A novel anti-C6 based approach for IgG avidity testing in Lyme borreliosis: A proof-of-concept study

Differentiating past from ongoing infection by serology is challenging. The sequential antibody response enables the detection of seroprogression on line immunoassays (LIA) but the different immunological age of antibodies, hence their different stage of affinity maturation also hinders IgG avidity determination. The conserved C6 segment of VlsE evokes an early, robust IgG response, thus might prove a suitable antigen for avidity measurements. Sera from 49 patients were tested (6 erythema migrans - EM, 22 post-primary - PP and 21 past infection). Paired sera were available from 24 patients in whom the diagnosis was confirmed by seroprogression and/or intrathecal antibody production (6 EM, 7 PP and 11 past infection). An in-house C6 avidity ELISA was used with 6 M urea solution and avidity-enhanced LIAs for representative samples. The avidity of reactive early and late antibodies differed on avidity-enhanced LIAs. Baseline C6 IgG intensity was higher in the PP group (EM vs. PP vs. past medians: 0.36 [IQR: 0.3-0.44] vs. 3.64 [IQR: 2.7-4.2] vs. 0.58 [0.36-1.1], p < 0.01, Kruskal-Wallis). C6 avidity at baseline did not differ in PP and past Lyme. Anti-C6 intensity and avidity evolved differently in the three groups between baseline and follow-up: in EM patients the avidity remained similarly low but the intensity increased (0.36 [0.3-0.44]) vs. 0.73 [0.62-0.87], p = 0.03, Wilcoxon); in PP Lyme the avidity increased (63.9 [42.1-73.6] vs. 74.7 [70-90.8], p = 0.03, Wilcoxon). Both the anti-C6 IgG avidity and intensity remained similar with residual antibodies from past infection. In conclusion, anti-C6 intensity and avidity changed as expected according to fundamental immunology. Anti-C6 IgG avidity testing might provide us with a useful, quantitative, supplementary tool in the future to differentiate past from active infection.