TLR3 Signaling Is Negatively Regulated By TAM Receptors In Sertoli Cells: Mechanism And Significance

TAM receptors belong to a subfamily of receptor tyrosine kinases, which contain 3 members—Tyro3, Axl and Mer (TAM). They are essential regulators of mammalian spermatogenesis. Mice lacking all three receptors produce no mature sperm, owing to the progressive death of differentiatinggerm cells. Toll-like receptors (TLRs) are "pattern-recognition receptors" that recognize conserved, pathogen-code molecular structures and play crucial roles in mediating innate and adaptive immunity. Several TLRs are expressed in Sertoli cells and can trigger testicular innate responses after activation by ligands. TLR signaling pathway must be tightly controlled because unrestrained TLR activationgenerates a chronic inflammatory milieu that often leads to pathogenesis of the host. However, the regulation of TLR signaling in Sertoli cells remains to be clarified. Here, we demonstrate that TAM receptors negatively regulate TLR3 signaling in Sertoli cells.All three TAM receptors are constitutively expressed in Sertoli cells. The activation of TLR3 triggers a TIR domain-containing adaptor inducing interferonβ(TRIF)-dependent pathway that activates the nuclear factorκB (NF-κB) and interferon regulatory factor 3 (IRF3), thereby resulting in the induction of inflammatory cytokines and type I interferons. We demonstrate that Sertoli cells from TAM triple mutant (TAM-/-) mice exhibit an excessive activation of TLR3 in response to its ligand Poly (I:C), resulting in the upregulation of inflammatory cytokines including IL-1β, IL-6, TNFa and type I interferons (IFNa and IFNβ). Gas6, a common ligand of TAM receptors, inhibits the TLR3-driven expression of cytokines in Sertoli cells. This TAM-mediated inhibition of TLR3 signaling in Sertoli cells is transduced through the upregulation of TLR signaling suppressors—suppressor of cytokine signaling 1 (SOCS1) and SOCS3 by Gas6. TAM signaling activates signal transducer and activator of transcription 1 (STAT1), thus upregulates SOCS1/3. Moreover, we provide evidence that TAM inhibition of inflammatory cytokine production by Sertoli cells have physiological significance in vivo.Our results illuminate a negative regulatory mechanism of TLR3 signaling in Sertoli cells. This regulatory system should be important in controlling the testicular innate immune responses to pathogens.