Mechanisms for increment of platelet associated IgG and platelet surface IgG and their implications in immune thrombocytopenia associated with chronic viral liver disease.

In addition to hypersplenism, immunological destruction of platelets by elevated platelet associated IgG (PAIgG) and platelet surface IgG (PSIgG) has been proposed as a causative factor for thrombocytopenia in chronic liver disease (CLD), although the implication of PAIgG may be debatable since recent investigations on idiopathic thrombocytopenic purpura disclosed the fact that PAIgG largely relates to the intra-platelet IgG in alpha-granules and not to PSIgG. Further, with regard to the elevated PSIgG of CLD, characterization as to whether it mainly represents anti-platelet glycoprotein (GP) antibodies or IgG contained in the immune complex has not been elucidated. Thirty-seven patients with chronic viral liver disease (CVLD); 31 hepatitis C and 6 hepatitis B were included in this study. First we monitored the changes in levels of PAIgG, alpha-granule IgG, PSIgG and mean platelet volume (MPV) during the course of partial splenic arterial embolization (PSE). The elevated level of PAIgG decreased after PSE, paralleling that of alpha-granule IgG, while PSIgG showed no change; MPV decreased reciprocally with the increase of platelet count. These results indicate that the increment of PAIgG in CVLD may be caused by accelerated destruction of platelets; this generally evokes hyperproduction of large-sized thrombocytes, which have an increased capability to uptake circulating IgG. To characterize PSIgG, we then tested CVLD patients for antiplatelet GP antibodies and found only a 5.4% positivity. It was also found that circulating immune complex levels in CVLD patients were clearly elevated, correlating with the levels of PSIgG. Thus, it was surmised that immune complexes bound to the platelet surface, and not platelet specific GP antibodies, may be playing a crucial role in platelet destruction of CVLD, possibly through phagocytosis by macrophages.