Paratope plasticity in diverse modes facilitates molecular mimicry in antibody response

The immune response against methyl-α -D-mannopyranoside mimicking 12-mer peptide (DVFYPYPYASGS) was analyzed at the molecular level towards understanding the equivalence of these otherwise disparate Ags. The Ab 7C4 recognized the immunizing peptide and its mimicking carbohydrate Ag with comparable affinities. Thermodynamic analyses of the binding interactions of both molecules suggested that the mAb 7C4 paratope lacks substantial conformational flexibility, an obvious possibility for facilitating binding to chemically dissimilar Ags. Favorable changes in entropy during binding indicated the importance of hydrophobic interactions in recognition of the mimicking carbohydrate Ag. Indeed, the topology of the Ag-combining site was dominated by a cluster of aromatic residues, contributed primarily by the specificity defining CDR H3. Epitope-mapping analysis demonstrated the critical role of three aromatic residues of the 12-mer in binding to the Ab. Our studies delineate a mechanism by which mimicry is manifested in the absence of either structural similarity of the epitopes or conformational flexibility in the paratope. An alternate mode of recognition of dissimilar yet mimicking Ags by the anti-peptide Ab involves plasticity associated with aromatic/hydrophobic and van der Waals interactions. Thus, antigenic mimicry may be a consequence of paratope-specific modulations rather than being dependent only on the properties of the epitope. Such modulations may have evolved toward minimizing the consequences of antigenic variation by invading pathogens

Epigenetic tethering of AID to the donor switch region during immunoglobulin class switch recombination

A complex of KAP1 and HP1 is needed to tether AID to the H3K9me3-marked donor switch region during CSR

Molecular mechanisms underlying AID target specificity during antibody diversification

Afin d'établir une immunité durable et adaptée, le répertoire des lymphocytes B est diversifié par les mécanismes d’hypermutation somatique (HMS) et commutation isotypique (CI). Ces processus sont initiés par des cassures à l’ADN induites par Activation Induced Cytidine Deaminase (AID). Lors de la CI, ces lésions sont reconnues par des facteurs de réparation de l’ADN et réparées par jonction d’extrémités non homologues. Cependant, malgré les multiples voies de réparation de l’ADN, les lésions générées par AID peuvent être réparées de manière aberrante et mènent à des recombinaisons illégitimes. C’est pourquoi de robustes mécanismes de régulation et ciblage d’AID sont requis afin de limiter les dommages à l’ADN au locus Igh dans les cellules B et prévenir les dommages collatéraux pouvant mener à une tumorogenèse étendue. Parmi ces mécanismes, des modifications épigénétiques du locus Igh corrélant avec la CI pourraient être importantes pour le ciblage spécifique d’AID au locus Igh. Aussi afin d’appréhender les mécanismes moléculaires qui sous-tendent le ciblage d’AID dans les loci Ig, avons-nous entrepris d’identifier les facteurs interagissant avec AID et étudier leur rôle fonctionnel au cours de l’HMS et CI.Nous avons montré que AID forme un complexe avec les facteurs KAP1 et HP1 qui est retenu au locus Igh, au niveau de la région de switch Sμ porteuse de la modification de l’histone H3 H3K9me3 in vivo. La disruption in vivo du complexe cause un défaut de formation des cassures dans la région Sμ et en un défaut concomitant de la CI, l’HMS n’étant pas altérée. Ainsi KAP1 et HP1 retiennent AID aux résidus H3K9me3 marquant la région Sμ afin de permettre une CI efficace.B lymphocytes diversify their antibody repertoire through somatic hypermutation (SHM) and class switch recombination (CSR), which require a single enzyme, Activation induced cytidine deaminase (AID). AID deaminates cytosines to uracils, to produce dU:dG mismatches that are recognized and processed by to result in SHM or CSR. AID-induced DNA lesions are processed to form DNA double-stranded breaks (DSBs) during CSR, recognized by components of the DNA damage response pathway and repaired through non-homologous end joining. Aberrant processing of such DNA breaks leads to increased levels of illegitimate recombination events, therefore tight regulatory and targeting mechanisms are required to restrict AID to the appropriate cell type and loci. Although, CSR correlates with epigenetic modifications at the Igh locus, and these modifications have been proposed to target the CSR machinery, the relationship between these and AID remains unknown. My results provide a mechanism linking AID to epigenetic modifications during CSR. We show that during CSR, AID forms a complex with KAP1 and HP1 that is tethered to the Igh locus (specifically to the donor switch region-S), bearing trimethylation at lysine 9 (H3K9me3) in vivo. Furthermore, in vivo disruption of this complex results in inefficient DSB-formation at S and a concomitant defect in CSR but not in somatic hypermutation. We thus propose a model in which KAP1 and HP1 tether AID to H3K9me3 residues that mark S in order to permit efficient CSR