Kinetic silver staining and quantification of proteins adsorbed to microtiter plates

Microbial Cryptotopes are Prominent Targets of B-cell Immunity

B-cell recognition of microbial antigens may be limited by masking of epitopes within three-dimensional structures (cryptotopes). Here we report that unmasking of cryptotopes by unfolding whole cytomegalovirus (CMV) antigen preparations with the chaotropic reagent Urea and probing with immune sera from healthy individuals (n = 109) increased ELISA signals by 36% in comparison to folded CMV antigens (P < 0.001). ELISA signals increased also significantly upon unfolding of S. aureus or E. coli antigens, whereas unfolded influenza H1N1 or respiratory syncitial virus antigens yielded reduced or unchanged reactivity in comparison to folded ones, respectively. Blocking of CMV cryptotope-specific Abs by incubation of an immunoglobuline preparation and three sera with unfolded CMV antigens enhanced clearly the neutralizing capacity of this immunoglobuline preparation against CMV infection. Thus, B-cell immunity frequently targets cryptotopes on CMV but these Abs are non-neutralizing, may reduce the neutralizing effectiveness of pathogen-specific Abs, and increase during immune maturation following primary CMV infection. The observation of functional consequences of Abs specific for cryptotopes may open whole new avenues to a better understanding of the humoral immune response to CMV and development of more effective vaccines and immunoglobuline preparations.

Recombinant human intelectin binds bovine lactoferrin and its peptides

Intelectin (IntL), a lectin that exists on the brush border membrane of the small intestine, plays a role in the innate immune response and also acts as a receptor for lactoferrin (LF), an iron-binding glycoprotein found in milk and other secretions. Similar to human LF (hLF), bovine LF (bLF) has been shown to induce proliferation and differentiation of human enterocytes and to modulate their cytokine productions. To evaluate the interaction between human IntL (hIntL) and bLF, recombinant hIntL (rhIntL) conjugated with a tag sequence was examined for its ligand-binding capacity by using microtiter plates coated with LF or other proteins. Interestingly, rhIntL showed higher binding for bLF than hLF. It also bound pepsin hydrolysate of bLF, but to a lower degree than native bLF. A very low binding of rhIntL was observed for bovine serum albumin or transferrin. These findings suggest that hIntL acts as a receptor for bLF and its digested fragments.

Carrier protein-modulated presentation and recognition of an N-glycan: observations on the interactions of Man(8) glycoform of ribonuclease B with conglutinin

Conglutinin is a serum lectin of the innate immune system, which binds high mannose N-glycans when these are appropriately presented on proteins. Here we use the conglutinin-ribonuclease B (RNaseB)-recognition system as a model to investigate the structural basis of selective recognition of protein-bound oligosaccharides by this carbohydrate-binding receptor. Conglutinin shows little binding to the isolated RNaseB-Man(8 )glycoform, and no binding to Man(5-6) glycoforms. In contrast, when the protein moiety is reduced and denatured we observe that conglutinin binds strongly to the isolated RNaseB-Man(8) glycoform and weakly to the Man(5-6) glycoforms. These results are in accord with observations on the binding to the N-glycans in the absence of carrier protein. NMR analyses of native RNaseB-Man(8) and -Man(5-6) glycoforms reveal that the three-dimensional structure of the protein moiety is essentially identical to that of non-glycosylated RNase (RNaseA). Thus there are no perceptible differences between the RNase protein forms that could account for differential availability of the N-glycan for conglutinin-binding. After reduction and denaturation, the NMR spectrum became typical of a non-structured polypeptide, although the conformational preferences of the N-glycosidic linkage were unchanged, and most importantly, the Man(8 )oligosaccharide retained the average conformational behavior of the free oligosaccharide irrespective of the carrier protein fold. This conformational freedom is clearly not translated into full availability of the oligosaccharide for the carbohydrate-recognition protein. We propose, therefore, that the differing bioactivity of the N-glycan is a reflection of the existence of different geometries of presentation of the carbohydrate determinant in relation to the protein surface within the glycan:carrier protein ensemble.

Adsorbed IgG: a potent adhesive substrate for human macrophages

Previous reports from our laboratory have demonstrated qualitatively that preabsorbed IgG can enhance long-term macrophage adhesion in vitro. This investigation further characterizes and quantifies the biological effect of adsorbed human IgG on human macrophages and probes the potential mechanisms. Ten-day human monocyte/macrophage cultures on Plastek M (PM), a normally poor cellular substrate for macrophages, confirmed the ability of preabsorbed IgG to dramatically enhance long-term macrophage adhesion. An adsorption solution concentration of 200 microg/mL of IgG was necessary to provide a consistent, optimal cellular response. (125)I adsorption studies indicated Langmuir-style IgG adsorption at low concentrations; however, no adsorption maximum was observed. Additional adsorption analysis revealed that the IgG fragments Fab, F(ab')(2), and Fc adsorb at levels only 20-40% that of the whole molecule. Despite the lower adsorption levels, both preabsorbed Fab and F(ab')(2) were shown to be as effective as whole molecule IgG at enhancing long-term macrophage adhesion. Surprisingly, the preabsorbed Fc fragment demonstrated no IgG-like activity, thereby eliminating the possibility of an Fc receptor-based mechanism. Other possible mechanisms, such as macrophage lectins, novel macrophage Fab receptors, and complement activation by adsorbed IgG and IgG fragments, are discussed.

The effect of formaldehyde, hydrogen peroxide and genetic detoxification of pertussis toxin on epitope recognition by murine monoclonal antibodies

The effect of detoxification of pertussis toxin (PT) for vaccine usage by either genetic manipulation, hydrogen peroxide or formaldehyde treatment on epitope recognition by a large collection of murine monoclonal pertussis toxin antibodies (PT MAbs) was assessed in a solid-phase and a soluble phase enzyme-linked immunosorbent assay (ELISA). The MAb binding patterns were found to be different in the two assays as the immobilization step appeared to cause conformational alterations in the native as well as the toxoided forms of PT. According to the solid-phase ELISA, genetic, hydrogen peroxide and 0.35% formaldehyde detoxification of PT resulted in reduced epitope binding in 2.9, 31.4 and 78.1% of the MAbs, respectively. In the soluble-phase ELISA, in which the MAbs were allowed to react with the toxoids or native toxin in solution, the percentages of MAbs showing decreased binding activity were 9.1, 50.0 and 71.4%, respectively. Stabilization of native PT and the genetically inactivated PT by 0.035% formaldehyde reduced the epitope binding activity in 50.0 and 8.7% of the MAbs, respectively. Increased antibody binding in the soluble-phase ELISA was observed in some of the toxoids: this ranged from 0% in the 0.35% formaldehyde-treated PT to 13.6% in the hydrogen peroxide-treated and 27.3% in the genetically detoxified PT. Regarding the effects of detoxification on epitopes recognized by PT-neutralizing MAbs in the soluble-phase ELISA, we found that treatment of PT with either 0.035%, 0.35% formaldehyde or hydrogen peroxide induced impairment of epitope binding in 72.7, 81.8 and 45.5% of the MAbs, respectively. In the genetically inactivated PT, the epitopes recognized by the neutralizing MAbs either appeared to remain intact or to show increased MAb binding activity. The epitope-binding patterns of several PT MAbs with mouse-protective properties varied considerably and were shown to be dependent on the detoxification procedure employed. The relevance of epitope alterations on PT as a vaccine component is discussed. The results of the present study may have important implications for future quality assessment of PT for use in acellular pertussis vaccines.