Salmonellae activate tumor necrosis factor alpha production in a human promonocytic cell line via a released polypeptide

Abstract

Invasive strains of Salmonella spp. cause both systemic and localized infections in humans. The ability to resist infection and some aspects of the tissue pathology associated with the presence of Salmonella in the gastrointestinal tract have been shown to be mediated in part by the induction of tumor necrosis factor alpha (TNF-α), a proinflammatory cytokine produced by activated macrophages and lymphocytes. Recent reports indicate that TNF-α is involved in the induction of human immunodeficiency virus replication by Salmonella in the latently infected human promonocytic cell line U1. In the present study, we investigated the effects of Salmonella on TNF-α production in U1 cells and a related cell line, U38. Unlike Escherichia coli or Yersinia enterocolitica, salmonellae rapidly induce TNF- α expression in these cells through a released factor(s). Time course experiments show that the kinetics of TNF-α production by U38 cells stimulated with Salmonella conditioned medium closely resemble those observed in response to live Salmonella. The observation that TNF-α levels are elevated by 60 rain after exposure to either bacteria or their conditioned medium suggests that the soluble inducer is continuously released or shed by the bacteria and that the signal acts rapidly to increase TNF-α production. Furthermore, the ability to produce the TNF-α inducer is shared by at least four Salmonella serotypes and does not correlate with the abilities to invade and to survive within phagocytes. Treatment of active conditioned medium with trypsin, but not low pH, high temperature, or urea, significantly inhibits its TNF-α-inducing effect on U38 cells, a finding which points to a polypeptide product of Salmonella as the mediator of TNF-α production. Gel filtration chromatography of Salmonella conditioned medium reveals two peaks of activity, consistent with molecular masses of approximately 150 and 110 kDa.