Complement activation in patients with immune thrombocytopenic purpura according to phases of disease course

Summary Immune thrombocytopenic purpura (ITP) is an autoimmune thrombocytopenia with shortened platelet survival and relative bone marrow failure. The pathogenesis involves antibody production, cytokine release, T cell impairment, complement activation and clearance of platelets. We measured plasma levels of C3, C4, C1q and sC5b‐9 in 80 ITP patients in acute phase, 50 ITP patients in complete (CR) or partial (PR) remission and 50 age‐ and sex‐matched healthy volunteers. Statistical analyses showed that acute ITP patients had higher plasma levels of sC5b‐9 and C1q than CR or PR patients (median = sC5b‐9: 200  versus  98 mg/dl, P ‐value < 0·001) (median C1q = 2·11 versus 1·00 mg/dl, P ‐value < 0·001). CR and PR ITP patients had sC5b‐9 and C1q plasma levels comparable to those observed in healthy volunteers. There was a significant correlation between sC5b‐9 and C1q plasma levels (Spearman’s rho correlation index on 130 ITP patients equal to 0·58, P ‐value < 0·001). We also found that sC5b‐9 plasma level is inversely correlated with the number of platelets. Furthermore, we divided acute ITP patients into subjects with detectable (24 of 80, 30%) or undetectable (56 of 80, 70%) anti‐platelet antibodies; patients with detectable anti‐platelet antibodies have significantly higher plasma levels of C1q and sC5b‐9. This research will potentially offer novel therapeutic strategies in light of new drugs affecting complement activation for monitoring therapy response.