Structure and function of gamma M macroglobulins

Predictors of symptomatic hyperviscosity in Waldenström macroglobulinemia

Symptomatic hyperviscosity is a well-established phenomenon in Waldenström macroglobulinemia (WM). Monoclonal IgM can variably impact intrinsic serum viscosity, leading to widely disparate symptomatic thresholds for development of hyperviscosity-related symptoms. Data regarding the predictors of symptomatic hyperviscosity and outcomes related to this complication remain scarce and a recent study proposed that IgM >6000 mg/dL be considered a new criterion for initiating therapy in otherwise asymptomatic (smoldering) WM to pre-empt hyperviscosity-related injury. Herein, we attempt to identify predictors of the development of symptomatic hyperviscosity and its impact in patients with WM. Of 997 WM patients evaluated from January, 1996 through June, 2017, symptomatic hyperviscosity was observed in 130 (13%) patients. Overall survival (OS) of these 130 patients was similar to that of patients without symptomatic hyperviscosity (median: 11.5 vs 11.6 years; P = 0.63). On multivariate-analysis, only viscosity >1.8 cp (risk ratio: 4.0, P = 0.02) assessed at the time of WM diagnosis was an independent predictor for the development of subsequent symptomatic hyperviscosity. Among patients with smoldering WM and IgM >6000 mg/dL at diagnosis (n = 13) who were managed expectantly, the median time-to-initial therapy was 6.9 years and only 15% developed hyperviscosity-related symptoms subsequently. In summary, the occurrence of symptomatic hyperviscosity does not impact OS. Serum viscosity at diagnosis of WM, and not IgM concentration, represents the single most important independent predictor for development of subsequent hyperviscosity-related symptoms. Patients with smoldering WM and high serum IgM can be safely observed in the absence of any indications per the Consensus recommendations to initiate WM-directed therapy.

IgM and its receptors: structural and functional aspects

This review combines the data obtained before the beginning of the 1990s with results published during the last two decades. The predominant form of the IgM molecule is a closed ring composed of five 7S subunits and a J chain. The new model of spatial structure of the pentamer postulates nonplanar mushroom-shaped form of the molecule with the plane formed by a radially-directed Fab regions and central protruding portion consisting of Cµ4 domains. Up to the year 2000 the only known Fc-receptor for IgM was pIgR. Interaction of IgM with pIgR results in secretory IgM formation, whose functions are poorly studied. The receptor designated as Fcα/µR is able to bind IgM and IgA. It is expressed on lymphocytes, follicular dendritic cells, and macrophages. A receptor binding IgM only named FcµR has also been described. It is expressed on T- and B-lymphocytes. The discovery of new Fc-receptors for IgM requires revision of notions that interactions between humoral reactions involving IgM and the cells of the immune system are mediated exclusively by complement receptors. In the whole organism, apart from IgM induced by immunization, natural antibodies (NA) are present and comprise in adults a considerable part of the circulating IgM. NA are polyreactive, germ-line-encoded, and emerge during embryogenesis without apparent antigenic stimuli. They demonstrate a broad spectrum of antibacterial activity and serve as first line of defense against microbial and viral infections. NA may be regarded as a transitional molecular form from invariable receptors of innate immunity to highly diverse receptors of adaptive immunity. By means of interaction with autoantigens, NA participate in maintenance of immunological tolerance and in clearance of dying cells. At the same time, NA may act as a pathogenic factor in atherosclerotic lesion formation and in development of tissue damage due to ischemia/reperfusion.

Spinal cord dysmyelination caused by an antiproteolipid protein IgM antibody: implications for the mechanism of central nervous system myelin formation

Antiglycolipid IgM antibodies are known to induce formation of "wide spaced" or "expanded" myelin, a distinctive form of dysmyelination characterized by a repeat period approximately two or three times normal, which is seen also in diseases, including multiple sclerosis. To determine whether an antibody directed against a myelin protein would cause equivalent pathology, we implanted O10 hybridoma cells into the spinal cord of adult or juvenile rats. O10 produces an IgM directed against PLP, the major protein of CNS myelin. Subsequent examination of the cords showed focal demyelination and remyelination. In addition, however, some juvenile cords, but none of the adult cords, displayed wide-spaced myelin with lamellae separated by an extracellular material comprising elements consistent with IgM molecules in appearance. Wide spacing tended to involve the outer layers of the sheath and in some cases alternated with normally spaced lamellae. A feature not seen previously consists of multiple expanded myelin lamellae in one sector of a sheath continuous with normally spaced lamellae in another, resulting in variation in sheath thickness around the axonal circumference. This uneven distribution of wide-spaced lamellae is most simply explained based on incorporation of IgM molecules into immature sheaths during myelin formation and implies a model of CNS myelinogenesis more complex than simple spiraling. The periaxonal space never displays widening of this kind, but the interface with adjacent myelin sheaths or oligodendrocytes may. Thus, wide spacing appears to require that IgM molecules bridge between two PLP-containing membranes and does not reflect the mere presence of immunoglobulin within the extracellular space.

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.

Unusual IgM fibrillar deposits in glomerulonephritis: ultrastructural and diffraction studies in a case report

Morphological examination of 2 renal biopsy specimens obtained from a 69-year-old woman with a nephrotic syndrome, high blood pressure, and a reduced glomerular filtration rate revealed, in ultrastructural study, a type of a glomerulonephritis with fibrillar deposits in a subendothelial position which were unusual in their immunoglobulin components (mainly IgM). The fibrillar components were of irregular size, 13 to 18 nm in diameter and presented a very particular "barbed wire" morphological aspect, not hitherto described. Diffraction studies and image analysis, revealed spiraled fibrils with regular alternating elements that we suggest may correspond to IgM molecules. The clinical (isolated renal symptoms) and laboratory (traces of 3 monoclonal components in the serum and 2 normal bone marrow biopsy specimens) data provided no evidence of hematopoietic malignancy, viral hepatitis or cryoglobulinemia.

Segmental flexibility and avidity of IgM in the interaction of polyvalent antigens

We prepared IgG and IgM with identical combining sites to a hapten, (4-hydroxy-3-nitrophenyl)acetic acid (NP), and used surface plasmon resonance to evaluate the association constants (Ka) in interactions of these antibodies (Abs) with antigens (Ags) which differed in the size of carriers and NP valence as well as in the stoichiometry of Ag to Ab in the immune complexes. It was found that IgM was unable to form an Ag1Ab1 complex with the highly haptenated Ag, NP(18.6)-bovine serum albumin (BSA), such that one NP(18.6)-BSA molecule was held by multiple contacts with Fab arms from five subunits, although IgM was capable of forming an Ag4Ab1 complex in which each subunit was bound to one NP(18.6)-BSA molecule. IgM was superior to IgG in interactions with large Ags of low hapten density. The Ka values of IgM to these Ags were estimated to be approximately 1x10(9) M(-1), about 20-fold higher than those of IgG. Reduction of inter-subunit and inter-chain disulfide bonds resulted in a decrease in Ka values to large Ags but no change in those to small Ags.

Dysmyelination induced in vitro by IgM antisulfatide and antigalactocerebroside monoclonal antibodies

Antiglycolipid antibodies cause a distinctive form of dysmyelination in vivo characterized by marked widening of the myelin period. Such "expanded" or "wide-spaced" myelin occurs in peripheral nerves in certain paraproteinemias and in the CNS in multiple sclerosis. We have used an in vitro system to reproduce this pathology under controlled conditions to assess the role of antibody specificity and class and the need for cofactors in generating this kind of lesion in peripheral myelin. Schwann cell myelin formed in vitro around dorsal root ganglion cell axons was exposed for 3-14 days to hybridoma cells that produce specific monoclonal antibodies. Typical wide-spaced myelin developed after exposure to either O4, which produces an IgM antisulfatide antibody, or O1, which produces an IgM antigalactocerebroside antibody. In both cases, the effect was apparent by three days in paranodal as well as internodal myelin, especially in the outer lamellae. This change did not depend on the presence of complement or macrophages in the cultures. Exposure to anti-GalC hybridoma cells, which produce an IgG3 antiglycolipid antibody, did not produce wide-spaced myelin, nor did exposure to hybridoma cells that secrete IgM antibodies directed against a non-myelin antigen. The location and rapidity of the pathologic changes seen after O4 or O1 are consistent with penetration of the antibodies through the external mesaxon of already formed myelin and then between compact lamellae, progressively spreading them apart in the centripetal direction. This in vitro model shows that either of two specific monoclonal IgM antiglycolipid antibodies can alone reproduce a well known form of myelin pathology under defined conditions.

The contribution of ER quality control to the biologic functions of secretory IgM

Secretory IgM provides a first line of defense against pathogens and is uniquely capable of enhancing the primary humoral immune response. Complement activation is especially important for these activities. Here, Padmalatha Reddy and Ronald Corley discuss how the 'quality control' mechanisms that regulate IgM assembly and secretion play important roles in the developmental progression of B cells and in B-cell function.

Production of IgM Hexamers by Normal and Autoimmune B Cells: Implications for the Physiologic Role of Hexameric IgM

Secreted IgM is predominantly found as pentameric molecules, but IgM can also be secreted as hexamers by B cell lines. Murine hexamers activate the complement cascade more efficiently than pentamers, but the physiologic significance of hexameric IgM remains unknown. Here, we report that IgM hexamers and pentamers are cleared from the circulation with similar kinetics, suggesting that the predominance of pentameric IgM in vivo reflects the regulation of polymer assembly and secretion in responding B cells. Normal IgM-secreting B cells, particularly those from the peritoneal cavity, are capable of secreting abundant hexameric IgM in vitro. The disparity between the ability of B cells to secrete IgM hexamers in vitro and the paucity of this polymer in vivo suggest that IgM hexamers might be deleterious. In support of this, we demonstrate that the autoantibodies from a number of patients with cold agglutinin (CA) disease include both IgM hexamers and pentamers. The CA IgM hexamers lyse human erythrocytes in the presence of human complement more efficiently than CA IgM pentamers, suggesting a potential role for hexameric IgM in the pathogenesis of this autoimmune syndrome.

Late events in assembly determine the polymeric structure and biological activity of secretory IgM

IgM antibodies can be secreted in at least two functional polymeric forms that can be distinguished according to subunit composition. While IgM hexamers comprise six H2L2 monomeric subunits, pentamers contain an additional polypeptide, the J chain. In the presence of high abundance J chain protein, IgM pentamers are preferentially assembled at the expense of hexamers. To determine the mechanism by which J chain regulates the assembly process, we defined the point at which J chain is added to assembling polymers. We found no evidence for the presence of J chain in small IgM assembly intermediates of IgM, suggesting that it was not stably associated with these complexes. However, J chain was found associated with large polymeric IgM complexes exhibiting sedimentation properties of intracellular pentameric structures. These complexes were frequently not completely covalently assembled; however, complete covalent assembly of J chain-containing pentameric complexes did occur prior to their maturation in the Golgi. These data argue that pentameric structures are the substrate for J chain incorporation into assembling IgM and suggest that the incorporation of J chain is thermodynamically favored over the addition of a sixth monomeric subunit into an assembling polymer. We conclude that late events in IgM polymer assembly, specifically the insertion of J chain, the exclusion of an additional monomeric subunit, and the covalent closure of the pentameric IgM molecule, determine the polymeric structure and, consequently, the biological activity of secreted IgM.

Spinal cord dysmyelination induced in vivo by IgM antibodies to three different myelin glycolipids

It was shown previously (Rosenbluth et al.: J. Neurosci. 16:2635-2641, 1996) that implantation of hybridoma cells that produce an IgM antigalactocerebroside into the spinal cord of young rats results in the development of myelin sheaths with a repeat period approximately 2-3x normal, similar to the abnormal peripheral myelin sheaths seen in human IgM gammopathies. We now present evidence that this effect can be reproduced in the spinal cord by implanting either of two other hybridomas, O4 and A2B5, that secrete, respectively, antisulfatide and antiganglioside IgM antibodies. The formation of expanded CNS myelin thus does not depend on antibodies to galactocerebroside specifically but can be mediated by IgM antibodies that react with other myelin glycolipids as well.

Paranodal structural abnormalities in rat CNS myelin developing in vivo in the presence of implanted O1 hybridoma cells

O1 hybridoma cells, which produce a monoclonal IgM antigalactocerebroside, were implanted into the spinal cords of immature and mature rats and the cords examined 5-24 days later. Study of the younger group, in which myelin was developing at the time of implantation, revealed examples of abnormal myelin sheaths in which the repeat period was markedly increased. The paranodal regions of these abnormal sheaths were superficially normal in configuration; i.e. myelin lamellae terminated one by one as 'terminal loops' that indented the axolemma and formed normal axoglial junctions displaying periodic 'transverse bands'. Neighbouring terminal loops are normally joined by tight junctions that block passage of tracers from the paranodal periaxonal space into the compact myelin, as seen after implantation of a control hybridoma. In the abnormal sheaths that developed after O1 implantation, in contrast, terminal loops were usually widely separated from each other. As a result, multiple pathways from the paranodal periaxonal space into the myelin sheath remained patent, forming potential routes for shunting nodal action currents. This subtle abnormality could thus compromise conduction, even though the sheaths might appear to be normally myelinated at the histological level. Equivalent abnormalities in human neurological diseases, including multiple sclerosis and paraproteinemic neuropathies, could underlie functional loss in the absence of frank demyelination.

IgM hexamers?

There has been a universal tendency to regard IgM antibodies as pentameric molecules comprising five immunoglobulin monomeric subunits joined by a single J-chain protein. Is this the only form of secreted IgM, or are the possibilities more complex? In this article, Joseph Brewer and colleagues propose that the IgM polymers secreted in primary immune responses may be more heterogeneous than previously believed and that, as a consequence, the biological activity of IgM may have considerable flexibility.

Selective removal of anti-acetylcholine receptor antibody in the low temperature operation of membrane plasma fractionation

The removal of anti-acetylcholine receptor antibody (anti-AChR Ab) by the plasma fractionator, Kuraray EVAL 2A, was measured as a function of the filtration temperature (4 and 37 degrees C). Plasmas procured from eight myasthenia gravis (MG) patients undergoing routine plasma exchanges (PE: membrane plasma filtration: n = 4, and centrifugation: n = 4) were used in the studies. Plasma flow rate was 20 ml/min, and plasma perfusion in single pass was terminated when the transmembrane pressure of the filter reached 300 mm Hg. Solute concentration data from before and after perfusion were used to assess the sieving coefficient (SC) of each solute. Results show that the SCs for anti-AChR Ab and albumin were significantly (P < .0002) lower at 4 degrees C (0.15 and 0.52, respectively) as compared to 37 degrees C (0.44 and 0.72, respectively). The SC of anti-AChR Ab (0.15) was significantly lower than the SC of IgG (0.42; P < .006) at 4 degrees C, even though it also belongs to the IgG class. The ratio of the SC of anti-AChR Ab to albumin at 4 degrees C (0.29) was significantly lower than that at 37 degrees C (0.57; P < .003). These data indicate that the selectivity of removal of anti-AChR Ab from albumin is higher at 4 degrees C than that at 37 degrees C. The volume treated at 4 degrees C was significantly lower than that treated at 37 degrees C, and it was less than that required for a clinical treatment; however, it was shown that filter backwashing is possible without loss of solute removal selectivity.(ABSTRACT TRUNCATED AT 250 WORDS)

19S and 7-8S forms of IgM antibody to hepatitis B core antigen in acute icteric hepatitis superimposed on hepatitis B surface antigen carriage

The 19S and 7-8S forms of IgM antibody to hepatitis B core antigen (IgM anti-HBc) were separated by rate-zonal centrifugation from the serum of 20 Greek hepatitis B surface antigen (HBsAg) carriers with a superimposed acute icteric hepatitis positive for IgM anti-HBc by a radioimmunoassay. Serological markers of hepatitis A virus (HAV), hepatitis B virus (HBV) and hepatitis D virus (HDV) infections were detected with radioimmunoassays and serum HBV DNA was detected with molecular hybridization techniques. Eighteen of the 20 carriers showed a predominance of one or the other form of IgM anti-HBc. Low molecular weight (7-8S) IgM anti-HBc was observed more frequently in HDV superinfection (5/9) and was related to a low mortality (1/9). In contrast, 19S IgM anti-HBc was observed more frequently in reactivation of chronic hepatitis B (6/9) and was related to a high mortality (5/9). These preliminary data show that in HBsAg carriers with a superimposed acute icteric hepatitis, predominance of 19S IgM anti-HBc is frequently associated with a severe clinical course; the opposite is true for predominance of 7-8S IgM anti-HBc.

Seronegative rheumatoid arthritis, rheumatoid factor cross reactive idiotype expression, and hidden rheumatoid factors

The major rheumatoid factor cross reactive idiotype (RCRI), defined by prototypic monoclonal rheumatoid factors (RFs), is expressed as a dominant idiotype by pokeweed mitogen induced plasma cells obtained from seropositive (RF+) patients with rheumatoid arthritis (RA). Some patients who meet clinical diagnostic criteria for RA set by the American Rheumatism Association fail to express RFs at any time during their clinical course. To determine if seronegative (RF-) patients with RA, so designated by the latex fixation, Rose-Waaler classic binding assays, or a RF enzyme linked immunosorbent assay (ELISA), express the RCRI in the absence of detectable RFs we examined pokeweed mitogen plasma cells from these patients by indirect immunofluorescence. In addition, we used an inhibition ELISA to detect RCRI bearing molecules in the sera of RF- patients with RA. Five of 10 RF- patients with RA had a high prevalence of RCRI+ plasma cells (16-49% of total pokeweed mitogen plasma cells in culture). Six of 20 RF- patients with RA had high serum concentrations of molecules marked by the RCRI, equivalent to 21-110 micrograms/ml of RCRI+ reference monoclonal IgM RF. Four of five patients who expressed the RCRI in high prevalence in pokeweed mitogen plasma cells, also demonstrated high concentrations of RCRI in their sera detected by inhibition ELISA. There was significant concordance of RCRI expression determined by the two different assays. Four RF- patients with RA who expressed RCRI in their whole sera had hidden RFs detected in their 19S and, in one case, 7S serum fraction. Detection of RF related molecules in whole sera by the expression of RCRI in RF- patients with RA identifies a subgroup of RF- patients with RA who possess hidden RFs. Some RF- patients with RA can express the major RCRI in pokeweed mitogen plasma cells and in their sera and therefore are related to patients with prototypic Waldenstrom's macroglobulinaemia, who produce RCRI+ 19S IgM monoclonal RFs.

Expression of the major rheumatoid factor cross-reactive idiotype in juvenile rheumatoid arthritis

The major rheumatoid factor cross-reactive idiotype (RF-CRI), defined by prototypic monoclonal IgM rheumatoid factors, is expressed in high frequency by pokeweed mitogen-derived plasma cells from patients with rheumatoid arthritis who express RF in their sera. Unlike adults with rheumatoid arthritis, most patients with juvenile rheumatoid arthritis are seronegative for RF, as detected by classic IgG binding assays. We report that approximately 50% of seronegative patients with juvenile rheumatoid arthritis express the RF-CRI in high frequency among their pokeweed mitogen-derived plasma cells, and that approximately 33% of patients express the RF-CRI in high titer in their sera. The possible mechanisms for expression of an idiotypic marker of RF without expression of IgG binding activity by classic assays are discussed.

The major rheumatoid factor cross-reactive idiotype in rheumatic disease

The major rheumatoid factor cross-reactive idiotype (RCRI), a tertiary structure formed by both light and heavy chains, is found on 60% of all monoclonal IgM kappa RFs. To determine if the RCRI is expressed in patients with rheumatic disease, we used polyclonal rabbit anti-idiotypic antibodies to detect RCRI in sera and in pokeweed mitogen cultures of blood mononuclear cells (PBM) from patients with rheumatoid arthritis (RA) and juvenile rheumatoid arthritis (JRA). We detected increased expression of RCRI+, plasma cells in PWM cultures, and in sera from these patients. We have determined that some 7S IgM molecules from RF+RA patients are RCRI+, and can bind IgG in a sensitive RF ELISA. We have also observed that the CD5+ B cell subset, which is responsible for autoantibody production, generates RCRI+ antibodies. We review these data and discuss the relationship of the idiotypic network of interacting antibodies with rheumatic disease.

Low and high molecular weight cytomegalovirus-specific immunoglobulin M antibody in renal transplant patients with cytomegalovirus infections

Sera from 27 renal transplant patients with primary and recurrent CMV infections and which were known to contain CMV-specific IgM antibodies were investigated by indirect immunofluorescence for the presence of virus-specific high molecular weight IgM (19S IgM) and low molecular weight IgM (7S IgM). After sucrose gradient fractionation of the sera, 19S IgM was found in all 27 patients, whereas 7S IgM was present in 11 out of 19 (56%) patients with primary CMV infection and in 1 out of 8 (12%) patients with recurrent CMV infection. The presence of 7S IgM was unrelated to the titre of the virus-specific IgM in whole serum. The presence of IgM rheumatoid factor was monitored by a sensitive fluorescence assay using measles virus antigen/antibody complexes. The absorption of the serum fractions with heat-aggregated gamma globulin failed to remove the specific IgM staining indicating that it was not due to IgM rheumatoid factor. On the other hand adsorption with protein A/sepharose removed the specific IgM staining from the 7S IgM fractions but not from the 19S IgM fractions. This suggests that specific 19S and 7S IgM antibodies may belong to different subclasses of IgM.

Ligand binding by murine IgM antibodies: intramolecular heterogeneity exists in certain, but not all, cases

The ligand binding properties of eight hybridoma-derived murine anti-DNP IgM(kappa) antibodies were analysed by equilibrium dialysis. Four of these proteins exhibited the expected valances of approximately 10 and relatively low affinities (less than or equal to 2.2 x 10(5) M-1). The remaining four proteins exhibited valences of considerably less than 10 (less than or equal to 8) and relatively high affinities (greater than 10(6) M-1). When these proteins were subjected to two cycles of lyophilization, those of the former group were observed to still exhibit approximately 10 sites per molecule with homogeneous affinities similar to those of the respective untreated molecules. However, molecules in the latter group (valences of less than or equal to 8) were observed to exhibit only five to six binding sites subsequent to lyophilization with no changes in affinities. When the reductive subunits from each of the IgM(kappa) proteins were subjected to trypsinization, two different patterns were observed in terms of the yields of Fab mu fragments. Each of the proteins originally exhibiting approximately 10 binding sites yielded greater than 90% of the expected Fab mu fragments. In contrast each of the proteins exhibiting less than or equal to 8 binding sites yielded only approximately 50% of the expected Fab mu fragments. Collectively these results indicate the existence of at least two different forms of murine IgM molecules, those with approximately 10 homogeneous, relatively stable sites and those with only approx. five stable sites. It is suggested that these intramolecular functional differences may be attributable to intramolecular conformational differences.

Thermodynamics of monoclonal and mixed cryoimmunoglobin solubilization

The direct calorimetric determination of heats of solution for four monoclonal and three mixed (IgM/IgG) cryoglobulins is described. Values obtained by differential scanning calorimetry (DSC) are compared to values of the apparent delta Hsol obtained by a polyethylene glycol (PEG) precipitation method. The four monoclonal cryoglobulins manifest heats of solution determined by DSC to be of the same order of magnitude as heats obtained by PEG precipitation, although DSC values were 25 to 125% lower than the corresponding van't Hoff enthalpies. Values of delta Hsol for mixed cryoglobulins were significantly greater than monoclonal cryoglobulins on a molar basis. These higher values are primarily attributed to the greater surface area of these complexes which results in more extensive contact between molecules in the solid phase. No evidence was found that conformational changes contributed to the calorimetric delta Hsol values employing a variety of spectroscopic methods.

Studies on the detrimental effects of bivalent binding in a microtitration plate ELISA and possible remedies

During the development of a microtitration plate enzyme-linked immunosorbent assay (ELISA) for 3-acetyldeoxynivalenol, using an immobilised hapten:protein conjugate, problems were encountered in obtaining an inhibition curve with hapten. Investigations into this phenomenon were conducted. Univalent Fab fragments were prepared and their binding compared with untreated antiserum on plates coated with different immobilised coating concentrations and with conjugate of a lower hapten:protein ratio. The studies showed that standard curves obtained with Fab fragments using the coating conjugate of high hapten ratio were more sensitive and showed a greater degree of inhibition. However with the conjugate of lower hapten ratio the untreated antiserum gave the more sensitive curve although inhibition was incomplete. From these results it is concluded that the antiserum probably contains two populations of antibodies, one of which binds bivalently to the plates at all concentrations of immobilised hapten.

The effect of interchain disulfide bond cleavage on the cold induced precipitation of cryoimmunoglobulins

Selective cleavage of the interchain disulfide bonds present in the two IgG1-kappa monoclonal cryoglobulins Ger and Muk results in a partial loss of cryoprecipitability of the parent proteins at 0 degree C. The progressive loss of cryoprecipitability which occurs as a function of increasing reductant concentration parallels the successive cleavage of interheavy-light and interheavy-heavy chain disulfides. Circular dichroism shows that reduction and alkylation of hinge region disulfides induces small conformational changes in the IgG molecules that could alter cryoprecipitability. The N-terminal amino acid sequence of the Fc component derived by restricted proteolysis with trypsin of protein Muk was found to be completely homologous with N-terminal Fc sequences of noncryoglobulin IgG reference proteins, indicating identical hinge regions. Reduction and alkylation of two monoclonal IgM cryoglobulins also reduces cryoprecipitability. After reduction and alkylation of either the monoclonal IgM rheumatoid factor or the polyclonal IgG component of two mixed cryoglobulins recombination results in decreased cryoprecipitation of the intact cryoglobulin complex. In all cases inhibition of cryoprecipitation is greater when iodoacetic acid rather than iodoacetamide is employed as the S-alkylating group. These results do not support a direct role for the hinge region in the precipitation of cryoimmunoglobulins.

Clinical significance of low molecular weight (7-8S) immunoglobulin M antibody to hepatitis B core antigen in chronic hepatitis B virus infections

Separation of 7-8S and 19S forms of serum immunoglobulin M (IgM) hepatitis B core antibody (anti-HBc) by rate-zonal centrifugation was carried out on serum from 80 American chronic carriers of hepatitis B surface antigen (HBsAg), all of whom were positive for IgM anti-HBc and had elevated levels of serum alanine aminotransferase (mean 164 IU/L). Seventy-three of the 80 sera showed a predominance of one or the other form of IgM anti-HBc. Fifty-four (68%) had predominantly 7-8S IgM anti-HBc, and 19 (24%) had predominantly 19S IgM anti-HBc. Sex, age, length of HBsAg-carrier state, mean alanine aminotransferase, mean total IgM anti-HBc level, presence of hepatitis B e antigen, and liver histology were similar in both groups. 19S IgM anti-HBc was detected in 11 (41%) of 27 male homosexuals compared with only 8 (17%) of 46 heterosexual patients (p = 0.03). Despite this apparent association, an explanation for the variable presence of 19S and 7-8S IgM anti-HBc predominance in chronic hepatitis B remains lacking.

A pre-translational defect in a case of human mu heavy chain disease

A patient (BW) was studied with Mu heavy chain disease (mu HCD) in whom a leukemic B-cell clone secreted a shortened monoclonal mu chain without associated light chain. The cells did, however, produce a normal-sized kappa light chain that was detected as urinary Bence-Jones protein. The cytoplasmic and secreted monomeric mu chain had an approximate mol. wt of 58,000. Radiochemical sequence analysis of the biosynthetically labelled mu chain revealed a protein that lacked the entire variable region. The sequence initiated at amino acid position 5 within the first constant region domain (CH1) of C mu. The primary in vitro translation product, the cytoplasmic and secreted proteins were all similarly truncated, thereby excluding extensive postsynthetic degradation. The mu RNA, that directed the synthesis of the truncated mu protein, was about 350 bp smaller than the normal mu RNA. Furthermore, by primer extension analysis it was possible to localize this deletion in the mu RNA to a region 5' of CH1. Thus, a defect at the level of Ig gene structure/assembly that deletes coding information or results in aberrant RNA processing must be responsible for the truncated mu HCD protein BW.

Analysis of the hepatobiliary transport of IgA with monoclonal anti-idiotype and anti-allotype antibodies

The processing and fate of mixed immune complexes is influenced by the antibody isotypes present. The hepatobiliary transport of mixed immune complexes containing the mouse IgA myeloma protein J558 and corresponding monoclonal IgG or IgM anti-J558 idiotype or monoclonal IgG anti-mouse IgA allotype antibodies has been studied. The anti-idiotype or anti-allotype antibodies were radiolabeled and injected into mice with or without mouse polymeric IgA (J558). IgG anti-idiotype antibodies to J558 IgA were selectively transported into bile by J558 IgA. This process occurred with a radiolabeled Fab preparation of the IgG anti-idiotype and was inhibitable with IgA of an irrelevant antigenic specificity. Thus, polymeric IgA influenced the fate of IgA-IgG idiotype-anti-idiotype serum immune complexes. A monoclonal anti-idiotype antibody of the IgM isotype (D8-3) was not selectively transported into bile by itself or as an IgA-IgM complex. A monoclonal IgG antibody (CB5-6) to a mouse allotype determinant in the Fc portion of IgA was not selectively transported into bile. This anti-allotype monoclonal antibody inhibited the hepatobiliary transport of 125I-polymeric J558 IgA and therefore appeared to directly or indirectly block the site in the Fc region of IgA recognized by the hepatic receptor.

Truncated mu (mu') chains in murine IgM. Evidence that mu' chains lack variable regions

Secreted IgM was shown to contain truncated mu (mu') chains with an apparent molecular mass of approximately 55 kD. The estimated percentage of IgM heavy (H) chains in the mu' form ranged from less than or equal to 1% in the case of one tumor IgM protein (104E) to greater than or equal to 30% in normal serum IgM. Serum mu' chains lacked antigenic determinants characteristic of immunoglobulin variable regions and showed a restricted isoelectric focusing pattern compared with that of conventional mu chains. Intracellular mu' chains were readily detected in bone marrow cells but not in spleen or lymph node cells; mu' chains were also detected in IgM-producing tumor cells and in a hybridoma cell line that deleted its productive mu allele. These results predict irregularities in IgM structure and recall an old controversy concerning the valence of IgM molecules.

Fragmentation and reduction of bovine secretory component. Preparation of a biologically active fragment and some evidence for a multiple-domain structure

A tryptic fragment (A) of Mr 25000 was prepared from bovine secretory component. The fragment binds polymeric immunoglobulin, although 9 times less effectively than secretory component on a molar basis. The fragment has four buried half-cystine residues and two exposed half-cystine residues. It gives rise to two fragments of Mr 11000-13000 on prolonged digestion with trypsin, and these do not bind polymeric immunoglobulin. It is proposed that fragment A consists of two immunoglobulin-like domains. Bovine secretory component was found to have 9-11 buried half-cystine residues and four exposed half-cystine residues. Reduction and alkylation of the exposed residues decreases the binding of polymeric immunoglobulin by 3-fold. Initial tryptic cleavage of bovine secretory component gives a fragment (Q) disulphide-bridged to a further fragment (T). Fragment Q is similar in size to a three-domain immunoglobulin fragment, and fragment T is similar in size to a two-domain immunoglobulin fragment. The two-domain fragment A is derived from fragment Q by further tryptic cleavage. The results are compatible with the proposal by Mostov, Friedlander & Blobel [(1984) Nature (London) 308, 37-43] that secretory component consists of multiple immunoglobulin-like domains. The results also indicate that optimal binding of polymeric immunoglobulin involves several domains stabilized by an exposed disulphide bridge.

Increase in the complement-fixing ability of murine IgG anti-lymphocyte antibodies by addition of monoclonal IgM rheumatoid factors

Mouse monoclonal IgM anti-IgG autoantibodies (rheumatoid factors) markedly enhanced the complement fixing ability of 2 murine IgG1 monoclonal antibodies against human lymphocytes (anti-Leu 4 and T305). The IgM rheumatoid factors were effective when added simultaneously with the monoclonal anti-lymphocyte antibodies, and were not themselves toxic towards human lymphocytes. Monoclonal IgM rheumatoid factors are simple reproducible reagents for amplifying complement fixation by murine IgG hybridoma antibodies.

The use of Fab-Fc recombinant antibodies for studying the mechanism of triggering the effector activities of immunoglobulins

Immunoglobulin species, recombined from Fab and Fc fragments with anti-red cell specificity were used as cytophilic antibodies in studies of triggering the effector activity of immunoglobulins. Recombinant immunoglobulins containing a single Fab' appeared inactive in binding to cell receptors and triggering antibody dependent cellular cytotoxicity, while those with two Fab' exhibited an activity comparable to intact antibody IgG. The experiments indicate that Fab fragment may interact with Fc and inhibit its binding to cell receptors.

Monoclonal IgM rheumatoid factors derived from arthritic MRL/Mp-lpr/lpr mice

MRL/lpr/lpr (MRL/l) mice develop a lupus-like syndrome and a disease histologically and serologically similar to human rheumatoid arthritis. Their sera contain polyclonal IgM rheumatoid factors (RF) reactive with all murine IgG subclasses (frequently strongest with IgG2a) and several heterologous IgG. To examine the repertoire and epitopic specificities of these RF, we fused splenocytes from 3.5-mo-old seropositive MRL/l mice with appropriate myeloma partners and derived 1,723 hybridomas of which 23 secreted IgMRF. These monoclonal IgMRF bound to murine IgG only, not to other murine isotypes. Eight murine IgG subclass-specific clonotypes were identified. Most clones reacted with either multiple IgG subclasses or with IgG2a alone. A few clones reacted solely with IgG2b but none reacted exclusively with IgG1 or IgG3. Monoclonal IgMRF with exclusively anti-IgG2a activity exhibited allotypic specificity, reacting, with few exceptions, with a, c, and e, but not b, d, or j IgG2a allotypes. Four clonotypes could be distinguished by cross-reactivity with IgG from species other than mice. Monoclonals possessing activity against several murine subclasses cross-reacted extensively with heterologous IgG, including all human IgG subclasses without allotypic restrictions. Monoclonal IgMRF specific for murine IgG2a or 2b did not cross-react with heterologous IgG. Based on the absence of cross-reactions by IgG2a-specific monoclonal autoantibodies, certain peptides of the IgG CH2 and CH3 domains appear to generate the antigenic determinants of the anti-IgG2a RF in MRL/l mice. All of the monoclonal RF bound to Fc and, with one exception, not to Fab fragments of murine IgG. Binding of the monoclonal RF to substrate IgG was not inhibited by Clq, thus excluding the Clq-binding site at the CH2 domain as one of the responsible epitopes in the induction of MRL/l RF. mIgMRF could be categorized as strongly, weakly, or noninhibitable by protein A, which interacts with IgG molecules at or near the CH2-CH3 junction. Inhibition appears to be caused by conformational changes and/or steric shielding of certain IgG areas distant from this junction and not by identical binding sites between protein A and RF. Certain of the mIgMRF that were weakly or not at all inhibitable by protein A were found to cross-react equally well with human Fc (CH2-CH3 domains) and pFc' (CH3 domain) fragments, indicating that the binding site for these monoclonals is at the CH3 domain. Monoclonal RF were devoid of anti-double-strand DNA, anticollagen, or antipeptidoglycan pentapeptide cross-reactivity, but one of the monoclonals cross-reacted with histones, four with single-strand DNA, and one with both histones and single-strand DNA.

The transient appearance of IgM antibodies in the bile of rats injected with Salmonella enteritidis

Following the intravenous injection of 2 X 10(8) killed Salmonella enteritidis into rats, high levels of specific antibodies were detected in the serum and bile after 4 days. The level in serum persisted while that in bile had virtually disappeared by day 7. On the basis of gel filtration on Ultrogel AcA22 the activity in bile was found to be associated with the IgM fraction. The transient nature of the biliary response was further evidenced by the association of the agglutinating activity with a four-fold increase in the concentration of IgM and the presence of complement-fixing bactericidal antibody. Initial evidence suggests that the source of the biliary IgM is from cells within the liver rather than transport of IgM from blood to bile. It is proposed that immune stimulation by blood-borne microorganisms can lead to a pulse of IgM in the bile, providing rapid augmentation of other local immune processes in the gastrointestinal tract.

The limited fragmentation of porcine haemolytic immunoglobulin M. Evidence that haemolytic activity depends on the number of Fab arms that can attach to the erythrocyte surface

Methods previously developed in this laboratory for the proteolytic removal of limited numbers of Fab arms from the IgM molecule have been used to study haemolytic activity. The gradual removal of arms produced a rapid decrease in complement-mediated lysis of erythrocytes but had a much slower effect on their agglutination. Therefore a stage was reached at which molecules with four Fab arms gave good agglutination but did not cause cell lysis. The same result was obtained whether Fab arms were removed randomly by pepsin or in pairs by trypsin. Hence the non-lytic nature of these molecules was independent of the arrangement of their remaining Fab arms. A sharp decrease in haemolytic activity was also obtained under conditions of high antibody concentration when the IgM molecular could attach only a few of its Fab arms to the erythrocyte surface. Possible explanations for the dependence of haemolytic activity on the number of Fab arms discussed.

Circular dichroism of immune complexes with rheumatoid factor activity

The circular dichroism (CD) of IgG complexes with RF activity and that of monomeric IgG without this activity, and isolated from the same individual, were compared. The IgG complexes had a significantly deviant CD spectrum in the near u.v. region, both before and after dissociation, whereas the monomeric IgG had a normal spectrum. Immunoglobulins were isolated from the same serum with the use of specific antiserum against unique determinants in some IgG complexes with RF activity. Both before and after dissociation the CD spectrum in the far u.v. region of these immunoglobulins differed significantly from that of the above-mentioned preparations. The results confirmed that the structure of IgG in the RF-active complexes differed from that of normal IgG. The immunoglobulins with the unique determinants had, in turn, a structure that was not found in the pool of the RF-active IgG molecules or in normal IgG.

Quantitative precipitin analysis of free and hidden RF IgM

Quantitative precipitin analysis using rabbit IgG anti-human Fc mu was performed with 16 RF IgM, six macroglobulinaemic IgM and four normal IgM. Abnormal precipitin curves were obtained for all RF IgM, even when the latter were not readily demonstrated with conventional serological tests for RF, and for macroglobulinaemic IgM with sedimentation rates greater than 19S. These IgM formed significantly more precipitates with IgG anti-Fcmu in the antigen excess zone than did normal IgM, but the precipitin curves for the other zones were similar for all IgM. The underlying mechanisms of some of the reactions were studied and discussed. Because the divergence in the precipitin reaction for normal IgM and RF IgM was so pronounced, a model precipitin curve was constructed. This could be used to detect RF IgM, even when not readily demonstrable with conventional serological tests for RF, by direct analysis of serum. The results obtained for RF IgM suggested that the method might be applied to RF IgG and intermediate complexes comprised of IgG. The mechanisms demonstrated here might be used to develop immunological methods for routine use.