Substrate analogue inhibitors of the IgA1 proteinases from Neisseria gonorrhoeae

Substrate analogues based on the amino acid sequence of the hinge region of human IgA1 around the cleavage site of the IgA1 proteinases secreted by Neisseria gonorrhoeae are competitive inhibitors of these enzymes. The octapeptide Thr-Pro-Pro-Thr-Pro-Ser-Pro-Ser, which occurs between residues 233 and 240, has an IC50 value of 0.26 mM for the type 1 proteinase and 0.50 mM for the type 2 enzyme. Acetylation of the octapeptide N-terminal amino group lowers affinity for the type 1 proteinase sixfold but does not change binding to the type 2 enzyme. Amidation of the C-terminal carboxyl group does not change binding to the type 1 proteinase but improves IC50 for the type 2 enzyme. Simultaneous blockade of both the N- and C-termini drastically lowers affinity of the octapeptide for both proteinases. Sequential replacement of the hydroxy amino acids in the blocked octapeptide with cysteine yields a series of inhibitors that generally bind to the neisserial IgA1 proteinases as well as or better than the unblocked octapeptide. The most effective inhibitor contains a cysteine residue at position 6 (P3') and has an IC50 value for the type 2 IgA1 proteinase of 50 microM. Dimerization of the cysteine-containing octapeptides significantly diminishes inhibitory properties. The substrate analogues described here are the first synthetic inhibitors of the neisserial IgA1 proteinases to be reported.

Studies on the specificity of the IgA-binding lectin, jacalin

The interactions of IgA with the jackfruit lectin, jacalin, were investigated with regard to the specificity of jacalin for species and subclasses of IgA. It was found that jacalin selectively bound to human IgA1, but not to human IgA2, mouse IgA or rat IgA. Binding studies with human IgA1 fragments produced by different IgA1 proteases revealed that jacalin bound to galactose-terminal oligosaccharides in the hinge region of human IgA1. Affinity chromatography employing jacalin-Sepharose provided a means to separate the subclasses of IgA in human whey.

The effect of an IgA1 protease on immunoglobulins bound to the sperm surface and sperm cervical mucus penetrating ability

A major site of impaired fertility in men with autoimmunity to sperm rests at the level of restricted sperm entry and motion within cervical mucus. We studied the effects of a protease derived from Neisseria gonorrhoeae, whose substrate specificity is limited to human IgA1, on the ability of antibody-bound sperm to penetrate human cervical mucus in vitro. IgA on the sperm surface, but not IgG, was degraded by IgA1 protease. A correlation was seen between the levels of IgA bound relative to IgG and the improvement in sperm cervical mucus penetrating ability after IgA1 protease exposure. These results provide evidence that antisperm autoantibodies of both IgA and IgG classes impair the ability of spermatozoa to populate the female reproductive tract. They implicate the Fc region of the immunoglobulin molecule in mediating this effect and offer the potential to restore male fertility by treating antibody-bound sperm in vitro with immunoglobulin-directed bacterial proteases, before insemination.

IgA1 protease cleaves heavy chains independently in dimeric human IgA1

Bacterial IgA1 proteases have substrate specificity for human IgA1 immunoglobulin, and cleave both the heavy (alpha) chains where they are paired by disulfide bonds in the hinge region. To determine if the close apposition of the alpha chains allows a single enzyme-substrate-binding event to cleave both hinge region peptides we quantitated the relative levels of intermediate products during the course of complete hydrolysis of an IgA1 paraprotein. The substrate had four Fab regions, analogous to a secretory IgA dimer. The experimental data were then compared to computer-generated models in which various levels of cooperativity among Fab regions were tested. The results most closely conformed to a model in which each individual alpha chain is proteolyzed independently, without regard to the total number of hinge region peptides available in the substrate IgA1. These results will be used to guide the design of IgA1 hinge region peptide analogues as IgA1 protease inhibitors.

Physical and genetic analysis of DNA regions encoding the immunoglobulin A proteases of different specificities produced by Haemophilus influenzae

The structural gene for immunoglobulin A protease (iga) from Haemophilus influenzae serotype d was cloned in pBR322. The gene was used as a probe for Southern hybridization analysis of chromosomal DNA from the five other H. influenzae serotypes (a, b, c, e, and f). In most cases strains from a single serotype exhibited a distinct pattern of restriction fragment(s) homologous to the iga gene probe which was unique for that serotype. Serotype f strains were unique in that they gave two distinct patterns of homologous restriction fragments which correlated well with the production of two different protease types by members of this group. An iga mutant of H. influenzae serotype d was isolated by introducing a 4-base-pair insertion into the cloned iga gene and using the altered DNA for transformation of an H. influenzae recipient. The resulting iga- mutant produced no immunoglobulin A protease but was otherwise indistinguishable from its iga+ parent in growth characteristics. Transformation of mutant cells with chromosomal DNA isolated from either a serotype d or a serotype c strain gave rise to iga+ transformants. Those obtained with serotype d DNA produced a type 1 protease, whereas those obtained with serotype c DNA produced either a type 1 protease (characteristic of serotype d) or a type 2 protease (characteristic of serotype c). Southern analysis of the latter transformants, using the iga gene probe, indicated that the type 1 transformants had a serotype d pattern of restriction fragments whereas the type 2 transformants had either a serotype c or a novel pattern of restriction fragments. These results indicate that there is considerable homology between the iga genes of the various serotypes and that the homologous sequences identified with the serotype d probe are the immunoglobulin A protease-coding sequences in each case.

Examination of Haemophilus pleuropneumoniae for immunoglobulin A protease activity

Haemophilus pleuropneumoniae, the etiological agent of porcine contagious pneumonia, was examined for the ability to produce an immunoglobulin A (IgA) protease specific for porcine IgA. No IgA protease activity against either porcine or human IgA was detected. Furthermore, no sequence homology was found between H. pleuropneumoniae chromosomal DNA and the gene which specifies IgA protease in Haemophilus influenzae.

Inhibition of microbial IgA proteases by human secretory IgA and serum

Microbial IgA proteases cleave human serum IgA1 immunoglobulin, but human secretory IgA is resistant to hydrolysis. We have found this resistance to be due to an inhibition of protease activity that is mediated by the Fab region of secretory IgA. The IgA proteases of the genus Neisseria are more sensitive to inhibition than is the protease of Streptococcus sanguis. There is also a serum inhibitor of Neisseria proteases that co-chromatographs with IgG. Monoclonal (myeloma) human IgG proteins and plasma protease inhibitors such as alpha-1-antitrypsin and alpha-2-macroglobulin do not inhibit. Human sera do not contain inhibitor to S. sanguis protease activity. We conclude that microbial IgA proteases are subject to inhibition by IgA in secretions and IgG in serum, and this activity is most consistent with being an anti-enzyme antibody. The insensitivity of S. sanguis IgA protease to inhibition is unexplained but provides further evidence that the IgA proteases are structurally diverse.

IgA1 proteases of Haemophilus influenzae: cloning and characterization in Escherichia coli K-12

Haemophilus influenzae is one of several bacterial pathogens known to release IgA1 proteases into the extracellular environment. Each H. influenzae isolate produces one of at least three distinct types of these enzymes that differ in the specific peptide bond they cleave in the hinge region of human IgA1. We have isolated the gene specifying type 1 IgA1 protease from a total genomic library of H. influenzae, subcloned it into plasmid vectors, and introduced these vectors into Escherichia coli K-12. The enzyme synthesized by E. coli was active and had the same specificity as that of the H. influenzae donor. Unlike that of the donor, E. coli protease activity accumulated in the periplasm rather than being transported extracellularly. The position of the protease gene in H. influenzae DNA and its direction of transcription was approximated by deletion mapping. Tn5 insertions, and examination of the polypeptides synthesized by minicells. A 1-kilobase probe excised from the IgA1 protease gene hybridized with DNA restriction fragments of all H. influenzae serogroups but not with DNA of a nonpathogenic H. parainfluenzae species known to be IgA1 protease negative.

Relationship between the specificity of IgA proteases and serotypes in Haemophilus influenzae

Haemophilus influenzae is one of five bacterial species known to produce IgA proteases, enzymes that specifically cleave the human IgA1 heavy chain. Strains of H. influenzae produce three distinct types of IgA proteases that cleave different peptide bonds within the IgA1 hinge region. Type 1 protease cleaves the prolyl-seryl bond at position 231-232; type 2 protease cleaves the prolyl-threonyl bond at position 235-236, the same bond attacked by Neisseria gonorrhoeae and Neisseria meningitidis type 2 proteases. Type 3 protease yields a unique double Fd cleavage pattern; the exact peptide bonds cleaved have not been determined. The type of protease produced correlates with the serotype, but not with the biotype, of the isolate; serotypes A, B, D, and F produce primarily type 1 protease, whereas serotypes C and E produce only type 2 enzyme. Each nontypable strain yields one of the three protease types. These data further extend our knowledge of the extreme specificity of the IgA proteases and suggest that IgA protease type may be useful in the taxonomy and epidemiology of H. influenzae.

J chain is covalently bound to both monomer subunits in human secretory IgA

Previous work has established that the secretory component (SC) in human secretory IgA is covalently linked to only one of the two IgA monomer subunits, but it has not been clear whether the J chain is covalently linked to one or to both of these subunits. In view of the asymmetry in the disulfide bonding between SC and the IgA subunits, an arrangement which follows disulfide interchange, several models for the disulfide linkage of J chain and the bonds between IgA subunits were envisaged and investigated. When sIgA was gel filtered through Sephadex G-200 in acetic acid, a single major symmetrical peak eluted at the front. This material contained SC, alpha and L chains, and all of the J chain. The greater resolution afforded by polyacrylamide gel electrophoresis in detergent confirmed that human sIgA contains no major noncovalently linked components in the 150,000-200,000 molecular weight range. In another series of experiments the Fc monomer, which is not covalently attached to SC, isolated after treatment of sIgA with IgA protease and cyanogen bromide, was investigated to learn whether alpha chain COOH-terminal octapeptides could be released by reduction. The results were negative. The available data thus favor a model in which J chain is disulfide-bonded to both IgA monomer subunits in sIgA.

Secretory immunity and the bacterial IgA proteases

The characteristics and functions of microbial IgA proteases are reviewed. These enzymes represent a structurally heterogeneous group of proteins that are secreted into the extracellular environment by bacteria capable of causing human disease. The IgA proteases, which vary in their requirements for metal ions, are neutral endopeptidases whose role in the infectious process is not known but whose pronounced substrate specificity for human proteins of the IgA1 subclass has repeatedly been demonstrated. As reagents, the IgA proteases are useful in cleaving IgA molecules to yield intact Fc alpha and Fab alpha fragments that will allow the study of the structure and function of the two large regions of IgA immunoglobulin proteins. The role, if any, of these enzymes in promoting infection by pathogenic members of the genera Neisseria, Hemophilus, and Streptococcus is not known, although the secretory immune system is primarily mediated by antibodies of the IgA isotype, among which are IgA1 subclass proteins, and these proteins are susceptible to cleavage by IgA protease. The determination of the role of these enzymes in the pathogenesis of human infection must await clearer understanding of antigenicity and antibody function at secretory sites and of the relative roles of the two subclasses of human IgA in immune defense.

IgA proteases of two distinct specificities are released by Neisseria meningitidis

Strains of Neisseria meningitidis produce two distinct extracellular IgA proteases that cleave the human IgA1 heavy chain at different points within the hinge region. Type 1 protease cleaves the prolyl-seryl peptide bond at position 237-238; type type 2 protease cleaves the prolyl-threonyl bond two residues amino terminal to that bond attacked by type 1 enzyme. Each meningococcal isolate elaborates only one of these two enzymes, and the type of protease produced correlates with certain serogroups: group A yielding only type 1, and groups X and Y only type 2 enzyme. In addition, analysis of amino acid sequences of human alpha-chain proteins reveals that the repeating octapeptide characteristic of the IgA1 hinge region is actually triplicated.

Specific proteolysis of human IgA by Streptococcus pneumoniae and Haemophilus influenzae

Streptococcus pneumoniae and Haemophilus influenzae are among the most common bacterial pathogens responsible for respiratory tract infections in otherwise healthy humans. Thirty-six strains of S. pneumoniae, 62 strains of H. influenzae, six hospital-acquired respiratory pathogens, and a strain of Streptococcus pyogenes were examined for production of IgA protease, a bacterial enzyme whose only known substrate is human IgA1. IgA protease was produced by 100% of the isolates of S. pneumoniae and 98% of the isolates of H. influenzae. The enzyme from both species cleaved human serum and secretory IgA1 proteins, but not human IgA2, IgG, or human serum albumin. None of the hospital-acquired pathogens had detectable IgA protease activity, a finding indicating that the production of this enzyme distinguishes S. pneumoniae and H. influenzae from the opporunistic respiratory pathogens.

IgA protease production as a characteristic distinguishing pathogenic from harmless neisseriaceae

IgA proteases are extracellular enzymes of bacteria that have human immunoglobulin A of the IgA1 subclass as their only known substrate. The identification of this enzyme in neisseria prompted us to determine whether IgA protease production correlates with pathogenicity within this genus. Multiple clinical isolates of Neisseria gonorrhoeae, N. meningitidis and eight species of non-pathogenic neisseria that commonly colonize the normal human nasopharynx were examined for IgA protease activity. All N. gonorrhoeae and N. meningitidis strains were enzyme positive; all non-pathogenic strains were negative. Among meningococci, the enzyme occurred in strains carried harmlessly in the nasopharynx as well as those isolated from systemic infections. Because mucosal immune defense is largely mediated by antibodies of the IgA isotype, the finding that IgA protease activity is linked specifically to the pathogenic neisseria suggests that the enzyme may be involved in the pathogenesis of neisserial infection.

Hepatic failure and death from erythropoietic protoporphyria

A 60-year-old white male presenting with a clinical picture of obstructive jaundice was subsequently found to have erythropoietic protoporphyria. The diagnosis was suspected because of a history of life-long photosensitivity and was confirmed by finding high levels of erythrocyte protoporphyrin. Liver biopsy revealed birefringent deposits of protoporphyrin by polarization microscopy accompanied by severe hepatic injury and fibrosis. The patient died rapidly from liver failure, and at autopsy the biliary tree was patent. Despite the autosomal dominant transmission of erythropoietic protoporphyria, we failed to detect any family members with the disease. This report is concluded with a brief discussion of the liver involvement in erythropoietic protoporphyria.

Assay and properties of IgA protease of Streptococcus sanguis

An assay procedure for streptococcal IgA protease is described which uses isotopically labelled human serum IgA as substrate. Enzyme activity was monitored by the radioactive counts in the Fab alpha product, which was separated from other components in the digestion mixture by electrophoresis. Cleavage of IgA was linear with respect to time using catalytic amounts of the enzyme. Km was calculated to be 5.5 X 10(-6)M, pH optimum 6.0-7.0 at 37 degrees C, and the enzyme was fully inactivated at low concentrations of the metal chelator ethylenediaminetetraacetic acid.

Loss of antibody activity in human immunoglobulin A exposed extracellular immunoglobulin A proteases of Neisseria gonorrhoeae and Streptococcus sanguis

Immunoglobulin A (IgA) proteases are extracellular enzymes elaborated by Neisseria gonorrhoeae, N. meningitidis, and Streptococcus sanguis. These enzymes each cleave human IgA1 at a critically situated prolyl-threonyl peptide bond to yield Fab alpha and Fc alpha fragments. To study their effect on the antibody activity of human IgA, we enzymatically digested a group of five human IgA monoclonal immunoglobulins with high-titer rheumatoid factor or cold agglutinin activity and human serum macroamylase, an amylase-IgA complex. In contrast to four control IgM rheumatoid factor monoclonal proteins, whose activity was unaffected by enzyme, gonococcal and streptococcal IgA proteases caused prompt, major reductions of IgA antibody activity to negligible levels and converted macroamylase activity to amylase of normal size, as determined by molecular sieve chromatography. In addition, both enzymes promptly deagglutinated sensitized cells that had been aggregated by IgA rheumatoid factors, indicating that IgA bound to antigen is also susceptible to enzyme cleavage. Fab fragments of Iga protein Chr, a rheumatoid factor, showed essentially no antigen-binding activity despite the high titers observed with the parent protein. These studies emphasize the high degree of specificity of the microbial proteases for IgA and their potential for interfering with antibody activity in the IgA1 subclass.