The immunoaugmenting properties of murine IgD reside in its C delta 1 and C delta 3 regions: potential role for IgD-associated glycans

Rethinking mucosal antibody responses: IgM, IgG and IgD join IgA

Humoral immune responses at mucosal surfaces have historically focused on IgA. Growing evidence highlights the complexity of IgA-inducing pathways and the functional impact of IgA on mucosal commensal bacteria. In the gut, IgA contributes to the establishment of a mutualistic host-microbiota relationship that is required to maintain homeostasis and prevent disease. This Review discusses how mucosal IgA responses occur in an increasingly complex humoral defence network that also encompasses IgM, IgG and IgD. Aside from integrating the protective functions of IgA, these hitherto neglected mucosal antibodies may strengthen the communication between mucosal and systemic immune compartments.

Secreted IgD Amplifies Humoral T Helper 2 Cell Responses by Binding Basophils via Galectin-9 and CD44

B cells thwart antigenic aggressions by releasing immunoglobulin M (IgM), IgG, IgA, and IgE, which deploy well-understood effector functions. In contrast, the role of secreted IgD remains mysterious. We found that some B cells generated IgD-secreting plasma cells following early exposure to external soluble antigens such as food proteins. Secreted IgD targeted basophils by interacting with the CD44-binding protein galectin-9. When engaged by antigen, basophil-bound IgD increased basophil secretion of interleukin-4 (IL-4), IL-5, and IL-13, which facilitated the generation of T follicular helper type 2 cells expressing IL-4. These germinal center T cells enhanced IgG1 and IgE but not IgG2a and IgG2b responses to the antigen initially recognized by basophil-bound IgD. In addition, IgD ligation by antigen attenuated allergic basophil degranulation induced by IgE co-ligation. Thus, IgD may link B cells with basophils to optimize humoral T helper type 2-mediated immunity against common environmental soluble antigens.

The enigmatic function of IgD: some answers at last

IgD emerged soon after IgM at the time of inception of the adaptive immune system. Despite its evolutionary conservation from fish to humans, the specific functions of IgD have only recently begun to be elucidated. Mature B cells undergo alternative mRNA splicing to express IgD and IgM receptors with identical antigenic specificity. The enigma of dual IgD and IgM expression has been tackled by several recent studies showing that IgD helps peripheral accumulation of physiologically autoreactive B cells through its functional unresponsiveness to self-antigens but prompt readiness against foreign antigens. IgD achieves this balance by attenuating IgM-mediated anergy while promoting specific responses to multimeric non-self-antigens. Additional research has clarified how and why certain mucosal B cells become plasmablasts or plasma cells specializing in IgD secretion. In particular, the microbiota has been shown to play an important role in driving class switch-mediated replacement of IgM with IgD. Secreted IgD appears to enhance mucosal homeostasis and immune surveillance by "arming" myeloid effector cells such as basophils and mast cells with IgD antibodies reactive against mucosal antigens, including commensal and pathogenic microbes. Here we will review these advances and discuss their implications in humoral immunity in human and mice.

New insights into the enigma of immunoglobulin D

Immunoglobulin D (IgD) has remained a mysterious antibody class for almost half a century. IgD was initially thought to be a recently evolved Ig isotype expressed only by some mammalian species, but recent discoveries in fishes and amphibians demonstrate that IgD was present in the ancestor of all jawed vertebrates and has important immunological functions. The structure of IgD has been very dynamic throughout evolution. Mammals can express IgD through alternative splicing and class switch recombination. Active cell-dependent and T-cell-independent IgM-to-IgD class switching takes place in a unique subset of human B cells from the upper aerodigestive mucosa, which provides a layer of mucosal protection by interacting with many pathogens and their virulence factors. Circulating IgD can bind to myeloid cells such as basophils and induce antimicrobial, inflammatory, and B-cell-stimulating factors upon cross-linking, which contributes to not only immune surveillance but also inflammation and tissue damage when this pathway is overactivated under pathological conditions. Recent research shows that IgD is an important immunomodulator that orchestrates an ancestral surveillance system at the interface between immunity and inflammation.

Semi-extended solution structure of human myeloma immunoglobulin D determined by constrained X-ray scattering

Human immunoglobulin D (IgD) occurs most abundantly as a membrane-bound antibody on the surface of mature B cells (mIgD). IgD possesses the longest hinge sequence of all the human antibody isotypes, with 64 residues connecting the Fab and Fc fragments. A novel rapid purification scheme of secreted IgD from the serum of an IgD myeloma patient using thiophilic (T-gel) and lectin affinity chromatography gave a stable, homogeneous IgD preparation. Synchrotron X-ray scattering and analytical ultracentrifugation of IgD identified the solution arrangement of its Fab and Fc fragments, and thereby its hinge structure. The Guinier X-ray radius of gyration R(G) of 6.9(+/-0.1)nm showed that IgD is more extended in solution than the immunoglobulin subclass IgA1 (R(G) of 6.1-6.2nm). Its distance distribution function P(r) showed a single peak at 4.7nm and a maximum dimension of 23nm. Velocity experiments gave a sedimentation coefficient of 6.3S, which is similar to that for IgA1 at 6.2S. The complete IgD structure was modelled using molecular dynamics to generate IgD hinge structures, to which homology models for the Fab and Fc fragments were connected. Good scattering curve fits were obtained with 18 semi-extended best fit IgD models that were filtered from 8500 trial models. These best-fit models showed that the IgD hinge does not correspond to an extended polypeptide structure. The averaged solution structure arrangement of the Fab and Fc fragments in IgD is principally T-shaped and flexible, with contribution from Y-shaped and inverted Y-shaped structures. Although the linear sequence of the IgD hinge is much longer, comparison with previous scattering modelling of IgA1 and IgA2(m)1 suggests that the hinge of IgA1 and IgD are more similar than might have been expected, Both possess flexible T-shaped solution structures, probably reflecting the presence of restraining O-linked sugars.

IgD-receptor (IgD-R) cross-linking partially protects murine T cells from dexamethasone-induced apoptosis

Based on our previous findings that immunoglobulin D (IgD) receptor (IgD-R) cross-linking with oligomeric IgD (IgD-R-xL) led to T cell activation, we examined the effect of IgD-R-xL on the expression of Fas antigen and apoptosis induction. In splenic T cells, IgD-R-xL followed by dexamethasone (dex) treatment resulted in a decreased percentage of Fas-positive cells as well as a decreased mean fluorescence intensity (P<0.05) when compared with cells treated with dex alone. There are significant differences in annexin-fluorescein isothiocyanate (FITC) and phosphatidylinositol (PI) staining between samples treated with dex alone and IgD-R-xL followed by dex-treated samples (P<0.05), suggesting a protective role for IgD-R-xL. No significant differences are seen in Fas antigen expression, annexin-FITC staining, and/or PI staining in murine T hybridoma (7C5) cells cultured under similar conditions (P<0.07). We hypothesize that ligation of IgD-R may predispose antigen-specific T lymphocytes for survival during primary immune responses when IgD-positive B cells serve as antigen-presenting cells.

IgD receptor-mediated signal transduction in T cells

Upregulation of immunoglobulin D-specific receptors (IgD-R) on CD4+ T cells may facilitate their interaction with specific carbohydrate moieties uniquely associated with membrane IgD on B cells. Previous studies have shown that upregulation of IgD-R facilitates cognate T-B cell interactions by mediating bidirectional signaling resulting in increased antibody responses and clonal expansion of antigen-specific T cells. Murine T hybridoma cells, 7C5, constitutively express IgD-R, as has been confirmed by staining with biotinylated IgD. Earlier studies have shown that inhibitors of protein tyrosine kinase (PTK) completely prevented upregulation of IgD-R in response to oligomeric IgD, suggesting that cross-linking of IgD-R may induce signal transduction and functional consequences through one or more PTK activation pathways, leading to upregulation of IgD-R. In the present study we show that cross-linking of IgD-R by oligomeric IgD indeed results in (a) T cell activation as seen by tyrosine phosphorylation of several intracellular proteins, (b) tyrosine phosphorylation of p56 Lck and PLC-gamma in 7C5 T hybridoma cells, and (c) phosphorylation of an approximately 29-kDa band that exhibits strong affinity for IgD. We analyzed tyrosine phosphorylation of p56 Lck and PLC-gamma in BALB/c splenic T cells that were exposed to oligomeric IgD both in vivo and in vitro. In vitro cross-linking as well as in vivo followed by in vitro cross-linking of IgD-R resulted in enhanced phosphorylation of p56 Lck and moderate tyrosine phosphorylation of PLC-gamma. These results suggest that interactions between IgD-R and IgD mediate signal transduction and support our previous findings that IgD-R+ T cells enhance cognate T cell-B cell interactions and antibody production.

Structural and functional properties of membrane and secreted IgD

More than 35 years ago, study of an unknown immunoglobulin (Ig) in the serum from a myeloma patient led to the discovery of IgD. Subsequently, the finding that it also exists as a membrane-bound Ig stimulated a large number of studies during the 70s. Then, the interest on IgD shrank, largely because of the lack of known function of secretory IgD (secIgD) and of a stagnating knowledge of the functions of surface IgD. In the recent years, very significant advances followed the tremendous accumulation of data on the physiology of the B cell receptor, of which IgD is the major component, on the role of secIgD in normal and diseased individuals. This review, which is focused on human IgD but integrates data in the mouse and other species when needed, summarizes present data on the structure, synthesis and functions of both membrane and secIgD, IgD receptors and the involvement of IgD in various diseases, especially the hyperIgD syndrome.