Structural and functional properties of membrane and secreted IgD

A new sensitive and specific enzyme-linked immunosorbent assay for IgD

We have developed a new highly specific ELISA for IgD, and then used it to measure levels of circulating IgD in the serum of 480 un-selected patients from the East Anglia region of UK. The assay is both extremely sensitive and specific, with a minimum detected IgD concentration of 30 pg/ml and more than 10,000-fold specificity for IgD over all other human immunoglobulins. The assay shows linear dilution characteristics with both purified IgD and human serum, and spiking of purified IgD into either purified immunoglobulins or human serum shows c. 100% recovery. Furthermore, intra-assay and inter-assay coefficients of variation for repeated measurements of the same samples are below 10% and 15% respectively. Measurement of IgD levels on the un-selected patient population showed levels to range from <300 pg/ml to over 100 microg/ml, with a geometric mean of 8 microg/ml. The distribution is approximately normal after log transformation. Levels of circulating IgD were higher in men than in women. There was a significant negative correlation between levels of IgD and age in women, but not in men. Moreover, after adjustment for age and sex, there were statistically significantly higher levels of circulating IgD in male (but not female) smokers, compared to their non-smoking counterparts. These results highlight the care that needs to be taken to control for age, sex and cigarette smoking when examining levels of circulating IgD in future studies.

Memory B cell subpopulations in the aged

The literature on immunosenescence has focused mainly on T cell impairment. With the aim of gaining insight into B cell immunosenescence, the authors investigated the serum IgD levels in 24 young and 21 old people and analyzed their relationship with the number of CD19+CD27+ memory cells. Serum IgD were quantified by the use of radial immunodiffusion and the lymphocyte population CD19+CD27+ was identified by a FACScan flow cytometer. Serum IgD levels were significantly lower (p < 0.0001) in old subjects, and the percentage of CD19+CD27+ lymphocytes were significantly increased (p = 0.01) in old subjects. Finally, a significant negative correlation was found (p = 0.01) between serum concentrations of IgD and CD19+CD27+. The present results show that the levels of IgD are negatively age-related to the amount of B memory cells. This suggests that the B repertoire available to respond to new antigenic challenges is decreased in the elderly. In fact, many memory IgD- B cells fill immunologic space, and the number of naïve IgD+ B cells is dramatically decreased. Therefore, these preliminary results suggest that a decrease of naïve IgD+CD27- B cells and a concomitant increase of memory IgD-CD27+ B cells could represent hallmarks of B immunosenescence, might provide biomarkers related to the lifespan of humans, and could be useful for the evaluation of antiaging treatments.

Diagnostic challenge of hyper-IgD syndrome in four children with inflammatory gastrointestinal complaints

OBJECTIVE

Hyper-IgD syndrome (HIDS) is a rare autosomal recessive disease characterized by recurrent fever, lymphadenopathy, diarrhoea, abdominal pain, headache, arthralgia and skin rash. Abdominal symptomatology may mimic inflammatory bowel disease. We report on four patients with HIDS who had been previously investigated for Crohn's disease (CD). The levels of IgD were measured in a series of patients with CD to evaluate the specificity of this assay in the differential diagnosis between the two conditions.

MATERIAL AND METHODS

Diagnosis of HIDS was based on clinical criteria as well as immunological or genetic data. IgD levels were measured in the four subjects affected by HIDS, in 59 patients with CD and in a group of 160 healthy controls.

RESULT

All patients underwent a variety of gastroenterological investigations because inflammatory bowel disease was suspected. Ultrasonography was pathologic in all the patients, showing enlargement of mesenteric lymph nodes. Abdominal leucocyte scintigraphy displayed diffuse signals of mild to moderate degree. IgD and IgA levels were elevated in three out of four patients. No difference in IgD values was found in CD patients as compared to the control group.

CONCLUSIONS

Gastrointestinal complaints associated with recurrent fever and mesenteric adenopathy warrant genetic investigation for HIDS, in order to avoid unnecessary invasive investigations and treatment.

Antibody- and Fc-receptor-based therapeutics for malaria

Abs (antibodies) are complex glycoproteins that play a crucial role in protective immunity to malaria, but their effectiveness in mediating resistance can be enhanced by genetically engineered modifications that improve on nature. These Abs also aid investigation of immune mechanisms operating to control the disease and are valuable tools in developing neutralization assays for vaccine design. This review explores how this might be achieved.

A pattern of anti-carbohydrate antibody responses present in patients with advanced atherosclerosis

We have previously shown that an antibody pool present in normal human serum binds cytokine receptors in vitro and may therefore interfere with assays that capture cytokines using their receptors. Here we show that this antibody pool is the same as the natural antibody termed anti-gal, that binds to the alpha-galactosyl carbohydrate epitope (alpha-gal) and which is the predominant obstacle to xenotransplantation. We report that there are high levels of IgD anti alpha-gal in most volunteers, in addition to the IgG2, IgA and IgM immunoglobulin isotypes against alpha-gal previously described. To determine if anti-gal may interfere with assays that depend on capture of cytokine with its receptor, we measured levels of several anti-carbohydrate antibodies in a cohort of patients with advanced atherosclerosis that had previously been used to measure levels of active TGF-beta using such an assay. For many isotype / carbohydrate combinations, there is a large and significant difference between the levels of anti-carbohydrate antibodies in patients with atherosclerosis and controls, after adjustment for age, sex and blood group. These results are similar to the previous data obtained for active TGF-beta, and therefore we cannot discount the possibility that anti-gal contributed to the previous data. Following further adjustment for several risk factors associated with cardiovascular disease, several anti-carbohydrate antibodies were still significantly different between patients and controls. Therefore, anti-carbohydrate antibodies may represent a new class of risk factors that may be associated with presence of advanced atherosclerosis, although larger studies will be required to confirm this hypothesis.

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.

Mucosal B cells: phenotypic characteristics, transcriptional regulation, and homing properties

Mucosal antibody defense depends on a complex cooperation between local B cells and secretory epithelia. Mucosa-associated lymphoid tissue gives rise to B cells with striking J-chain expression that are seeded to secretory effector sites. Such preferential homing constitutes the biological basis for local production of polymeric immunoglobulin A (pIgA) and pentameric IgM with high affinity to the epithelial pIg receptor that readily can export these antibodies to the mucosal surface. This ultimate functional goal of mucosal B-cell differentiation appears to explain why the J chain is also expressed by IgG- and IgD-producing plasma cells (PCs) occurring at secretory tissue sites; these immunocytes may be considered as 'spin-offs' from early effector clones that through class switch are on their way to pIgA production. Abundant evidence supports the notion that intestinal PCs are largely derived from B cells initially activated in gut-associated lymphoid tissue (GALT). Nevertheless, insufficient knowledge exists concerning the relative importance of M cells, major histocompatibility complex class II-expressing epithelial cells, and professional antigen-presenting cells for the uptake, processing, and presentation of luminal antigens in GALT to accomplish the extensive and sustained priming and expansion of mucosal B cells. Likewise, it is unclear how the germinal center reaction in GALT so strikingly can promote class switch to IgA and expression of J chain. Although B-cell migration from GALT to the intestinal lamina propria is guided by rather well-defined adhesion molecules and chemokines/chemokine receptors, the cues directing preferential homing to different segments of the gut require better definition. This is even more so for the molecules involved in homing of mucosal B cells to secretory effector sites beyond the gut, and in this respect, the role of Waldever's ring (including the palatine tonsils and adenoids) as a regional inductive tissue needs further characterization. Data suggest a remarkable compartmentalization of the mucosal immune system that must be taken into account in the development of effective local vaccines to protect specifically the airways, eyes, oral cavity, small and large intestines, and urogenital tract.

Regional induction of adhesion molecules and chemokine receptors explains disparate homing of human B cells to systemic and mucosal effector sites: dispersion from tonsils

Ethical constraints restrict direct tracking of immune-cell migration throughout the human body in vivo. We, therefore, used deletion of the immunoglobulin M (IgM) heavy-chain constant-gene (Cμ) segment as a marker to provide a dispersal signature of an effector B-cell subset (IgD+IgM-CD38+) induced selectively in human tonsils. By DNA analysis, the Cμ deletion identified dissemination of such blasts and their plasma-cell progeny to peripheral blood, lymph nodes, and bone marrow, as well as to mucosae and glands of the upper airways. Also the endocervix was often positive, while the small intestine was mainly negative, as could be expected from the identified homing-molecule profile of the marker cells, with relatively low levels of integrin α4β7 and CC chemokine receptor 9 (CCR9). Of further importance for vaccine design, the circulating cells expressed abundantly CD62L (L-selectin) and CCR7, which provided a mechanism for integration of respiratory and systemic immunity. Most mucosal vaccines are at present administered perorally, and our results suggested that the nasal route is no alternative for vaccination against rotavirus or other small-intestinal infections in humans. However, immunization of nasopharynx-associated lymphoid tissue clearly appears preferable to target respiratory pathogens and may to some extent also protect against infections of the female genital tract. (Blood. 2005;106:593-600)

The CD19 Molecule is Crucial for MID-Dependent Activation of Tonsillar B Cells from Children

The Moraxella immunoglobulin (Ig) D-binding protein (MID) induces a strong proliferative response in human peripheral blood IgD+ B cells from adults isolated by positive selection using anti-CD19-conjugated microbeads. Here, we show that tonsillar B cells from children isolated with positive selection are unable to respond to MID stimulation. The proliferative response was very low or absent at various concentrations of MID tested and at different time points analysed, whereas the MID response of tonsillar B cells from adults isolated with positive selection was considerably higher. Tonsillar B cells from children isolated with positive selection responded to formalin-fixed preparations of Moraxella catarrhalis and Staphylococcus aureus Cowan strain I. In comparison to cells isolated with positive selection, a much higher proliferative response was recorded in tonsillar B cells from children isolated with negative selection, indicating that occupation of the CD19 molecule (i.e. positive selection) inhibited the response. Indeed, the addition of anti-CD19 monoclonal antibodies (MoAb) to MID-activated tonsillar B cells from children isolated with negative selection strongly inhibited the proliferative response. In contrast, anti-CD21 MoAb at the same concentration did only show a minor inhibition on the MID-induced response. Pre-incubation of tonsillar B cells isolated from children with anti-CD19 or anti-CD21 MoAb did not affect the binding of biotin-conjugated MID as analysed by flow cytometry. These results suggest that MID-activated tonsillar B cells from children have a strong requirement for signalling through the CD19 molecule. Future experiments will further reveal the importance of CD19 and possibly other molecules for optimal activation of tonsillar B cells isolated from both children and adults.

Immunoglobulin genetics of Ornithorhynchus anatinus (platypus) and Tachyglossus aculeatus (short-beaked echidna)

In this paper, we review data on the monotreme immune system focusing on the characterisation of lymphoid tissue and of antibody responses, as well the recent cloning of immunoglobulin genes. It is now known that monotremes utilise immunoglobulin isotypes that are structurally identical to those found in marsupials and eutherians, but which differ to those found in birds and reptiles. Monotremes utilise IgM, IgG, IgA and IgE. They do not use IgY. Their IgG and IgA constant regions contain three domains plus a hinge region. Preliminary analysis of monotreme heavy chain variable region diversity suggests that the platypus primarily uses a single VH clan, while the short-beaked echidna utilises at least 4 distinct VH families which segregate into all three mammalian VH clans. Phylogenetic analysis of the immunoglobulin heavy chain constant region gene sequences provides strong support for the Theria hypothesis. The constant region of IgM has proven to be a useful marker for estimating the time of divergence of mammalian lineages.

Deposition of IgD, alpha-1-antitrypsin and alpha-1-antichymotrypsin on Demodex folliculorum and D. brevis infesting the pilosebaceous unit

A total of seven routinely processed biopsy specimens of facial skin lesions with infestation of Demodex folliculorum or D. brevis were immunostained for plasma proteins and secretory proteins. The cuticular layer of the mites located within the pilosebaceous unit was selectively immunoreactive for IgD (delta chain), alpha-1-antitrypsin and alpha-1-antichymotrypsin. Negative results were obtained for IgG, IgA, IgM, IgE, albumin, fibrinogen, C3, amyloid P component, prealbumin, lysozyme and lactoferrin. These findings suggest a novel function of IgD and serum protease inhibitors as a protective host response to the mite.

Identification of a novel immunoglobulin delta transcript and comparative analysis of the genes encoding IgD in Atlantic salmon and Atlantic halibut

Atlantic salmon possesses two parallel Ig heavy chain gene complexes, A and B, most probably as a result of ancestral tetraploidy. Consequently, there are two distinct IgD heavy chain (delta) subvariants in this species. The Igdelta(B) gene was characterised in a previous study. In the present work the Igdelta(A) gene was amplified by PCR and sequenced. Both Igdelta genes in salmon have a structure like delta1-(delta2-delta3-delta4)(2)-delta5-delta6-delta7-TM1-TM2 and show a high degree of sequence identity (approximately 95%). 3'RACE and RT-PCR analyses performed in the present study indicate that Igdelta transcripts of membrane type are dominating in Atlantic salmon and Atlantic halibut. However, a different transcript, originating from the Igdelta(B) gene in salmon, was identified by PCR. This RNA fragment is spliced between the regular donor/acceptor sites in delta6 and TM2. Cloning and characterisation of cDNA encoding the membrane form of halibut IgD revealed an overall Ig domain structure equivalent to that in salmon. Corresponding duplications of delta2-delta3-delta4 have now been found in three teleost fishes: salmon, halibut and catfish. The tandem duplicated fragments are highly similar within each species, while not being especially conserved between the species. Thus, the duplicated gene fragments have either arisen independently in each species or are subjected to homogenisation by some means.

Biased Iglambda expression in hypermutated IgD multiple myelomas does not result from receptor revision

Normal IgM(-)IgD(+) CD38(+) B cells and IgM(-)IgD(+) multiple myelomas (MM) are characterized by Cmu deletion, biased Iglambda expression and hypermutated IgV regions. The predominant Iglambda usage has been proposed as resulting from secondary Ig gene rearrangements during extensive clonal expansion in the germinal center environment. Here, four cases of IgDlambda MM were studied to address the question of light chain receptor revision in a 'single cell' model. Detailed analyses of both IGK and IGL alleles of each case were performed by Southern blotting, (RT-) PCR, and sequencing. The expressed IgV genes were extensively mutated and Cmu deletion was confirmed in two cases. In addition, in the four MM a total of six non-functional deletional IGK rearrangements were identified, which proved to be unmutated. We conclude that IgD myelomas indeed originate from (post) germinal center B cells in which, in spite of the fact that they are hypermutated, there is no evidence of receptor revision.

Familial Mediterranean Fever: association of elevated IgD plasma levels with specific MEFV mutations

Familial Mediterranean Fever (FMF) is a recessively inherited disorder, characterized by episodic fever, abdominal and arthritic pain, as well as other forms of inflammation. Some FMF patients present higher IgD serum levels, and it is not yet known whether such an elevation is related to specific genotypes or correlated with a specific phenotype. In order to evaluate the association between known FMF-related mutations and IgD levels in confirmed patients, as well as the correlation between those levels and the presence of specific clinical signs, genotypic analysis and IgD plasma measurements were performed for 148 Lebanese and Jordanian FMF patients. Most common mutational patterns were M694V heterozygotes (19%) and homozygotes (17%), and V726A heterozygotes (18%) and homozygotes (5%), with an additional 11% combining both mutations. Twenty-one patients had higher IgD levels (superior to 100 microg/ml). The risk for higher IgD levels was significantly associated with M694V homozygote status (OR = 6.25) but not with heterozygotic one (OR = 1). Similarly, the risk for higher IgD was also found with V726A homozygotes (OR = 2.2) but not with heterozygotes (OR = 1.05). The use of colchicine was not statistically associated with IgD levels. Clinically, hyper IgD was also found significantly associated with arthritis (OR = 18). Thus, homozygotic status for M694V, and to a lesser extent V726A, is associated with increased risk for higher IgD plasma levels, regardless of colchicine use. Elevated IgD plasma levels are also correlated with the severity of FMF manifestations, and especially with arthritis.