Differential response of macrophage subpopulations to soleus muscle reloading after rat hindlimb suspension

The hypothesis that distinct populations of macrophages are associated with muscle necrosis and regeneration was examined in Wistar rat soleus muscle after 10 days of hindlimb suspension and 2, 4, and 7 days after the resumption of weight bearing. Necrosis was identified using histological features, such as muscle fiber infiltration, and regeneration was identified using immunohistochemical techniques for developmental myosin heavy chain (dMHC). Light-microscopic observations show that necrotic fibers in 2-day reloaded soleus muscle were invaded by ED1+ and Ia+ macrophages. The number of invaded fibers in muscles reloaded for 2 days increased to 2.8/mm2 compared with 0.2/mm2 in age-matched normal muscle but returned to control values by the 4th day of resumed weight bearing. In the interstitial spaces of 2-day recovery muscle, ED1+ and Ia+ macrophages numbered 369 and 332/mm2, respectively, compared with 12 and 72/mm2, respectively, in control soleus. After 7 days of reloading, the number of ED1+ cells was similar to that of control. Ia+ macrophages decreased to 240/mm2 at 4 days but after 7 days rose above control values to 429/mm2. ED2+ macrophages in 4- and 7-day reloaded soleus increased 70-80% in the interstitial spaces compared with control but were not observed to infiltrate necrotic muscle fibers at any time points. Immunohistochemistry and immunoblots using a monoclonal anti-dMHC antibody demonstrate a greater proportion of myofibers expressing dMHC isoforms after 4 and 7 days of reloading. These findings indicate that macrophage subpopulations are associated with distinct stages during the recovery process from hindlimb suspension: ED1+ macrophages are associated with muscle necrosis, whereas ED2+ cells are associated with muscle regeneration