacity and surface markers characteristic of mononuclear phagocytic cells. We cultured thymic cells derived from various mouse strains on extracellular matrix coated tissue culture dishes, in the presence of conditioned medium. These cells developed into discrete TDM phi colonies and demonstrated a different proliferation rate. The TDM phi bearing haplotype H-2k (C3H/Crgl, CBA/LAC) proliferated more rapidly than TDM phi derived from mouse strains bearing the haplotypes of H-2b (C57BL/6), H-2d (BALB/c) and H-2k/d (A/J). To evaluate more precisely the number of precursor cells which generated TDM phi colonies and to study their growth kinetics, thymic cells were cultured in soft agar. A relatively large number of TDM phi precursor cells was derived from thymuses of H-2k haplotype origin. These precursor cells manifested a high rate of proliferation as compared with precursor cells of other murine haplotypes studied (H-2b, H-2d). These observations suggest that macrophage precursor cells are also located in the thymus and their numbers are controlled by genetic factors. We also observed a correlation between the number of precursor cells and the age of the mice. The percentage of TDM phi in new born and young mice was approximately four times higher than that found in older mice. It, therefore, seems that the degree of thymic involution affects the number of thymic macrophage precursor cells, which might indirectly affect thymocyte maturation and/or differentiation.