Alternative non-immune F(ab')2-mediated immunoglobulin binding to group C and G streptococci

Bioaffinity-based surface immobilization of antibodies to capture endothelial colony-forming cells

Maximizing the re-endothelialization of vascular implants such as prostheses or stents has the potential to significantly improve their long-term performance. Endothelial progenitor cell capture stents with surface-immobilized antibodies show significantly improved endothelialization in the clinic. However, most current antibody-based stent surface modification strategies rely on antibody adsorption or direct conjugation via amino or carboxyl groups which leads to poor control over antibody surface concentration and/or molecular orientation, and ultimately bioavailability for cell capture. Here, we assess the utility of a bioaffinity-based surface modification strategy to immobilize antibodies targeting endothelial cell surface antigens. A cysteine-tagged truncated protein G polypeptide containing three Fc-binding domains was conjugated onto aminated polystyrene substrates via a bi-functional linking arm, followed by antibody immobilization. Different IgG antibodies were successfully immobilized on the protein G-modified surfaces. Covalent grafting of the protein G polypeptide was more effective than surface adsorption in immobilizing antibodies at high density based on fluorophore-labeled secondary antibody detection, as well as endothelial colony-forming cell capture through anti-CD144 antibodies. This work presents a potential avenue for enhancing the performance of cell capture strategies by using covalent grafting of protein G polypeptides to immobilize IgG antibodies.

Multimerization is required for antigen binding activity of an engineered IgM/IgG chimeric antibody recognizing a skin-related antigen

Monoclonal antibodies offer great tools for research. We encountered a potentially useful mouse IgM monoclonal antibody whose antigen is expressed in normal skin but lost in human skin cancer. Because IgM is difficult to work with and the antigen was unknown, we decided to convert the IgM (µ) to IgG (γ) version. After cDNA for the antibody was obtained by RACE PCR, we made a series of molecules with different combinations of IgM and IgG domains. Whereas V_H-Cµ1-Cµ2-Cγ3 and V_H-Cµ1-Cµ2-Hinge-Cγ2-Cγ3 functionally bound to the antigen, V_H-Cγ1-Hinge-Cγ2-Cγ3, V_H-Cµ1-Hinge-Cγ2-Cγ3, and V_H-Cµ1-Cµ2-Cγ2-Cγ3 did not. Gel filtration analyses revealed that the functional molecules tend to form multimers and the multimeric forms retained antigen binding activity. Furthermore, the mutation of amino acid residue p.309Q > C of mouse IgG and addition of IgM tailpiece to the C-terminus of the molecules induced multimer formation, dramatically enhanced antibody functionality and all non-functional molecules became strongly functional. The functional molecules could be bound by protein A/protein G and other IgG specific reagents and therefore should be useful for further characterization of the antigen. Our study revealed that multimerization of converted IgM is functionally important for antigen binding activity of engineered IgM/IgG chimeric antibodies.

Preparation of Fab' from murine IgG2a for thiol reactive conjugation

Lysyl endopeptidase (LE) from Achromobacter lyticus M497-1 (EC 3.4.21.50) was utilized to prepare F(ab')2 fragments from mouse anti-P-glycoprotein IgG2a obtained from the UIC2 hybridoma. This report describes a novel single step purification procedure for F(ab')2 fragments that eliminates residual LE activity responsible for secondary cleavage of F(ab')2 to Fab fragments. The purification of F(ab')2 and Fc fragments was accomplished utilizing protein G affinity chromatography and either gradient or step changes in the pH/ionic strength for elution of the Fc and F(ab')2 fragments. Residual LE was eluted from the protein G column with buffer containing 200 mM L-lysine prior to elution of F(ab')2 and Fc fragments. The activity of LE was monitored using the fluorogenic substrate Boc-Val-Leu-Lys-7-amido 4-methyl coumarin. A similar purification procedure for F(ab')2 fragments produced following pepsin digestion of IgG2a is also outlined. The ability of Fab' fragments, from reduced F(ab')2 fragments following LE digestion of IgG2a, to conjugate to thiol reactive groups was demonstrated using N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer-meso chlorin e6 mono (N-2-aminoethylamide) (Mce6) conjugates containing reactive maleimide groups. The biological activity of the Fab' targeted HPMA copolymer-Mce6 conjugates was tested against the P-glycoprotein expressing human ovarian carcinoma A2780/AD cell line utilizing a cell survival assay. Fab' targeted HPMA copolymer-Mce6 conjugate demonstrated significantly higher cytotoxicity than either a monoclonal antibody (mAb) targeted HPMA copolymer-Mce6 conjugate or a non-targeted HPMA copolymer-Mce6 conjugate, p < 0.05.

Nonimmune interaction of the SfbI protein of Streptococcus pyogenes with the immunoglobulin G F(ab')(2) fragment

Fibronectin-binding protein I (SfbI) of Streptococcus pyogenes binds to mouse immunoglobulin G (IgG) but not to IgA or IgM in a nonimmune fashion. The fibronectin-binding domains of SfbI were responsible for this activity, which was targeted to the IgG F(ab')(2) fragment. SfbI also binds to B cells but not to CD4(+) or CD8(+) lymphocytes.

Immunoglobulin Fab fragment-binding proteins

Five molecules are known to bind the Fab fragments of human immunoglobulins (Ig). Microbial protein A and protein G are primarily Fc-binding molecules but can also bind other structures of the heavy chain, which are located in the variable domain of the third subgroup (VH3) and in the first constant domain of IgG (CH1 gamma), respectively. In contrast, the two other microbial receptors have a sole Ig-binding site, directed to chi chains (protein L) or to Ig polymers (protein P). Protein Fv is synthesized by human liver cells and released in the digestive lumen, where it forms large complexes with secretory Ig after binding to the VH domains. These five molecules, in the main, bind cleaved Ig and most of them recognize all classes of antibodies. Bacterial molecules are, or can be, used as reagents to purify and detect Ig and fragments. Furthermore, a possible use in human therapy or vaccination is envisaged, and the human protein Fv is a key-factor in immune protection against intraluminal pathogens of the gut.

Purification of recombinant chimeric B72.3 Fab' and F(ab')2 using streptococcal protein G

Streptococcal protein G has been used extensively for the purification of antibodies using the interaction of the Fc region with protein G. Many antibodies also interact with protein G through a low-affinity binding site for the Fab region. The exploitation of this low-affinity interaction for the purification of Fab' fragments is described here. Chimeric mouse-human B72.3 Fab' and F(ab')2 fragments were expressed by CHO cells and purified from CHO cell supernatant using protein G-Sepharose. Since chimeric B72.3 Fab' bound weakly to the protein G-Sepharose it could be separated from F(ab')2 and eluted with a pH 7 wash whereas B72.3 F(ab')2 required elution at pH 2. Both Fab' and F(ab')2 were recovered with full immunoreactivity and could be further purified using gel-filtration chromatography to greater than 99% purity. This method allows the simple purification of directly expressed Fab' or F(ab')2 fragments from CHO cell supernatant.

Human antibody responses to bacterial antigens: studies of a model conventional antigen and a proposed model B cell superantigen

We have investigated the human antibody repertoires that bind to two different classes of bacterial antigens. Immunization with the conventional antigen, type b capsular polysaccharide of Haemophilus influenzae Hib PS, uniformly induces IgA and IgG responses dominated by clones that use heavy chains structurally related to two subsets of VH3 genes, while in a minority of subjects antibodies from the VH1 or VH4 families are co-induced. In contrast, the "alternative binding site" of Staphylococcal Protein A (SPA) represents an unconventional determinant, because; (i) SPA is bound by a large proportion of non-immune IgM, IgA and IgG F(ab')2, (ii) SPA is bound only to Fab from the VH3 family, which can be encoded by at least four different germline genes, (iii) SPA binding is independent of VL usage, (iv) by flow cytometry SPA is bound by > 15% of tonsilar B cells, but not to T cells. (v) In vitro stimulation with an SPA containing mitogen induces the preferentially production of Ig bearing a VH3 marker. Taken together, these studies characterize a VH family restricted binding interaction that is distinct from the properties associated with conventional antigens such as Hib PS. Based on these data we propose that SPA represents a prototype for a B cell superantigen.

Structural and functional analysis of the human IgG-Fab receptor activity of streptococcal protein G

Streptococcal protein G (SPG) shows specific binding activity to IgGs and serum albumins from various species. In order to investigate the structural domains of SPG responsible for the specific interaction with human IgG-Fab, the binding characteristics of a collection of recombinant receptors were analysed. The study includes receptors comprising different parts of the SPG molecule as well as chimeric receptors containing IgG-binding domains of staphylococcal protein A (SPA) fused to the N-terminal AB-region of SPG, which has been claimed to interact with human IgG-Fab. Purified defined gene products were allowed to compete for the binding to human IgG, human IgG-F(ab')2 fragments and human serum albumin (HSA) in several sets of competitive binding experiments. The results demonstrate that the C-terminal C domains have both IgG-Fc- and IgG-Fab-binding capacities, whereas the N-terminal AB region is responsible for the HSA-binding only. These results, which are in conflict with previous work, demonstrate that the binding to both the IgG-Fc and the IgG-Fab region is mediated by the same structurally distinct receptor region of SPG.

Isolation of a 30 kDa immunoglobulin binding protein from Pseudomonas maltophilia

We have demonstrated that Pseudomonas maltophilia (ATCC No. 13637) possesses an exposed, immunologically accessible protein which binds to the Fc region of several species of immunoglobulins. Whole bacteria suspensions were incubated for 18 h with purified 125I-labelled antibodies with and without added non-labelled immunoglobulins. The suspensions were centrifuged for 30 min and the pellet containing bacteria was assessed for radioactivity. Using this crude assay, the whole organism bound 125I-labelled rabbit and mouse immunoglobulins and the purified Fc portion of human IgG. All of these labelled preparations were competitively displaced by unlabelled rabbit and mouse immunoglobulins, and Fc of human IgG, as well as human immunoglobulin subclasses. The organism was sonicated to solubilize this immunoglobulin binding protein. Using this sonicated preparation, it was shown that unlabelled Fc of IgG, unlabelled mouse and rabbit immunoglobulins, all competitively displaced 125I-labelled human Fc of IgG in a dose-response manner. A partially purified protein was prepared by Sephacryl S-300 followed by Sephadex G-100 column chromatography. This preparation was incubated with 125I-Fc gamma and with the following purified unlabelled preparations: F(ab')2 of IgG, Fc of IgG, murine monoclonal IgA, IgG1, IgG2, IgG3, and IgG4. All except F(ab')2 of IgG produced dose response competitive displacement. The molecular weight, as estimated by SDS-PAGE and Western blot, was 30,000 daltons. In Western blots, Fc gamma, murine monoclonal IgA, and human immunoglobulin subclasses, all showed affinity for the immobilized protein. Human F(ab')2 fragments did not show affinity for the protein. Radioiodinated pseudomonal Ig-binding protein showed affinity for human IgG coupled to Sepharose, and was displaced by unlabelled pseudomonal Ig-binding protein. Scatchard analysis of binding showed two binding affinities: two distinct types of Ig-binding proteins were obtained, a high affinity with Kd = 1.54 x 10(-10) and a lower affinity with Kd = 2.36 x 10(-8). This immunoglobulin binding protein may be useful in immunoglobulin purification or identification.

Electron microscopic localization of lipoteichoic acid on group A streptococci

The location of lipoteichoic acid (LTA) on the surface of group A streptococci was studied by immunoelectron microscopic and ultrastructural cytochemical methods, i.e. by means of LTA antibodies labelled with ferritin, or concanavalin A labelled with ferritin or colloidal gold. All these methods proved the LTA to be located on the outer cell surface of most group A streptococcus strains. The differences in the intensity of labelling paralleled the hydrophobicity of the strains, being substantially higher in the strains exhibiting a high degree of hydrophobicity. Treatment of streptococci with pronase or trypsin led to a complete loss of surface-located LTA. On the other hand, pepsin treatment of streptococci under mild conditions resulted in an increased amount of surface-located LTA in some strains. On the isolated cell walls, LTA could be demonstrated only on the outer surface of the walls. These findings correlated well with the presumed role of group A streptococcus LTA in the adherence of streptococci to the epithelial cells which is accomplished with the aid of surface-located LTA molecules.

Non-immune Fab- and Fc- mediated interactions of avian Ig with S. aureus and group C and G streptococci

Serum samples from 19 avian species representing 8 orders were tested for their capacity to inhibit the Fab- and Fc-mediated immunoglobulin binding to protein A-carrying S. aureus and protein G-carrying group C and G streptococci. Four species (mallard, dunlin, starling and blackbird) belonging to three different orders showed a high degree of Fc-mediated protein A- and protein G-reactivity. Five species demonstrated a high level and nine species exhibited a low level of Fab-mediated protein A-reactivity. The four species identified as Fc-reactive were capable of Fab-mediated immunoglobulin binding with streptococcal surface proteins but incapable of Fab-mediated protein A binding. SDS-PAGE analysis confirmed that the protein A-Sepharose affinity purified material contained proteins corresponding to immunoglobulin chains. Inhibition results by avian sera were confirmed by direct binding of protein A-reactive proteins to bacteria, by precipitation in gel and by Western blot analysis of binding to protein A and protein G, respectively.

Streptococcal protein G has affinity for both Fab- and Fc-fragments of human IgG

We have analyzed the binding of IgG and fragments of IgG [Fc, F(ab')2, and Fab] to group C and G streptococci and to protein G, the IgG binding cell wall protein of these bacteria. A direct correlation (r = 0.87, P less than 0.0001) was observed when the binding of radiolabelled, polyclonal IgG F(ab')2- and Fc-fragments to 23 group C and G streptococcal strains was compared. One strain (G-148) was treated with increasing amounts of pepsin, trypsin or papain and the Fab-binding structure was found to be much more sensitive to the enzymes as compared to the Fc-binding. A 35 K fragment of protein G was coupled to Sepharose, and both radiolabelled IgG F(ab')2- and Fc-fragments bound to the Sepharose beads. Binding of IgG fragments was inhibited by intact IgG or by the homologous IgG fragment, whereas Fc-fragments did not inhibit Fab binding or vice versa. Two radiolabelled protein G-fragments (28 and 35 K) showed different binding to polyclonal IgG, IgG F(ab')2-, IgG Fab- and IgG Fc-fragments. Thus, in a dot binding assay the 35 K fragment bound all IgG fragments tested, whereas the 28 K protein G fragment bound only intact IgG and IgG Fc-fragments. These results indicate two independent and separate binding sites for Fab- and Fc-fragments on protein G. Different binding sites on protein G were also indicated by Western blot analysis of four different protein G-fragments (28, 35, 42 and 65 K). In these experiments the 28 K fragment showed affinity only for Fc-fragments, while the higher mol. wt protein G preparations bound both IgG Fab- and Fc-fragments.

Bindings of plasma proteins to streptococci of serological group L with special reference to their immunoglobulin G Fc-receptor activity

Of 33 streptococcal cultures belonging to serological group L, all bound human immunoglobulin (Ig) G, fibrinogen, and fibronectin; 32 bound bovine IgG; 31 bound alpha 2-macroglobulin; 5 bound albumin; and none bound either haptoglobin or IgA. The binding sites for IgG could be isolated from the L streptococci by trypsinization and purified by affinity chromatography on human IgG-Sepharose. The purified Fc receptors reacted with IgG subclasses 1, 2, 3, 4 of humans, 1 and 2 of bovines, ovines, and caprines as well as a, b, c, and T of equines. They had a molecular mass of approximately 49,000 Da. Thus, the Fc receptors from L streptococci corresponded to type III Fc receptors of Streptococcus dysgalactiae.

Mechanism of activation of human basophils by Staphylococcus aureus Cowan 1

We investigated the capacity of Staphylococcus aureus Cowan 1 and S. aureus Wood 46 to induce histamine release from human basophils in vitro. S. aureus Cowan 1 (10(5) to 10(7)/ml), which synthesizes protein A (Staph A), stimulated the release of histamine from basophils, whereas S. aureus Wood 46 (10(5) to 2 X 10(7)/ml), which does not synthesize Staph A, did not induce histamine secretion. Soluble Staph A (10(-3) to 10 micrograms/ml), but not staphylococcal enterotoxin A, induced histamine secretion from human basophils. Staph A binds through its classical site to the Fc region of human immunoglobulin G (IgG) and through its alternative site to the Fab portion of the different human immunoglobulins. Hyperiodination of Staph A, which destroys over 90% of the original Fc reactivity without altering the Fab-binding site, did not alter the ability of the protein to induce histamine release. The stimulating effect of Staph A was dose dependently inhibited by preincubation with human polyclonal IgG (0.3 to 100 micrograms/ml) and a human monoclonal IgM (0.3 to 100 micrograms/ml) which have F(ab')-Staph A reactivity. In contrast, rabbit IgG, which possesses only Fc-Staph A reactivity, and a Staph A-unreactive human monoclonal IgM did not inhibit Staph A activity. Similar results were obtained with intact S. aureus Cowan 1. Preincubation with either Staph A or anti-IgE (rabbit anti-Fc epsilon) resulted in complete desensitization to a subsequent challenge with the homologous stimulus. Staph A and anti-IgE induced partial cross-densensitization to the heterologous stimulus. Cells preincubated with anti-IgG (rabbit anti-Fc gamma) lost a small but significant part of their ability to release with Staph A but did not lose their response to anti-IgE. Basophils from which IgE had been dissociated by brief exposure to lactic acid no longer released histamine in response to anti-IgE and Staph A. When basophils from which IgE had been dissociated were incubated with human polyclonal IgE, they regained their ability to induce histamine in response to Staph A and anti-IgE. In contrast, two monoclonal IgEs which do not bind to Staph A did not restore the basophil responsiveness to Staph A. Furthermore, there was complete cross-desensitization between soluble Staph A and S. aureus Cowan 1, while cells desensitized to S. aureus Wood 46 released normally with Staph A and S. aureus Cowan 1.(ABSTRACT TRUNCATED AT 400 WORDS)

Non-immune F(ab')2- and Fc-mediated interactions of mammalian immunoglobulins with S. aureus and group C and G streptococci

The distribution among mammalian species of non-immune F(ab')2- and Fc-mediated immunoglobulin interactions with surface proteins of S. aureus (protein A) and of group C and G streptococci was studied. Serum samples from 48 mammalian species representing 15 orders were first tested for their capacity to inhibit streptococcal F(ab')2-mediated binding; 26 of these sera were also tested for streptococcal IgG Fc-mediated binding. Analogous inhibition experiments were then carried out with staphylococci. All mammalian species studied inhibited both types of immunoglobulin binding to streptococci, viz the serum samples contained both F(ab')2- and Fc-reactive immunoglobulins. The reactivity was equal to that of human serum in 26 out of 47 mammalian sera. Seven sera showed a low degree of inhibition compared to human serum. The inhibiting capacities of the two streptococcal non-immune interactions showed a direct correlation (r = 0.91, p less than 0.0001 for the r-value) for individual species. The inhibition patterns observed with S. aureus differed from the profiles recorded with the streptococcal strains, suggesting that these organisms interact with separate sites on the immunoglobulin molecules. Isolated F(ab')2-binding was recorded in 5 out of 24 sera, and Fc-binding alone was noted in 7 sera. Taken together, the present studies demonstrate that mammalian immunoglobulins possess F(ab')2- and Fc-binding sites for protein A and for receptors on group C and G streptococci. The F(ab')2-mediated binding to streptococci is associated with Fc-reactivity, in contrast to protein A which may interact exclusively with a complementary structure in either the F(ab')2- or the Fc-portion of the immunoglobulins.

Semi-quantitative determination of IgG-binding structures on bacteria by direct fluorescence technique

A simple semiquantitative method for determination of IgG-binding structures on bacteria by direct fluorescence technique is described. The fluorescence of bacterial bound IgG-FITC-conjugate was measured in a paste-like bacterial sediment by using a fluorescence microscope photometer unit. For this purpose sharply centrifuged IgG-FITC conjugate treated bacteria, from which the washing fluid was carefully removed, were transferred to a glass slide and fluorescence was measured at the contact layer of the adhered drop on the inverted slide. The measured fluorescence intensity area was found to be correlated with the amount of bound IgG-FITC/cell, if bacteria had been incubated with an excess of fluorescein labeled IgG. The IgG-binding of different streptococcal strains was compared with the average IgG-binding of strain Cowan I resulting from 13 different cultivations. For strain Cowan I 9.4 X 10(4) IgG-molecules were estimated to bind on one staphylococcal cell. For a screening of IgG-binding bacterial strains the method did not demand a standardization of bacteria by cell counting.

Independent binding of native and aggregated IgG in group A streptococci

Irrespective of IgG Fc-receptor activity, earlier characterized, many group A streptococci were recently found to bind aggregated IgG Fab and/or light chains. In the present study, binding of glutaraldehyde-aggregated, radiolabelled, intact human IgG (a*IgG) to group A streptococci was tested, and strains representing several M-types were found reactive. In particular, high binding was observed among type M12 strains, earlier found devoid of Fc-receptors for monomeric IgG; accordingly, unlabelled, native IgG had little influence on the binding. The sites binding a*IgG were highly sensitive to trypsin and relatively resistant to heat treatment. The binding to M12 was inhibited by human fibrinogen and, to a lesser extent, by heat-aggregated serum albumin. Rabbit antiserum to M12 was more inhibitory than antiserum to a heterologous type of group A streptococci or normal rabbit serum. Our results indicate that streptococcal M-protein binds a*IgG by a multipoint requiring interaction of low specificity and that previously described Fc-receptors binding native IgG are not involved. For comparison, in Cowan I staphylococci and one strain of group G streptococci tested, high binding of a*IgG was also observed; however, this binding was inhibited by native IgG, indicating that protein A and group G streptococcal Fc-receptor, earlier known to bind untreated IgG, also bound a*IgG.

Two separate non-immune interactions between staphylococcal protein A and immunoglobulins are mediated by structures on gamma chains

The present investigation was undertaken to determine whether the light or the heavy immunoglobulin chain is involved in the alternative, non-immune F(ab')2-mediated binding to staphylococcal protein A. Purified human polyclonal IgG was mildly reduced with dithiothreitol and alkylated with iodoacetamide. Intact IgG, purified light and heavy chains of polyclonal immunoglobulin G were tested in an inhibition assay for alternative non-immune F(ab')2-mediated binding to the protein A-carrying S. aureus, strain Cowan I. The IgG Fc-mediated binding to protein A was studied in parallel inhibition experiments. Heavy chains inhibited both the alternative F(ab')2- and the classical Fc-mediated binding to protein A. Isolated light chains were non-reactive. Intact IgG molecules were more potent inhibitors than isolated heavy chains tested in equimolar concentrations. Our results indicate that the alternative non-immune interaction between staphylococcal protein A and human immunoglobulins is mediated by structures expressed on the heavy immunoglobulin G chain. Thus, there are two separate protein A binding sites on gamma chains.

A non-immune interaction between the light chain of human immunoglobulin and a surface component of a Peptococcus magnus strain

The immunoglobulin-binding capacity of a Peptococcus magnus strain was studied in a sensitive binding assay using purified human immunoglobulin preparations. The P. magnus strain 312 was capable of binding 48% of polyclonal IgG. Twenty-four of 40 purified myeloma proteins (60%) representing immunoglobulin classes A, G and M showed definite reactivity with an uptake level ranging from 45 to 90%. The remaining 16 monoclonal proteins were non-reactive, binding less than 15%. One myeloma protein with antistaphylolysin and two with antistreptolysin O specificity, i.e. monoclonal proteins with defined antigen specificity, were highly reactive. Binding capacity was observed in all four IgG subclasses and in Ig classes A and M. Twenty-three of 27 myeloma proteins of kappa type were reactive but only one of 13 myeloma proteins of lambda type interacted with the P. magnus strain. Isotope-labelled Fab gamma, F(ab')2 gamma and F(ab')2 alpha fragments were effectively bound by the strain. IgG Fc fragments were completely non-reactive. Isolated light immunoglobulin chains inhibited in a dose-dependent way the uptake of intact IgG to bacteria. Purified heavy chains were non-inhibitory. Isotope-labelled antistaphylolysin IgG F(ab')2 fragments preincubated with staphylolysin were as reactive as free antibody fragments, suggesting that the bacterial binding structure is located outside the antibody-combining site. The immunoglobulin reactivity of P. magnus was not affected by heating the bacteria to 80 degrees C for 5 min nor by treatment with trypsin or sodium metaperiodate. Digestion of 2 X 10(9) organisms with 100 micrograms of pepsin and papain reduced the binding by 58 and 90%, respectively. These data indicate that the binding of immunoglobulin to P. magnus is a non-immune reactivity mediated by a heat-stable surface protein interacting with specific sites on the light chain of the immunoglobulin molecule.

Binding of aggregated human serum albumin to M12 and some other types of group A streptococci

In radiobinding tests many group A, C and G streptococci react with IgG and IgA, irrespective of the antigen-combining sites, as well as with various other serum proteins, e.g. human serum albumin (HSA). The present study demonstrated that glutaraldehyde-aggregated, radiolabelled HSA (a*HSA), in comparison to monomeric HSA, binds more avidly to streptococci. Of group A streptococci, strains representing types M6, M12, M18, M46, M55 and M57 displayed pronounced binding of a*HSA whereas a number of other serotypes were non-reactive. The streptococcal sites involved proved to be relatively heat-resistant and highly sensitive to trypsin treatment. Human fibrinogen counteracted the binding of a*HSA. The uptake by M12 was inhibited strongly by rabbit antiserum raised against M12, whereas other antisera were less active. The results suggest that the bacterial structure binding a*HSA is a protein and that, in at least one serotype, M12, the binding occurs to the M-protein.

Non-immune IgG F(ab')2 binding to group C and G streptococci is mediated by structures on gamma chains

The present investigation was designed to determine whether the heavy or the light immunoglobulin chain is involved in the non-immune binding of IgG F(ab')2 fragments to specific surface receptors on human group C and G streptococci. Purified human polyclonal IgG was mildly reduced with dithiothreitol and alkylated with iodoacetamide. Light (L) and heavy (H) chains were separated. Intact IgG and purified L and H chains of polyclonal immunoglobulin G were tested in an inhibition assay for non-immune IgG F(ab')2-mediated binding to group C and G streptococci. H chains inhibited the uptake of isotope-labelled IgG F(ab')2 fragments. Isolated L chains were non-reactive. Intact IgG molecules were more potent inhibitors than isolated H chains tested in equimolar concentrations. These results indicate that the non-immune interaction between human group C and G streptococci and F(ab')2 fragments of human IgG is mediated by reactive sites exposed on the immunoglobulin G H chains. The observation that intact IgG on a molar basis was more inhibitory than purified gamma chains suggests that the L chains may contribute to the reactivity, presumably by passive stabilization of the immunoglobulin molecule.