Investigating endogenous immune-mediated monocyte memory in rheumatoid arthritis.

OBJECTIVES

Inflammation triggered by endogenous stimuli that signal cellular stress or tissue injury must be tightly controlled to balance robust protection from intrinsic danger while avoiding catastrophic destruction of healthy tissues. Here, we assess the contribution of innate memory to this balance.

METHODS

Memory evoked by the extracellular matrix protein tenascin-C, a damage-associated, toll-like receptor 4 (TLR4) agonist, was compared to that induced by the pathogenic TLR4 agonist lipopolysaccharide (LPS) by transcriptomic and epigenetic profiling of monocytes from healthy individuals or people wirh rheumatoid arthritis (RA), and tissue macrophages from the RA synovium.

RESULTS

Tenascin-C reprograms monocyte response to subsequent threats, inducing concomitantly suppressed and enhanced responses to rechallenge. Comparative analysis of tenascin-C and LPS revealed common and distinct gene expression signatures, effects controlled transcriptionally and associated with stimulus-specific epigenetic mediators. Altered responses following rechallenge after priming with tenascin-C were not limited to subsequent TLR4 activation but were evident in response to various pathogenic and endogenous stimuli detected by different receptors. In healthy monocytes primed with tenascin-C, rechallenge with stimuli found at high levels in the joints of people with RA resulted in trained responses that were not induced by LPS, including genes associated with chronic inflammation, tissue destruction, altered metabolism, and poor treatment response in RA. The expression of a large subset of these genes was elevated in monocytes from people with RA in the absence of any stimulation and in RA synovial macrophage populations associated with disease flare. Moreover, higher levels of permissive complexes within key epigenetic nodes and increased bivalent modification creating poised loci within endogenously trained genes were observed in RA cells.

CONCLUSIONS

These data highlight how innate reprogramming during 'sterile' inflammatory diseases contributes to chronicity, uncovering pathways unique to endogenous immune triggers that could provide disease-specific points of intervention without engendering global immune suppression.