NF-kappaB-mediated self defense of macrophages faced with bacteria

NF-kappaB is a ubiquitous transcription factor that is extensively exploited by immune cells involved in host defense mechanisms. Macrophages participate in the first line of defense against microorganisms, but little is known about whether and how NF-kappaB is involved in the handling of microbes by macrophages. To explore this issue, NF-kappaB-inactive macrophages, NIKMAC(NR), were created by overexpression of a super-repressor mutant of IkappaB alpha. When co-cultured with Escherichia coli, the NIKMAC(NR) macrophages exhibited impairment of bactercidal activity. Microscopic analysis revealed that NIKMAC(NR) cells faced with bacteria underwent rapid and fulminant apoptosis. Similary, NIKMAC(NR) macrophages cultured in the presence of a bacterial component, lipopolysaccharide, showed dramatic apoptosis. Inhibition of RNA synthesis or protein synthesis failed to block the apoptosis of NIKMAC(NR) cells, indicating that macrophages possess a pre-existing, apoptotic pathway that can be triggered by bacteria. Apoptosis was not observed in NIKMAC(NR) macrophages exposed to non-microbial stimuli including phorbol ester and opsonized zymosan. However, NIKMAC(NR) cells were more susceptible to apoptosis triggered by TNF-alpha and reactive oxygen intermediates, both of which are produced abundantly by macrophages when faced with bacteria. These data suggest a critical role for NF-kappaB in the survival of macrophages at the site of bacterial infection.