CHANGES IN NEUTROPHIL ACTIN AND SHAPE DURING SEQUESTRATION INDUCED BYCOMPLEMENT FRAGMENTS IN RABBITS

Complement fragment-induced sequestration of neutrophils within the lu ngs may be mediated by stimulus-induced decreases in the deformability of neutrophils, prolonging their lung capillary transit times, As cha nges in deformability often occur through changes in cytoskeletal prot eins, this study determined whether the distribution of actin within i ntracapillary neutrophils runs altered by intravascular complement fra gments and whether sequestered neutrophils were less deformed. Ultrath in cryosections of lung tissue from rabbits given an infusion of compl ement fragments or saline were immunolabeled with anti-actin antibodie s. The number of gold particles/microvillus and the density of gold pa rticles/mu m(2) cytoplasm in the submembrane and the central region of intracapillary neutrophils was quantitated. Neutrophil shape was eval uated using laser confocal microscopy. In control rabbits, the ratio o f submembrane/central gold was always greater than one and most neutro phils were elongated, 97% having shape factors > 1.10, The ratio of su bmembrane/central gold was greater in complement-treated rabbits (5.1 +/- 0.9) than controls (2.6 +/- 0.4; P < 0.026), The number of gold pa rticles/microvillus was also increased in complement-treated rabbits ( 2.3 +/- 0.5) compared with controls (2.3 +/- 0.5; P < 0.045). Neutroph ils were more often spherical when rabbits received complement fragmen ts for 1.5 minutes than in control lungs or after 15-minute infusions, These data suggest that complement fragments induce a rapid redistrib ution of actin from the central to the submembrane region and the micr ovilli and result in more round neutrophils. This redistribution may d ecrease the deformability of neutrophils by altering the stiffness of the submembrane region and/or by preventing the microvilli from flatte ning.