Exploring the relationship between natural and cryptic polyreactivity in human antibodies.

Antibody polyreactivity, defined as the ability of a single antibody molecule to recognize multiple unrelated antigens, is a common feature of the human antibody repertoire and contributes to immune defense and homeostasis. In addition to naturally occurring polyreactivity, antibodies can acquire inducible polyreactivity following exposure to environmental factors such as heme, ferrous ions, reactive oxygen species, or protein-destabilizing conditions. Although this "cryptic" polyreactivity is thought to arise in inflammatory and hemolytic settings, its relationship with natural polyreactivity remains poorly understood. In this commentary, we examine whether cryptic polyreactivity represents an amplification of pre-existing antigen-binding promiscuity or instead emerges de novo in otherwise monospecific antibodies. Analyses of clinical-stage therapeutic antibody repertoires indicate that heme- or ferrous ion-induced polyreactivity does not correlate with baseline natural polyreactivity, suggesting that these phenomena are largely mechanistically distinct. However, heme can enhance the promiscuous antigen recognition of certain naturally polyreactive broadly neutralizing anti-HIV-1 antibodies, underscoring the complexity of this phenomenon. A better understanding of the relationship between natural and cryptic antibody polyreactivity may help identify hidden liabilities in therapeutic antibodies and provide new insight into the adaptive flexibility of humoral immunity.