Genetic basis for the breadth and potency of HIV-1 V2-specific antibodies

Abstract

Some HIV infected people develop broadly neutralizing antibodies (bNAbs) that block many diverse unrelated strains of HIV from infecting target cells, and through passive immunization, protect animals from SHIV infection. Therefore, understanding the development of bNAbs and their neutralization can inform the design of an HIV vaccine. We investigated the genetic basis for the neutralization capacity of members of a bNAb family that target the variable loop 2 (V2) of the HIV-1 Envelope trimer. This lineage, CAP256-VRC26, contains bNAbs and closely related “off-track” members that lack breadth. We found that few mutations in the third complementarity-determining region of the heavy chain (CDRH3), which forms most of the paratope, restricts the breadth of two off-track antibodies. Furthermore, we established that a globally rare viral variant guided one of these antibodies away from breadth, indicating that vaccine immunogens should represent common circulating strains. Analysis of the sequence of the unmutated common ancestor (UCA) of the CAP256-VRC26 lineage showed that the off track mutations were predicted to occur with a higher probability than the breadth-conferring mutations. Most of the improbable mutations were located in the CDRH3 and we found that transferring the CDRH3 from bNAb CAP256.25 into the CAP256.UCA introduced breadth and tolerance for emerging viral variants. In addition, we showed that the framework and light chain contributed to potency and that the second complementarity-determining region of the light chain forms part of the paratope of CAP256.25. Together, approximately half of the mutations in CAP256.25 were necessary for broad neutralization, indicating that immunogens that target affinity maturation to key sites, including the CDRHs and light chain, could rapidly elicit breadth. In addition to neutralization, antibodies mediate effector functions via the Fc domain which is important for both active and passive immunization. By determining the breadth and effector functions of newly identified IgG4 class switched members of the CAP256-VRC26 lineage, we showed that effector functions can vary dramatically across related antibodies and the Fab sequence can impact these functions. Furthermore, we demonstrated that IgG4-mediated phagocytosis was greater than IgG1 matched antibodies which may be important for passive immunization since both IgG1 and IgG4 cross into the placenta. Together, we have identified key regions and mutations that confer breadth and potency, and modulate Fc effector function for a V2-specific bNAb lineage. This informs the design of a vaccine that directs affinity maturation and the formulation of functional monoclonal antibodies for passive immunization