Inhibition of bacterial IgA proteases by human secretory IgA and serum

Impact of the molecular form of immunoglobulin A on functional activity in defense against Streptococcus pneumoniae

Antibodies of the immunoglobulin A (IgA) class react with capsular polysaccharides of Streptococcus pneumoniae and support complement-dependent opsonophagocytosis (OPC) of the organism by phagocytes. We characterized the biologic impact of the molecular forms of human monoclonal capsule-specific IgA (monomeric IgA [mIgA], polymeric IgA [pIgA], and secretory IgA [SIgA]) on OPC and susceptibility to cleavage by IgA1 protease. The efficiency of SIgA in support of OPC of S. pneumoniae was comparable to that of pIgA, and both forms exceeded that of mIgA by a fivefold margin. This structure-function relationship was associated with three factors. First, the avidities, or functional affinities, of both pIgA and SIgA for pneumococcal capsules exceeded those of mIgA. Second, both pIgA and SIgA required less complement to achieve similar levels of bacterial OPC than did mIgA, indicating that secretory component does not hinder the effect of complement. Third, both pIgA and SIgA mediated agglutination of the organism, whereas mIgA did not. All three forms of capsule-specific IgA showed comparable susceptibilities to cleavage and functional inhibition by bacterial IgA1 protease, demonstrating that secretory component does not prevent the proteolytic degradation of IgA1 by IgA1 protease. IgA1 cleavage results in formation of identical Fab fragments for each of the molecular forms, thereby abolishing the contribution of multivalence of pIgA and SIgA. In summary, the polymeric forms of IgA (both pIgA and SIgA) provide a substantial advantage in binding, agglutination, and OPC of the organism.

Type V protein secretion pathway: the autotransporter story

Gram-negative bacteria possess an outer membrane layer which constrains uptake and secretion of solutes and polypeptides. To overcome this barrier, bacteria have developed several systems for protein secretion. The type V secretion pathway encompasses the autotransporter proteins, the two-partner secretion system, and the recently described type Vc or AT-2 family of proteins. Since its discovery in the late 1980s, this family of secreted proteins has expanded continuously, due largely to the advent of the genomic age, to become the largest group of secreted proteins in gram-negative bacteria. Several of these proteins play essential roles in the pathogenesis of bacterial infections and have been characterized in detail, demonstrating a diverse array of function including the ability to condense host cell actin and to modulate apoptosis. However, most of the autotransporter proteins remain to be characterized. In light of new discoveries and controversies in this research field, this review considers the autotransporter secretion process in the context of the more general field of bacterial protein translocation and exoprotein function.

Antibody reactivity of a standardized human serum protein solution against a spectrum of microbial pathogens and toxins: comparison with fresh frozen plasma

In this study, we compared a standardized solution of human serum protein (HSP) and fresh frozen plasma (FFP) with regard to the antibody specificity against a number of microbial pathogens and some important pathogenicity factors of bacterial pathogens. Due to the clinical use of HSP and FFP for therapeutical plasma exchange, we have chosen a spectrum of microbial pathogens for serological analysis that is critical in clinical settings. With the enzyme-linked immunosorbent assay technique, we could show that HSP contains marked IgG antibody reactivity against antigens of Escherichia coli, Campylobacter jejuni, Enterobacter sakazakii, Proteus mirabilis, Pseudomonas aeruginosa, Klebsiella pneumoniae, Staphylococcus aureus, S. epidermidis, Streptococcus pneumoniae, Enterococcus faecalis, Chlamydia pneumoniae, and Candida albicans. Although no IgM antibodies against the pathogens tested could be detected in HSP, moderate IgA reactivity was found against 4 of 12 microbial antigens. Immunoblot analysis demonstrated specific IgA and IgG responses against the endoproteinase Glu-C and the superantigens enterotoxin A and B of S. aureus, the IgA-protease of Neisseria gonorrhoeae, and Shiga toxin 2 of enterohemorrhagic E. coli. By using 3 different HSP batches in parallel, we could demonstrate antibody reactivity against important microbial pathogens and toxins. This antibody profile is essentially more homogeneous than that of 3 batches of FFP.

Immunoglobulins in nasal secretions of healthy humans: structural integrity of secretory immunoglobulin A1 (IgA1) and occurrence of neutralizing antibodies to IgA1 proteases of nasal bacteria

Certain bacteria, including overt pathogens as well as commensals, produce immunoglobulin A1 (IgA1) proteases. By cleaving IgA1, including secretory IgA1, in the hinge region, these enzymes may interfere with the barrier functions of mucosal IgA antibodies, as indicated by experiments in vitro. Previous studies have suggested that cleavage of IgA1 in nasal secretions may be associated with the development and perpetuation of atopic disease. To clarify the potential effect of IgA1 protease-producing bacteria in the nasal cavity, we have analyzed immunoglobulin isotypes in nasal secretions of 11 healthy humans, with a focus on IgA, and at the same time have characterized and quantified IgA1 protease-producing bacteria in the nasal flora of the subjects. Samples in the form of nasal wash were collected by using a washing liquid that contained lithium as an internal reference. Dilution factors and, subsequently, concentrations in undiluted secretions could thereby be calculated. IgA, mainly in the secretory form, was found by enzyme-linked immunosorbent assay to be the dominant isotype in all subjects, and the vast majority of IgA (median, 91%) was of the A1 subclass, corroborating results of previous analyses at the level of immunoglobulin-producing cells. Levels of serum-type immunoglobulins were low, except for four subjects in whom levels of IgG corresponded to 20 to 66% of total IgA. Cumulative levels of IgA, IgG, and IgM in undiluted secretions ranged from 260 to 2,494 (median, 777) microg ml(-1). IgA1 protease-producing bacteria (Haemophilus influenzae, Streptococcus pneumoniae, or Streptococcus mitis biovar 1) were isolated from the nasal cavities of seven subjects at 2.1 x 10(3) to 7.2 x 10(6) CFU per ml of undiluted secretion, corresponding to 0.2 to 99.6% of the flora. Nevertheless, alpha-chain fragments characteristic of IgA1 protease activity were not detected in secretions from any subject by immunoblotting. Neutralizing antibodies to IgA1 proteases of autologous isolates were detected in secretions from five of the seven subjects but not in those from two subjects harboring IgA1 protease-producing S. mitis biovar 1. alpha-chain fragments different from Fc(alpha) and Fd(alpha) were detected in some samples, possibly reflecting nonspecific proteolytic activity of microbial or host origin. These results add to previous evidence for a role of secretory immunity in the defense of the nasal mucosa but do not help identify conditions under which bacterial IgA1 proteases may interfere with this defense.

Evaluation of immunoglobulin A1 (IgA1) protease and IgA1 protease-inhibitory activity in human female genital infection with Neisseria gonorrhoeae

Immunoglobulin A1 (IgA1) protease, an enzyme that selectively cleaves human IgA1, may be a virulence factor for pathogenic organisms such as Neisseria gonorrhoeae. Host protection from the effects of IgA1 protease includes antibody-mediated inhibition of IgA1 protease activity, and it is believed that the relative balance between IgA1 protease and inhibitory antibodies contributes to the pathogenesis of disease caused by IgA1 protease-producing organisms. We have examined the levels of these two opposing factors in genital tract secretions and sera from women with uncomplicated infection with N. gonorrhoeae. When IgA1 in cervical mucus was examined by Western blotting, no evidence of cleavage fragments characteristic of IgA1 protease activity was seen in gonococcus-infected or control patients. Cleavage fragments typical of IgA1 protease were detected, however, after the addition of exogenous IgA1 protease to cervical mucus. Degraded IgA1 was detected in some vaginal wash samples, but the fragment pattern was not typical of IgA1 protease activity. All N. gonorrhoeae isolates from the infected patients produced IgA1 protease in vitro. All but two serum samples and 16 of 65 cervical mucus samples displayed inhibitory activity against gonococcal IgA1 protease, but there was no significant difference in the level of inhibitory activity between gonococcus-infected and noninfected patients in either cervical mucus or serum. There was no difference in the levels of IgA1 protease-inhibitory activity in serum or cervical mucus collected from patients at recruitment and 2 weeks later. These results suggest that cleavage of IgA1 by gonococcal IgA1 protease within the lumen of the female lower genital tract is unlikely to be a significant factor in the pathogenesis of infections by N. gonorrhoeae.

IgA1 protease production by bacteria colonizing the upper respiratory tract

Thirty-eight clinical isolates of Haemophilus influenzae and ten clinical isolates of Streptococcus pneumoniae were examined for IgA1 protease production. A suspension of surface material of each individual strain was incubated with human secretory IgA; IgA1 cleavage products were detected by SDS-PAGE and immunoblotting. The high incidence of IgA1 protease-positive strains (68.4% of the examined H. influenzae and 100% of the examined S. pneumoniae strains) confirms that IgA1 protease activity is a frequent characteristic of these two species. Yet the presence of this enzyme is, if at all, only a minor decisive factor for the induction of symptomatic infections of the upper respiratory tract by IgA1 protease-positive bacteria.

Inhibition of Prevotella and Capnocytophaga immunoglobulin A1 proteases by human serum

Oral Prevotella and Capnocytophaga species, regularly isolated from periodontal pockets and associated with extraoral infections, secret specific immunoglobulin A1 (IgA1) proteases cleaving human IgA1 in the hinge region into intact Fab and Fc fragments. To investigate whether these enzymes are subject to inhibition in vivo in humans, we tested 34 sera from periodontally diseased and healthy individuals in an enzyme-linked immunosorbent assay for the presence and titers of inhibition of seven Prevotella and Capnocytophaga proteases. All or nearly all of the sera inhibited the IgA1 protease activity of Prevotella buccae, Prevotella oris, and Prevotella loescheii. A minor proportion of the sera inhibited Prevotella buccalis, Prevotella denticola, and Prevotella melaninogenica IgA1 proteases, while no sera inhibited Capnocytophaga ochracea IgA1 protease. All inhibition titers were low, ranging from 5 to 55, with titer being defined as the reciprocal of the dilution of serum causing 50% inhibition of one defined unit of protease activity. No correlation between periodontal disease status and the presence, absence, or titer of inhibition was observed. The nature of the low titers of inhibition in all sera of the IgA1 proteases of P. buccae, P. oris, and P. loescheii was further examined. In size exclusion chromatography, inhibitory activity corresponded to the peak volume of IgA. Additional inhibition of the P. oris IgA1 protease was found in fractions containing both IgA and IgG. Purification of the IgG fractions of five sera by passage of the sera on a protein G column resulted in recovery of inhibitory IgG antibodies against all three IgA1 proteases, with the highest titer being for the P. oris enzyme. These finding indicate that inhibitory activity is associated with enzyme-neutralizing antibodies.

Pulmonary disposition of antimicrobial agents: methodological considerations

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Mucosal immunity: the immunology of breast milk

The mammary glands represent one part of the mucosal immune system, a definable, subunit of humoral and cellular immune functions in man that appears to have developed particular qualities well suited to guard our interface with the environment. As our understanding of secretory immunoglobulins and lymphocyte migration patterns continues to develop, the immunologic components found in breast milk appear increasingly likely to play a specific immunologic role in the protection of the nursing infant. The biologic basis for the observed protective effect of breast-feeding is reviewed with an emphasis on the mechanisms involved in the development and maintenance of mucosal immunity in general.

Proteases and their inhibitors in chronic inflammatory periodontal disease

This article reviews the current knowledge of the sources, function and interactions of proteolytic enzymes and their inhibitors in chronic inflammatory periodontal disease. Proteolytic tissue degradation is a typical phenomenon in chronic inflammatory periodontal disease. The proteolytic enzymes can be both host- and bacteria-derived. The proteases of the inflammatory cells are aimed for digestion of bacteria, enhanced locomotion through connective tissue, demarcation of the site of infection and tissue remodeling. Uncontrolled release of proteases in inflammation causes self-digestion and tissue destruction. The potential of the bacterial proteases in degradation of connective tissue is not yet known. Biochemical and immunologic mediators of inflammation are released by proteolytic reactions. Immunoglobulin-cleaving proteases present a specific mechanism in perturbation of host defenses. The 2 main protease inhibitors in serum, alpha-1-antitrypsin and alpha-2-macroglobulin, are also present in the gingival tissue fluid guarding the function of proteases. It has been suggested, although not confirmed, that deficiency in serum protease inhibiting capacity could be correlated with susceptibility to periodontal disease. Mucous secretions contain local low molecular weight protease inhibitors, but their possible rôle in saliva is not known. Bacteria-derived, antiproteolytic short peptides may prove to be useful in pharmacological control of tissue destruction at inflammatory sites.

Survey of immunoglobulin A protease activity among selected species of Ureaplasma and Mycoplasma: specificity for host immunoglobulin A

Because immunoglobulin A (IgA) is the predominant immunoglobulin at mucosal surfaces, IgA proteases produced by pathogenic bacteria are considered potential virulence factors for organisms that cause disease or gain entry at mucous membranes. To determine the role of IgA protease in the pathogenicity of mycoplasmal disease, a variety of human and animal mycoplasma and ureaplasma species were examined for IgA protease activity with human, murine, porcine, and canine IgA. None of the mycoplasma species examined showed detectable IgA protease activity with any of the IgAs tested. Twenty-eight strains of Ureaplasma urealyticum isolated from human urogenital tissues cleaved human IgA1, but no cleavage of human IgA2 or murine, porcine, or canine IgA was observed. Ureaplasmas isolated from nonhuman hosts (feline, canine, avian, and bovine [Ureaplasma diversum]) did not cleave human IgA1. Two strains of canine ureaplasmas were able to cleave canine IgA, but not murine IgA. Thus, ureaplasmas from other species can produce IgA protease, but the specificity of the enzyme was restricted to the IgA of the appropriate host. This finding suggests that IgA proteases could play a role in the selective host specificity of mucosal pathogens.