Factors influencing fetal macrophage development: I. Reactions of the tumor necrosis factor-alpha cascade and their inhibitors.

BACKGROUND

When fetal rat lungs are explanted to organ culture, precursor angular cells soon convert to nascent macrophages that multiply rapidly as they mature into efficient phagocytes. The present study examines the influence of proinflammatory early cytokines of the tumor necrosis factor-alpha (TNF alpha) cascade on this initial expression of the macrophage phenotype.

METHODS

Fourteen- and 15-day fetal rat lungs were grown for varying periods on an agar-solidified medium with and without test factors added singly or in combination. Growth of the macrophage population was followed daily by light microscopy and quantified by measuring the area of coronas formed as cells emerged from explants.

RESULTS

TNF alpha interleukin-1 beta (IL-1 beta) stimulated growth of the macrophage population, as had macrophage- and granulocyte-macrophage colony-stimulating factors (M- and GM-CSFs) in prior studies. Inhibition was obtained by exposure to IL-1 receptor antagonist and antibodies neutralizing the CSFs. Only the effects of TNF alpha were sufficiently delayed to discount possible influence on conversion and growth of nascent macrophages. Two transcription blockers, dexamethasone and pyrrolidine dithiocarbamate (PDTC), an inhibitor of nuclear factor NF-kappa B, both profoundly suppressed macrophage growth without preventing conversion of precursors. Effects of dexamethasone were significantly ameliorated by IL-1 beta alone and combined with GM-CSF; those of PDTC were mitigated by M-CSF and a combination of IL-1 beta and TNF alpha but not by GM-CSF.

CONCLUSIONS

IL-1 beta, M-CSF, and GM-CSF all promote growth of the young macrophage population. TNF alpha is effective only later on, likely because early-stage cells lack its receptors which normally use intracellular signalling pathways similar to those for IL-1. The severity of PDTC inhibition to population growth indicates that NF-kappa B is important for transmitting proliferative signals in these cells.