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 ¹¹¹In-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.