Antibodies elicited by influenza virus hemagglutinin fail to bind to synthetic peptides representing putative antigenic sites

Masking terminal neo-epitopes of linear peptides through glycosylation favours immune responses towards core epitopes producing parental protein bound antibodies

Glycosylation of hydrophobic peptides at one terminus effectively increases their water-solubility, and conjugation through the opposing end to a carrier protein, renders them more immunogenic. Moreover, the glycosylation minimizes antibody responses to potentially deleterious, non-productive terminal neo-epitope regions of the peptides, and consequently shifts peptide immunogenicity towards the core amino acid residues. As proof of concept, glycopeptide-protein conjugates related to influenza hemagglutinin (HA), neuraminidase (NA), and the dimerization loop region of human epidermal growth factor receptor 2 (Her2), demonstrated a favorable production of core peptide specific antibodies as determined by ELISA studies. Furthermore, glycosylated Her2 peptide conjugate antisera were also shown to recognize full length Her2 protein by ELISA and at the cell surface through flow cytometry analysis. In contrast, unmasked peptide conjugates generated significant antibody populations that were specific to the terminal neo-epitope of the peptide immunogen that are notably absent in parental proteins. Antibodies generated in this manner to peptides in the dimerization loop of Her2 are also functional as demonstrated by the growth inhibition of Her2 expressing SKBR3 carcinoma cells. This method provides a technique to tailor-make epitope-specific antibodies that may facilitate vaccine, therapeutic and diagnostic antibody development.

Globular Head-Displayed Conserved Influenza H1 Hemagglutinin Stalk Epitopes Confer Protection against Heterologous H1N1 Virus

Significant genetic variability in the head region of the influenza A hemagglutinin, the main target of current vaccines, makes it challenging to develop a long-lived seasonal influenza prophylaxis. Vaccines based on the conserved hemagglutinin stalk domain might provide broader cross-reactive immunity. However, this region of the hemagglutinin is immunosubdominant to the head region. Peptide-based vaccines have gained much interest as they allow the immune system to focus on relevant but less immunogenic epitopes. We developed a novel influenza A hemagglutinin-based display platform for H1 hemagglutinin stalk peptides that we identified in an epitope mapping assay using human immune sera and synthetic HA peptides. Flow cytometry and competition assays suggest that the identified stalk sequences do not recapitulate the epitopes of already described broadly neutralizing stalk antibodies. Vaccine constructs displaying 25-mer stalk sequences provided up to 75% protection from lethal heterologous virus challenge in BALB/c mice and induced antibody responses against the H1 hemagglutinin. The developed platform based on a vaccine antigen has the potential to be either used as stand-alone or as prime-vaccine in combination with conventional seasonal or pandemic vaccines for the amplification of stalk-based cross-reactive immunity in humans or as platform to evaluate the relevance of viral peptides/epitopes for protection against influenza virus infection.

Antigenic characterization of recombinant hemagglutinin proteins derived from different avian influenza virus subtypes

Since the advent of highly pathogenic variants of avian influenza virus (HPAIV), the main focus of avian influenza research has been the characterization and detection of HPAIV hemagglutinin (HA) from H5 and H7 subtypes. However, due to the high mutation and reassortation rate of influenza viruses, in theory any influenza strain may acquire increased pathogenicity irrespective of its subtype. A comprehensive antigenic characterization of influenza viruses encompassing all 16 HA and 9 neuraminidase subtypes will provide information useful for the design of differential diagnostic tools, and possibly, vaccines. We have expressed recombinant HA proteins from 3 different influenza virus HA subtypes in the baculovirus system. These proteins were used to generate polyclonal rabbit antisera, which were subsequently employed in epitope scanning analysis using peptide libraries spanning the entire HA. Here, we report the identification and characterization of linear, HA subtype-specific as well as inter subtype-conserved epitopes along the HA proteins. Selected subtype-specific epitopes were shown to be suitable for the differentiation of anti-HA antibodies in an ELISA.

Increased antibodies against unfolded viral antigens in the elderly after influenza vaccination

Objective  Our studies aimed to measure the quality of antibody response to influenza vaccines in the elderly. The frequency of significant rise in hemagglutination inhibition (HAI) titer in the elderly is low and although annual vaccination reduces morbidity and mortality, better correlates of vaccine efficacy in the elderly are needed.

Methods  We measured the amount and avidity of serum antibodies against native H3N2 influenza glycoproteins or denatured virus (unfoldons) in pre‐ and post‐vaccinated sera of 36 elderly subjects.

Results  Eighty percent of subjects had high pre‐immunization antibody levels and only 13% showed ≥2fold increase after vaccination, but 33% showed ≥2fold increase in avidity. With increasing dosage there was a significant increase in avidity against unfoldons with 50% of subjects showing ≥2fold increase at the highest dose. Elderly subjects given subunit vaccine showed higher reactivity with unfoldons (78% of native) than younger subjects studied earlier who were given inactivated whole virus vaccine (19% of native).

Conclusion  The clear inverse relationship between pre‐immunization antibody levels and antibody increase after vaccination implies that a major reason for the low frequency of antibody responses in elderly subjects is simply because they have high pre‐immunization antibody levels. Only low reactivity was observed with earlier viruses. The increased proportion and avidity of antibodies against unfoldons is of concern, as these are not protective, and vaccine developers need to be aware of the role of age or vaccine formulation in inducing anti‐unfoldon antibodies.

Immunogenically fit subunit vaccine components via epitope discovery from natural peptide libraries

Antigenic peptides that bind pathogen-specific Abs are a potential source of subunit vaccine components. To be effective the peptides must be immunogenically fit: when used as immunogens they must elicit Abs that cross-react with native intact pathogen. In this study, antigenic peptides obtained from phage display libraries through epitope discovery were systematically examined for immunogenic fitness. Peptides selected from random peptide libraries, in which the phage-displayed peptides are encoded by synthetic degenerate oligonucleotides, had marginal immunogenic fitness. In contrast, 50% of the peptides selected from a natural peptide library, in which phage display segments of actual pathogen polypeptides, proved very successful. Epitope discovery from natural peptide libraries is a promising route to subunit vaccines.

Conformation-dependent antibody response to Pseudomonas aeruginosa outer membrane proteins induced by immunization in humans

Outer membrane proteins (OMPs) of pathogenic bacteria have been used as protective antigens in developing bacterial vaccines. In the present study, we compared the antibody responses to a Pseudomonas aeruginosa OMP vaccine elicited in humans and rabbits by immunization. Immunization with the vaccine induced high titers of serum IgG antibody both in rabbits and humans but reactivities of the induced antibodies with the OMPs were different. The rabbit immune sera recognized most of the OMPs in the vaccine both in immunoblot and immunoprecipitation analyses. In contrast, a great variation in band pattern and intensity was observed among the human immune sera in immunoblot analysis, but not in immunoprecipitation analysis. Denaturation of the OMPs did not affect the binding activity of the rabbit immune sera as determined by ELISA, but substantially reduced those of the human immune sera and anti-OMP IgG purified from a pooled normal human plasma. These data suggest that antibody response to P. aeruginosa OMPs elicited by immunization in humans is mainly directed against discontinuous or conformation-dependent epitopes, which should be taken into account in developing vaccines, especially for OMP-derived synthetic peptides.

Liposome-induced conformational changes of an epitopic peptide and its palmitoylated derivative of influenza virus hemagglutinin

The conformation of synthetic HA317-329-NH2 representing the major B- and T-cell epitopic region of influenza virus hemagglutinin, its palmitoylated derivative (HA317-329-Thr(Pal)-NH2), and the intersubunit peptide (HA317-341-NH2) comprising also the fusion peptide, were studied in aqueous buffer and in the presence of neutral and negatively charged liposomes. The free peptide is unordered in aqueous solution, even in the presence of liposomes. However, grafting the palmitic acid or the fusion peptide onto the C-terminus of the peptide enables the hydrophilic HA317-329 to adopt folded (turn) and beta-strand structure on the surface of neutral and negatively charged liposomes, respectively. The results emphasize the importance of some kind of anchor for achieving a specific conformation of epitopic peptide HA317-329-NH2 on the surface of liposomes.

Analysis of the interaction between a synthetic peptide of influenza virus hemagglutinin and monoclonal antibodies using an optical biosensor

The interaction between two monoclonal antibodies and their corresponding Fab' fragments with a synthetic peptide, corresponding to the C-terminal 23 residues of the HA1 chain of influenza virus hemagglutinin against which they were generated, has been examined using an optical biosensor employing the detection principal of surface plasmon resonance (Pharmacia BIAcore). The data obtained has been analysed in detail by linear transformation of the primary data and nonlinear regression analysis, as well as by analysis of equilibrium binding data. The 2/1 antibodies and their Fab' fragments displayed higher affinity than the corresponding 1/1 proteins. The IgGs were found to have equilibrium association constants (KA) 10-20-fold higher than the corresponding Fab' fragments. This appears largely to be due to differences in the dissociation rate constant (kd) and probably reflects increased avidity due to bivalent binding.

Protection of mice against lethal viral infection by synthetic peptides corresponding to B- and T-cell recognition sites of influenza A hemagglutinin

Previously, we reported 12 synthetic T- and B-cell recognition regions representing surface areas of the hemagglutinin (HA) of X31 influenza virus. In the present study, four of these peptides were examined in Balb/c mice for their ability to produce protective immunity against lethal infection with a dose equivalent to 10 LD50 of influenza virus. These peptides corresponded to the following sequences: 23-36 (HA1-1); 138-152 (HA1-3); 183-199 (HA1-6) and 1-11 (HA2-10). Each of the selected peptides, in their free form, evoked anti-peptide antibodies that cross-react with intact X31 virus. Two of the peptides, HA1-1 and HA1-3, also elicited virus-specific delayed type hypersensitivity (DTH) responses. These two peptides, when injected into mice, not only failed to protect the immunized mice against challenge with influenza virus, but in fact caused greater susceptibility to viral infection as compared to control animals that had been injected with saline. In contrast, peptides HA1-6 and HA2-10, which were unable to induce adequate virus-specific DTH responses, conferred 42-46% and 54-73% protection, respectively, compared to the control group that received only saline (P < 0.03 to P < 0.01).

A common neutralizing epitope conserved between the hemagglutinins of influenza A virus H1 and H2 strains

When mice were immunized with the A/Okuda/57 (H2N2) strain of influenza virus, a unique monoclonal antibody designated C179 was obtained. Although C179 was confirmed to recognize the hemagglutinin (HA) glycoprotein by immunoprecipitation assays, it did not show hemagglutination inhibition activity to any of the strains of the three subtypes of influenza A virus. However, it neutralized all of the H1 and H2 strains but not the H3 strains. Moreover, it inhibited polykaryon formation induced by the H1 and H2 strains but not by the H3 strains. Two antigenic variants against C179 were obtained, and nucleotide sequence analysis revealed that amino acid sequences, from 318 to 322 of HA1 and from 47 to 58 of HA2, conserved among H1 and H2 strains were responsible for the recognition of C179. Since the two sites were located close to each other at the middle of the stem region of the HA molecule, C179 seemed to recognize these sites conformationally. These data indicated that binding of C179 to the stem region of HA inhibits the fusion activity of HA and thus results in virus neutralization and inhibition of cell-cell fusion. This is the first report which describes the presence of conserved antigenic sites on HA not only in a specific subtype but also in two subtypes of influenza A virus.

The immune response to synthetic peptides of human protamines HP1 and HP2

Peptides representing the amino-terminal sequence of protamines HP1 (sequence 1-12) and HP2 (sequence 1-11), the two major nuclear proteins of human sperm, have been synthesized. Rabbits were immunized either with peptide conjugated with a carrier or with free peptide. The resulting antisera were examined for their capacities to bind the homologous peptide, other peptides from protamines HP1, HP2, from ram protamine, a protein resembling HP1, and finally with the whole protamine. Only free peptides were immunogenic. Antisera were found to react with the homologous peptide, but also with some other peptides. More especially, antibodies to peptide HP1 1-12 were found to recognize an epitope shared by the homologous peptide, peptide HP1 37-49 and peptide 1-12 of ram protamine. The common antigenic determinant seems to depend on the conformation of the peptides, rather than strictly related to common sequences. Anti-peptide antibodies react poorly and in a non-specific manner with the parent protein. The failure of reactivity with the protamines strongly suggest that these small basic proteins are folded and probably globular molecules in contrast with the totally random model postulated by several previous works.

Probing the idiotype/anti-idiotype antibody interaction with a set of synthetic peptide homologues

Anti-idiotypic (anti-Id) antibodies were raised against two murine monoclonal antibodies (mAb 1/1 and mAb 2/1) which recognise two distinct and well-characterised epitopes on a 24-residue synthetic peptide representing part of the haemagglutinin (HA) of influenza virus. A monoclonal anti-Id antibody, specific for mAb 2/1, could bind to mAb 2/1 when the paratope of the latter was occupied with peptide, indicating that this anti-Id antibody is directed to a framework idiotope. In contrast, an anti-Id mAb derived from mAb 1/1-immunised mice was inhibited in its binding to Id by the parent peptide and also by the heptapeptide NVPEKQT which constitutes the epitope recognised by mAb 1/1. The small size of this synthetic peptide eliminates the possibility of significant steric inhibition in the system, and establishes that this mAb is a true paratope-directed anti-Id antibody. The interaction of this anti-Id mAb with the paratope of mAb 1/1 in the presence of a set of peptide homologues of the epitope was also examined. A peptide as short as 5 residues, which contains two of the three irreplaceable residues of the epitope, could inhibit binding between the two mAbs.

Neutralizing epitopes of human parainfluenza virus type 3 are conformational and cannot be imitated by synthetic peptides

The possibility that linear epitopes on the haemagglutinin-neuraminidase (HN) surface glycoprotein of human parainfluenza virus type 3 (PIV-3) might induce neutralizing antibodies after virus infection was investigated. Thirty-seven peptides, representing 64% of the extramembranous portion of the HN molecule of PIV-3, were synthesized. Their ability to bind to 14 neutralizing murine monoclonal antibodies (mAbs) specific for HN or 26 high-titre human serum samples were tested in a direct enzyme-linked immunosorbent assay (ELISA) and in an indirect competition ELISA. None of the synthetic peptides reacted with any of the mAbs or serum samples in the direct test and none of 11 synthetic peptides tested blocked mAbs from binding to HN in the competition ELISA. These findings suggest that synthetic peptides cannot be used to imitate the known neutralizing epitopes on the HN. Analyses of reduced and non-reduced HN in ELISA and immunoblot assays confirmed that protein folding and tertiary structure are essential for epitope formation in these neutralizing sites. However, some children's sera analysed by immunoblotting contained antibodies to an uncharacterized linear epitope(s) not recognized by our panel of mAbs, raising the possibility that a neutralizing linear epitope does exist on the HN of PIV-3.

Antigenic properties and protective capacity of a cyclic peptide corresponding to site A of influenza virus haemagglutinin

Two cyclic peptide analogues corresponding to residues 139-146 (site A) of influenza A virus haemagglutinin (strain X31) were synthesized. The ability of these peptides to react with anti-influenza virus antibodies was found to depend on the conformation of the loop and on the orientation in which the peptide was presented to antibodies. Antibodies raised to the peptides were able to bind in ELISA with influenza virus antigen that had been allowed to dry on the microtitre plate. When OF1 mice were immunized with cyclic peptides, approximately 80% of the animals were protected against an intranasal challenge with influenza virus.

Development and characterization of monoclonal antibodies with specificity for metallic radioisotope chelators linked to antibodies and other proteins

Monoclonal antibodies with specificity for the diethylenetriaminepentaacetic acid (DTPA) portion of the GYK-DTPA (glycyl-tyrosyl-lysine-DTPA) linker structure used to chelate metals to immunoglobulins have been prepared. A significant proportion of these antibodies were lambda light chain isotype. Competition assays demonstrated that DTPA, rather than GYK, was the binding site of the antibodies tested. These monoclonal antibodies should be useful reagents for use in assays specific for the presence of the common linker structure used to chelate metallic radioisotopes to monoclonal antibodies. The antibodies tested did not discriminate between chelated and unchelated DTPA.

Class II-restricted T-cell clones to a synthetic peptide of influenza virus hemagglutinin differ in their fine specificities and in the ability to respond to virus

Fifteen T-cell clones were derived from BALB/c or DBA/2 mice immunized with a synthetic peptide corresponding to the C-terminal 24 residues (residues 305 to 328) of the HA1 chain of H3 subtype influenza virus hemagglutinin. All of the clones proliferated when the peptide was presented in association with I-Ed. By using shorter homologs, it was shown that the T-cell response was focused predominantly on the region at the N-terminal end of the peptide encompassed by residues 306 to 319. Individual clones recognizing this region differed in their absolute requirements for residues at the extremities of the site and also in their patterns of efficiency of recognition of shorter homologs. One particular clone defined another site of T-cell recognition within residues 314 to 328. The response of the clones to peptide analogs identified certain residues within the sites that were critical for recognition, with the substitution Gln-311----Ser having a differential effect on clones responding to the N-terminal site. Only one of the clones responded well to influenza virus itself. This clone also required relatively low concentrations of the parent peptide for optimum stimulation and was suppressed by higher concentrations. The data demonstrate striking heterogeneity in the T-cell response even to a short synthetic peptide, with different T-cell clones recognizing slightly different but overlapping areas of the molecule.

Minimum requirements for immunogenic and antigenic activities of homologs of a synthetic peptide of influenza virus hemagglutinin

Synthetic peptides of increasing length and corresponding in sequence to the C-terminal end of the HA1 molecule of influenza virus were constructed and examined for their immunogenic and antigenic properties. Peptides containing at least the four C-terminal amino acids, when coupled to keyhole limpet hemocyanin, were capable of eliciting antibody in BALB/c mice that bound to the 24-residue parent peptide H3 HA1 (305 to 328). In the absence of a carrier, the C-terminal decapeptide was the shortest peptide capable of eliciting antibody. The specificity of this antibody was indistinguishable from that of a monoclonal antibody to the parent peptide which recognizes an epitope encompassed by the C-terminal seven residues. All peptides containing at least the C-terminal four residues were able to inhibit completely the binding of this monoclonal antibody to the parent peptide. Taken together, these results indicate that (i) the tetrapeptide is capable of eliciting specific antibody when coupled to a carrier, (ii) this tetrapeptide possesses all of the antigenic information necessary to occupy the paratope of a monoclonal antibody elicited by the longer parent peptide, and (iii) the decapeptide contains all of the information necessary to elicit a specific immune response and therefore carries an epitope recognized by T cells as well as one recognized by B cells.

Genetic control and fine specificity of the immune response to a synthetic peptide of influenza virus hemagglutinin

The immune response to a synthetic peptide, H3 HA1(305-328), representing the C'-terminal 24 amino acid residues of the HA1 chain of the hemagglutinin of the H3 subtype of influenza virus is controlled by genes in the I region of the major histocompatibility complex. Mice of the H-2d haplotype are high responders and produce antibody for several months after a single injection of peptide without carrier. Mice of the H-2b, H-2k, and H-2q haplotypes are low antibody responders. Investigation of recombinant and congenic mouse strains revealed that high responsiveness requires the genes that encode the I-Ed molecule. Immunoassays, involving direct binding to analogs of this peptide and inhibition by both these analogs and synthetic epitopes, were used to analyze the specificity of the polyclonal response. In BALB/c mice, the primary antibody response is directed principally against the antigenic site 314-LKLAT-318, whereas the secondary response after a boost is predominantly directed to a distinct site, 320-MRNVPEKQT-328. The T-cell response to the peptide H3 HA1(305-328), as measured by antigen-induced proliferation of primed T cells in vitro, is also I-Ed restricted in high-responder H-2d mice and is directed against an antigenic site that does not require the four C-terminal residues unique to the H3 influenza subtype. A different epitope appears to be recognized by T cells from CBA (H-2k) mice, which proliferate to a moderate extent on exposure to the peptide but, nevertheless, do not provide help for an antibody response.

Simultaneous binding of two monoclonal antibodies to epitopes separated in sequence by only three amino acid residues

Two monoclonal antibodies recognizing distinct epitopes the outer boundaries of which are separated by only three amino acid residues, a maximum of 10A, were demonstrated to bind simultaneously to a short synthetic peptide. The affinity of binding of the two monoclonal antibodies and of Fab' fragments derived from them was determined. The stoichiometry of the interaction was analysed by velocity sedimentation and by gel permeation chromatography experiments. The results indicate that the immune complexes formed are composed of two antibody molecules in association with one or two peptide molecules.

Antigenic determinants of influenza virus hemagglutinin. XII. the epitopes of a synthetic peptide representing the C-terminus of HA1

A synthetic peptide comprising the C-terminal 24 amino acids of the heavy chain (HA1) of influenza virus hemagglutinin was constructed and examined for antigenic and immunogenic activity. Monoclonal antibodies as well as polyclonal antisera raised against the synthetic peptide were able to bind to intact virus. This binding was greatly enhanced if the virus was first subjected to pH 5, suggesting that this treatment exposes the C-terminus of HA1. Using synthetic analogs of the native sequence it was shown that the epitope recognized by one of the monoclonal antibodies encompasses one or more of the C-terminal four amino acids of HA1 (residues 325-328), which are conserved within subtypes but differ between subtypes, while the other monoclonal antibody recognizes a different epitope which involves at least one of the five variable residues at positions 311-315.

Peptides as antigens. Importance of orientation

Factors known to be important in producing protein-reactive peptide antibodies include the accessibility of the region from which the peptide sequence is derived, the hydrophilic-phobic character of the sequence, and the length of the peptide. The data presented here indicate that the orientation of the peptide coupled to a carrier protein also influences the binding pattern of peptide antibodies. An octapeptide, representing a sequence from the alpha chain of the human acetylcholine receptor, was coupled either through an N- or C-terminal cysteine-glycine-glycine linker to a carrier protein and used to immunize rabbits. The resulting antisera reacted at comparable titers to the uncoupled immunizing peptides, but did not crossreact with the identical but opposite-linked peptide. Characterization of the binding to other homologous peptides showed that immunization with the N-terminal-linked peptide induced antibodies reactive specifically with the C-terminal amino acid(s). Immunization with the C-linked peptide resulted in antibodies reactive with a site of the peptide near the C-terminus.

Immunogenicity of loop-structured short synthetic peptides mimicking the antigenic site A of influenza virus hemagglutinin

In order to assess the relevance of conformation for the antigenic site A of the hemagglutinin of influenza virus we synthesized two peptides, comprising two variant sequences of the central part of site A (amino acids 140 - 146 of subunit HA1) inserted into an artificial peptide skeleton, which imposes a loop-like structure on the respective sequence stretch. Assuming that the loop structure in the synthetic peptides would roughly approximate to the structure of the cognate protein sequence we tried to raise protein-reactive anti-peptide antibodies. The antibodies obtained indeed showed reactivity against influenza virus, although the discriminating specificity with regard to a mutation at position 144 was lost for virus binding in contrast to the highly specific peptide binding. Considering the failures in raising anti-hemagglutinin antibodies against the site A by immunization with short flexible peptide our results support the hypothesis that conformation makes a major contribution to the immunogenic and antigenic characteristics of site A in influenza hemagglutinin.

Development of a Genetically-Engineered, Candidate Polio Vaccine Employing the Self-Assembling Properties of the Tobacco Mosaic Virus Coat Protein

A synthetic gene coding for the coat protein of tobacco mosaic virus (TMVCP) was expressed in E. coli under the direction of the lacUV5 promoter. Modification of the 3' end of the TMVCP gene by insertion of a region coding for an antigenic epitope from poliovirus type 3 resulted in the production of a hybrid TMVCP (TMVCP-polio 3). Both the E. coli-produced TMVCP and TMVCP-polio 3 were shown to assemble into virus-like rods under acidic conditions in E. coli extracts. Their purification was accomplished in a single step by chromatography on Sepharose 6B. TMVCP-polio 3 induced the formation of poliovirus neutralizing antibodies following injection into rats. The level of immune response was related to the degree of polymerization of the TMVCP-polio 3 preparations.

Dissection of the human antigammaglobulin idiotype system with monoclonal antibodies

Murine monoclonal antibodies (MoAb) were prepared by immunizing mice with human monoclonal rheumatoid factors from patients with mixed cryoglobulinaemia. Indirect solid phase radioimmunoassay and haemagglutination inhibition were used to screen the MoAb. Reactivity patterns of MoAb were determined using (a) polyclonal and monoclonal IgM proteins, (b) monoclonal IgM proteins from patients with neuropathy, (c) monoclonal and polyclonal IgM antigammaglobulins, and (d) various isolated VkIIIb-positive immunoglobulins. Several patterns were obtained: MoAb reacting with private idiotypic determinants, with VkIIIb-related determinants, and with cross-reactive idiotypes (CRI). Two MoAb of the second type reacted with VkIIIb-positive immunoglobulins and light chains regardless of their antigenic activity. Another MoAb reacted with VkIII light chains only when in association with mu heavy chains. MoAb of the third type defined distinct CRI systems. One of these was restricted to antigammaglobulins and another also involved neuropathy-associated monoclonal IgM proteins. All MoAb clearly reacted with a minor population of normal polyclonal IgM, except for the MoAb detecting private idiotypic determinants. Studies using inhibition of binding by enzyme-linked immunosorbent assay showed that polyclonal IgM antigammaglobulins may carry a CRI determinant detected by one of the MoAb. This CRI system, defined by the MoAb Glo 86.3, was similar to but not identical with the previously described Wa CRI, which is widely prevalent among IgM antigammaglobulins of rheumatoid arthritis.