Summary The nature of the antigens recognized by γδ T cells and their potential recognition of major histocompatibility complex (MHC)-like molecules has remained unclear. The CD1 family of lipid-presenting molecules are suggested ligands for Vδ1 TCR-expressing γδ T cells, the major γδ lymphocyte population in epithelial tissues. We crystallized a Vδ1 TCR in complex with CD1d and the self-lipid sulfatide, revealing an unusual recognition of CD1d by germline Vδ1 residues spanning all complementarity determining region (CDR) loops, with sulfatide recognition separately encoded by non-germline CDR3δ residues. Binding and functional analysis showed that CD1d presenting self-lipids, including sulfatide, was widely recognized by gut Vδ1 + γδ T cells. These findings provide structural demonstration of MHC-like recognition of a self-lipid by γδ T cells and reveal the prevalence of lipid recognition by innate-like T cell populations.

CD1c is abundantly expressed on human dendritic cells (DC) and B cells, where it binds and displays lipid antigens to T cells. Here we report that CD1c tetramers carrying M. tuberculosis phosphomycoketide bind γδ T cell receptors (TCRs). An unbiased method of ligand-based TCR selection detects interactions only with Vδ1+ TCRs, and mutational analyses demonstrate a role of the Vδ1 domain during recognition. These results strengthen evidence for a role of CD1c in the γδ T cell response, providing biophysical evidence for CD1c-γδ TCR interactions and a named foreign antigen. Surprisingly, TCRs also bind CD1c complexes formed with diverse lipids such as lysophosphatidylcholine, sulfatide or mannosyl-phosophomycoketide, but not lipopeptide ligands. Dissection of TCR interactions with CD1c carrying foreign antigens, permissive ligands and non-permissive lipid ligands clarifies the molecular basis of the frequently observed but poorly understood phenomenon of mixed self and foreign antigen reactivity in the CD1 system.

J-chain (Joining chain) is a small polypeptide that regulates multimerization of secretory IgM and IgA, the only two mammalian Igs capable of forming multimers. J-chain also is required for polyIg-receptor (pIgR)-mediated transport of these Ig classes across the mucosal epithelium. It is generally assumed that all plasma cells express J-chain regardless of expressed isotype, despite the documented presence of J-chain-negative plasma cells in mammals, specifically in all monomeric IgA- and some IgG-secreting cells. Compared to most other immune molecules, J-chain has not been extensively studied, due, in part, to technical limitations. Even the reported phenotype of the J-chain knockout mouse is often misunderstood or underappreciated. In this short review, we will discuss J-chain in light of the various proposed models of its expression and regulation, with an added focus on the evolutionary significance of J-chain and its expression in different B cell lineages/differentiation states.

Sharks and other cartilaginous fish are the phylogenetically oldest living organisms that rely on antibodies as part of their adaptive immune system. They produce the immunoglobulin new antigen receptor (IgNAR), a homodimeric heavy chain-only antibody, as a major part of their humoral adaptive immune response. Here, we report the atomic resolution structure of the IgNAR constant domains and a structural model of this heavy chain-only antibody. We find that despite low sequence conservation, the basic Ig fold of modern antibodies is already present in the evolutionary ancient shark IgNAR domains, highlighting key structural determinants of the ubiquitous Ig fold. In contrast, structural differences between human and shark antibody domains explain the high stability of several IgNAR domains and allowed us to engineer human antibodies for increased stability and secretion efficiency. We identified two constant domains, C1 and C3, that act as dimerization modules within IgNAR. Together with the individual domain structures and small-angle X-ray scattering, this allowed us to develop a structural model of the complete IgNAR molecule. Its constant region exhibits an elongated shape with flexibility and a characteristic kink in the middle. Despite the lack of a canonical hinge region, the variable domains are spaced appropriately wide for binding to multiple antigens. Thus, the shark IgNAR domains already display the well-known Ig fold, but apart from that, this heavy chain-only antibody employs unique ways for dimerization and positioning of functional modules. protein evolution | antibody structure | protein folding | protein engineering

γδ T cells are a prominent epithelial-resident lymphocyte population, possessing multi-functional capacities in the repair of host tissue, pathogen clearance, and tumor surveillance. Although three decades have now passed since their discovery, the nature of γδ TCR-mediated ligand recognition remains poorly defined. Recent studies have provided structural insight into this recognition, demonstrating that γδ T cells survey both CD1 and the presented lipid, and in some cases are exquisitely lipid specific. We review these findings here, examining the molecular basis for and the functional relevance of this interaction. We discuss potential implications on the notion that non-classical MHC molecules may function as important restricting elements of γδ TCR specificity, and on our understanding of γδ T cell activation and function.