Transgenic Expression of Human Platelet FcγRIIA (CD32A) in Mice Does Not Affect the Thrombocytopenia Associated with WASp Deficiency.

Wiskott-Aldrich Syndrome (WAS) is an X-linked hematopoietic disorder that is characterized by immune deficiency, eczema and severe thrombocytopenia with small platelets. Platelets isolated from WAS patients are cleared rapidly from the circulation when transfused autologously. However, the role of WASp, the protein mutated or absent in WAS, is unclear since platelets isolated from WAS patients function normally. WASp knockout (KO) mice only have a mild thrombocytopenia. Because 1) human, but not mouse platelets express the Fc receptor for IgGs, FcγRIIA (CD32A), and 2) platelet-associated IgGs (PAIgGs) are often observed in WAS patients, we sought to determine whether FcγRIIA expression in humans was involved in the severe thrombocytopenia associated with WAS. WASp KO mice expressing human FcγRIIA on the surface of their platelets were generated by breeding female mice carriers for WASp deficiency with heterozygous FcγRIIA transgenic (TG) males. A total of 221 offsprings were obtained, of which only 97 were males. WASp KO / FcγRIIA TG males were a minority, with a total of 19 animals, compared to 24 WASp WT / FcγRIIA WT, 30 WASp WT / FcγRIIA TG and 24 WASp KO / FcγRIIA WT males. WASp KO / FcγRIIA WT and WASp KO / FcγRIIA TG males had about 70% of normal platelet count compared to WASp WT / FcγRIIA WT and WASp WT / FcγRIIA TG mice (Table 1). Thus, FcγRIIA expression did not affect the thrombocytopenia associated with WASp deficiency. PAIgGs were not detected on the surface of WASp KO / FcγRIIA TG platelets, as evaluated by flow cytometry using an anti-mouse IgG antibody. Spleen weight was increased in WASp KO / FcγRIIA TG compared to WASp WT / FcγRIIA WT and WASp WT / FcγRIIA TG males, but similar to that of WASp KO / FcγRIIA WT males (Table 1). Age of the mice was not involved since similar results were obtained with 6-, 12- or 24-weeks old mice. However, an increased population of macrophages appeared in the spleen of mice lacking WASp as they aged, as evidenced by flow cytometry using anti-mouse Mac-1 (CD11b) and Gr-1 (Ly-6G) antibodies. Our data indicate that expression of platelet FcγRIIA alone does not explain the difference observed between the severe thrombocytopenia of WAS patients and the mild thrombocytopenia of WASp KO mice. WASp deficiency may affect the surface organization of platelets such that clearance is accelerated by spleen macrophages.