Allotypic and isotypic aspects of human immunoglobulin A

Preparation and properties of recombinant Clostridium ramosum IgA proteinase. Isolation of Fc-SC and Fab fragments of human secretory IgA

Immunoglobulin A (IgA) proteinase from Clostridium ramosum is the enzyme which cleaves IgA of both subclasses; in contrast, the other bacterial proteinases cleave only IgA1 proteins. Previous reports characterized the activity of proteinase naturally secreted by C. ramosum specific for the normal human serum IgA of IgA1 and IgA2m(1) subclasses and also for secretory IgA (SIgA). Its amino acid sequence was determined, and the recombinant proteinase which cleaved IgA of both subclasses was prepared. Here we report the optimized expression, purification, storage conditions and activity testing against purified human milk SIgA. The recombinant C. ramosum IgA proteinase isolated in the high degree of purity exhibited almost complete cleavage of SIgA of both subclasses. The proteinase remained active upon storage for more than 10 month at -20 °C without substantial loss of enzymatic activity. Purified SIgA fragments are suitable for studies of all antigen-binding and Fc-dependent functions of SIgA involved in the protection against infections with mucosal pathogens.

Solution Structure Determination of Monomeric Human IgA2 by X-ray and Neutron Scattering, Analytical Ultracentrifugation and Constrained Modelling: A Comparison with Monomeric Human IgA1

Immunoglobulin A (IgA), the most abundant human immunoglobulin, mediates immune protection at mucosal surfaces as well as in plasma. It exists as two subclasses IgA1 and IgA2, and IgA2 is found in at least two allotypic forms, IgA2m(1) or IgA2m(2). Compared to IgA1, IgA2 has a much shorter hinge region, which joins the two Fab and one Fc fragments. In order to assess its solution structure, monomeric recombinant IgA2m(1) was studied by X-ray and neutron scattering. Its Guinier X-ray radius of gyration R(G) is 5.18 nm and its neutron R(G) is 5.03 nm, both of which are significantly smaller than those for monomeric IgA1 at 6.1-6.2 nm. The distance distribution function P(r)for IgA2m(1) showed a broad peak with a subpeak and gave a maximum dimension of 17 nm, in contrast to the P(r) curve for IgA1, which showed two distinct peaks and a maximum dimension of 21 nm. The sedimentation coefficients of IgA1 and IgA2m(1) were 6.2S and 6.4S, respectively. These data show that the solution structure of IgA2m(1) is significantly more compact than IgA1. The complete monomeric IgA2m(1) structure was modelled using molecular dynamics to generate random IgA2 hinge structures, to which homology models for the Fab and Fc fragments were connected to generate 10,000 full models. A total of 104 compact best-fit IgA2m(1) models gave good curve fits. These best-fit models were modified by linking the two Fab light chains with a disulphide bridge that is found in IgA2m(1), and subjecting these to energy refinement to optimise this linkage. The averaged solution structure of the arrangement of the Fab and Fc fragments in IgA2m(1) was found to be predominantly T-shaped and flexible, but also included Y-shaped structures. The IgA2 models show full steric access to the two FcalphaRI-binding sites at the Calpha2-Calpha3 interdomain region in the Fc fragment. Since previous scattering modelling had shown that IgA1 also possessed a flexible T-shaped solution structure, such a T-shape may be common to both IgA1 and IgA2. The final models suggest that the combination of the more compact IgA2m(1) and the more extended IgA1 structures will enable human IgA to access a broader range of antigens than either acting alone. The hinges of both IgA subclasses appear to show reduced flexibility when compared to their equivalents in IgG, and this may be important for maintaining an extended IgA structure.

Nontypeable Haemophilus influenzae: pathogenesis and prevention

In this paper, we describe the ability of nontypeable Haemophilus influenzae (NTHi) to coexist with the human host and the devastating results associated with disruption of the delicate state of balanced pathogenesis, resulting in both acute and chronic respiratory tract infections. It has been seen that the strains of NTHi causing disease show a marked genetic and phenotypic diversity but that changes in the lipooligosaccharide (LOS) and protein size and antigenicity in chronically infected individuals indicate that individual strains of NTHi can remain and adapt themselves to avoid expulsion from their infective niche. The lack of reliance of NTHi on a single mechanism of attachment and its ability to interact with the host with rapid responses to its environment confirmed the success of this organism as both a colonizer and a pathogen. In vitro experiments on cell and organ cultures, combined with otitis media and pulmonary models in chinchillas, rats, and mice, have allowed investigations into individual interactions between NTHi and the mammalian host. The host-organism interaction appears to be a two-way process, with NTHi using cell surface structures to directly interact with the mammalian host and using secreted proteins and LOS to change the mammalian host in order to pave the way for colonization and invasion. Many experiments have also noted that immune system evasion through antigenic variation, secretion of enzymes and epithelial cell invasion allowed NTHi to survive for longer periods despite a specific immune response being mounted to infection. Several outer membrane proteins and LOS derivatives are discussed in relation to their efficacy in preventing pulmonary infections and otitis media in animals. General host responses with respect to age, genetic makeup, and vaccine delivery routes are considered, and a mucosal vaccine strategy is suggested.

Evaluation of monoclonal antibodies with specificity for human IgA, IgA subclasses and allotypes and secretory component. Results of an IUIS/WHO collaborative study

51 monoclonal antibodies (McAb) with putative specificity for human IgA, the IgA subclasses, Am allotypes or secretory component (SC) were evaluated for immunoreactivity and specificity by nine laboratories employing immunodiffusion, agglutination, immunohistological assays, immunoblotting and direct binding and competitive inhibition enzyme immunoassays. McAbs specific for IgA PAN (n = 24), IgA1 (n = 7), IgA2 (n = 3), IgA2m(2) (n = 2), non-IgA2m(2) (n = 4) and SC or secretory IgA (n = 5) were identified that were immunoreactive and specific in the assays employed. The McAbs identified as IgA- or SC-reactive were shown to be non-reactive to human IgG, IgM, IgD, IgE, kappa and lambda by direct binding and competitive inhibition immunoassays. Interestingly, no McAbs with restricted specificity for IgA2m(1) were identified. Some McAbs displayed higher affinity and/or better performance in one or several of the assay groups. The IgA- and SC-specific McAbs identified in this international collaborative study have potential as immunochemical reference reagents to identify and quantitate monomeric and polymeric IgA in human serum and secretions.

Biological significance of IgA1 proteases in bacterial colonization and pathogenesis: critical evaluation of experimental evidence

IgA1 protease activity, which allows bacteria to cleave human IgA1 in the hinge region, represents a striking example of convergent evolution of a specific property in bacteria. Although it has been known since 1979 that IgA1 protease is produced by the three leading causes of bacterial meningitis in addition to important urogenital pathogens and some members of the oropharyngeal flora, the exact role of this enzyme in bacterial pathogenesis is still incompletely understood owing to lack of a satisfactory animal model. Cleavage of IgA1 by these post-proline endopeptidases efficiently separates the monomeric antigen-binding fragments from the secondary effector functions of the IgA1 antibody molecule. Several in vivo and in vitro observations indicate that the enzymes are important for the ability of bacteria to colonize mucosal membranes in the presence of S-IgA antibodies. Furthermore, the extensive cleavage of IgA sometimes observed in vivo, suggests that IgA1 protease activity results in a local functional IgA deficiency that may facilitate colonization of other microorganisms and the penetration of potential allergens. It has been hypothesized that IgA1 protease activity of Haemophilus influenzae, Neisseria meningitidis, and Streptococcus pneumoniae, under special immunological circumstances, allows these bacteria to take advantage of specific IgA1 antibodies in a strategy to evade other immune factors of the human body. The decisive factor is the balance between IgA antibodies against surface antigens of the respective bacteria and their IgA1 protease. Recent studies have shown that serine-type IgA1 proteases of H. influenzae, meningococci, and gonococci belong to a family of proteins used by a diverse group of Gram-negative bacteria for colonization and invasion.

Mucosal IgA elaboration

Secretory IgA is the main immunoglobulin present along mucosal surfaces. It is elicited best by oral rather than parenteral administration of specific antigens. The role of antigen form on the development of a secretory IgA response is still unclear. IgA protects by preventing attachment of microorganisms or their toxic products to the surface epithelium. A wide variety of regulatory T cells are now known to be of considerable importance in optimizing the secretory IgA response. This regulation is at least partly due to the elaboration of small polypeptide products (lymphokines). These lymphokines have been shown to be key signals during the maturation of IgA precursor B cells to IgA-secreting plasma cells. By studying models of the mucosal immune system which closely approximate the natural mucosal immune response, it should be possible to develop vaccines against many pathogenic microorganisms, their toxic products, and to toxicants and carcinogens within the environment.

An enzyme-linked immunosorbent assay for the determination of the IgA subclass distribution of antigen-specific antibodies

An ELISA for the determination of the IgA subclass distribution of antigen-specific antibodies was developed using commercially available monoclonal anti-IgA1 anti-IgA2 subclass antibodies. Furthermore an anti-A2m allotype-specific antibody was included in the study. The specificity and sensitivity of the monoclonal anti-immunoglobulin antibodies used was analyzed using sera from normal and IgA class- or subclass-deficient individuals (with or without homozygous C alpha 1 subclass gene deletions). Human IgA1 and IgA2 hybridoma antibodies were also used. In this particular assay, only two out of four tested anti-IgA1 and two out of three tested anti-IgA2 antibodies proved to be specific for their corresponding IgA subclass. The anti-A2m(2) monoclonal antibody was shown to be specific for the corresponding allotype. These ELISA methods may facilitate further work on the regulation of IgA subclass production in man.

Activation of complement by human serum IgA, secretory IgA and IgA1 fragments

Activation of the complement (C) system by human IgA was studied. Both subclasses of IgA, IgA1 and IgA2, and secretory IgA were shown to activate C, as determined by deposition of C3 on glutaraldehyde-activated microwells coated with IgA. The activation of the C system occurred in the presence of MgEGTA and not in D-deficient serum. In addition to C3, deposition of properdin (P) but not of C4 was detected. These results indicate that C activation, as determined by measuring deposition of C3 and P, occurred by the alternative pathway (AP). The data further show that the major part of the hinge region, which is deleted in IgA2 as compared with IgA1 and which forms the major structural difference between the two subclasses, is not involved in C activation. Reduction and alkylation destroyed the ability of IgA to activate C, as has also been demonstrated for IgG. In order to define the C activating region of the IgA molecule, several fragments of IgA1 were tested. The four-chain molecules F(ab')2 and F(abc)2 were shown to activate the AP. No activation was observed with the two-chain fragments Fab and Fc. The Fc fragment of IgA also did not activate the CP, as does the Fc fragment of IgG. This indicates that activation of the AP of C by IgA is dependent on the presence of the F(ab')2 fragment.

IN CONCLUSION

human IgA does activate C by the AP. This activation requires an intact F(ab')2 fragment.

Monoclonal antibodies against different domains of human IgA: specificities determined by immunoblotting and haemagglutination-inhibition

The specificity of 14 monoclonal antibodies has been determined by immunoblotting (IB) and haemagglutination-inhibition (HAI) analysis using IgA1 and IgA2 myeloma proteins and eight different IgA1 fragments. Two antibodies probably recognized epitopes on the CH1 domain of IgA. They reacted with all Fab-containing fragments irrespective of whether these originated from the same or different IgA proteins. Seven antibodies were directed against epitopes on the CH2 domain. These antibodies were reactive with F(abc)2 fragments. They failed to react with Fab, Fab' and F(ab')2 fragments. Two out of these seven antibodies did not react with two-chain IgA half-molecules and Fabc fragments containing a single heavy and a single light chain. This suggests that these two antibodies recognized an epitope whose structure is dependent on disulfide linked heavy chains. Five other antibodies showed specificity for the CH3 domain. They were reactive with all CH3-containing molecules, irrespective of whether they comprised one or two alpha chains. Our study demonstrates that IB is an appropriate technique to determine domain specificity of monoclonal anti-immunoglobulin reagents. Although the IB tests were performed on denatured proteins the results agreed surprisingly well with those of the HAI analyses. Moreover, the IB technique could be used on fragments which could not be purified well enough for HAI analyses.

Monoclonal antibodies against isotypic and isoallotypic determinants of human IgA1 and IgA2: fine specificities and binding properties

We have analysed and compared the fine specificity and behavior in various immunoassays of 10 mouse monoclonal antibodies, from three independent laboratories, directed against IgA1, IgA2 or non-IgA2m(2). The following observations were made. (1) Although all of the monoclonal antibodies were specific for a particular IgA subclass or isoallotype in a radioimmunoassay, three of them were not specific when tested in indirect immunofluorescence on plasma cells derived from pokeweed-activated peripheral blood lymphocytes. In this highly sensitive system, contrary to direct immunofluorescence previously performed using formalin-fixed lymphoid tissue, the anti-IgA1 69.114 reacted with some of the IgA2 plasma cells, the anti-IgA2 DLDB7 reacted with some of the IgA1 plasma cells and the anti-IgA2 16.512 dimly reacted with all IgM plasma cells. (2) Among the eight anti-IgA subclass antibodies, seven were directed against the CH2 domain of IgA whereas the anti-IgA1 1-155-1 recognised an epitope destroyed by Streptococcus sanguis IgA1 protease and localised in the hinge region of IgA1. The two anti-isoallotype antibodies were directed against epitope(s) probably localised in the 65 C-terminal amino acid residues of the alpha-CH3 domain. All of the 10 antibodies were able to react with endogeneously produced surface IgA on B-cells. (3) Using monoclonal anti-IgA subclass antibodies in radioimmunoassay may be hazardous in the absence of knowledge of their affinity constants and of careful control experiments: some of the antibodies were not sensitive in radioimmunoassays designed to measure the serum titer of specific IgA1 and IgA2 antibodies. Moreover, major differences were observed between the different monoclonal reagents with respect to the influence of the size of IgA on a solid-phase sandwich radioimmunoassay. While three of the anti-IgA1 underestimated dimeric IgA relative to monomeric IgA, the fourth anti-IgA1 and all the anti-IgA2 overestimated dimeric IgA relative to monomeric IgA, by a factor sometimes close to 7.

Monoclonal antibodies to distinct epitopes on human IgA and their use to IgA determination

Several monoclonal antibodies (MAbs) against human IgA have been obtained which specifically bind to human myeloma and polyclonal IgA. Three of these MAbs have been purified and studied further. They recognize both IgA subclasses and define three distinct epitopes on the IgA molecule. These MAbs were used to develop a solid-phase radioimmunoassay (RIA) in which one MAb was immobilized and the other two were labeled with 125I. The assay has a sensitivity in the nanogram range. A good correlation was found (r = 0.97, P less than 0.001) when the solid-phase RIA was compared with a commercially available immunodiffusion technique for the determination of IgA levels in serum samples.

Assay of human IgA subclass antibodies in serum and secretions by means of monoclonal antibodies

Solid-phase radioimmunoassays have been developed for the detection and quantification of human serum and secretory IgA antibodies to a variety of food, bacterial and viral antigens. Monoclonal antibodies specific for IgA1 and IgA2 and capable of binding to serum and secretory IgA were used. The assays were calibrated by reference to standard serum or purified myeloma proteins bound to solid-phase anti-immunoglobulin reagents, and sigmoid calibration curves were constructed by means of computer programs using 4-parameter logistic or weighted logit-log principles. Polymeric and monomeric forms of IgA antibodies were assayed in fractions separated by high performance size exclusion chromatography. These techniques have demonstrated the expected predominance of IgA1 antibodies in serum, and these included polymeric forms. Saliva contained both IgA1 and IgA2 antibodies, and increased proportions of IgA2 antibodies to lipopolysaccharides and lipoteichoic acid were observed.

Gene conversion in human immunoglobulin gamma locus shown by unusual location of IgG allotypes

The constant region of the gamma 1, gamma 2 and gamma 3 heavy chains of the human IgG1, IgG2 and IgG3 immunoglobulins carries antigenic determinants or G1m, G2m and G3m allotypes, which are genetic markers of these subclasses. The exceptional presence on gamma 1 and gamma 2 chains of Gm allotypes usually located on the CH3 domain of gamma 3 shows an unexpected clustering of base changes and subsequent identity of short DNA sequences in the CH3 exon of the non-allelic gamma 1, gamma 2 and gamma 3 genes. Such clusters of substitutions are not easily explained on the classical basis of point mutations. A gene conversion, which substituted a segment of the gamma 1 or gamma 2 gene with the homologous region of the non-allelic gamma 3 gene, is more likely. Other examples of possible conversion involving the gamma genes are described. The conservation or the restoration of short sequences produced by the conversion events might be related to the biological properties of the constant region of the heavy chains.

Immunogenic and antigenic epitopes of immunoglobulins. X: Monoclonal antibodies specific for human IgA, the IgA1 and IgA2 subclasses and an nA2m(2) iso-allotypic epitope

Monoclonal antibodies (McAb) specific for human IgA, the IgA1 and IgA2 subclasses and the iso-allotypic epitope nA2m(2) have been produced. Three distinct McAb recognize an IgA1-specific epitope expressed in the C alpha 2 domain or the hinge region whilst a further McAb is directed possibly to an IgA1 hinge region epitope. The McAb having nA2m(2) specificity recognizes an iso-allotypic epitope expressed within the Fab region. IgA1 and IgA2 epitopes were detected in gorilla but not rhesus or baboon sera suggesting that the IgA subclasses represent a recent gene duplication even. However, these epitopes were also detected in some non-primate sera.

Secretion of immunoglobulins and plasma proteins from the jejunal mucosa. Transport rate and origin of polymeric immunoglobulin A

Parameters of secretion of IgA and several other plasma proteins from the jejunal mucosa were investigated in 11 individuals who had a normal distribution of Ig-containing cells in the lamina propria and in one patient who was totally deficient in jejunal IgA and IgM plasmacytes. Jejunal samples were collected during segmental gut perfusion. The following results were obtained: (a) The secretion of polymeric IgA (p-IgA, mean equals 217 micrograms/40 cm per min) exceeded those of albumin (132 micrograms), IgG (35 micrograms), and monomeric IgA (m-IgA, 15 micrograms, or 6.4% of total IgA). About 35% of IgA was IgA2 in the jejunal secretion, compared with approximately 23% in serum. This closely corresponds to the 35 and 24% of IgA2 plasmocytes in jejunal mucosa and peripheral lymph nodes, respectively. (b) For each protein, a relative coefficient of excretion (RCE) was calculated (jejunum to serum concentration ratio expressed relative to that of albumin). RCEs of 1.41 for orosomucoid, 1.0 for albumin, 0.83 for IgG, and 0.74 for IgE and, in the deficient patient, of 0.64 for m-IgA and 0.016 for IgM were obtained. This was inversely related to the molecular weight of these proteins and indicated their predominantly passive transport into the jejunum. However, in normal individuals, the RCE of transferrin (approximately 1.11 greater than 1, P greater than 0.05), alpha 2-macro globulin (approximately 0.77), m-IgA (approximately 1.98), and p-IgA (approximately 218) exceeded the value expected from simple seepage from plasma, thus pointing to an additional role of either local gut synthesis and/or active transepithelial transport. (c) Approximately 98% of p-IgA, approximately 99% of IgM, and approximately 68% of m-IgA in jejunal secretions were derived from local production in the gut wall, as determined by 125I-p-IgA specific activities and/or by comparison between the RCE values of the deficient patient to the values of controls. Therefore, the jejunal production of p-IgA (approximately 312 mg/d per 40 cm vs. approximately 54 mg/d from bile) contributes the majority of upper intestinal IgA in humans. The active transport of plasma p-IgA across the intestinal mucosa (approximately 0.08 mg/40 cm per kg per d) contributes less than 2% of the total amount of p-IgA (4.5 mg/kg per d) that is cleared daily from plasma.

Human immunoglobulin heavy chain A2 gene allotype determination by restriction fragment length polymorphism

The human immunoglobulin heavy chain alpha 2 genes have two allelic forms or allotypes called A2m(1) and A2m(2). Previously, these allotypic markers have only been distinguishable by serology. Studies of the alpha 2 genes, however, show that it is possible to differentiate between the allotypes by restriction enzyme site polymorphisms, both in the protein coding regions and in flanking regions. These polymorphic sites have been used to determine the alpha 2 allotypes of several human DNAs.