Figure 7. FAPs phagocytize cellular debris in regenerating muscle.

(A) FAPs phagocytize necrotic cells in vitro. FAPs isolated from regenerating muscles were fed CFSE-labeled thymocytes for 1h, n=3. LTs: live thymocytes, NTs: necrotic thymocytes, ATs: apoptotic thymocytes, OTs: opsonized thymocytes. (B) Confocal microscopy of WT FAPs phagocytizing necrotic thymocytes (magnification, X1200). PDGFRα staining (red) identifies FAPs, whereas CFSE (green) represents necrotic thymocytes. Blue is DAPI. (C, D) FAPs are efficient at phagocytizing necrotic debris in vivo. (C) Three days after CTX-induced injury, TA muscles were injected with CFSE-labeled necrotic thymocytes and phagocytosis was enumerated 1h later. Data is plotted as percent uptake in the indicated cellular population, n=3. (D) Confocal microscopy of FAPs in regenerating muscle (magnification, X600). Red-PDGFRα, Green-IgG. (E, F) Representative images of IL-4Rα^f/f and IL-4Rα^f/fPdgfrα^Cre TA muscles 8 days after injury CTX, n=4–5 per genotype. (E) Evans Blue staining of CTX-injured muscles (magnification, X100). (F) Hematoxylin and eosin staining of representative section (magnification, X200). (G, H) Quantification of CFSE⁺ FAPs in WT, ΔdblGATA, and IL-4Rα^−/− mice, n=6–8 per genotype. (I, J) IL-4 promotes clearance of necrotic debris in regenerating muscle. Representative images of IgG deposition (magnification, X580) (I) or calcified necrotic debris as visualized by Alizarin Red S stain (magnification, X40) (J), n=5 per treatment. (K) Model for functions of type 2 immunity in regenerating muscles. Error bars represent SEM. See also Figure S6 and S7.