LAUSR

It is unknown under what conditions and to what extent weak/non‐complement (C)‐binding IgG subclasses (IgG2/IgG4) can block C1q‐binding triggered by C‐binding IgG subclasses (IgG1/IgG3). Therefore, we investigated in vitro C1q‐binding induced by IgG subclass mixtures targeting the same HLA epitope. Various mixtures of HLA class II specific monoclonal antibodies of different IgG subclasses but identical V‐region were incubated with HLA DRB1*07:01 beads and monitored for C1q‐binding. The lowest concentration to achieve maximum C1q‐binding was measured for IgG3, followed by IgG1, while IgG2 and IgG4 did not show appreciable C1q‐binding. C1q‐binding occurred only after a critical amount of IgG1/3 has bound and sharply increased thereafter. When both, C‐binding and weak/non‐C‐binding IgG subclasses were mixed, C1q‐binding was diminished proportionally to the fraction of IgG2/4. A 2‐ to 4‐fold excess of IgG2/4 inhibited C1q‐binding by 50%. Very high levels (10‐fold excess) almost completely abrogated C1q‐binding even in the presence of significant IgG1/3 levels that would usually lead to strong C1q‐binding. In sensitized renal allograft recipients, IgG subclass constellations with ≥ 2‐fold excess of IgG2/4 over IgG1/3 were present in 23/66 patients (34.8%) and overall revealed slightly decreased C1q signals. However, spiking of patient sera with IgG2 targeting a different epitope than the patient's IgG1/3 synergistically increased C1q‐binding. In conclusion, if targeting the same epitope, an excess of IgG2/4 is repressing the extent of IgG1/3 triggered C1q‐binding in vitro. Such IgG subclass constellations are present in about a third of sensitized patients and their net effect on C1q‐binding is slightly inhibitory.