IgG Subclass Profiles of HLA Antibodies Enhance Prediction of C1q-Binding in Kidney Transplant Recipients

<b>Background/Objectives</b>: While standard Luminex single antigen bead (SAB) detects total IgG antibodies, qualitative differences among IgG subclasses may influence their immunologic risk. In particular, complement fixing ability, assessed via C1q binding, is linked to poor transplant outcomes. This study aimed to evaluate the relationship between IgG subclasses and C1q-binding activity in HLA antibodies and to define clinically relevant subclass-specific mean fluorescence intensity (MFI) thresholds for predicting complement binding. <b>Methods</b>: We analyzed 4189 HLA IgG bead reactions from sera of 37 kidney transplant recipients using SAB assays for total IgG, IgG1-4 subclasses, and C1q-binding. IgG subclasses were assessed using a modified SAB assay with subclass-specific monoclonal secondary antibodies. <b>Results</b>: IgG reactivity (MFI ≥ 1000) was observed in 15.3% of beads (639/4189), with 31.0% (198/639) also positive for C1q binding. IgG⁺C1q⁺ beads exhibited significantly higher MFIs compared with IgG⁺C1q^- beads. IgG1 showed positive correlations with both total IgG (<i>r_s</i> = 0.5439, <i>p</i> < 0.0001) and C1q MFIs (<i>r_s</i> = 0.4042, <i>p</i> < 0.0001), with the strongest correlations at HLA-DQ. Among subclass-positive beads, IgG1 predominated and was strongly associated with C1q binding, whereas isolated IgG2 or IgG4 positivity was rarely C1q-binding. ROC analysis identified an IgG1 MFI threshold of >837 to predict C1q positivity with 73.2% sensitivity and 92.3% specificity, while the cutoff for total IgG MFI was >7881 with 85.4% sensitivity and 88.9% specificity. At the patient level, IgG1-positive immunodominant DSAs were more frequent in antibody-mediated rejection than in non-rejection biopsies <b>Conclusions</b>: IgG1 predominates among complement-fixing antibodies and correlates strongly with total IgG and C1q binding. Quantitative IgG subclass assessment, especially IgG1, may serve as a useful predictor of complement activation.