Distribution of actin-binding protein and myosin in polymorphonuclear leukocytes during locomotion and phagocytosis

Polymorphonuclear (PMN) leukocytes, highly motile ameboid cells vital for mammalian defense against infection, acquire a distinct polarized morphology during locomotion and phagocytosis. An organelle-excluding pseudopod extends in the direction of movement and surrounds objects during phagocytosis. The anterior pseudopod contains a three-dimensional network of actin filaments. Actin-binding protein (ABP) and myosin cause the crosslinking and contraction, respectively, of actin filaments in vitro. We used indirect immunofluorescence to study the redistribution of myosin and ABP molecules in rabbit PMN leukocytes during locomotion and phagocytosis. In unpolarized PMN leukocytes, ABP and myosin had a diffuse distribution with some predilection for the cortex. In polarized PMN leukocytes crawling toward yeast particles, myosin and ABP staining concentrated in the anterior pseudopod. In PMN leukocytes fixed during phagocytosis of the yeast particles, antimyosin and anti-ABP staining concentrated strikingly in the distal portions of the pseudopod embracing the yeasts. Staining for catalase, a cytoplasmic protein in PMN leukocytes, for lactoferrin, a protein of specific granules, and for myeloperoxidase, a protein of azurophilic granules, was not concentrated in pseudopods. Taken together with available morphologic and biochemical information, these findings are consistent with a mechanism wherein interactions of actin, ABP and myosin redistribute cortical cytoplasm into pseudopods involved in locomotion and phagocytosis.