Noncytocidal mechanisms of action of tumor necrosis factor-alpha on human tumor cells: enhancement of HLA gene expression synergistic with interferon-gamma.

In this report, we present evidence that tumor necrosis factor alpha (TNF-alpha) exerts regulatory activity on the expression of HLA genes in human tumor cell lines at the level of mRNA expression and at a posttranscriptional level. These mechanisms are independent of direct cytotoxic/cytostatic effects, as enhancement of HLA antigen expression was observed in both sensitive cell lines and in cell lines resistant to TNF-mediated growth inhibition. In a priori HLA-negative cells, a strong TNF-alpha-mediated enhancement of the Interferon gamma (IFN-gamma) induced expression of HLA genes was revealed by Northern blot and immunofluorescence analysis. A distinct mechanism of TNF-alpha action is suggested from enhancement of constitutively expressed HLA genes. In this case, TNF-alpha raised HLA antigen density without apparent enhancement of cytoplasmic mRNA levels. This indicates that TNF-alpha influences HLA expression also at the level of translation or at a posttranslational level. TNF-alpha treatment of HLA-negative cells did not by itself result in HLA gene induction, suggesting that TNF-alpha acts as an enhancer and not as an inducer of HLA gene expression.