IN-VITRO REACTIVITY OF CRYOGLOBULIN IGM AND IGG IN HEPATITIS-C VIRUS-ASSOCIATED MIXED CRYOGLOBULINEMIA

Background/Aims: Mixed cryoglobulinemia is frequently associated with chronic hepatitis C virus infection. We aimed to clarify the mechanism , kinetics and participating proteins in cryoprecipitate formation, wh ich are still being debated, Methods: Eighteen patients with cryoglobu linemia were studied, Isolated serum cryoprecipitates and purified cry oglobulin IgM and IgG fractions were analyzed in vitro by turbidimetry for temperature-dependent complex formation, Immunoglobulin reactivit y, i.e. in cryoprecipitates and in cryoglobulin-free sera, was studied using immunoblot and enzyme immunoassays. HCV RNA was detected by rev erse transcriptase/polymerase chain reaction, Results: By turbidimetry , purified cryo-IgM precipitated (in the absence of HCV RNA) with cryo -IgG as well as with non-cryoglobulin IgG and with IgG Fc or F(ab')(2) fragments, In contrast, purified cryo-IgG did not precipitate with no n-cryoglobulin IgM, Anti-HCV IgG reactivity was found in cryoglobulin- free sera, in cryoprecipitates and in purified cryoglobulin IgG fracti ons, The respective titers were similar, Purified cryo-IgM did not rea ct to HCV-encoded proteins. Binding of cryo-IgM to heterologous IgG wa s inhibited by intact IgG (up to a mean of about 52%) as well as by Ig G Fc (33%) and F(ab')(2) fragments (17%), Binding of cryo-IgM to IgG w as enhanced at low temperature (4 degrees C vs. 37 degrees C), particu larly for type III cryoglobulin IgM. Conclusions: In hepatitis C virus -associated cryoglobulinemia the in vitro precipitate formation depend ed on cryo-IgM, while IgG appeared to act as an unspecific antigenic p artner, Hepatitis C viral particles were probably not required, Cryo-I gM binding occurred primarily to intact IgG, Anti-HCV reactivity of ei ther cryo-IgM or cryo-IgG was not necessary for precipitate formation, Regarding the pathogenesis, a direct hepatitis C virus protein-depend ent stimulation of B-cells producing cryo-IgM seems to be unlikely.