Importance of circulating monocyte subpopulations and Fcγ receptors in immune thrombocytopenia

Monocytes have been linked to the pathogenesis of immune thrombocytopenia (ITP) because of their role in autoantibody-mediated platelet phagocytosis. However, monocytes constitute unique populations with major differences in expression for surface Fc & gamma; receptors (Fc & gamma;Rs). Thus, we evaluated monocytes in whole blood samples from patients with newly diagnosed and chronic ITP. Monocyte subpopulations were identified phenotypically by flow cytometry and defined according to the surface expression of CD14 (lipopolysaccharide receptor) and of CD16 (low-affinity Fc & gamma; receptor III) into classical (CLM), intermediate (INTM) and nonclassical (non-CLM) monocytes. We also examined the expression of Fc & gamma;RI/CD64 and Fc & gamma;RIII/CD16 by monocyte subpopulations.Newly diagnosed patients showed a decrease in non-CLM, expressed as a relative percentage of total monocytes compared with controls and chronic ITP patients. Both non-CLM and INTM of newly diagnosed patients closely correlated with platelet count. These monocyte subpopulations showed significantly enhanced CD64 expression in newly diagnosed patients. On the contrary, patients with chronic ITP presented higher non-CLM in percentage than controls and concomitant lower CLM and total monocytes, in percentage and number. The expression of CD64 was increased by all monocyte subpopulations, CLM, INTM, and non-CLM in chronic patients. In conclusion, differences in monocyte subpopulations, together with enhanced expression of Fc & gamma;RI/CD64 are evident in patients with ITP.