Characterizing the skeletal muscle immune microenvironment for sarcopenia using transcriptome analysis and histological validation

Sarcopenia is a condition characterized by the age-related loss of skeletal muscle mass and function. The pathogenesis of the disease is influenced by chronic low-grade inflammation. However, the specific changes in the immune landscape changes of sarcopenic muscle are not yet fully understood. To gain insights into the immune cell composition and interactions, we combined single-nuclei RNA sequencing data, bulk RNA sequencing datasets, and comprehensive bioinformatic analyses on skeletal muscle samples from young, aged, and sarcopenic individuals. Histological staining was then performed on skeletal muscles to validate the distribution of immune cells in clinical samples. Overall, we analyzed the transcriptomes of 101,862 single nuclei, revealing a total of 10 major cell types and 6 subclusters of immune cell types within the human skeletal muscle tissues. Among the immune cells, macrophages constituted the largest immune fraction. A specific marker gene LYVE1 for skeletal muscle macrophages was further identified. Cellular subclasses included four distinct groups of resident macrophages, which play a different role in physiological or non-physiological conditions. Using bulk RNA sequencing data, we identified strong enrichment for a macrophage- rich inflammation in sarcopenia. Our findings demonstrate age-related changes in the composition and cross-talk of immune cells, which contribute to chronic inflammation. Furthermore, macrophages emerge as a potential therapeutic target, thus advancing our understanding of the pathogenesis of sarcopenia.