HIV-1 broadly neutralizing antibodies demonstrate a high propensity for binding to heme.

Polyreactivity is the ability of antibodies to bind to various unrelated antigens with low affinities and is a frequent feature of HIV-1 broadly neutralizing antibodies (bNAbs). Besides naturally occurring polyreactivity, human immune repertoires contain antibodies that can acquire polyreactivity de novo, by their interaction with the heterocyclic cofactor molecule heme. Since polyreactivity could influence antibody functions, we investigated here the impact of cofactor-induced polyreactivity on HIV-1 bNAbs. To this end, we evaluated the binding to heme of 38 HIV-1 bNAbs and 43 influenza virus (Flu) neutralizing antibodies. The majority of HIV-1 bNAbs were heme reactive, whereas only few human anti-Flu antibodies interacted with this cofactor. Molecular modeling and mutagenesis further showed that heme interacts with regions rich in aromatic and positively charged amino acid residues in bNAbs' paratopes. Strikingly, heme interaction with bNAbs strongly enhanced their intrinsic polyreactivity, while not altering their HIV-1 binding and neutralization potentials. Together, these findings contribute to a better understanding of the molecular properties of HIV-1-neutralizing Abs and underscore the importance of the interaction of bNAbs with heme under certain pathological conditions.