*Purpose: Cytomegalovirus (CMV) is one of the most common pathogens infecting solid organ transplant recipients. We recently showed that (1) CMV infection impairs the induction of transplant tolerance, and (2) Receptor Tyrosine Kinase MerTK signaling in macrophages is critical for tolerance induction. The current study attempted to determine if CMV impairs tolerance via its effect on macrophage MerTK expression and/or signaling, and if so, what potential measures could be taken to rescue MerTK signaling in setting of CMV infection.

*Methods: B6 bone marrow cells were harvested for differentiation of bone marrow derived macrophages (BMDMs) by M-CSF (20ng/ml in medium). Murine CMV virus (MCMV) was used for in vitro infection. Poly I:C(10ug/ml) was used to activate TLR3 signaling. MRT67307(2uM), an inhibitor of interferon related factor 3 (IRF3) phosphorylation, was used to block TLR3 signaling in BMDMs. GW3965(1uM), an agonist to the transcription factor liver X receptor (LXR) upstream of MerTK, was used to enhance MerTK expression on BMDMs.

*Results: MCMV infection downregulated BMDM MerTK expression and upregulated their IFN-α production. Similarly, poly I:C treatment of BMDMs also resulted in downregulation of their MerTK expression. As expected, MCMV infection significantly enhanced TLR3 as well as phospho-IRF3 expression. Interestingly, LXR, the nuclear transcription factor known to be upstream of MerTK, remained stable after MCMV infection, suggesting that MCMV-induced TLR3 activation impaired MerTK expression via an LXR-independent pathway. Therapeutically, combining the kinase inhibitor MRT67307 with the LXR agonist GW3965 in MCMV-infected BMDMs resulted in rescued MerTK expression in setting of MCMV infection and inhibition of MCMV-induced IFN-α production.

*Conclusions: MCMV infection inhibits MerTK expression and its downstream effect via activating the TLR3/IRF3 pathway. Combination of IRF3 inhibitor and LXR agonist rescues MerTK expression and signaling, and may be a promising therapeutic strategy for protecting transplant tolerance in setting of CMV infection.

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