Leukocyte Kinetics in Acute Lung Injury (Ali)-Associated Skeletal Muscle Wasting and Recovery

Rationale: Critically ill patients undergo severe skeletal muscle wasting.Muscle biopsies show infiltration of leukocytes, yet their function remains uncertain.This project uses a murine acute lung injury (ALI)-associated skeletal muscle injury model to define leukocyte composition over the timecourse of muscle injury and recovery, and their relationship to muscle function.Methods: Our lab has previously demonstrated that muscle wasting is partially rescued by 2 days of treadmill exercise during lung injury.ALI was induced in male and female C57BL/6J and MaFIA mice using intratracheal Escherichia coli LPS.A subset of ALI-mice then underwent two days of early moderate-intensity exercise.The tibialis anterior (TA) muscles were dissociated into single cell suspension.Ly6C high and Ly6C low macrophage/monocytes (MΦ/Mo), neutrophils, and dendritic cells were analyzed by flow cytometry at pre-injury (control), ALI days 3, 10, and 25-28.MΦ/Mo were profiled as proinflammatory with MHCII and anti-inflammatory with CD206 and CD163.Ex vivo isolated soleus absolute and specific tetanic contractile force measurements were performed.Results: Skeletal muscle hematopoietic-derived leukocytes (CD45 + ) decreased following lung injury and did not return to baseline by day 25-28 (late ALI) (p=0.049 by ANOVA), whereas neutrophil infiltration peaked at day 3 of ALI (ALI Day 3: 34.93% vs Control: 13.59%, p=0.003) and returned to control levels by day 10 (p<0.001).This neutrophil peak occurred simultaneously with nadir maximal absolute tetanic force of the soleus muscle (p=0.006).Dendritic cells decreased and returned to baseline by late ALI (ALI Day 3: 11.56% vs ALI Day 25-28: 26.07%, p=0.0003), and had increase in late ALI with early exercise compared to non-exercised ALI mice (p=0.009).Ly6C low MΦ/Mo cells decreased through late ALI (p=0.0006) and had a decreasing percentage of pro-inflammatory (Control: 30.1% vs ALI Day 10: 8.2%, p=0.041) and increasing anti-inflammatory phenotype expression (ALI Day 3 14.8%Ly6C low MΦ/Mo vs Late ALI 36.1%Ly6Clow MΦ/Mo, p=0.015).Soleus muscle maximal absolute tetanic force demonstrated dynamic changes over the timecourse (p=0.002) with decreased force generation during acute ALI followed by recovery in late ALI.Conclusions: We found dynamic changes in muscle hematopoietic-derived leukocyte populations over the course of LPS-induced acute lung injury and recovery, with persistent changes even 28 days later.The changes in Ly6C low MΦ/Mo functional phenotype percentage suggests that macrophages may be participants in the muscle functional response.Early exercise modulates late muscle function and muscle-associated leukocyte composition.Interestingly, dendritic cells may increase with early exercise suggesting exercise may attenuate muscle wasting by altering the muscle immune cell milieu.