Biosynthesis of immunoglobulin A (IgA) and immunoglobulin M (IgM). Control of polymerization by J chain

Cell suspensions of mouse plasma-cell tumours secreting IgA (immunoglobulin A) and IgM (immunoglobulin M) were incubated with radioactive leucine for various periods of time. The secreted immunoglobulins were precipitated from the culture medium with specific rabbit antisera to determine the relative distribution of radioactivity among the different molecular species, and to estimate the fraction of total radioactivity in the J chain. For IgM-secreting cells there is a balanced synthesis of 7S subunits and J chains, and the secreted product is uniformly assembled to the pentamer. In cells secreting IgA, however, the results demonstrate that the pool of intracellular J chain is less than the intracellular IgA pool. The concentration of J chain is therefore limiting and is less than the requirement for complete polymerization. The major factor that determines whether an intracellular monomer is secreted as such or is polymerized with the addition of J chain is therefore the amount of intracellular J chain. When this is limiting, as it is in cells secreting IgA, then monomer will be secreted.     

Serum immunoglobulin M (IgM) during early development of masu salmon (Oncorhynchus masou).

1. The development of serum immunoglobulin M (IgM) was studied by measuring IgM concentration in early developmental stages of masu salmon (Oncorhynchus masou) using single radial immunodiffusion. 2. IgM concentration was measurable from 88 days after hatching (46.5 micrograms/ml) and was maintained at a relatively constant level (less than 100 micrograms/ml) until 235 days after hatching. 3. IgM concentration increased significantly to a level of 691 +/- 37 micrograms/ml (mean +/- SE, N = 59) during the period from 251 to 429 days after hatching. The highest level of serum IgM was maintained from 429 days to 489 days after hatching. 4. The increase of IgM concentration was not accompanied by parallel increases in total serum protein concentration.     

Biosynthesis of immunoglobulin A (IgA). Secretion and addition of carbohydrate to monomer and polymer forms of a mouse myeloma protein

Cell suspensions of mouse plasma-cell tumour MOPC 315 secreting predominantly IgA (immunoglobulin A) monomer and dimer were incubated with radioactive leucine, mannose, galactose and fucose for various periods of time. The amounts of secreted and intracellular immunoglobulins were measured by co-precipitation with specific antibody, and the molecular species present were assessed by electrophoresis in polyacrylamide gels. Analysis of the secreted myeloma protein demonstrated that monomer and dimer IgA molecules are identical with respect to carbohydrate composition and rate of secretion. Within the cell, the myeloma protein is almost entirely accounted for by monomer units which either leave the cell as such or are polymerized with the addition of J chain close to the time of secretion. The results support the concept of a stepwise addition of carbohydrate residues to IgA immunoglobulin during the process of secretion. Similar patterns of carbohydrate assembly were found for the monomer or dimer molecules. Mannose residues are added at an early stage, whereas fucose is added close to the time of secretion. Galactose is also added early, but some may also be incorporated at a later stage. Control of IgA polymerization is considered unlikely to reflect regulation at the level of carbohydrate addition, and it is suggested that the critical controlling factor is the J chain.     

Biosynthesis of J-Chain in Mouse IgA and IgM

RECENT evidence^1,2 suggests that the polymeric immunoglobulins, IgM, serum IgA and secretory IgA, but not IgG or monomeric serum IgA contain a third polypeptide chain, the J-chain, of molecular weight about 23,000¹ or 26,000². Each polymeric molecule irrespective of the number of monomeric (H₂L₂) subunits, is thought to contain only one molecule of J-chain. It is distinguishable from light chain by its amino-acid composition, tryptic peptide map and fast electrophoretic mobility on alkaline-urea-polyacrylamide gels^1,2, but J-chains isolated from IgM and secretory IgA are indistinguishable by these criteria².     

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.     

Primary structure of a human IgA1 immunoglobulin. V. Amino acid sequence of a human IgA lambda light chain (Bur).

The sequence of the lambda light chain of the Bur IgA1 molecule has been determined. It comprises 214 amino acid residues with a blocked NH2 terminus and lacks carbohydrate. The V-region sequence is of the VlambdaII subgroup and contains the coupled interchanges Arg-7 and Cys-87. The Lv3 region is comparatively short and hydrophobic in nature and lends support for the designation of this area as a hypervariable deletion region. The C-region exhibits the Mcg+ Kren+ Oz- isotypes. These appear coupled with substitution at position 100 (in the V-region). The pattern of nonrandom association of V- and C-regions and H and L chains is discussed in terms of the generation of antibody diversity. With the companion papers in this series, the complete primary structure of a human IgA1 molecule is established.     

The incorporation of J chain into reassembled human immunoglobulin M

Human IgM (immunoglobulin M) was reduced with 24mm-mercaptoethylamine. This atreatment resulted in complete dissociation to IgMs subunits and free J chain. Intr-subunit interchain disulphide bonds remained intact. The mixture then was encouraged to reoxidize. The schlieren pattern of the reoxidized mixture showed the presence of a considerable quantity of IgM in addition to residual IgMs. The isolated reassembled IgM did not dissociate in 5m-guanidinium hydrochloride. It apparently contained the same amount of covalently attached J chain as did native IgM. The J chain was a part of the high-molecular-weight Fc fragment obtained from the reassembled IgM.     

Investigations of the structural function of the J chain in human immunoglobulin M

Human IgM molecules were treated with Na(2)SO(3) or mercaptoethylamine in concentrations ranging from 2 to 14mm or 2 to 22mm respectively. The dissociation of IgM to IgM(s) varied from 0% to 100%. At the intermediate concentrations of either reagent the amount of freed J chains was less than expected. In an attempt to find an explanation for this, IgM was partially dissociated to IgM(s) with mercaptoethylamine. The IgM(s) isolated by gel filtration was divided according to the ascending and descending portions of the elution curve. These portions were treated with 24mm-mercaptoethylamine and analysed for the presence of J chains. Only the ascending portion contained free J chains. Thus, after mild reduction where not all the IgM molecules are dissociated to IgM(s), some J chains remain covalently attached to some IgM(s) molecules although most of the J chains are freed. It was concluded that the J chain could serve as a ;hitch' for IgM(s) molecules forming intact IgM.     

Characterization of human class-switched polymeric (immunoglobulin M [IgM] and IgA) anti-human immunodeficiency virus type 1 antibodies 2F5 and 2G12

We have previously generated human monoclonal anti-human immunodeficiency virus type 1 (anti-HIV-1) antibodies 2F5IgG and 2G12IgG with an exceptional cross-clade neutralizing potential. 2F5IgG and 2G12IgG passively administrated to macaques were able to confer complete protection from both intravenous and mucosal challenge with pathogenic HIV-simian immunodeficiency virus chimeric strains and have shown beneficial effects in a phase-1 clinical trial. We now class-switched 2F5 and 2G12 to the immunoglobulin M (IgM) or IgA isotype, to enforce features like avidity, complement activation, or the potential to neutralize mucosal transmission. For this purpose we expressed functional polymeric 2F5 and 2G12 antibodies in CHO cells and evaluated their anti-HIV-1 activity in vitro. The class switch had a strong impact on the protective potential of 2F5 and 2G12. 2G12IgM inhibited HIV-1 infection of peripheral blood mononuclear cell cultures up to 28-fold-more efficiently than the corresponding IgG and neutralized all of the primary isolates tested. The 2F5 and 2G12 antibodies of all isotypes were able to interact with active human serum to inhibit viral infection. Furthermore, we demonstrated that polymeric 2F5 and 2G12 antibodies but not the corresponding IgGs could interfere with HIV-1 entry across a mucosal epithelial layer in vitro. Although polymeric 2F5 antibodies had only limited potential in the standard neutralization assay, the results from the mucosal assay suggest that 2F5 and 2G12 antibodies may have a high potential to prevent natural HIV-1 transmission in vivo.     

Polymeric immunoglobulin M is secreted by transfectants of non-lymphoid cells in the absence of immunoglobulin J chain.

Plasmids were constructed in which expression of genes encoding the heavy and light chains of a hapten-specific IgM antibody is under control of a heat shock promoter. Glioma, phaeochromocytoma and other non-lymphoid cell lines transfected with the plasmids were able to process and secrete immunoglobulin following heat induction. The glioma transfectants were studied in detail and were shown to secrete polymeric IgM in a yield similar to that obtained with a plasmacytoma. However, the glioma IgM was not associated with J chain and was largely composed of pentamers and hexamers. Thus, neither J chain nor other lymphoid-specific proteins are required for assembly and secretion of polymeric IgM although the absence of J chain may encourage hexamer formation.     

Non-specific adsorption of fish immunoglobulin M (IgM) to blocking reagents on ELISA plate wells

Enzyme-linked immunosorbent assay (ELISA) is a popular technique for quantifiable detection of specific antibodies in warm-blooded animals, but it has not been accepted for detection of fish antibodies because of its low reproducibility, which is due in part to high background optical density (OD) measurements. In the present study, we report that the high background of a fish antibody-detection ELISA resulted from non-specific adsorption of fish immunoglobulin M (IgM) to blocking reagents on the ELISA plate wells. Four fish sera (from rainbow trout Oncorhynchus mykiss, masu salmon O. masou, Japanese flounder Paralichthys olivaceus and koi Cyprinus carpio) were poured into ELISA plate wells pre-blocked with several blocking reagents (skim milk, soybean milk, bovine serum albumin, fetal bovine serum, gelatin and Roche BlockingReagent) and then washed out in order to measure the remaining fish IgM on the ELISA plate wells. Significant amounts of fish IgMs (OD absorbance at 492 nm: 0.3 to 1.1) remained on the ELISA plate wells with no antigenic protein except blocking reagents. The amount of remaining fish IgMs on the ELISA plate wells decreased significantly following treatment of fish sera with skim milk. However, the specific immuno-reactivity of fish IgM was not reduced by such treatment. Thus, we conclude that treatment of fish sera with skim milk is useful in reducing the high background OD often observed in fish IgM detection ELISA.