The structure and biology of human IgD

hIgDFc-Ig inhibits B cell function by regulating the BCR-Syk-Btk-NF-κB signalling pathway in mice with collagen-induced arthritis

Rheumatoid arthritis (RA) is an autoimmune disease targeting the synovium. Previous studies have found that IgD may be a potential target for the treatment of RA. We designed a new type of fusion protein, hIgDFc-Ig (DG), to block the binding of IgD to IgD receptor (IgDR). In this study, we found that DG has a significant therapeutic effect in mice with collagen-induced arthritis (CIA). DG improved the claw of irritation symptoms in these mice, inhibited the pathological changes in spleen and joint tissues, and had a moderating effect on B cell subsets at different inflammatory stages. Moreover, DG could also decrease the levels of IgA, IgD, IgM and IgG subtypes of immunoglobulin in the serum of mice with CIA. In vitro, B cell antigen receptor (BCR) knockout Ramos cells were established using the CRISPR/Cas9 technology to further study the activation of BCR signalling by IgD and the effect of DG. We found that the therapeutic effect of DG in mice with CIA may be achieved by inhibiting the activation of BCR signalling by IgD, which may be related to the activation of Igβ. In summary, DG may be a potential biological agent for the treatment of RA and it has broad application prospects in the future.

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.

MAINTENANCE OF SERUM ANTIBODY LEVELS

In vertebrates, serum antibodies are an essential component of innate and adaptive immunity and immunological memory. They also can contribute significantly to immunopathology. Their composition is the result of tightly regulated differentiation of B lymphocytes into antibody-secreting plasma blasts and plasma cells. The survival of antibody-secreting cells determines their contribution to the immune response in which they were generated and to long-lasting immunity, as provided by stable serum antibody levels. Short-lived plasma blasts and/or plasma cells secrete antibodies for a reactive immune response. Short-lived plasma blasts can become long-lived plasma cells, probably by competition with preexisting plasma cells for occupation of a limited number of survival niches in the body, in a process not yet fully understood. Limitation of the number of long-lived plasma cells allows the immune system to maintain a stable humoral immunological memory over long periods, to react to new pathogenic challenges, and to adapt the humoral memory in response to these antigens.

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.

Effect of glycosylation on antibody function: implications for genetic engineering

Antibodies are able to both bind antigens and trigger the responses that eliminate them from circulation. All antibodies are glycosylated at conserved positions in their constant regions, and the presence of carbohydrate can be critical for antigen clearance functions such as complement activation. The structure of the attached carbohydrate can also affect antibody activity. Antibody glycosylation can be influenced by the cell in which it is produced, the conformation of the antibody and cell culture conditions. These variables should be considered in the design and production of antibodies with selected specificity and function.

Identification of a dog IgD-like molecule by a monoclonal antibody

IgD has not been identified in dogs. We produced a monoclonal antibody (mAb) designated 9B during the production of hybridomas to dog IgE. Using Western blot analysis under non-reducing conditions, the mAb (9B) recognized a predominant protein band of 185 kDa which was also recognized by anti-dog IgG F(ab')2, suggesting that this 185 kDa protein is an immunoglobulin (Ig) containing light chains. Under reducing conditions, the mAb (9B) recognized only one protein band of 55 kDa which presented a distinct molecular weight (MW) and immunoreactivity from the dog tau, mu, alpha, and epsilon chains. The 55 kDa band did not react with anti-dog IgE, IgM, IgA, and IgG, but did react with the mAb (9B). The MW was 75 kDa for the epsilon chain, 77.5 kDa for the mu chain, 58 kDa for the alpha chain, and 52 kDa for the tau chain. Further, by immunofluorescent staining, this Ig recognized by the mAb (9B) was found on the surface of dog lymphocytes. Studies of this dog Ig with the mAb revealed that this Ig bound to protein A and protein G-Sepharose, and that its enzyme-linked immunosorbent assay (ELISA) activity as measured by the mAb (9B) did not change after heating at 56 degrees C for 2 h. Ragweed-specific IgG, IgE, and this newly defined Ig significantly increased when dogs were immunized with ragweed extract. These data suggest that this Ig is a previously unrecognized IgD-like molecule in dogs.

Detection of allogeneic Qa/TL and Ly specificities on murine tumor cells with IgD in tumor-regressor serum

Serum from C3H/He mice, which show regression of MM2 tumor cells after transplantation and removal (regressor serum, RS) contains non-gammaglobulin components that cross-react with various tumor cells of mice [22, 23]. In addition to tumor cells, various allogeneic lymphocytes are also susceptible to an RS-dependent lymphocyte-mediated cytotoxic reaction. To identify tumor cell surface antigens that cause the cross-reactive host response, the serum components were analyzed by absorption of RS with allogeneic lymphocytes. RS components were found to recognize allogeneic lymphocyte antigens including Qa-2 and Ly6.2. Specificity for the Qa-2 antigen was further tested using Qa-2-congenic mice. The expression of Qa-2 antigen was detected on the surfaces of MM2 and other tumor cells derived from H-2k mice (seven among nine cell lines tested) by a membrane immunofluorescence method using a Qa-2-specific mAb. Physical characteristics of the Qa-2-specific component in RS were determined and found to differ from those of regular IgGs but to be similar to those of IgDs. Using an enzyme-linked immunosorbent assay with an IgD-specific mAb and Qa-2-lacZ fusion protein, the existence of IgD in RS with specificity for Qa-2 was confirmed. These results suggest that the RS component with Qa-2 specificity is an IgD, the specificity and physiological role of which are unknown.

A new qualitative method for detecting IgD in unconcentrated cerebrospinal fluid

We report a method which is capable of demonstrating the isoelectric focusing (IEF) pattern of immunoglobulin D in unconcentrated cerebrospinal fluid (CSF) samples containing as little as 0.1-0.5 ng of total IgD. The method used was an immuno-sandwich technique, with alkaline phosphatase enzyme amplification. Oligoclonal and polyclonal IgD patterns were seen in CSF samples. No cross-reactivity with other immunoglobulins (IgG, IgA and IgM) was detected.

Intrathecal synthesis of IgG, IgA, IgM and IgD in untreated multiple sclerosis and controls

The intrathecal production (ITP) of the immunoglobulins (Ig) G, A, M and D was examined in untreated patients with multiple sclerosis (MS) and controls. Sensitive sandwich enzyme-linked immunosorbent assay systems were applied to the determination of IgA, IgM and IgD levels in the cerebrospinal fluid and plasma from a group of 61 consecutive MS patients (41 relapsing-remitting and 20 secondary chronic progressive MS). Age-related reference limits for all Ig variables were defined in a group of 57 patients with tension headache. ITP of IgG was demonstrated in 85% of the MS patients, ITP of IgA in 41%, of IgM in 44% and of IgD in 18%. Among 9 MS patients with normal IgG index, 3 displayed ITP of IgA, 3 of IgM and 1 of IgD. Plasma IgA was elevated in 20% and plasma IgM in 24% of the MS cases. No significant variation of Ig ITP was demonstrated in a different group of 27 untreated MS patients examined during exacerbation and remission. Among control patients with other inflammatory nervous system diseases, ITP of IgA, IgM and IgD was found more frequently, and of IgG in a smaller percentage compared with MS patients.

Class switch from mu to delta is mediated by homologous recombination between sigma mu and sigma mu sequences in human immunoglobulin gene loci

Class switch of immunoglobulin from mu to gamma occurs by recombination between two repetitive switch sequences: S mu and S gamma. However, there are no such sequences in the mu-delta introns of human and mouse genomes. Although the frequency of IgD-secreting cells is extremely low in mouse about 1% of patients with myeloma produce IgD in human. In a previous report (Nucleic Acids Res. 1988. 16: 9497) we reported that a 442-bp DNA sequence located in the JH-mu intron (defined as sigma mu) was inserted into the mu-delta intron (defined as sigma mu) in human genome. There is no such insertion in mouse. We analyzed Ig H chain gene loci of two human IgD myelomas: one was analyzed by cloning and sequencing and the other by Southern hybridization. We found that recombination had occurred between these two homologous DNA sequences, resulting in loss of the DNA segment from sigma mu to sigma mu. On the other hand, in a Burkitt lymphoma, Daudi, the DNA fragment from sigma mu to sigma mu was duplicated. These results suggest that homologous recombination between sigma mu and sigma mu sequences mediates class switch from mu to delta in human and that it occurs via unequal crossing-over between sister chromatids or daughter chromosomes.

Peripheral neuropathy in IgD myeloma. Cerebrospinal fluid paraprotein analysis in three cases

High resolution polyacrylamide gel isoelectric focusing (IEF), followed by direct immunofixation with anti-delta chains monospecific antibodies, were used to detect and identify IgD paraprotein in the cerebrospinal fluid (CSF) of 3 patients affected by IgD myeloma. Two of these patients presented a paraproteinemic neuropathy. Blood-brain barrier damage was investigated by means of CSF/serum albumin ratio. IgG index and CSF and serum IgD/albumin ratio were also evaluated. An intrathecal origin of the IgD paraprotein was excluded. The correlation between the presence of the paraprotein in the CSF and the possible neurological involvement was also examined.

Quantitative IgD measurement for discrimination of human bloodstains

The IgD concentrations of eluates of artificial bloodstains and of the corresponding sera from 40 subjects of different ages were measured by single radial immunodiffusion. IgD was found in bloodstains stored up to 53 days, i.e. IgD storage stability is sufficient for forensic purposes. Since serum IgD concentrations of individuals, in particular of adults, are almost invariable and serum levels of different individuals can vary by more than a 1000-fold, the discrimination of bloodstains on the basis of IgD is generally possible. Thus IgD constitutes a further marker in antibody profiling of bloodstains.

A 30-kDa protein is disulfide linked to IgM on normal and neoplastic murine B lymphocytes

Four cloned continuously cultured mouse B lymphoma cells of the lines WEHI-279, BCL1 and 38C-13 were used to study the surface presentation of IgM. Cells of all lines expose, in addition to mu 2L2, various, disulfide-linked subunits as already shown for normal B lymphocytes (Koch, N. and Haustein, D., Mol. Immunol. 1982. 19:477). In contrast to normal B cells, B lymphoma cells also carry IgM structures of higher molecular weight than mu 2L2 on their surface. Furthermore, B lymphoma cells of all investigated lines expose a mu 2L2 structure to which a 30-kDa protein is disulfide-linked. This 30-kDa protein could also be identified on normal mouse B lymphocytes when more drastic conditions were employed. mu 2L2 disulfide linked to this 30-kDa protein is the main IgM structure on normal and neoplastic B cells whereas only small amounts of mu 2L2 (unlinked to this protein) can be detected.

Activation of mouse lymphocytes by anti-immunoglobulin. IV. Stimulation with soluble heterologous anti-delta antibodies

Mouse spleen cells were stimulated to proliferate in vitro by soluble affinity-purified heterologous antibodies to mouse delta. Antibodies from goat or rabbit antisera to TEPC 1017, a mouse IgD myeloma protein, were purified on an affinity column of TEPC 1033, a second mouse IgD myeloma protein. Maximum uptake of [3H]thymidine in the range of 60,000 cpm was obtained after 48 hr of culture with anti-delta at concentrations of 50 micrograms/ml. In contrast, the hybridoma 10-4.22 anti-delta was nonmitogenic at similar concentrations. The proliferative response was not impaired upon removal of T cells by treatment with an anti-thymocyte serum (ATS), nor by removal of adherent cells by passage of spleen cells over Sephadex G-10 columns and counter-flow centrifugation. Splenic lymphocytes isolated on the fluorescence activated cell sorter (FACS) with intermediate-to-high amounts of surface IgD (sIgD) were responsive to soluble anti-delta, while IgD-negative cells, or cells with low amounts of sIgD, were unresponsive. Spleen cells from mice less than 4 weeks of age, or from mice carrying the X-linked B cell defect (xid), were unresponsive to anti-delta. These results indicate that anti-delta acts similarly to anti-mu in stimulating a proliferative response by later maturing B cells, which are characterized by a high density of sIgD.

Complete covalent structure of a human immunoglobulin D: sequence of the lambda light chain

We have determined the amino acid sequence of the lambda light chain of human IgD WAH. Together with the sequence of the delta heavy chain already reported, this establishes the complete covalent structure of a human immunoglobulin D. The sequence determination was greatly aided by our ability to use high-pressure liquid chromatography to purify large peptides, including one large fragment extending from Ser-81 through the carboxyl terminus. The IgD molecule is a four-chain monomer of Mr approximately equal to 176,000; it consists of two lambda chains (each of 214 residues, Mr = 22,893) and two delta chains (each of 512 residues, Mr approximately equal to 65,000, including carbohydrate), and, unlike murine IgD, it contains two C delta 2 domains. A computer-assisted search using the J (joining) segment of the WAH lambda chain as a test piece showed a close evolutionary relationship of human and mouse J lambda regions and suggested that germ-line J lambda genes in the two species are very similar if not identical. DNA segments encoding J lambda, J kappa, and JH appear to have had a common evolutionary origin, and, surprisingly, JH seems closer to J lambda than does J kappa.

The export of immunoglobulin D by human neoplastic B lymphocytes

An investigation has been made into the ability of human neoplastic B lymphocytes expressing surface IgM and IgD to export IgD in culture. Cells that expressed surface Ig of the lambda light chain type frequently exported IgD (10/12 patients), whereas cells expressing surface Ig of the kappa light chain type exported no IgD, although most (8/11 patients) were able to export IgM. It appears, therefore, that in most of the 23 cases studied, cells synthesizing IgD with lambda light chains can both express and export IgD, whereas those synthesizing IgD kappa can only insert it into the surface membrane. This finding and the known preponderance of lambda in plasma IgD imply that the possession of a lambda chain facilitates the IgD secretory pathway, a conclusion that implicates a control mechanism subsequent to the surface/secretory dichotomy arising from different splicings of heavy chain messenger RNA.

Amino acid sequence of the first constant region domain and the hinge region of the delta heavy chain of human IgD

We have determined the amino acid sequence of the first constant (C) region domain (C delta 1) and the hinge region of the delta heavy chain of human IgD WAH and also the sequence of the adjacent COOH-terminal portion of the variable (V) region, including the JH region. Together with the sequence of the Fc fragment already reported, this establishes the complete amino acid sequence of the C region of the human delta chain and confirms the presence of three C region domains in human IgD. Although the CH1 domains of the five classes of human heavy chains have the expected degree of homology (approximately 30%), the homology of the C delta 1 domains of the human and mouse chains is less than that exhibited by the CH1 domains of other pairs of human and mouse heavy chains. The hinge region of the human delta chain has an unusual structure; the NH2-terminal half has four (or five) GalN oligosaccharides attached, whereas the COOH-terminal half lacks carbohydrate, is dissimilar in sequence, and has a high charge. A computer search verified that the GalN-rich segment has a high degree of identity in sequence with the middle portion of the human C mu 2 domain and that the high-charge segment is related to the same sequence. We propose that the two segments of the human delta hinge have a common evolutionary origin and arose by duplication and independent mutation of a hinge exon derived from the ancestral gene for the C mu 2 domain.

Rat IgD myeloma protein: cell-free translation of the delta mRNA and biochemical analysis of intracellular and membrane delta chain

The analysis of IgD molecules produced by the tumor-derived IR 731 cell line showed that these molecules are not secreted and are only present at the cell surface as covalent dimers of 48 000 dalton delta chains. The original IR 731 tumor secreted complete IgD; thus, the disppearance of secretion in the cell line was correlated with the absence of light chain synthesis. The mRNA coding for the cell line delta chain was isolated and translated. The analysis of the translated delta chain (mol. wt.: 40 000) confirmed the absence of a region having the size of a domain as previously shown by partial amino acid sequence. The observation that the absence in delta chain of this region, which comprises most of the CH2 domain, could be a general feature of murine species is discussed.

Mouse immunoglobulin D: messenger RNA and genomic DNA sequences

The molecular structure of a mouse immunoglobulin D from a plasmacytoma tumor and that of the normal mouse gene coding for immunoglobulin D are presented. The DNA sequence results indicate an unusual structure for the tumor delta chain in two respects: (i) Only two constant (C) region domains, termed C delta 1 and C delta 3 by homology considerations, are found; the two domains are separated by an unusual hinge region C delta H that lacks cysteine residues and thus cannot provide the covalent cross-links between heavy chains typically seen in immunoglobulins. The two domains and hinge are all coded on separate exons. (ii) At the carboxyl end of the delta chain there is a stretch of 26 amino acids that is coded from an exon located 2750 to 4600 base pairs downstream from the rest of the gene. Analogy with immunoglobulin M suggests that this distally coded segment C delta DC may have a membrane-binding function; however, it is only moderately hydrophobic. A fifth potential exon (C delta AC), located adjacent to the 3' (carboxyl) end of C delta 3, could code for a stretch of 49 amino acids. The tumor's expression of the delta gene may be aberrant, but the simplest interpretation would be that this tumor expresses one of the several biologically significant forms of the delta chain.

Cell surface immunoglobulin mu and gamma chains of human lymphoid cells are of higher apparent molecular weight than their secreted counterparts

Selected human lymphoma-derived and lymphoblastoid cell lines have been characterized and shown to be useful model systems for the expression of either cell surface Ig, or secretory Ig, or both simultaneously. Daudi and Raji cell lines were shown to express surface IgM with no secretion of IgM. The lymphoblastoid cell lines Tay-3 and RPMI 1788 are high-rate secretors of IgM. The lymphoma cell line BJAB synthesizes both cell surface IgM and secretory IgM. In the secreting cells, we have identified an intracellular mu chain with an apparent molecular weight approximately 2000 less than secreted mu chain. In cells synthesizing surface IgM, a surface mu chain has been identified which has an apparent mol. wt. of approximately 2000 more than secreted mu chain. The intracellular mu chain in these cells is indistinguishable in size from the mature surface mu chain; the Daudi cell line is exceptional in that surface deposition of the intracellular mu chain is preceded, or accompanied, by an increase in apparent mol. wt. The lymphoblastoid cell line Bec-11 synthesized both cell surface IgG and secretory IgG. In Bec-11 cells, a surface form of gamma chain, having a higher apparent mol. wt. than secreted gamma chain, has been resolved. There is no detectable difference between L chains associated with any of the different forms of H chain. A biosynthetic relationship between the different forms of H chain is proposed.

Characterization of IgD. I. Isolation of two molecular forms from human serum

Human IgD present in the serum of normal individuals or of patients with Hodgkin's disease (having high IgD concentrations) was characterized and compared with five IgD myeloma proteins. IgD was isolated using a highly specific anti-delta insoluble immunoabsorbent from which the bound material was eluted with sodium dodecyl sulphate (SDS) or urea. The latter reagent could be removed by extensive dialysis, thus making possible the renaturation of the eluted molecules. The purity of the IgD thus isolated was confirmed by antigenic analysis. Both kappa and lambda light chain determinants were present on serum IgD, although lambda light chain was predominant with a ratio over the kappa chain of 2:1. SDS-polyacrylamide slab gel electrophoresis analysis revealed two different molecular forms of serum IgD, one (IgD) migrating identically to monoclonal IgD, the other (IgD2) having a faster mobility. The difference between the two molecules was entirely, due to the different sizes of their constituent delta chains. Peptide mapping of the two chains (delta1 and delta2 respectively) and of the delta chain of an IgD myeloma protein was carried out using 125I-labelled material. The three molecules displayed a high degree of homology, the delta2 chain differing by the presence of three characteristic extra peptides. The significance of these extra peptides is discussed in the light of the peptide mapping technique employed.

Characterization of human lymphocytes bearing Fc receptors for IgE isolated from blood and lymphoid organs

Human lymphocytes isolated from adult peripheral blood and cord blood, tonsils, adenoids and spleens were analysed for Fc receptors for IgE (Fcepsilon) by rosette assays. The Fcepsilon+ cells were also characterized for their class of surface immunoglobulin (sIg), complement receptors, receptors for sheep erythrocytes (E), and Helix pomatia A haemagglutinin (HP), and their abilities to phagocytoze and adhere. The average number of Fcepsilon+ cells was in adult peripheral blood 1.2 +/- 0.4%, in cord blood 3.0 +/- 1.3%, in tonsils 4.2 +/- 5.2%, in adenoids 5.8 +/- 4.2%, and in spleens varied from 0.8% to 15.8% among individual patients. Overnight culturing of the lymphocytes under conditions that allowed detection of Fc receptors for IgM (Fcmu) usually lowered the number of Fcepsilon+ cells. Neuraminidase treatment caused no change. Rosette formation was inhibited by IgE myeloma proteins and their Fc fragments, but not by mildly reduced and alkylated and heated (56 degrees C) IgE, indicating that the receptors are specific for the native configuration of the IgE Fc fragment. Double cell surface marker analyses with fluoresceinated F(ab')2 fragments of purified anti-mu, delta, and gamma antibodies used to label the Fcepsilon rosette-forming lymphocytes from peripheral adult and cord blood showed that 50-80% were sIgM+ but only 0-28% were sIgD+. In contrast, approximately 80% of the Fcepsilon+ cells from tonsils, adenoids and spleens were sIgM+ and sIgD+. The Fcesilon+ lymphocytes represented 10-20% of the sIgM+ lymphocytes in both the peripheral adult and cord blood. Depletion and enrichment experiments indicated that most of the Fcepsilon+ cells bear complement receptors. Lymphocytes having both E and Fcepsilon receptors were not found. Furthermore, the lymphocytes with HP receptors, a marker for T cells and immature B cells, were Fcepsilon-. Monocytes and neutrophil granulocytes did not form Fcepsilon rosettes. These data indicated that a minor population of human B lymphocytes have Fcepsilon receptors. The majority of the Fcepsilon+ lymphocytes in the blood differ from those in tonsils, adenoids and spleen in that the majority of the former are sIgM+/sIgD- and the latter sIgM+/sIgD+. The sIgM+/sIgD- Fcepsilon+ cells in the blood are probably relatively mature lymphocytes since they lacked HP receptors, which are found on immature B cells.