STRESS-INDUCED EXTRACELLULAR HSP72 IS A FUNCTIONALLY SIGNIFICANT DANGER SIGNAL TO THE IMMUNE SYSTEM

Abstract

Extracellular heat-shock proteins (eHSP), such as those belonging to the 70-kDa family of HSP (e.g., eHSP72), have been hypothesized to act as a “danger signal” to immune cells, promote immune responses, and improve host defense. The current studies examined this hypothesis. Adult, male F344 rats were exposed to an acute laboratory stressor (100, 5-s, 1.6 mA inescapable tailshocks; IS) and challenged with E.coli. The ability to clear bacteria from the site of injection, levels of eHSP72, the immune response to eHSP72 and E. coli derived lipopolysaccharide (LPS) and the amount of time required to recover from bacterial challenge were measured. IS exposure improved bacterial clearance at the site. Rats exposed to IS had elevated eHSP72 that was induced rapidly (25 min) and remained elevated in the circulation and at the inflammatory site (2 hrs after stressor termination). Both IS exposure and eHSP72 administration in the absence of stress resulted in a facilitated pattern of recovery following bacterial inflammation induced by subcutaneous E. coli injection. In vitro stimulation of rat splenocytes and macrophages with eHSP72 elevated nitric oxide (NO), tumor necrosis factor alpha (TNF-α), interleukin (IL)-1beta, and IL-6 and this effect was specific to eHSP72 as it was not diminished by polymyxin B and was reduced by prior heat-denature treatment. Stimulation of cells with eHSP72 combined with LPS resulted in a greater NO and cytokine response than that observed following stimulation with eHSP72 or LPS alone. In vivo at the inflammatory site, the bacterial-induced NO response was potentiated by stress, and NO inhibition (L-NIO) reduced the stress-induced facilitation, but had no effect on the control kinetics, of bacterial inflammation recovery. Thus these results lend support to the hypothesis that stressor exposure increases eHSP72, which acts as a “danger signal” to potentiate the NO response to bacterial challenge, and facilitate recovery from bacterial inflammation.