A possible role for L-selectin in the release of polymorphonuclear leukocytes from bone marrow

Previous studies from our laboratory have shown that the expression of L-selectin on polymorphonuclear leukocytes (PMN) is higher in the bone marrow than in peripheral blood. The present study was designed to determine the location of this L-selectin loss as the PMN pass from the hematopoietic tissue into venous sinusoids of the bone marrow. Bone marrow and peripheral blood samples were collected at the beginning, during, and just after five normothermic cardiopulmonary bypass procedures in which there was active marrow release and compared with five hypothermic procedures in which marrow release was suppressed by lowering body temperature to 27°C. L-selectin expression was measured on PMN in the hematopoietic tissue and venous sinusoids in the bone marrow using quantitative histology and immunocytochemistry. At baseline there was more L-selectin on PMN in the bone marrow hematopoietic tissue than in the sinusoids (24.7 ± 3.5 vs. 10.3 ± 2.5%, P < 0.004). Bone marrow release during normothermic cardiopulmonary bypass procedure was associated with a rise in peripheral blood band cells (0.18 ± 0.7 vs. 2.98 ± 0.56 X 10 9 /I, P < 0.01) and a further reduction of L-selectin expression on PMN in the sinusoids (P < 0.03). Hypothermia (27°C) prevented both the rise in peripheral blood band cells and the reduction in L-selectin on PMN in the sinusoids. In vitro studies showed that lowering the temperature had a similar effect on shedding of L-selectin from PMN. We conclude that PMN shed L-selectin as they move from the hematopoietic compartment into the venous sinusoids of the bone marrow and postulate that this could control the release of PMN from the marrow.