Human IgM molecules that bind staphylococcal protein A contain VHIII H chains

Staphylococcal protein A (SPA) is a bacterial membrane protein which has distinct binding sites for Fc gamma and for the Fab region of some IgM, IgG, IgA, and IgE molecules. This study establishes a structure-function correlation responsible for the binding of Ig Fab regions to SPA. Binding of 24 isolated human monoclonal IgM proteins to SPA was measured in a solid phase RIA. VH and V kappa subgroups of each IgM were determined by SDS-PAGE, transfer blotting, and detection with antisera prepared against specific first framework region peptides. Binding to SPA was seen with 10 of 11 VHIII IgM, but none of the 7 VHI or 6 VHII. Similarly, polyclonal IgM fractionated on a SPA-Sepharose CL4B column showed nearly complete partition of VHIII molecules into the SPA-binding fraction, and VHI and VHII subgroup proteins into the fall-through. We conclude that SPA binding is a functional marker for VHIII H chains in human IgM molecules.     

Shared N-terminal sequences in monclonal IgMkappa and IgGkappa proteins from a patient with a complex multiple paraprotein disorder.

The N-terminal sequence analyses were performed on the heavy (H) and light (L) chains of the idiotypically identical IgM kappa and IgG kappa paraproteins isolated from the serum of patient, Cam. The N-terminal 39 residues of the kappa chains of the IgM and IgG were identical and belonged to the human V kappa III subgroup. This sequenced stretch included the first L chain hypervariable region. The N-terminal 27 residues of the variable regions (VH) of the respective mu and gamma heavy chains were also identical and belonged to the human VHIII subgroup. These identical VH sequences were unique with lysine residues at positions 13 and 19. This dual lysine substitution has not been seen in 37 other human VHIII sequences reported in the literature. This N-terminal sequence homology in the V-regions of Cam IgM kappa and IgG kappa paraproteins and the shared idiotypy expressed by Cam IgM, IgG, and IgA proteins strongly suggest the existence of complete structural homology in the variable regions of the and L chains of these Ig molecules of three separate Ig classes. At the cellular and genetic level, these results point toward a common clonal origin for the idiotypically related Ig molecules and suggest that identical V-region (VH and VL) genes were utilized by the Cam lymphoid clone in the biosynthesis of the respective IgM, IgC, and IgA proteins.     

A study of the variable heavy chain (VH) region of membrane-bound Ig on human chronic leukemic lymphocytes.

Lymphocytes from 20 patients with chronic lymphocytic leukemia (CLL) were studied for membrane staining by direct immunofluorescence by employing anti-F(ab')2, anti-VHI, anti-VHII, anti-VHIII subgroup-specific antisera, as well as light chain-specific antisera. Some lymphocyte preparations were also studied in indirect immunofluorescence with an antiserum raised against a fragment (VH) corresponding to the variable region of the heavy chain of a human IgG3 myeloma protein (Kup). Lymphocytes from each CLL patient demonstrated a restriction of VH subgroups expressed on the cell membrane; six were restricted to the VHI subgroup, seven to VHII, and seven to the VHIII subgroup. This restriction gave further evidence for monoclonality of the membrane-bound Ig and the leukemic cell proliferation. Antiserum to the VH fragment stained closely similar percentages of CLL lymphocytes to that obtained with anti-F(ab')2 antiserum. Furthermore, double staining revealed that the same cells were stained with anti-VH antiserum as were stained with anti-F(ab')2 antiserum, i.e., only the B lymphocytes.     

Protein L. A novel bacterial cell wall protein with affinity for Ig L chains

A novel Ig-binding protein has been isolated from the surface of bacteria belonging to the anaerobic species Peptococcus magnus. To solubilize the protein, peptococci were treated with different proteolytic enzymes (papain, pepsin, and trypsin) or with mutanolysin, a bacteriolytic agent known to digest the cell walls of streptococci. Papain, trypsin, and mutanolysin all solubilized peptides showing affinity for radiolabeled human IgG in Western blot analysis. Compared with papain and trypsin, mutanolysin liberated a more homogeneous material, which also had a higher m.w. This mutanolysin-solubilized protein (Mr 95 kDa) was obtained highly purified by a single isolation step on IgG-Sepharose, and the molecule was found to exhibit unique Ig-binding properties. Thus, in dot blots and in Western blots, human IgG, F(ab')2 and Fab fragments of IgG, and human kappa and lambda L chains all showed affinity for the protein. Moreover, the molecule also bound human IgM and IgA, whereas no binding was recorded for IgG-Fc fragments or IgG H chains. Finally, the protein bound to human polyclonal Ig L chains immobilized on polyacrylamide beads. These different data demonstrate that the isolated peptococcal protein binds Ig through L chain interaction. The name protein L is therefore suggested for this novel Ig-binding bacterial cell wall protein.     

Nonimmune binding of Ig to Clostridium perfringens. Preferential binding of IgM and aggregated IgG

In an attempt to find a bacterial IgM receptor, a large number of bacterial strains of different species were screened for the ability to bind human IgM. Certain strains of the anaerobic bacterium Clostridium perfringens were found to bind a major fraction of polyclonal IgM. One bacterial strain showed a particularly high binding capacity and was studied in more detail. This strain is also able to bind a minor fraction of polyclonal IgA and IgG. Inhibition experiments indicate that the different Ig classes bind to one and the same R structure. The ability of the strain to bind polyclonal Ig is correlated to the number of subunits in the Ig. This correlation can most simply be explained by increasing avidity with increasing number of subunits. In agreement with this hypotheses, experiments with aggregated IgG show that binding ability increases with aggregate size. Experiments with Ig fragments indicate that the binding structure in Ig is located in the F(ab')2 region. The ability of this bacterial strain to bind a majority of IgM molecules as well as aggregated IgG is potentially useful in immunologic work and represents a new type of Ig binding to bacteria.     

Expression of VHIII-associated cross-reactive idiotype on human B lymphocytes. Association with staphylococcal protein A binding and Staphylococcus aureus Cowan I stimulation.

It has been demonstrated that staphylococcal protein A (SPA) has an "alternative" binding site with specificity for human Ig H chain V region of the VHIII subgroup. Because the major mitogenic component of Staphylococcus aureus Cowan I (SAC) is SPA, it is possible that SAC stimulates a subpopulation of B cells expressing Ig of the VHIII H chain subgroup. In the present study, we have investigated further the relationship between SPA binding and the expression of VHI- or VHIII-associated cross-reactive idiotype (CRI) on the surface of tonsillar B lymphocytes enriched for the expression or nonexpression of the CRI, and we examined the Ig secreted by cell lines established from these populations of B cells by EBV transformation. The VHIII CRI (D12)-enriched population yielded 21 cell lines, with 67% of them secreting SPA-reactive Ig; in contrast, only 6% (1 of 16) of VHI CRI-expressing lines secreted SPA-reactive Ig. The CRI-negative B cell population yielded 54 cell lines, of which 20% secreted SPA-reactive Ig, as might be anticipated because a majority of VHIII Ig+ B cells will be CRI-. SAC stimulation of CRI+ and CRI- populations showed preferential stimulation of the D12 population. These data support the proposal that SAC stimulation of human B cells is mediated through binding of SPA by its alternative binding site to IgV regions of the VHIII subgroup.     

Idiotypic interactions in type II mixed cryoglobulins

An analysis of interactions between immunoglobulin molecules within cryoglobulins has been carried out in 18 patients with type II mixed cryoglobulinemia. In this series, there was a prevalence of VH I and VK I variable regions subgroups in the monoclonal IgM component. Our analysis of the IgG component indicated a particular selection of IgG molecules during cryoprecipitation. There was a prevalence of IgG3 and of the VH I subgroup and the isoelectrofocusing pattern revealed a very restricted spectrotype in two thirds of these IgG. These results which suggested a restricted reactivity between cryo-IgM and IgG fractions were confirmed by the analysis of the interaction between each IgG and each IgM from the cryoprecipitates. All IgM reacted with intact IgG or Fc fragments but another reaction was observed between cryo-IgM and Fab fragments from a limited number of cryo-IgG, with a pattern suggestive of idiotypic specificity. Results of the absorption of each cryo-IgM on Fc or on Fab fragments from the corresponding cryo-IgG also suggested the existence of a reaction between IgM and IgG Fab in addition to that involving IgM Fab and IgG Fc. The coexistence of the 2 reactions should confer a higher stability to the IgM-IgG complex. Therefore, it is possible that the proliferation of one clone of IgM-RF producing B cells would be followed in certain cases by a relatively restricted anti-idiotypic IgG response. The IgM-RF would preferentially react with these anti-idiotypic IgG.     

Mouse Ig coded by VH families S107 or J606 bind to protein A

Twenty-five monoclonal mouse Ig (5 IgA, 7 IgM, and 13 IgG1) were tested for binding to staphylococcal protein A. They were allowed to attach to protein A Sepharose column at pH 8.0 and were then eluted with a pH gradient from approximately 7.5 to 3.0. Five of them (IgM or IgA) did not bind. Ten came off with pH approximately 6. They were all IgG1, and were probably bound (weakly) via the Fc portion. The remaining 10 (3 IgA, 4 IgM, and 3 IgG1) were more firmly bound; they came off with pH-values ranging from 5.0 to 3.5. They all expressed VH genes of families J606 or S107, whereas all the 15 Ig that were not firmly bound expressed VH genes of six other families. The VL domains seem to be unimportant for protein A binding inasmuch as a firmly binding and a weakly binding IgG1 antibody share identical VK domains. VH sequences of protein A-binding and nonbinding Ig were compared. No likely peptide sequences were found that might make the ligand for protein A.     

Purification of antibody Fab and F(ab')2 fragments using Gradiflow technology

The Gradiflow, a preparative electrophoresis instrument designed to separate molecules on the basis of their size and charge, was used to purify antibody Fab and F(ab')2 fragments. The method described is charge based, utilizing the difference in the pI between the antibody Fab/F(ab')2 fragments and antibody Fc fragments that occur after enzyme digestion of whole antibody molecules. This method of purification was successful across a range of monoclonal and polyclonal antibodies. In particular, F(ab')2 fragments were purified from a number of mouse monoclonal antibodies (both IgG1 and IgG2a isotypes) and Fab fragments were purified from egg yolk IgY polyclonal antibodies. This is a rapid purification method which has advantages over alternative methods that usually comprise ion exchange and gel filtration chromatography. This method may be applicable to most antibody digest preparations.     

Roles of plasma proteins and surface negative charge of erythrocytes in erythrocyte aggregation

The effect of plasma proteins (and IgG fragments) and sialic acid content of erythrocytes on the aggregation of human erythrocytes was quantitatively examined by using a rheoscope combined with a television image analyser and a computer. (1) The velocity of erythrocyte aggregation by plasma proteins was increased with increasing in their molecular weight, i.e., IgG less than IgA less than fibrinogen less than IgM. F(ab')2. Fab and Fc could not induce the aggregation. (2) The aggregation induced by fibrinogen was accelerated by IgG and its peptic fragment, F(ab')2, but was unaffected by the plasmic fragments, Fab and Fc. The accelerating effect by IgG and F(ab')2 was inhibited by Fab and Fc. (3) The aggregation of erythrocytes was accelerated by decreasing the sialic acid content (due to the reduction of the electrostatic repulsive force among erythrocytes), and the effect of desialylation on the IgG-induced aggregation was greater than that of desialylation on the fibrinogen-induced aggregation. (4) The roles of plasma proteins and of sialic acid content of erythrocytes on the aggregation of erythrocytes were discussed.     

Recombinant, truncated CD4 molecule (rT4) binds IgG

CD4 is a cell surface glycoprotein that identifies the subset of human T lymphocytes that induces sIg+ B lymphocytes to differentiate and secrete Ig after intimate T-B cell contact. In the course of studying a recombinant, truncated form of CD4 (rT4) we noticed that goat antibodies of apparently irrelevant specificities bound to immobilized rT4. To directly study whether rT4 interacts with Ig molecules, purified human IgG was added to rT4-coated wells and a dose-dependent interaction between IgG and rT4 was observed by ELISA. Purified myeloma IgG proteins bound to immobilized rT4 with the same avidity as polyclonal IgG that suggests that rT4-IgG binding was not due to the presence of anti-rT4 antibodies in the IgG fraction. IgG from 6 sera bound to rT4 in concentration dependent manner similar to purified IgG. Immobilized rT4 specifically bound IgG, and not IgM, IgA, IgD, or beta 2-microglobulin. The specific interaction of rT4 and IgG was also observed when IgG or IgM were immobilized, demonstrating that IgG binding was not a unique property of immobilized rT4. As with low affinity receptors for IgG, rT4 bound heat aggregated IgG with increased avidity. Neither anti-CD4 mAb nor dextran sulfate inhibited rT4-IgG binding. rT4 bound Fc but not F(ab)2 fragments. Each of the purified IgG subclasses; IgG1, 2, 3, and 4 bound to rT4 with similar avidity. Taken together, these data suggest that rT4 specifically interacts with a public structure on IgG Fc.     

Differential IgG-binding characteristics of staphylococcal protein A, streptococcal protein G, and a chimeric protein AG

Various Gram-positive bacteria express different types of IgG-binding receptors, each of which displaying certain unique binding properties. To evaluate specificity and avidity aspects of the differential binding pattern, a set of competitive binding assays was employed, by using staphylococcal protein A (SPA), streptococcal protein G (SPG), and a chimeric protein AG. These receptors were analyzed, in a reciprocal fashion, for binding and inhibition of binding to a selected panel of polyclonal and monoclonal Ig. Results of the study reveal that a majority of the determinants on human and bovine IgG, recognized by SPA and SPG, are either coextensive or closely overlapping. Accordingly, a minor portion of the determinants appear to be unique in the sense that a particular determinant(s) is selectively identified by one of the two receptors. Binding assays involving purified Fc fragments from human IgG, suggest that SPG shows exclusive specificity for an Fab region determinant(s) not recognized by SPA, whereas the Fc determinants for SPA and SPG are identical or overlapping. Furthermore, one of the IgG subclasses of bovine origin appears to be seen by the SPG receptor only. The competition study also demonstrates that the novel chimeric protein AG receptor shows higher or equal avidity for variants of human IgG molecules compared to the best of its parental constituents. It can thus be deduced that chimeric receptors might be useful as optimized tools for immunologic applications.     

Asymmetrically glycosylated IgG isolated from non-immune human sera

When human IgG or its F(ab')2 fragment purified from a pool of non-immune sera was passed through a Con A-Sepharose column, 12% of the molecules bound to concanavalin A. While 44% of Fab' and 72% of Fd' fragments obtained from F(ab')2 retained by concanavalin A and eluted with methyl alpha-D-mannoside bound to concanavalin A, the Fab' and Fd' fragments obtained from non-retained F(ab')2 and the L chains and Fc fragments did not interact with the lectin. Only Fd' fragment obtained from the F(ab')2 retained by concanavalin A inhibited the fixation of guinea-pig erythrocytes to concanavalin A. These results are similar to those previously observed for IgG antibodies of different animal species and indicate that partial asymmetric glycosylation is a general phenomenon that is not restricted exclusively to IgG molecules with known specificity.     

Human IgA1 blockade of IgG-initiated lysis of Neisseria meningitidis is a function of antigen-binding fragment binding to the polysaccharide capsule

We have recently shown that human IgA1 can initiate lysis of group C Neisseria meningitidis via the classical C pathway when bound to specific outer membrane proteins, but that IgA1 can also function as a blocking antibody when bound to the polysaccharide capsule of meningococci. In this report, we further characterized IgA1 blockade by examining the effect of IgA1 on IgG-initiated immune lysis of group C meningococci. We purified IgG and monomeric IgA1 from either convalescent group C meningococcal case sera or tetravalent (A, C, Y, W135) polysaccharide vaccinate sera. In the absence of IgA1, IgG initiated complete lysis (greater than 99%) of strains 118V (C:P3,4:L2,4) 126E (C:P3:L1,8), and 35E (C:P5:L2). Addition of IgA1 to the bactericidal reaction mixture completely blocked the lytic function of IgG. Removal of the Fc portion of IgA1 with either pepsin or IgA1 protease did not affect blockade. Both the F(ab')2 and Fab derivatives of IgA1 blocked lysis quantitatively as well as intact IgA1. The Fc fragment produced by IgA1 protease cleavage neither increased nor decreased Fab-mediated blockade. IgA1 and its Fab and F(ab')2 fragments blocked IgG-initiated lysis via either the classical pathway in factor B-depleted and in properdin-deficient serum, the alternative pathway in MgEGTA-chelated serum, or both pathways combined. Absorption of the IgA1 and IgG with alum-bound group C polysaccharide completely removed blocking and lytic activity, respectively, indicating that both the blocking IgA1 and the lytic IgG were specific for the group C capsule. Blocking by IgA1 was a linear function of the polysaccharide Ag-binding capacity (ABC) ratio of blocking IgA1 to lytic IgG. Complete blockade was observed at an ABC ratio of 5.5. At ABC ratios of 3.3 and 4.4, IgA1 affected significant blockade whether added previous to, concurrent with, or subsequent to sensitization of the organisms with IgG. With the use of a C polysaccharide ELISA, we found that the binding of IgA1 to the group C capsule in the presence of IgG exhibited positive cooperativity and therefore that blockade was independent of the ability of IgA1 to directly compete with IgG for binding to epitopes within the group C capsule. We conclude that IgA1, when bound to the group C polysaccharide capsule, can block IgG-initiated lysis of group C meningococci through either the classical or the alternative pathway before or after the organism is exposed to IgG, and that blockade is an Fc-independent event.     

Polyclonal immunoglobulin response of thymic,hepatic and splenic lymphocytes from fetal,germ-free and conventionally reared pigs to different B-cell activators

Immunoglobulin (Ig) response to different polyclonal B-cell activators was measured by ELISA in cell culture media of thymocytes, splenocytes and liver cells isolated from pig fetuses, 8-d-old germ-free piglets and conventionally reared pigs. Both in fetal and in postnatal life polyclonally stimulated lymphocytes were found to produce predominantly the IgM isotype; the first IgM formation was detected in 50-d-old fetal liver (gestation in pigs lasts 114 d). Surprisingly, 73-d-old fetal thymic cells were shown to be induced to Ig synthesis and secretion. In contrast to splenocytes of the same age, which secreted exclusively IgM, fetal thymocytes produced IgM, IgG and IgA. Polyclonally stimulated splenic cells as compared with thymic cells started to produce IgA later in fetal ontogeny, whereas the IgG response was not detectable in splenic cell culture media during the whole embryonal development and appeared only after birth. The earliest and the highest Ig stimulation was found after cultivation of lymphocytes withNocardia delipidated cell mitogen. Interestingly, the moderate stimulatory effect of 65-kDa heat shock protein (Hsp-65) in polyclonal IgM response of fetal splenocytes was observed. We showed that thymic B lymphocytes represent probably the first maturing B cell population detectable in fetal life, which is able to differentiate after polyclonal stimulation into IgM as well as IgA and IgG producing cells. Dedicated to Professor J. Šterzl on the occasion of his 70th birthday     

Ligand-loaded but not free complement receptors for C3b/C4b and C3d co-cap with cross-linked B cell surface IgM and IgD

We have performed experiments to investigate possible physical interactions between C receptors (CR) and surface Ig (sIg) on the B cell plasma membrane. These molecules were found to be independent, non-linked, B cell surface structures, because capping CR1, CR2, sIgM, or sIgD with a specific antibody did not affect the distribution of the remainder of these molecules. Both CR1 and CR2, if bound by antibodies that did not independently cap CR, however, became associated with cross-linked sIg because CR that have been bound by intact anti-CR antibodies or their Fab fragments co-capped with sIgM or sIgD that had been bound by divalent anti-IgM or anti-IgD antibody. CR1 that had bound C3b similarly co-capped with sIg when sIg was cross-linked. Ligand-bound or even cross-linked CR did not associate with non-cross-linked sIg because sIgD, bound by a univalent Fab fragment of anti-IgD antibody, did not co-cap with CR that had been cross-linked by a sandwich of mouse anti-CR antibody and goat anti-mouse Ig. Other surface molecules, such as B1 and HLA-DR Ag, when bound by specific antibodies, did not cap with cross-linked sIg, and sIgD, when bound by a univalent Fab fragment of anti-IgD antibody, did not co-cap with cross-linked sIgM. Interactions between CR and sIg were not mediated by an association with IgG FcR because co-capping of CR and sIg was observed when F(ab')2 fragments of both anti-CR and anti-Ig antibodies were used. These results demonstrate that B cell surface CR can become associated with sIg, but only if sIg is cross-linked and CR is bound by anti-CR antibody or has bound its natural ligand.     

Induction of apoptosis by monoclonal antibody anti-APO-1 class switch variants is dependent on cross-linking of APO-1 cell surface antigens.

Apoptosis, programmed cell death, was previously shown to be induced by the mAb anti-APO-1 (IgG3, kappa) by binding to the APO-1 cell surface Ag, a new member of the nerve growth factor/TNF receptor superfamily. To investigate the role of the Ig H chain Fc regions we compared induction of apoptosis by the original mAb IgG3 anti-APO-1 with anti-APO-1 F(ab')2 fragments and different anti-APO-1 isotypes (IgG1, IgG2b, IgG2a, and IgA) isolated by sequential sublining. We found that IgG3 was the most active isotype; IgG1, IgG2a, and IgA showed intermediate activity, and IgG2b and F(ab')2 were inactive. Cytotoxic activity of the inactive or less active antibody preparations was fully reconstituted by protein A, anti-mouse Ig, or anti-mouse Ig F(ab')2, respectively. Thus, APO-1-mediated induction of apoptosis was dependent on efficient cross-linking of APO-1 cell surface Ag, indirectly augmented by anti-APO-1 Fc-Fc self-aggregation. Because of their different in vitro activity we selected IgG3-, IgG2b-, and IgA anti-APO-1 to test their antitumor activity against solid human B lymphoblastoid tumors in SCID mice. The isotypes showed a different serum half-life (IgG3: 9.2-10.4 days, IgG2b: 1.9-2.6 days, and IgA: 14.1-29.2 h) and a different initial tumor localization 4 h after i.p. injection (IgG3 around the blood vessels, IgG2b homogeneously, and IgA heterogeneously distributed in the tumor). All antibody preparations induced tumor regression by induction of apoptosis, even IgG2b anti-APO-1 inactive in vitro without cross-linking. The activity of IgA anti-APO-1, which did not mediate complement-dependent cytotoxicity or antibody-dependent cellular cytotoxicity indicates that apoptosis may be used as the main if not the only mechanism of induction of tumor regression in vivo. As with in vitro, IgG3 anti-APO-1 was the most effective isotype also in vivo. This result suggests that cross-linking of APO-1 on the tumor cell surface may also be required for tumor regression by apoptosis in vivo. Taken together, our data show that selective targeting of apoptosis to tumors may be an efficient antitumor mechanism.     

Some T-cell leukemia lines express surface markers related to a restricted set of human VH determinants

Serological studies were carried out to obtain information regarding the relationship of the VH-related determinants expressed by certain permanent in vitro T-cell leukemia lines and corresponding determinants expressed by characterized human serum immunoglobulins. A panel of conventional (goat and rabbit) antisera, produced against various Fab-related fragments of monoclonal human Waldenstrom macroglobulins and polyclonal IgG molecules, bound to certain in vitro T-cell leukemia lines, notably, 70-N2, MT-1, YT4E, and HUT78, as shown by microhemagglutination. Inhibition studies using characterized myeloma proteins to inhibit this agglutination indicated the expression of a restricted VH-related determinant by these T-cell lines. Parallel studies performed using conventional (rabbit) and murine monoclonal/hybridoma antibodies produced against the isolated 68,000-Da VH-related product synthesized by the 70-N2 line showed that the determinant expressed by this molecule was restricted in expression, comprising 2-3% of the normal, polyclonal human Fab pool, and that the determinants found on the other positive T-cell leukemias were cross-reactive rather than identical. The inhibition studies suggest that the determinant resides between residue 22 and the end of the VH region. These results further define the antigenic nature of the VH-related marker found on the surfaces of certain normal and neoplastic T-cell lines.     

Normal human sera contain antibodies directed at Fab of IgA

Serum samples from 26 normal volunteers were evaluated by isotype-specific ELISA for the presence of IgG and IgM antibodies directed at IgA. Although there were wide variations in antibody levels, anti-IgA antibodies of both isotypes were found in all individuals tested. The anti-IgA activity was detected against a variety of polymeric and monomeric IgA1 and IgA2 myeloma proteins containing both kappa and lambda light chains. By using Fab and Fc fragments generated by incubation of an IgA1 myeloma protein with IgA1 protease, it was shown that the anti-IgA activity was specific for the Fab portion of the IgA molecule. It was also demonstrated that the serum of two individuals contained both IgG and IgM activity directed at autologous affinity-purified IgA. IgM antibody levels against both whole IgA and Fab of IgA were significantly higher than IgG antibody levels. Cells producing anti-IgA antibodies of both isotypes were detected in lipopolysaccharide-stimulated human spleen.     

Properties of an immunoglobulin binding factor from human seminal plasma.

A soluble factor (IBF) in human seminal plasma that binds serum immunoglobulins (Ig) of various species was purified to homogeneity by ammonium sulfate precipitation, preparative isoelectrofocusing, and gel filtration chromatography. The purified IBF interacted weakly with Fc and F(ab')2 fragments and not with Fab. It interacted with anti-Leu 11b and polyclonal anti-Fc gamma RIII antibodies, but not with other anti-Fc gamma R antibodies (32.2, IV.3 and 3G8). IBF is probably a non-glycosylated protein with isoelectric point ranging from 5.1 to 5.8. The estimated Mr of the purified native IBF is 27 kD, determined by sodium dodecyl sulfate polyacrylamide electrophoresis (SDS-PAGE) under non-reducing condition. In its native form, IBF did not bind Ig or interact with anti-Fc gamma R antibodies. Following SDS-PAGE under reducing condition, IBF migrated as a single protein with an estimated Mr of 16 kD and interacted with Ig of various species and with anti-Leu 11b antibodies. When carboxymethylated, however, IBF did not bind IgG. The present results suggest that free sulfhydryl groups of IBF is required for Ig binding.