Analysis of peritoneal macrophages at 5 days post injection showed that expression of full-length MT4-MMP yielded normal cell-surface αM integrin levels in infected MT4-MMP-null macrophages (GFP+Itgam+F4/80+); in contrast, infection with the catalytic dead mutant yet resulted in higher surface αM integrin levels than observed in wild-type macrophages, demonstrating that MT4-MMP catalytic activity regulates αM integrin cell-surface levels in vivo (Fig. 3c). Here, MT4 is linked to infection.