HSP60 induces self-tolerance to repeated HSP60 stimulation and cross-tolerance to other pro-inflammatory stimuli

In addition to their primary function as intracellular chaperone proteins, the immunomodulatory properties of heat shock proteins (HSP), including their role as adjuvants for vaccines, have become a focus of intense research interest. Interestingly, the effect of chronic exposure to an endogenous immunomodulator and initiator of inflammation such as autologous HSP60 has as yet remained uncharacterized. In this study, we demonstrate that pretreatment of monocytes with human HSP60 results in a suppression of TNF-alpha production on restimulation with HSP60. Furthermore, desensitization with HSP60 inhibits TNF-alpha expression in these cells in response to LPS stimulation, thereby inducing "cross-tolerance". In contrast to TNF-alpha suppression, IL-1beta expression was augmented in HSP60-pretreated monocytes on restimulation, while being suppressed in THP-1 cells. Addition of an anti-IL-10 neutralizing antibody had no significant effect on HSP60- or LPS-induced tolerance.HSP60 priming of monocytes also results in significant down-regulation of HLA-DR, CD86 and Toll-like receptor 4 expression, but minimally up-regulates CD80 expression, similar to that previously reported with LPS. By identifying a previously unrecognized "tolerizing" effect of extended exposure to autologous HSP60 on the innate immune system, as opposed to its recently identified pro-inflammatory stimulatory capacity, this study highlights a further level of complexity of our understanding of the biological activities of HSP.