L- and P-selectin and CD11/CD18 in intracapillary neutrophil sequestrationin rabbit lungs

Infusion of complement fragments induces rapid sequestration of neutrophils within pulmonary capillaries. This study examined the mechanisms through w hich this sequestration occurs, as well as the effect of complement fragmen ts on the expression of L-selectin and CD11/CD18 using ultrastructural immu nohistochemistry. Studies using anti-P-selectin antibodies, fucoidin, L-sel ectin-depleted neutrophils, and anti-CD18 antibodies showed that selectins and CD18 were not required for neutrophil sequestration. However, maintaini ng the sequestered neutrophils within the pulmonary capillaries required bo th L-selectin and CD11/CD18. Neutrophils in the pulmonary capillaries of ra bbits given complement fragments expressed 72% less L-selectin and 98% more CD11/CD18 than did those in rabbits given saline. Shedding of L-selectin o ccurred preferentially from the microvillar processes of the plasma membran e rather than from the flat intervening regions. About 28% of L-selectin st ill remained on intracapillary neutrophil membranes after 15 min and was li kely available for binding. Shedding of L-selectin appeared slower in vivo than in vitro. These studies indicate that neutrophil sequestration induced by complement fragments requires at least two sequential steps, one that d oes not require recognized adhesion molecules followed by a second that req uires L-selectin and CD11/CD18.