Immunoglobulin A-metallo-type specific prolyl endopeptidases

The immunoglobulin A1 proteinase fromStreptococcus pneumoniaeis inhibited by tetracycline compounds

Streptococcus pneumoniae produces a zinc-dependent proteinase that cleaves human immunoglobulin (Ig) A1 in the hinge region. This metalloproteinase is hypothesized to act as a virulence factor by allowing S. pneumoniae to evade the protection provided by IgA1, thus enhancing its ability to colonize the human nasopharyngeal region. No biologically compatible inhibitors of this enzyme have been identified. We determined that doxycycline and a chemically modified tetracycline inhibit this enzyme in vitro at low micromolar concentrations.

Characterization of igaB, a second immunoglobulin A1 protease gene in nontypeable Haemophilus influenzae

Nontypeable Haemophilus influenzae is an important respiratory pathogen, causing otitis media in children and lower respiratory tract infection in adults with chronic obstructive pulmonary disease (COPD). Immunoglobulin A1 (IgA1) protease is a well-described protein and potential virulence factor in this organism as well as other respiratory pathogens. IgA1 proteases cleave human IgA1, are involved in invasion, and display immunomodulatory effects. We have identified a second IgA1 protease gene, igaB, in H. influenzae that is present in addition to the previously described IgA1 protease gene, iga. Reverse transcriptase PCR and IgA1 protease assays indicated that the gene is transcribed, expressed, and enzymatically active in H. influenzae. The product of this gene is a type 2 IgA1 protease with homology to the iga gene of Neisseria species. Mutants that were deficient in iga, igaB, and both genes were constructed in H. influenzae strain 11P6H, a strain isolated from a patient with COPD who was experiencing an exacerbation. Analysis of these mutants indicated that igaB is the primary mediator of IgA1 protease activity in this strain. IgA1 protease activity assays on 20 clinical isolates indicated that the igaB gene is associated with increased levels of IgA1 protease activity. Approximately one-third of 297 strains of H. influenzae of diverse clinical and geographic origin contained igaB. Significant differences in the prevalence of igaB were observed among isolates from different sites of isolation (sputum > middle ear > nasopharynx). These data support the hypothesis that the newly discovered igaB gene is a potential virulence factor in nontypeable H. influenzae.

ZapA, a possible virulence factor from Proteus mirabilis exhibits broad protease substrate specificity

The opportunistic bacterium Proteus mirabilis secretes a metalloprotease, ZapA, considered to be one of its virulence factors due to its IgA-degrading activity. However, the substrate specificity of this enzyme has not yet been fully characterized. In the present study we used fluorescent peptides derived from bioactive peptides and the oxidized beta-chain of insulin to determine the enzyme specificity. The bradykinin- and dynorphin-derived peptides were cleaved at the single bonds Phe-Ser and Phe-Leu, with catalytic efficiencies of 291 and 13 mM/s, respectively. Besides confirming already published cleavage sites, a novel cleavage site was determined for the beta-chain of insulin (Val-Asn). Both the natural and the recombinant enzyme displayed the same broad specificity, demonstrated by the presence of hydrophobic, hydrophilic, charged and uncharged amino acid residues at the scissile bonds. Native IgA, however, was resistant to hydrolysis by ZapA.

Interaction of antigens and antibodies at mucosal surfaces

Infections often involve the mucosal surfaces of the body, which form a boundary with the outside world. This review focuses on immunoglobulin A (IgA) antibodies because IgA is the principal mucosal antibody class. IgA is synthesized by local plasma cells and has a specific polymeric immunoglobulin receptor-mediated transport mechanism for entry into the secretions. By serving as an external barrier capable of inhibiting attachment of microbes to the luminal surface of the mucosal epithelial lining, IgA antibodies form the first line of immune defense. In addition to this traditional mode of extracellular antibody function, recent evidence suggests that IgA antibodies can also function in a nontraditional fashion by neutralizing viruses intracellularly, if a virus is infecting an epithelial cell through which specific IgA antibody is passing on its way to the secretions. IgA antibodies are also envisaged as providing an internal mucosal barrier beneath the mucosal lining. Antigens intercepted by IgA antibodies here can potentially be ferried through the epithelium and thereby excreted. In addition to the polymeric immunoglobulin receptor on mucosal epithelial cells, IgA antibodies can bind to receptors on a variety of leukocytes and have been shown, in some experimental systems, to be capable of activating the alternative complement pathway, making IgA antibodies potential participants in inflammatory reactions. Although the relationship of IgA antibodies to inflammation is not entirely clear, the bias presented is that IgA is basically noninflammatory, perhaps even anti-inflammatory. According to this view, the major role of the Fc portion of IgA antibodies is to transport IgA across mucosal epithelial cells and not, as in the case of the other classes of antibody, to activate secondary phenomena of the kind that contribute to inflammation. Because of IgA's key role as an initial barrier to infection, much current research in mucosal immunology is directed toward developing new vectors and adjuvants that can provide improved approaches to mucosal vaccines. Finally, because of advances in monoclonal antibody technology, topical application of antibodies to mucosal surfaces has significant potential for preventing and treating infections.

Localization of the binding site for the monocyte immunoglobulin (Ig) A-Fc receptor (CD89) to the domain boundary between Calpha2 and Calpha3 in human IgA1

Immunoglobulin (Ig) A serves as the first line of humoral defense at all mucosal surfaces and is present in large quantities of blood. In playing its role in humoral immunity, IgA interacts with a variety of effector molecules present both in serum and on the surfaces of immune and inflammatory cells. To study these interactions, we previously established expression of human IgA1 in insect cells using recombinant baculoviruses and showed that the expressed antibody is a structurally and functionally intact polypeptide useful for examining the molecular properties of IgA. Indeed, since the C alpha 2 N-linked glycosylation site lies near the Fab-distal pole of C alpha 2, the inability of a mutant IgA1 lacking C alpha 2 N-glycosylation to bind its cognate receptor suggested that the monocyte Fc alpha receptor (mFcalphaR) recognizes IgA at a hinge-distal site encompassing the boundary between the C alpha 2 and C alpha 3 domains. In this report, we utilize both domain-swapped IgA/IgG and point-mutated IgA chimeras to verify the above hypothesis. Using an antigen-specific rosetting assay and a mFc alpha R-expressing cell line, we show that (a) C alpha 2 and C alpha 3 together are necessary and sufficient for binding; (b) neither the IgA hinge nor the tailpiece is necessary for binding; (c) mutations away from the interdomain boundary do not affect binding; and (d) mutations located near the three-dimensional boundary between C alpha 2 and C alpha 3 completely disrupt binding. Taken together, these results localize the mFc alpha R recognition site on IgA to the boundary region between the second and third constant domains--a site analogous to that recognized by Staphylococcus aureus protein A on IgG. The use of this hinge-distal site is, to date, unique among Fc receptors of the Ig superfamily.

Analysis of the specificity of bacterial immunoglobulin A (IgA) proteases by a comparative study of ape serum IgAs as substrates

Immunoglobulin A (IgA) proteases are bacterial enzymes with substrate specificity for human serum and secretory IgAs. To further define the basis of this specificity, we examined the ability of IgA proteases of Clostridium ramosum, Streptococcus pneumoniae (EC 3.4.24.13), Neisseria meningitidis (EC 3.4.21.72), and Haemophilus influenzae (EC 3.4.21.72) to cleave serum IgAs of gorillas, chimpanzees, and orangutans. All enzymes cleaved the IgAs of the three apes despite differences in ape IgA1 hinge sequence relative to the human prototype. To directly compare the ape and human hinge cleavage sites, the sites were identified in eight ape IgA digests. This analysis confirmed that ape proteins were all cleaved in the IgA hinge region, in all but one case after proline residues. The exception, C. ramosum protease, cleaved gorilla and chimpanzee IgAs at peptide bonds having no proline, but the scissile bonds were in the same hinge location as the Pro-221-Val-222 cleaved in human IgA1. These data indicate that proline is not an invariant substrate requirement for all IgA proteases and that the location of the scissile bond, in addition to its composition, is a critical determinant of cleavage specificity.