Positive Feedback Cycle of TNFα Promotes Staphylococcal Enterotoxin B-Induced THP-1 Cell Apoptosis
Recombinant interferon-gamma (IFN-gamma) induced HLA-DR gene expression in both U937 and THP-1 human monocytic cell lines, although the former was only very weakly inducible. Combination of recombinant tumor necrosis factor (TNF) and IFN-gamma resulted in a synergistic enhancement of DR mRNA and protein induction in both cell lines. TNF alone increased the constitutive expression of the DR gene in THP-1 cells. In the HLA class II-negative U937 cells, TNF used alone was not able to induce DR gene expression. Such a negative result was not due to a lack of TNF receptor expression in U937 cells, since TNF clearly induced HLA class I and TNF gene expression in this cell line. THP-1, but not U937, cells secreted TNF under the influence of IFN-gamma. Neutralization of TNF by a specific antibody decreased IFN-gamma-induced DR antigen expression in THP-1 cultures. These observations indicate that TNF is not able to directly induce DR gene expression, but rather amplifies ongoing expression of this gene, whether constitutive or induced by IFN-gamma. In the two cell lines tested, the level of DR inducibility under the influence of IFN-gamma used alone depended on a different inducibility of TNF secretion by IFN-gamma. Altogether, our observations indicate that TNF, whether exogenous or endogenously produced under the influence of IFN-gamma, amplifies DR gene expression in monocytes, a phenomenon that may provide to such antigen-presenting cells a selective sensitivity to the DR-inducing effects of IFN-gamma.