The carboxyl-terminal domains of IgA and IgM direct isotype-specific polymerization and interaction with the polymeric immunoglobulin receptor

Mucosal surfaces are protected by polymeric immunoglobulins that are transported across the epithelium by the polymeric immunoglobulin receptor (pIgR). Only polymeric IgA and IgM containing a small polypeptide called the "joining" (J) chain can bind to the pIgR. J chain-positive IgA consists of dimers, and some larger polymers, whereas only IgM pentamers incorporate the J chain. We made domain swap chimeras between human IgA1 and IgM and found that the COOH-terminal domains of the heavy chains (Calpha3 and Cmu4, respectively) dictated the size of the polymers formed and also which polymers incorporated the J chain. We also showed that chimeric IgM molecules engineered to contain Calpha3 were able to bind the rabbit pIgR. Since the rabbit pIgR normally does not bind IgM, these results suggest that the COOH-terminal domain of the polymeric immunoglobulins is primarily responsible for interaction with the pIgR. Finally, we made a novel chimeric IgA immunoglobulin, containing the terminal domain from IgM. This recombinant molecule formed J chain-containing pentamers that could, like IgA, efficiently form covalent complexes with the human pIgR ectodomain, known as secretory component.

Cysteine residues required for the attachment of the light chain in human IgA2

In humans, there are two subclasses of IgA, IgA1 and IgA2, with IgA2 existing as three allotypes, IgA2m(1), IgA2m(2) and IgA2(n). In IgA1, Cys(133) in C(H)1 forms the disulfide bond to the L chain. Our previous studies indicated that in IgA2 lacking Cys(133), a disulfide bond forms between the alpha-chain and the L chain when Cys(220) is followed by Arg(221), but not when Cys(220) is followed by Pro(221), suggesting that the Cys in C(H)1 might be involved in disulfide bonding to the L chain. However, here we show that covalent assembly of the H and L chains in IgA2(n) requires hinge-proximal Cys(241) and Cys(242) in C(H)2 and not Cys(196) or Cys(220) in C(H)1. Using pulse-chase experiments, we have demonstrated that wild-type IgA2(n) with Arg(221) and Cys(241) and Cys(242) assembles through a disulfide-bonded HL intermediate. In contrast, the major intermediate for IgA2 m(1) with Pro(221) assembly was H(2) even though both Cys(241) and Cys(242) were present. Only a small fraction of IgA2 m(1) assembles through disulfide-bonded HL. Overall, our studies indicate that for IgA2 covalent assembly of the H and L chains requires the hinge-proximal cysteines in C(H)2 and that the structure of C(H)1 influences the efficiency with which this covalent bond forms.

Reactivity and isotype profiling of monoclonal antibodies using multiple antigenic peptides

The characterisation of monoclonal antibodies (MAbs) is essential for the development of assay systems particularly where antigens have been developed using synthetic peptides. Indeed some peptide-carrier conjugates fail to induce immune responses and may not generate antibodies that bind to native protein. As an alternative to peptide-carrier conjugates, multiple antigenic peptides (MAPs) have been used for immunization strategies, but with little regard to the characteristics of the MAbs produced. In this study, we used 3 MAPs of Epstein-Barr virus (EBV) latent membrane protein 1 (LMP1) to immunise BALB/c mice. Overall, the polyclonal antibody responses from tail bleeds showed that MAPs evoked B-cell responses. However, on screening 144 hybridomas, 24 MAb supernatants exhibited weak to moderate reactivity in enzyme-linked immunosorbant assay (ELISA) and against cell cytospin preparations (B95.8 and AG876 LCL), respectively. Isotype profiling of hybridoma supernatants also showed that 11 out of 24 were IgM. Further characterization of 6 MAbs in Western blotting showed reactivity to recombinant LMP1 and only one MAb (B28D) showed weak reactivity to the malignant cells (Hodgkin/Reed-Sternberg; HRS cells) of an EBV+ Hodgkin's lymphoma using paraffin-embedded tissue. It is probable that these MAPs failed to augment T-cell help and contributed to the production of low affinity (IgM) antibodies. These observations may be of importance to future immunization strategies, where MAPs are used in the production of monoclonal reagents.

Comparative analyses of immunoglobulin genes: surprises and portents

The study of immunoglobulin genes in non-mouse and non-human models has shown that different vertebrate groups have evolved distinct methods of generating antibody diversity. By contrast, the development of T cells in the thymus is quite similar in all of the species that have been examined. The three mechanisms by which B cells uniquely modify their immunoglobulin genes -- somatic hypermutation, gene conversion and class switching -- are increasingly believed to share some fundamental mechanisms, which studies in different vertebrate groups have helped (and will continue to help) to resolve. When these mechanisms are better understood, we should be able to look to the constitutive pathways from which they have evolved and perhaps determine whether the rearrangement of variable, diversity and joining antibody gene segments -- V(D)J recombination -- was superimposed on an existing adaptive immune system.

Phage display based cloning of proteins interacting with the cytoplasmic tail of membrane immunoglobulins

The reduced quantity and quality of serum immunoglobulins (sIgs) in mutant mice expressing truncated cytoplasmic tails of IgE and IgG1 indicate an active role for the cytoplasmic domains of mIgG1 and mIgE. We used phage display technology to identify candidate proteins able to interact with the cytoplasmic tail of mIgE. Using a murine cDNA B cell library displayed on the surface of phage as prey and the 28 amino acid long cytoplasmic tail of IgE as bait, we isolated phage encoding the murine hematopoietic progenitor kinase 1 (HPK1). Surface plasmon resonance analysis measurements confirmed affinity of HPK1 to the mIgE cytoplasmic tail and revealed association to other immunoglobulin isotypes as well. Immunoprecipitation experiments, using lysates from two B cell lines expressing nitrophenyl (NP) specific mIgE molecules showed co-precipitation of IgE and HPK1. The interaction of HPK1 with the cytoplasmic domains of membrane immunoglobulins indicate an active role of the tails as part of an isotype specific signal transduction, independent from the Igalpha/Igbeta heterodimers, and may represent a missing link to upstream regulatory elements of HPK1 activation.

Unraveling of the polymorphic C lambda 2-C lambda 3 amplification and the Ke+Oz- polymorphism in the human Ig lambda locus

Two polymorphisms of the human Ig(lambda) (IGL) locus have been described. The first polymorphism concerns a single, 2- or 3-fold amplification of 5.4 kb of DNA in the C(lambda)2-C(lambda)3 region. The second polymorphism is the Mcg(-)Ke(+)Oz(-) isotype, which has only been defined via serological analyses in Bence-Jones proteins of multiple myeloma patients and was assumed to be encoded by a polymorphic C(lambda)2 segment because of its high homology with the Mcg(-)Ke(-)Oz(-) C(lambda)2 isotype. It has been speculated that the Mcg(-)Ke(+)Oz(-) isotype might be encoded by a C(lambda) gene segment of the amplified C(lambda)2-C(lambda)3 region. We now unraveled both IGL gene polymorphisms. The amplification polymorphism appeared to result from a duplication, triplication, or quadruplication of a functional J-C(lambda)2 region and is likely to have originated from unequal crossing over of the J-C(lambda)2 and J-C(lambda)3 region via a 2.2-kb homologous repeat. The amplification polymorphism was found to result in the presence of one to five extra functional J-C(lambda)2 per genome regions, leading to decreased Ig(kappa):Ig(lambda) ratios on normal peripheral blood B cells. Via sequence analysis, we demonstrated that the Mcg(-)Ke(+)Oz(-) isotype is encoded by a polymorphic C(lambda)2 segment that differs from the normal C(lambda)2 gene segment at a single nucleotide position. This polymorphism was identified in only 1.5% (2 of 134) of individuals without J-C(lambda)2 amplification polymorphism and was not found in the J-C(lambda)2 amplification polymorphism of 44 individuals, indicating that the two IGL gene polymorphisms are not linked.

Autoantigen microarrays for multiplex characterization of autoantibody responses

We constructed miniaturized autoantigen arrays to perform large-scale multiplex characterization of autoantibody responses directed against structurally diverse autoantigens, using submicroliter quantities of clinical samples. Autoantigen microarrays were produced by attaching hundreds of proteins, peptides and other biomolecules to the surface of derivatized glass slides using a robotic arrayer. Arrays were incubated with patient serum, and spectrally resolvable fluorescent labels were used to detect autoantibody binding to specific autoantigens on the array. We describe and characterize arrays containing the major autoantigens in eight distinct human autoimmune diseases, including systemic lupus erythematosus and rheumatoid arthritis. This represents the first report of application of such technology to multiple human disease sera, and will enable validated detection of antibodies recognizing autoantigens including proteins, peptides, enzyme complexes, ribonucleoprotein complexes, DNA and post-translationally modified antigens. Autoantigen microarrays represent a powerful tool to study the specificity and pathogenesis of autoantibody responses, and to identify and define relevant autoantigens in human autoimmune diseases.

Isolation and partial characterisation of immunoglobulin from southern bluefin tuna Thunnus maccoyii Castelnau

Specific and total serum immunoglobulins were extracted by immunoaffinity, mannan-binding protein and Protein A affinity chromatography from southern bluefin tuna (Thunnus maccoyii Castelnau) immunised with rabbit IgG, and from non-immunised southern bluefin tuna. SDS-PAGE in 10% reducing gels revealed two heavy chains with molecular weights of approximately 74.6 +/- 1.3 kDa and 71.2 +/- 0.9 kDa, and two light chains with molecular weights of approximately 29 +/- 1.2 kDa and 28 +/- 1.0 kDa. Under non-reducing, but denaturing, conditions in 4% and 5% SDS-PAGE gels, a high molecular weight and a low molecular weight fraction were demonstrated. By gel filtration using Sephacryl HR 300 a molecular weight of 845 kDa, consistent with a tetramer, was obtained for the high molecular weight fraction, and a molecular weight of 168 kDa, consistent with a monomer, was obtained for the low molecular weight fraction. The extinction coefficient at A280 for the purified immunoglobulin (Ig) was determined to be 1.24. Tuna a-rabbit IgG Ig was reactive with all non-reduced mammalian IgG antigens tested, suggesting that common conformational antigenic determinants were recognised.

Somatic diversity of the immunoglobulin repertoire is controlled in an isotype-specific manner

We have studied two aspects of the IgE immune response. First, we have compared the kinetics of the IgE response to the T cell-dependent antigen ph-Ox coupled to ovalbumin with that of the IgG1 response and we have assessed the quality of the IgE response. Second, we have studied the generation of somatic diversity, understood as the combined effect of somatic mutation and the selection of D(iversity) and J(oining) elements, in germinal center B cells at the molecular level, using the germ-line sequence of the prototype anti-ph-Ox heavy chain variable element V(H)Ox1 as reference. We evaluated sequences derived from mu-, gamma 1- and epsilon-variable elements and showed that somatic diversification was different for all isotypes studied. We further compared the IgE responses of wild-type mice with those of mice expressing a truncated cytoplasmic IgE tail (IgE(KVK Delta tail)). IgE(KVK Delta tail) mice showed a more diverse sequence pattern. We corroborated previous results suggesting that short CDR3 regions are indicative for high-affinity antibodies by measuring relative affinities of phage-expressed Fab fragments with prototype long and short CDR3 regions. Therefore, the composition of the antigen-receptor is responsible for the selection process and the expansion of antigen-specific cells, leading to an isotype-specific antibody repertoire.

Three-dimensional structure of the Fab from a human IgM cold agglutinin

Cold agglutinins (CAs) are IgM autoantibodies characterized by their ability to agglutinate in vitro RBC at low temperatures. These autoantibodies cause hemolytic anemia in patients with CA disease. Many diverse Ags are recognized by CAs, most frequently those belonging to the I/i system. These are oligosaccharides composed of repeated units of N:-acetyllactosamine, expressed on RBC. The three-dimensional structure of the Fab of KAU, a human monoclonal IgM CA with anti-I activity, was determined. The KAU combining site shows an extended cavity and a neighboring pocket. Residues from the hypervariable loops V(H)CDR3, V(L)CDR1, and V(L)CDR3 form the cavity, whereas the small pocket is defined essentially by residues from the hypervariable loops V(H)CDR1 and V(H)CDR2. This fact could explain the V(H)4-34 germline gene restriction among CA. The KAU combining site topography is consistent with one that binds a polysaccharide. The combining site overall dimensions are 15 A wide and 24 A long. Conservation of key binding site residues among anti-I/i CAs indicates that this is a common feature of this family of autoantibodies. We also describe the first high resolution structure of the human IgM C(H)1:C(L) domain. The structural analysis shows that the C(H)1-C(L) interface is mainly conserved during the isotype switch process from IgM to IgG1.

Isotype switch variants reveal clonally related subpopulations in diffuse large B-cell lymphoma

Primary diffuse large B-cell lymphomas (DLBCLs) are aggressive tumors accounting for approximately 40% of B-cell malignancies. The immunoglobulin (Ig) variable region genes have undergone rearrangement and are commonly somatically mutated. The majority show intraclonal variation which indicates that somatic mutation has continued after transformation. Typically, cells of DLBCLs express Ig of a single isotype, but there may be accompanying cells that express alternative isotypes. To probe the status of the isotype switch process in DLBCL, 4 cases of tumor-derived constant region transcripts of all isotypes were investigated. Following the identification of the VDJ sequences, the presence of the major isotype expected from immunohistochemical analysis was confirmed at the RNA level. Another 3-4 alternative isotypes were revealed in all cases, some of which could also be detected by immunohistochemistry. All cases were somatically mutated with an intraclonal variation. In 2 cases there were clearly distinct patterns of somatic mutation between isotypes, which was consistent with independent evolution of the tumor subpopulations. There was apparent clustering of mutational patterns into either an IgMD/IgG3/IgA set or an IgG1/IgA set, indicating that the switch to IgA can occur by different routes. Alternative isotype expression is evident in DLBCL at both the RNA and protein levels. The pattern of mutation indicates that switching is occurring in subpopulations of the tumor after malignant transformation. The findings support the concept that isotype switch events may be a feature of DLBCL.

Distribution of immunoglobulin isotypes and subisotypes in equine guttural pouch (auditory tube diverticulum)

To clarify the functions of the equine guttural pouch, the distribution of various immunoglobulin isotypes and subisotypes in the guttural pouch mucosa were examined in healthy horses. IgGa was present in the mucosa of guttural pouch, mucosal lymph nodules and submucosal lymph nodules. IgM was scattered in the mucosal lymph nodules and in the germinal centers of the submucosal lymph nodules. IgGc was recognized only in the submucosal lymph nodules. These immunoglobulin isotypes and subisotypes were found in lymphocytes and plasma cells. On the other hand, IgA was detected in glandular epithelial cells and the surface layer of the mucosal epithelium, as well as in free cells. This finding suggests that IgA is secreted through the glandular epithelium. Based on the above findings, we conclude that the guttural pouch has phylactic ability.

The thermal stability of immunoglobulin: unfolding and aggregation of a multi-domain protein

The denaturation of immunoglobulin G was studied by different calorimetric methods and circular dichroism spectroscopy. The thermogram of the immunoglobulin showed two main transitions that are a superimposition of distinct denaturation steps. It was shown that the two transitions have different sensitivities to changes in temperature and pH. The two peaks represent the F(ab) and F(c) fragments of the IgG molecule. The F(ab) fragment is most sensitive to heat treatment, whereas the F(c) fragment is most sensitive to decreasing pH. The transitions were independent, and the unfolding was immediately followed by an irreversible aggregation step. Below the unfolding temperature, the unfolding is the rate-determining step in the overall denaturation process. At higher temperatures where a relatively high concentration of (partially) unfolded IgG molecules is present, the rate of aggregation is so fast that IgG molecules become locked in aggregates before they are completely denatured. Furthermore, the structure of the aggregates formed depends on the denaturation method. The circular dichroism spectrum of the IgG is also strongly affected by both heat treatment and low pH treatment. It was shown that a strong correlation exists between the denaturation transitions as observed by calorimetry and the changes in secondary structure derived from circular dichroism. After both heat- and low-pH-induced denaturation, a significant fraction of the secondary structure remains.

The interaction between F3 immunoglobulin domains and protein tyrosine phosphatases zeta/beta triggers bidirectional signalling between neurons and glial cells

F3, a mouse glycosyl-phosphatidylinositol anchored molecule of the immunoglobulin superfamily, is known to influence axonal growth and fasciculation via multiple interactions of its modular immunoglobulin-like domains. We prepared an Fc chimeric molecule (F3IgFc) to identify molecules interacting with these domains and characterize the functional impact of the interactions. We affinity-isolated tenascin-C and isoforms of the proteoglycan-type protein tyrosine phosphatases zeta/beta (PTPzeta/RPTPbeta) from extracts of developing mouse brain. We showed that both PTPzeta/RPTPbeta and tenascin-C can bind directly to F3, possibly in an exclusive manner, with the highest affinity for the F3-PTPzeta/RPTPbeta interaction. We observed a strong binding of F3IgFc-coated fluorospheres to astrocytes in neural primary cultures and to C6 astrocytoma cells, and demonstrated, in antibody perturbation experiments, that F3-Ig binding on astrocytes depends on its interaction with PTPzeta/RPTPbeta. We also found by confocal analysis that tenascin-C and PTPzeta/RPTPbeta were colocalized on astrocytes which suggests a complex interplay of interactions between PTPzeta/RPTPbeta, tenascin-C and F3. We showed that the interaction between PTPzeta/RPTPbeta and F3-Ig-like domains can trigger bidirectional signalling. C6 glia-expressed PTPzeta/RPTPbeta stimulated neurite outgrowth by cortical and cerebellar neurons, whereas preclustered F3IgFc specifically modified the distribution of phosphotyrosine labelling in these glial cells. Both effects could be prevented and/or mimicked by anti-F3 and anti-6B4PG antibodies. These results identify F3 and PTPzeta/RPTPbeta as potential mediators of a reciprocal exchange of information between glia and neurons.

Extensive junctional diversity in Ig light chain genes from early B cell progenitors of mu MT mice

Nontemplated (N) nucleotide additions contribute significantly to the junctional diversity of all Ag receptor chains in adult mice except Ig light (L) chains, primarily because terminal deoxynucleotidyl transferase (TdT) expression is turned off at the time of their rearrangement in pre-B cells. However, because some Ig L chain gene rearrangements are detectable earlier during B cell ontogeny when TdT expression is thought to be maximal, we have examined the junctional processing of kappa- and lambda-chain genes of CD45(B220)+CD43+ pro-B cells from mu MT mice. We found that both kappa and lambda coding junctions formed in these B cell precursors were extensively diversified with N-region additions. Together, these findings demonstrate that Ig L chain genes are equally accessible to TdT in pro-B cells as Ig heavy chain genes. Surprisingly, however, the two L chain isotypes differed in the pattern of N addition, which was more prevalent at the lambda-chain locus. We observed the same diversity pattern in pre-B cells from TdT-transgenic mice. These results suggest that some aspects of TdT processing could be influenced by factors intrinsic to the sequence of Ig genes and/or the process of V(D)J recombination itself.

Varied redox forms of teleost IgM: an alternative to isotypic diversity?

Teleosts (bony fish) are thought to primarily or exclusively possess a single structural form of immunoglobulin (Ig), a tetrameric IgM. However, in species wherein intact Ig has been electrophoretically analyzed under denaturing, non-reducing conditions, a significant degree of structural diversity has been revealed. This IgM molecule appears to be assembled with great latitude in the degree of disulfide crosslinking between monomeric or halfmer subunits composing the complete IgM molecule. This heterogeneity in the basic structure (herein referred to as redox forms) is not due to isotypic differences as each B cell produces this heterogeneity within its immunoglobulin product. Additionally, in the case of the catfish, a single fish/mouse chimeric Ig H gene is capable of producing IgM with a comparable amount of structural heterogeneity within the mouse cell. Thus, the piscine B lymphocyte routinely assembles a variety of redox forms from one IgM chain. This has both profound biosynthetic implications for macromolecular assembly processes as well as intriguing possibilities for the generation of teleost Ig functional diversity.

Evaluation of the role of Fc gamma and complement receptors in the decreased phagocytosis of hereditary haemochromatosis patients

Hereditary haemochromatosis (HH) monocytes have a decreased antibody mediated phagocytosis of rabbit erythrocytes and Staphylococcus aureus compared to control monocytes. In order to investigate whether this decrease could be attributed to a different level of expression of Fc gamma receptors (Fc gamma R) or complement receptors (CR), which cooperate even in the absence of complement, the surface expression of these receptors was determined on monocyte-enriched suspensions. In contrast to what was expected, HH monocytes displayed a significantly higher level of Fc gamma RI and Fc gamma RIIa as compared to healthy donor monocytes, but these differences were very small. The expression of the other receptors studied were similar for both groups. The heat-inactivated mouse serum used for opsonizing the erythrocytes mainly contained mouse IgG1. Two genetically different forms of Fc gamma RIIa are known, each with a different affinity for mouse IgG1 antibodies. Therefore, the Fc gamma RIIa polymorphism in monocytes (MN) of both groups was also investigated. A similar distribution was found for patients and healthy donors. In addition, the extent of erythrophagocytosis of both donors and patients was independent of Fc gamma RIIa allotype. Our results indicate that the altered phagocytosis by HH monocytes cannot be attributed to a different level of expression of receptors involved in phagocytosis or to Fc gamma RIIa polymorphism.

Restricted use of cationic germline V(H) gene segments in human Rh(D) red cell antibodies

The human red cell Rh(D) antigen elicits the production of high-affinity IgG antibodies, which can prevent blood transfusion and cause hemolytic disease of the newborn. It has been known for 20 years that Rh(D) antibodies are among the most positively charged human serum IgGs. Analysis by IEF of 9 human anti-Rh(D) monoclonal antibodies showed that their isoelectric points (pI) (8.3 to 8.6) were also significantly higher than the average pI of serum IgGs (7.0 to 8.5). Sequencing of the anti-Rh(D) H and L chains cDNAs showed a preferential use of V(H)1, V(H)3, J(H)6, and V(kappa)1 gene segments. The high pIs in IEF were correlated with a higher number of cationic amino acid residues in the H chain V regions without clustering in the complementary determining region. Computer analysis indicated that the germline V(H) used in anti-Rh(D) was selected among the most cationic segments available in the human V(H) repertoire or expressed in normal B cells. These results indicate that the selection of cationic V(H) segments may be an important early step in the formation of clinically relevant anti-Rh(D) and other red cell antibodies, possibly to facilitate epitope binding in the negatively charged red cell membrane environment.

Is Xenopus IgX an analog of IgA?

Using class specific monoclonal antibodies we analyzed the tissue distribution of B cells expressing the three immunoglobulin (Ig) isotypes (IgM, IgX, IgY) in Xenopus. Large numbers of IgM- and IgX-, but not IgY-, positive B cells are located in the gut epithelium of the intestine. In this organ up to 60% of all B cells can be IgX positive, while in the spleen or liver they are hardly detectable. The majority of IgX-producing cells resemble plasma cells. IgY-producing cells are found in the liver and spleen but not in the intestine. In contrast to IgY, the expression of IgM and IgX is thymus independent. Upon systemic immunization, a several-fold increase of specific IgM and IgY, but not IgX, antibodies was detected in the sera. This and its association with the mucosae of the intestine resembles results reported for mammalian IgA; therefore, IgX of Xenopus might be considered an analog of IgA.

Comparison of surface properties of human IgA, IgE, IgG and IgM antibodies with identical and different specificities

In this paper, the authors report the use of liquid-liquid partition chromatography (LLPC) in an aqueous polyethylene glycol (PEG)/dextran two-phase system to compare the surface properties (partition properties) of human antibodies and fragments thereof. The surface properties of all the monoclonal antibodies of different classes and subclasses investigated were within the same broad range as that observed for the polyclonal antibodies and no relationship was found between the exposed surfaces of the immunoglobulins (Ig) and their heavy chain isotype. Moreover, Fc fragments from various IgG1, 2 and 4 myeloma proteins were found to exhibit similar surface properties. Employing chimeric antibodies with identical variable regions the authors found that intact IgG1, 2 and 4 displayed identical surface properties, while the corresponding IgA1, IgA2, IgG3, IgE and IgM antibodies differed both from each other and from the IgGs. The surface properties of chimeric IgG3 could be made similar to those of the IgG1, 2 and 4 chimers by partially reducing the length of the hinge section, but new differences in surface properties appeared when their hinges were of similar length. Thus, LLPC can be used to detect differences or similarities in the surface properties of the antigen-binding regions as well as the Fc part in the various isotypes. This can shed light on biological activities such as antigen binding and effector function.

Bias in murine IgG isotype immobilisation. Implications for IgG glycoform analysis ELISA procedures

This study investigates immobilisation of murine IgG in various ELISA procedures. Monoclonal murine IgG isotypes and polyclonal IgG from sera were studied. Similar binding curves to plastic were found for all four individual murine IgG isotypes. Single isotypes displayed different affinities for both protein A and protein G, in particular IgG1 was poorly and IgG3 strongly bound to both of these proteins. When mixtures of the isotypes were bound to either plastic, protein A or protein G, competition was observed in which IgG3 was dominant. Paradoxically, studies on the binding rates of single isotypes direct to plastic revealed that IgG3 had the slowest binding rate. Heating of bound IgGs resulted in significant but isotypically non-selective losses from the plates. The data demonstrate that despite obtaining equivalent individual IgG isotype binding curves, mixtures of IgG isotypes behave very differently, with competition for binding occurring even on plastic. The IgG isotype levels of murine sera were measured for individual mice, and the capture efficiency of each IgG isotype by protein A determined at different serum dilutions. Comparisons were made between the observed capture levels of IgG isotypes and their known serum levels. At all dilutions tested, greater than expected binding of IgG3, IgG2b and IgG2a was observed. At a serum dilution of 1/100 the binding of these three isotypes was increased 16-, 2.9- and 0.4-fold, respectively. These increases were balanced by a decrease in IgG1 binding which was the most prevalent serum IgG isotype. The results described above suggest that capture techniques are biased and unlikely to provide a coating of IgG isotypes that accurately reflects that of the serum. This bias is derived from the specificity of the individual isotypes for either protein A or protein G, and the errors further compounded by direct competition between isotypes whatever the capture surface. Induced coalescence of IgG3 may explain the latter observations.

Peptides derived from IgE heavy chain constant region induce profound IgE isotype-specific tolerance

(BALB/c x SJL)F1 mice, perinatally injected with peptide-N-glyconase F-treated, deglycosylated IgE heavy chain or recombinant IgE heavy chain (CH epsilon 2-CH epsilon 4), were profoundly inhibited in antigen-specific IgE production. There exist minimally two tolerogenic IgE peptides, residing in the CH epsilon 2 and CH epsilon 4 domains. Peptide I, generated by V8 protease, comprises 39 amino acids within CH epsilon 2, beginning at amino acid 103. Peptide E begins at amino acid 312 of the CH epsilon 4 domain and extends through the CH epsilon 4 domain. The total lack of antigen-specific IgE responses in IgE peptide-treated mice was not due to overproduction of interferon-gamma, nor lack of interleukin (IL)-4, as predicted by the Th2/IL-4 paradigm for IgE production. IgE-tolerant mice exhibited comparable levels of circulating anti-IgE antibodies to those of PBS-treated control mice. IgG obtained from sera of both sources failed to inhibit IgE responses in vitro. Moreover, IgE responses of spleen cells from IgE peptides-treated mice were restored by CD4+ T cells from PBS-treated control mice. We hypothesize that regulation of antigen-specific IgE responses is mediated by CD4+ T cells which normally recognize IgE peptides on IgE precursor B cells, and can be rendered tolerant by perinatal IgE peptide treatment.

Structural features of the extracellular portion of membrane-anchoring peptides on membrane-bound immunoglobulins

Membrane-bound immunoglobulins, mIgs, are displayed as transmembrane proteins on the surface of B cells, where they serve as antigen receptors. The mIgs are anchored to the membrane through a carboxy-terminal extension of the immunoglobulin heavy chain. Three distinct structural regions of these membrane-anchor peptides, of mouse and human mIgs, have been delineated: (1) a central conserved stretch of 25 hydrophobic, unchanged amino acid residues, which spans the membrane lipid bilayer; (2) a C-terminal hydrophilic region of 3-28 amino acids, which is intracytoplasmic; and (3) an N-terminal extracellular hydrophilic region of 13-67 amino acids, which is isotype-specific. Here we report predicted secondary and tertiary structures of the third structural region of the membrane anchoring peptide along with corroborating experimental evidence. The predictions of secondary and tertiary structure indicate that most of these regions can assume an chi-helical conformation. Circular dichroism spectroscopy of corresponding synthetic peptide confirms this essential feature. The choice of solvent and pH have dramatic effects on peptide helicity; solvent conditions consistent with a membrane-proximal environment promote helicity. Additional studies suggest that the two adjacent extracellular peptides may be stabilized through coiled-coil interactions similar to those described for some other transmembrane proteins.

Membrane exon sequences of the three Xenopus Ig classes explain the evolutionary origin of mammalian isotypes

We have cloned and sequenced the genes corresponding to the membrane exons of the three immunoglobulin (Ig) heavy chain isotypes (mu, upsilon and chi) of Xenopus. Among membrane Ig (mIg) polypeptides, the transmembrane domain are the most highly conserved. The transmembrane and cytoplasmic domains of Xenopus mIgM are similar to the corresponding domains of all known vertebrate mIgM molecules, supporting the idea that amphibian mu gene is homologous, not just analogous, to the mu gene of higher vertebrates. The membrane forms of the two other Ig isotypes mIgX and mIgY exhibit the specific structure found in all Ig membrane exons, but are not homologous with any specific mammalian non-mu Ig isotype; they are most similar to Xenopus mIgM. Based on the conserved transmembrane domains of Xenopus mIgX, mIgY, we suggest that first the upsilon and later the chi genes arose by duplication from the original mu gene. The transmembrane and the 37-amino-acid-long cytoplasmic domains of Xenopus mIgY have conserved residues found in avian mIgY and mammalian mIgG and mIgE, suggesting that the modern isotypes might share a common ancestor with amphibian mIgY. However, while the sequence similarity between the membrane exons of avian mIgY and mammalian mIgG and IgE is striking, the overall similarity with Xenopus mIgY is very low. Thus, the genes giving rise to Xenopus mIgY and those eventually leading to avian mIgY and mammalian mIgG and mIgE must have diverged early in evolution, probably at the level of the primitive amphibians or before.