Unique MyD88/TRAM signaling post TLR2/6 recognition of conserved viral architectures results in anti-viral immunity and improved clearance of secondary bacterial infection

Abstract

Abstract Respiratory virus infections can either reduce or increase host susceptibility to subsequent secondary bacterial infections (SBI). Recently, we discovered that the anti-viral immune response initiated by the recognition of exogenous pattern of virus architectures, namely the repeating protein subunit pattern (RPSP), results in improved clearance of S. aureus during SBI in mice. The RPSP is likely conserved in all viruses, suggesting a generalized mode of viral pattern recognition not specific to any single virus. Thus, we identified RPSP as a new PAMP and found it to be recognized by the TLR2/6 heterodimer on the surface of macrophages. We found that this RPSP recognition occurs prior to particle internalization and independent of virus infection. TLRs are known to signal from either the cell surface, through MyD88/Mal, or from the endosome, through TRAM/TRIF, for the induction of inflammatory responses or type I IFNs, respectively. We found that when compared to WT macrophages, tram−/− macrophages exhibited diminished RPSP internalization by confocal microscopy and flow cytometry. However, blocking endosomal acidification by Bafilomycin A1 did not reduce S. aureus killing by RPSP-treated macrophages. This suggests that a non-canonical endosomal signaling pathway may be induced upon RPSP exposure. Indeed, using knockout mice we found that type I IFN, but also MyD88 signaling are critical for improved S. aureus clearance post RPSP. Our results indicate that the recognition of RPSP by TLR2/6 activates a unique intracellular signaling pathway, specifically a combination of the two known pathways, that results in type I IFN-dependent improvement in S. aureus killing by macrophages.