Increased uptake by splenic red pulp macrophages contributes to rapid platelet turnover in WASP(–) mice

Thrombocytopenia caused by rapid platelet consumption contributes to the severe thrombocytopenia of Wiskott-Aldrich syndrome (WAS) and to the milder thrombocytopenia seen in murine WAS. We show that rapid clearance of 111 In-labeled murine WASP(–) platelets correlates with enhanced splenic uptake. Using platelets labeled with a pH-sensitive fluorescent marker (pHrodo), we quantify normal platelet uptake by red pulp macrophages (RPMs), and demonstrate its enhancement after in vivo opsonization of platelets. The spleens of WASP(–) mice contain an increased number of RPM, and rapid clearance of WASP(–) platelets in WASP(–) mice in turn generates an increased number of pHrodo(+) splenic RPMs. To separately assess the platelet intrinsic and recipient-dependent functions involved in the clearance and splenic phagocyte uptake of WASP(–) platelets, we performed "crossed" pHrodo(+) platelet injection studies (wild type [WT] to WASP(–), WASP(–) to WT). We show that an extrinsic effect of recipient WASP deficiency on the clearance of WASP(–) platelets correlates with increased platelet uptake by RPMs. An intrinsic effect of platelet WASP deficiency on platelet clearance does not, however, correlate with increased total uptake by WT or WASP(–) RPMs. In contrast to other published findings, we find no evidence of a baseline or antibody-induced increase in phosphatidyl serine exposure on WASP(–) platelets. Our findings suggest that an increased number of RPMs in WASP(–) mice contributes significantly to the increased platelet consumption rate in WASP(–) mice. This might explain the consistent efficacy of splenectomy in murine and clinical WAS.