SITES OF LEUKOCYTE SEQUESTRATION IN THE PULMONARY MICROCIRCULATION

The location and mechanisms of leukocyte sequestration in the pulmonar y circulation have been investigated by using high-magnification in vi vo videomicroscopy to record the passage of unlabeled native leukocyte s through canine pulmonary capillaries. Of 650 leukocytes traversing c apillary networks, 46 +/- 6% (SE) of the leukocytes passed through wit hout stopping, 42 +/- 9% stopped in segments between junctions, and 12 +/- 4% stopped in junctions. Leukocytes rolling along arteriolar wall s were nearly spherical, as 94% had aspect ratios (major axis divided by minor axis) less than or equal to 1.25. To pass through the capilla ry bed, the leukocytes deformed into elongated shapes. Many leukocytes remained elongated after entering the venules (53% had aspect ratios greater than or equal to 1.25). Venular rolling was blocked by fucoidi n (blocking both L- and P-selectin) but not by anti-P-selectin antibod ies alone, indicating that rolling leukocytes adhered to the venular e ndothelium by L-selectin. These observations demonstrate that leukocyt es deform to transit the capillary bed, that they stop more frequently in segments than in junctions, and that rolling leukocytes in the ven ular marginated pool adhere via L-selectin.