5 fluorouracil (
5FU) has been a first-choice
chemotherapy drug for several
cancer types (e.g., colon, breast, head, and neck); however, its efficacy is diminished by patient acquired resistance and pervasive side effects.
Leukopenia is a hallmark of
5FU; however, the impact of 5FU-induced
leukopenia on healthy tissue is only becoming unearthed. Recently, skeletal muscle has been shown to be impacted by
5FU in clinical and preclinical settings and weakness and
fatigue remain among the most consistent complaints in
cancer patients undergoing
chemotherapy. Monocytes, or more specifically macrophages, are the predominate immune cell in skeletal muscle which regulate turnover and homeostasis through removal of damaged or old materials as well as coordinate skeletal muscle repair and remodeling. Whether 5FU-induced
leukopenia extends beyond circulation to impact resident and infiltrating skeletal muscle immune cells has not been examined. The purpose of the study was to examine the acute effects of
5FU on resident and infiltrating skeletal muscle monocytes and inflammatory mediators. Male C57BL/6 mice were given a physiologically translatable dose (35 mg/kg) of
5FU, or PBS, i.p. once daily for 5 days to recapitulate 1 dosing cycle. Our results demonstrate that
5FU reduced circulating leukocytes, erythrocytes, and thrombocytes while inducing significant
body weight loss (>5%). Flow cytometry analysis of the skeletal muscle indicated a reduction in total CD45+ immune cells with a corresponding decrease in total CD45+CD11b+ monocytes. There was a strong relationship between circulating leukocytes and skeletal muscle CD45+ immune cells. Skeletal muscle Ly6cHigh activated monocytes and M1-like macrophages were reduced with
5FU treatment while total M2-like CD206+CD11c- macrophages were unchanged. Interestingly,
5FU reduced bone marrow CD45+ immune cells and CD45+CD11b+ monocytes. Our results demonstrate that
5FU induced
body weight loss and decreased skeletal muscle CD45+ immune cells in association with a reduction in infiltrating Ly6cHigh monocytes. Interestingly, the loss of skeletal muscle immune cells occurred with bone marrow cell cycle arrest. Together our results highlight that skeletal muscle is sensitive to
5FU's off-target effects which disrupts both circulating and skeletal muscle immune cells.