Is skeletal muscle a functional immunosensory organ of pathogens? A murine model of polymicrobial sepsis.

Abstract

The body’s ability to sense lipopolysaccharide (LPS) is critical in the inflammatory response to bacterial infection. LPS is a ligand for toll‐like receptors (TLRs), expressed on immune and many other cell types. When activated, they lead to secretion of cytokines such as IL‐6 and IL‐1ß. These cytokines, in turn, promote the activity and trafficking of inflammatory cells to the site of infection. We hypothesize that skeletal muscle (skm), which also expresses TLRs, plays a role in directing inflammatory cells via cytokine secretion during sepsis. In order to test this, we developed an inducible skeletal muscle‐specific knockdown of MYD88 (an adaptor protein necessary for TLR function) using the Mer‐CRE‐Mer system.METHODSMale and female mice on skmCRE background were injected with raloxifene to induce skmMYD88 knockdown vs. vehicle control (no CRE induction). Three weeks after CRE induction, sepsis was induced via intraperitoneal injection of a calibrated dose of cecal slurry. The animals were sacrificed 6 and 12 hours post‐injection. Plasma and peritoneal lavage were collected at the time of sacrifice, and analyzed for the presence of various inflammatory cells via a HESKA hematological analyzer.RESULTSskmMYD88−/− females showed a significant increase in the %neutrophils in the lavage at both the 6h (P<0.05) and 12h time points (P<0.0001) compared to skmMYD88+/+. The % of lavage basophils were elevated in female skmMYD88−/− at 12h (P< 0.01), as were %eosinophils (P<0.0001). Female lavage %monocytes increased at 12h in skmMYD88−/−. There were no differences in lavage %lymphocytes. No differences between the male skmMYD88−/− were observed for any cell types in the peritoneal lavage. In the blood, the only significant differences were found in males, in which the skmMYD88−/− group displayed increased %monocytes and decreased %lymphocytes at 12h.DISCUSSIONThe data suggest that skeletal muscle is capable of selectively governing the direction and presence of immune cells at the site infection, i.e. the peritoneum. The cause of this influence appears to be, in part, due to skeletal muscle TLR activation. While total leukocytes remain unchanged in the peritoneal lavage, knocking down skmMYD88 resulted an altered immune cell profile during the time‐course of infection. In the case of neutrophils, skeletal muscle appears to play a major suppressive regulatory role. Overall, these results implicate skeletal muscle as a significant player in pathogen recognition and response.Support or Funding InformationSupported by NIH 1R01GM118895 (TLC) and the BK Betty Stevens Endowment