Escherichia coli Prevents Phagocytosis-Induced Death of Macrophages via Classical NF- B Signaling, a Link to T-Cell Activation

NFB is a crucial mediator of macrophage inflammatory responses, but its role in the context of pathogeninduced adaptive immune responses has yet to be elucidated. Here, we demonstrate that classical NFB activation delays phagocytosis-induced cell death (PICD) in Raw 264.7 and bone marrow-derived macrophages (BMDMs) upon ingestion of bacteria from the Escherichia coli laboratory strain Top10. By expression of a nondegradable form of I B (superrepressor) and pyrrolidine dithiocarbamate treatment, prolonged activation of NFB upon bacterial coculture is suppressed, whereas initial induction is only partially inhibited. This activation pattern results in partial inhibition of cellular activation and reduced expression of costimulatory CD86. Notably, suppression of classical NFB activation does not influence bacterial uptake rates but is followed by increased production of oxygen radicals and enhanced intracellular killing in Raw macrophages. This is associated with reduced expression of NFB-dependent antiapoptotic c-IAP-2 and a loss of the mitochondrial transmembrane potential. Accordingly, NFB inhibition in Raw cells and BMDMs causes increased apoptotic rates within 12 h of bacterial ingestion. Interestingly, accelerated eradication of E. coli in NFB-inhibited macrophages is associated with reduced antigen-specific T-cell activation in macrophagelymphocyte cocultures. These data suggest that E. coli inhibits PICD of macrophages via classical, antiapoptotic NFB activation and thus facilitates signaling to T cells. Subsequently, a proper adaptive immune response is likely to be generated. Conclusively, therapeutic inhibition of classical NFB activation in macrophages may hamper the initiation of adaptive immunity.