Antibodies Use Heme as a Cofactor to Extend Their Pathogen Elimination Activity and to Acquire New Effector Functions

Abstract

Various pathological processes are accompanied by release of high amounts of free heme into the circulation. We demonstrated by kinetic, thermodynamic, and spectroscopic analyses that antibodies have an intrinsic ability to bind heme. This binding resulted in a decrease in the conformational freedom of the antibody paratopes and in a change in the nature of the noncovalent forces responsible for the antigen binding. The antibodies use the molecular imprint of the heme molecule to interact with an enlarged panel of structurally unrelated epitopes. Upon heme binding, monoclonal as well as pooled immunoglobulin G gained an ability to interact with previously unrecognized bacterial antigens and intact bacteria. IgG-heme complexes had an enhanced ability to trigger complement-mediated bacterial killing. It was also shown that heme, bound to immunoglobulins, acted as a cofactor in redox reactions. The potentiation of the antibacterial activity of IgG after contact with heme may represent a novel and inducible innate-type defense mechanism against invading pathogens.

A Cryptic Polyreactive Antibody Recognizes Distinct Clades of HIV-1 Glycoprotein 120 by an Identical Binding Mechanism

“…Heme Binds to Ab21-Our previous studies indicate that induction of polyreactivity of antibodies by heme occurs through direct binding of the macrocyclic cofactor to the immunoglobulin molecule (34). To figure out whether this is also valid for acquisition of binding polyreactivity by Ab21, we performed spectroscopy analyses (Fig.…”

“…In addition to intrinsically polyreactive Abs, normal immune repertoires contain Abs that can acquire antigen-binding polyreactivity post-translationally. Thus, exposure of these Abs to biologically relevant redox agents, such as reactive oxygen species, heme, iron ions, etc., results in a structural reorganization of their antigen-binding sites and acquisition of antigen-binding specificities to multiple unrelated antigens (32)(33)(34). Importantly, the substances that reveal the cryptic antibody polyreactivity are usually released in vivo at sites of inflammation or tissue damage and thus might modulate the antigen-binding properties of the susceptible Abs that are present in the immediate microenvironment (35).…”

Cofactor binding and enzymatic activity in an unevolved superfamily of de novo designed 4‐helix bundle proteins

“…Promiscuous heme binding was highlighted by a recent study of antibodies, which demonstrated that immunoglobulins have an intrinsic propensity to bind heme. 33 The immune system takes advantage of this binding to acquire new antigenic specificities. Moreover, sequestering free heme by antibodies is advantageous in fighting various pathological states that are accompanied by release of free heme into the circulation.…”

Diversification post-traductionnelle de la spécificité des immunoglobulines

“…Dans la circulation sanguine de tous les individus sains, on trouve des anticorps circulants monoréactifs qui sont capables de devenir polyréactifs. L'acquisition de cette polyréactivité survient de manière post-traductionnelle, après exposition des anticorps à des molécules de bas poids moléculaire, telles que l'hème ou les ions ferreux que l'on retrouve dans différentes conditions physiopathologiques [5]. Plusieurs équipes de recherche, dont la nôtre, ont démontré que l'exposition à l'hème d'Ig humaines de donneurs sains augmentait drastiquement la capacité de ces Ig à lier (K D de l'ordre du nanomolaire) différents antigènes du soi ou exogènes (bacté-ries et virus) [5].…”

“…L'acquisition de cette polyréactivité survient de manière post-traductionnelle, après exposition des anticorps à des molécules de bas poids moléculaire, telles que l'hème ou les ions ferreux que l'on retrouve dans différentes conditions physiopathologiques [5]. Plusieurs équipes de recherche, dont la nôtre, ont démontré que l'exposition à l'hème d'Ig humaines de donneurs sains augmentait drastiquement la capacité de ces Ig à lier (K D de l'ordre du nanomolaire) différents antigènes du soi ou exogènes (bacté-ries et virus) [5]. Ainsi, environ 20 % des immunoglobulines circulantes d'un individu sain possèdent cette polyréac-tivité inductible qui peut être révélée après interaction avec l'hème.…”