Tumor necrosis factor alpha is an autocrine growth regulator during macrophage differentiation

Previous experiments have revealed the expression of tumor necrosis factor alpha (TNF-alpha) transcripts in all murine bone marrow-derived macrophage colonies isolated from days 5 through 9 of differentiation in vitro. These results implicated a role for TNF-alpha gene expression during macrophage differentiation. Antisense oligomers to the initiation region of the TNF-alpha message were used to inhibit its expression, thus allowing the role of TNF-alpha gene expression in controlling the differentiation of macrophages to be determined. Results showed that TNF-alpha regulated the proliferation of macrophages during differentiation. Cells isolated on day 3 were exclusively vulnerable to the effects of blocking TNF-alpha gene expression, displaying a 30% increase in proliferation over control cells or sense oligomer-treated cells. Thus, in the absence of TNF-alpha gene expression, cells maintained proliferation instead of undergoing terminal differentiation. Exogenous TNF-alpha was capable of rescuing day 3 antisense-treated cells, therefore maintaining normal levels of proliferation. In contrast, blocking interleukin 1 beta gene expression by antisense oligonucleotide treatment had no effect on proliferation. Addition of exogenous recombinant murine or human TNF-alpha decreased the total cell number 25-50% regardless of whether cells were grown in medium containing colony-stimulating factor 1 (CSF-1) or granulocyte-macrophage colony-stimulating factor (GM-CSF). These results suggested that exogenous TNF-alpha suppressed proliferation of early hematopoietic progenitors, whereas endogenous TNF-alpha regulated proliferation of macrophage progenitors. The number of differentiated, adherent macrophages on day 5 of differentiation in vitro was increased by TNF-alpha treatment of GM-CSF-induced macrophages but was suppressed in CSF-1-induced macrophages. These findings suggest that distinct TNF receptor expression and/or signaling is induced in differentiating macrophages stimulated with either growth factor.