Characterising antibody avidity in individuals of varied Mycobacterium tuberculosis infection status using surface plasmon resonance

There is increasing evidence supporting a role for antibodies in protection against tuberculosis (TB), with functional antibodies being described in the latent state of TB infection. Antibody avidity is an important determinant of antibody-mediated protection. This study characterised the avidity of antibodies against Ag85A, an immunodominant Mycobacterium tuberculosis (M.tb) antigen and constituent of several anti-TB vaccine candidates, in individuals of varied M.tb infection status. Avidity of Ag85A specific antibodies was measured in 30 uninfected controls, 34 individuals with latent TB infection (LTBI) and 75 active pulmonary TB (APTB) cases, employing the more commonly used chaotrope-based dissociation assays, and surface plasmon resonance (SPR). Chaotrope-based assays indicated that APTB was associated with a higher antibody avidity index compared to uninfected controls [adjusted geometric mean ratio (GMR): 1.641, 95% confidence interval (CI): 1.153, 2.337, p = 0.006, q = 0.018] and to individuals with LTBI [adjusted GMR: 1.604, 95% CI: 1.282, 2.006, p < 0.001, q <0.001]. SPR assays showed that APTB was associated with slower dissociation rates, an indication of higher avidity, compared to uninfected controls (adjusted GMR: 0.796, 95% CI: 0.681, 0.932, p = 0.004, q = 0.012) and there was also weak evidence of more avid antibodies in the LTBI compared to the uninfected controls (adjusted GMR: 0.871, 95% CI: 0.763, 0.994, p = 0.041, q = 0.123). We found no statistically significant differences in anti-Ag85A antibody avidity between the APTB and LTBI groups. This study shows that antibodies of increased avidity are generated against a principle vaccine antigen in M.tb infected individuals. It would be important to determine whether TB vaccines are able to elicit a similar response. Additionally, more research is needed to determine whether antibody avidity is important in protection against infection and disease.