Unphosphorylated STAT1 represses apoptosis in macrophages during Mycobacterium tuberculosis infection

In murine macrophages infected with Mycobacterium tuberculosis (Mtb), the level of phosphorylated STAT1 (P-STAT1), which drives the expression of many pro-apoptosis genes, increases quickly but then declines over a period of hours. By contrast, infection induces a continued increase in the level of unphosphorylated STAT1 that persists for several days. Here, we found that the level of unphosphorylated STAT1 correlated with the intracellular bacterial burden during the later stages of infection. To investigate the significance of a high level of unphosphorylated STAT1, we increased its concentration exogenously, and found that the apoptosis rate induced by Mtb was sufficiently decreased. Further experiments confirmed that unphosphorylated STAT1 affects the expression of several immune-associated genes and lessens the sensitivity of macrophages to CD95 (FAS)-mediated apoptosis during Mtb infection. Furthermore, we characterized 149 proteins that interacted with unphosphorylated STAT1 and the interactome network. The cooperation between unphosphorylated STAT1 and STAT3 results in downregulation of CD95 expression. Additionally, we verified that unphosphorylated STAT1 and IFIT1 competed for binding to eEF1A. Taken together, our data show that the role of unphosphorylated STAT1 differs from that of P-STAT1, and represses apoptosis in macrophages to promote immune evasion during Mtb infection.