Factor In Macrophage Colony-Stimulating

ABSTRACT-Infection in vitro of freshly explanted N:NIH(S) mouse bone marrow with ecotropic murine leukemia viruses produced an increase over control uninfected cultures in the 50 or more cell granulocyte-macrophage (GM) colonies and 10-49 cell clusters detected after 7 days of incubation in 0.3 % agar at 37 ° C and 7 % CO 2, This effect was observed only at plating densities above 5.0X10 · cells/ml and was not observed with macrophage-depleted populations of colony-forming units of GM progenitor cells (GM-CFUc) purified by isopyknic density gradient centrifu-gation of nonadherent cells harvested from long-term bone marrow cultures. Fewer virus-infected, compared to uninfected, peritoneal exudate macrophages were required to stimulate the same number of GM colonies and clusters in a given number of purified GM-CFUc. In contrast, murine leukemia virus infection of T-Iymphocytes or NIH/3T3 embryo fibroblasts did not stimulat