Infections caused by Mycoplasma genitalium, a human genital tract pathogen, often persist for months to years . Antigenic variation of the immunodominant adhesin proteins, MgpB and MgpC, is thought to enhance persistence by avoiding specific antibody-dependent immune clearance. We used surface plasmon resonance (SPR) to study the binding of antibodies from immunized rabbits and experimentally infected primates to recombinant MgpB protein fragments. Primate antibodies collected from 2 weeks before to 2, 4, and 8 weeks after infection associated specifically with two different domains of the MgpB adherence protein (variable region B and a conserved C-terminal region) bound via amine coupling to the surfaces of CM5 sensor chips. Association and dissociation kinetics were measured and used to calculate dissociation constants (KD values). Association kinetics varied by 80-fold over the 14 antibody-antigen combinations tested, while dissociation kinetics varied by only 4-fold, suggesting that association kinetics describe the interactions better than dissociation. In general, antibodies from later time points in the immune response bound antigens more tightly and more quickly, as expected from the process of affinity maturation. Antibodies bound best to the conserved C-terminal region, increasing in affinity from 300 nM to 5 nM for antibodies 4 or more weeks after infection. Binding to variable region B also increased from 300 nM to 5 nM, but highest affinity was measured 2 weeks after infection, after which affinity decreased. Binding was lowest to a variant B region that predominated 8 weeks after infection. Antibodies before infection bound with weak (1 μM) affinity, increasing to around 300 nM after infection. None of the antibody samples bound the variant B region with better than high-nanomolar affinity. Overall, we measured that antibody affinity increased by 7- to 20-fold after infection, but that this could be counteracted by sequence variation that reduced peak affinity by 2- to 3-fold.

Antigenic variation in Mycoplasma genitalium reduces antibody-antigen association kinetics

ACS Fall 2021

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