MOUSE MACROPHAGE CLONES IMMORTALIZED BY RETROVIRUSES ARE FUNCTIONALLY HETEROGENEOUS

Murine macrophage clones were generated from thymus, spleen, brain, and bone marrow by in vitro immortalization with recombinant retroviruses carrying an avian v-myc oncogene. The cloned cell lines express F4/80 molecules, exert phagocytosis, have nonspecific esterase activity, and express class II molecules after interferon-gamma activation. The macrophage clones are diploid and their karyotypes have remained stable for > 3 years in culture. After the macrophage clones were activated, their pattern of cytokine production was investigated. Functional heterogeneity in cytokine transcription was demonstrated: one of six liposaccharide-activated macrophages was unable to transcribe interleukin 1-alpha, whereas all of the liposaccharide-activated clones were able to transcribe tumor necrosis factor-alpha. Interleukin 6 production was detected in three of six clones. The production of nitrite and tumor necrosis factor-alpha as effector molecules of cytotoxicity was detected in all clones, thus showing that a single macrophage can exert more than one cytotoxic mechanism. The results indicate that immortalized and cloned macrophages have a differentially regulated expression of cytokine genes, adding further evidence for the existence of functional heterogeneity among cloned macrophages. This heterogeneity seems to derive from differentiation-related mechanisms rather than from external constraints.