Resident myeloid-derived immune cells contribute to early lipopolysaccharide-induced cytokine secretion in mouse soleus muscle
Skeletal muscles secrete a variety of cytokines in response to inflammatory stimuli such as lipopolysaccharide (LPS); however, the contributions of resident macrophages or other non-muscle cells to the secretory responses are not well understood. Here we tested the LPS responsiveness of mouse soleus when a critical toll receptor adapter protein (Myd88) was knocked down only in myeloid-derived cells (e.g. resident macrophages). The phenotype is referred to as LyzMyd88-/- ; the litter mate controls were Myd88fl/fl. In solei from LyzMyd88-/-mice, cytokine secretory rates for interleukin-6 (IL-6) and keratinocyte-derived cytokine (KC, CXCL1) were significantly reduced to 56.3%, and 60.6% of control, respectively, over the first hour of LPS exposure. In the second hour, the secretion of granulocyte colony stimulating factor (G-CSF), IL-6, KC(CXCL1) and monocyte chemoattractant protein-1 (MCP-1, CCL2) were greatly elevated by 5-10-fold in both phenotypes compared to the first hour. However, only MCP-1 secretion was decreased to 70.6% of control in the second hour. We also tested the secretory response to buffer containing 1% sterile mouse plasma because dilute plasma is known to amplify the responses of macrophages to LPS. Treatment with 1% plasma alone affected baseline measures of some cytokines but resulted in no further increases in secretion during either hour of exposure. However, small and gradual increases in secretory rates were observed for several cytokines over the study period, with or without plasma, with the largest responses seen in IL-6 and KC. Overall, the results are consistent with a significant early contribution of myeloid-derived, resident immune cells to the cytokine secretory responses of intact oxidative skeletal muscle. In addition, small quantities of plasma in the buffer have no independent stimulatory effects on cytokine secretion.