Macrophages, dendritic cells, and neutrophils use phagocytosis to capture and 
clear off invading pathogens. The process is triggered by the interaction of 
ligands on the pathogens' surface with specific phagocytic receptors, including 
immunoglobulin (FcR) and complement C3bi (CR3) receptors (integrin 
alpha(M)beta2, Mac1) . Localized actin-filament assembly that acts as the 
driving force for particle engulfment is controlled by Rho-family small GTPases 
. RhoA regulates CR3-mediated phagocytosis through a mechanism that is still 
unclear . Mammalian Diaphanous-related (mDia) formins participate in the 
generation of a diverse set of actin-remodeling events downstream of RhoA , and 
mDia1 is recruited around fibronectin-coated beads in a RhoA-dependent manner in 
fibroblasts . Here, we set out to examine whether mDia proteins are involved in 
CR3-mediated phagocytosis in macrophages. We show that the RhoA effector mDia1 
is recruited early during CR3-mediated phagocytosis and colocalizes with 
polymerized actin in the phagocytic cup. Interfering with mDia activity inhibits 
CR3-mediated phagocytosis while having no effect on FcR-mediated phagocytosis. 
These results indicate a new function for mDia proteins in the regulation of 
actin polymerization during CR3-mediated phagocytosis.