Polymorphonuclear leukocytes L-selectin expression decreases as they age in circulation

S. F. Van Eeden, S. Bicknell, B. A. Walker and J. C. Hogg University of British Columbia, Pulmonary Research Laboratory, St. Paul's Hospital, Vancouver, Canada. We recently reported that L-selectin expression increases on circulating polymorphonuclear leukocytes (PMN) during active bone marrow release, which suggests that older cells in the circulation have lower levels of L-selectin than those recently released from the bone marrow. The present study was designed to test the hypothesis that L-selectin expression reduces on PMN as they age in the circulation. In vitro studies using flow cytometry showed that PMN L-selectin decreased to 14.6 +/- 2.3% of baseline during a 24-h incubation at 37 degrees C. To test this hypothesis in vivo, rabbit PMN, labeled in vivo with 5'-bromo-2-deoxyuridine (PMN-brdU) in donor animals, were infused into recipient rabbits as whole blood (n = 5) and followed over 24 h in the circulation with a double immunolabeling technique. These results showed that the fraction of the L-selectin-negative PMN-BrdU in the circulation increased with time (P < 0.001), and nearly all of the PMNBrdU in the circulation were L-selectin negative after 24 h. Removal of L-selectin from the surface of PMN-BrdU with chymotrypsin before infusion did not change their rate of removal from the circulation (half-life or t 1/2 262 vs. 296 min, P = NS). We conclude that there is a continuous loss of L-selectin from PMN during their life span in the circulation, which supports the hypothesis that L-selectin expression decreases on PMN as they age.