Human milk lactoferrin is a serine protease that cleaves Haemophilus surface proteins at arginine-rich sites

Lactoferrin is a member of the lactotransferrin family of non-haem, iron-binding glycoproteins and is found at high concentrations in all human secretions, where it plays a major role in mucosal defence. In recent work, we observed that lactoferrin has proteolytic activity and attenuates the pathogenic potential of Haemophilus influenzae by cleaving and removing two putative colonization factors, namely the IgA1 protease protein and the Hap adhesin. Experiments with protease inhibitors further suggested that lactoferrin may belong to a serine protease family. In the present study we explored the mechanism of lactoferrin protease activity and discovered that mutation of either Ser259 or Lys73 results in a dramatic decrease in proteolysis. Examination of the crystal structure revealed that these two residues are located in the N-terminal lobe of the protein, adjacent to a 12-15 A cleft that separates the N-lobe and the C-lobe and that can readily accommodate large polypeptide substrates. In additional work, we found that lactoferrin cleaves IgA1 protease at an arginine-rich region defined by amino acids 1379-1386 (RRSRRSVR) and digests Hap at an arginine-rich sequence between amino acids 1016 and 1023 (VRSRRAAR). Based on our results, we conclude that lactoferrin is a serine protease capable of cleaving arginine-rich sequences. We speculate that Ser259 and Lys73 form a catalytic dyad, reminiscent of a number of bacterial serine proteases. In addition, we speculate that lactoferrin may cleave arginine-rich sequences in a variety of microbial virulence proteins, contributing to its long-recognized antimicrobial properties.

Human lactoferrin proteolytic activity: analysis of the cleaved region in the IgA protease of Haemophilus influenzae

Human lactoferrin proteolytically cleaves and inactivates two colonization factors of non-typable Haemophilus influenzae, the IgA protease precursor protein (Iga), and Hap, the non-pilus adhesin by which microoganisms adhere to host epithelial cells and form microcolonies. Iga and Hap are homologous proteins that are members of the autotransporter family of secreted proteins expressed by gram-negative bacteria. Studies of Iga cleaved by lactoferrin, reported here, show that proteolysis occurred within the helper region of Iga (Iga(beta)) domain which anchors the autotransporter within the Haemophilus outer membrane. The amino-terminus of the extracted Iga protein was not modified. The location of the proteolytic active site in human lactoferrin is under study. Lactoferrin proteolysis may attenuate pathogenicity of H. influenzae, an important cause of otitis media.

Clinical pathology of foodborne diseases: notes on the patient with foodborne gastrointestinal illness

The symptoms and signs in persons with food- or waterborne infections provide clues to the nature of the infecting microbe. Proper treatment of the affected individual, and protection of those exposed to the same source, is dependent on time-honored methods of diagnosis: exposure history, and physical examination. Laboratory testing may help to identify the responsible agent. Spontaneous recovery is the most likely outcome once supportive measures such as fluid and electrolyte replacement are addressed. Antibiotics are often unnecessary and may prolong fecal excretion of certain microorganisms. In immunosuppressed persons or those weakened by marginal nutrition, foodborne infection can be more severe, mandating more specific therapy. Management requires knowing the level of tissue invasion and organ infected by each of the commonly encountered microbes. Some of the most life-threatening infections (cholera, for example) are associated with no visible tissue injury, yet they have a profound impact on gut function. In contrast, salmonellosis and shigellosis can cause severe gut injury, and when foodborne infections extend beyond the confines of the gut, skilled care is essential. Examples are hemolytic uremic syndrome of Escherichia coli infections, or listeriosis, both of which require urgent attention. Long-term consequences of gut infections such as the paralytic Guillain-Barre syndrome following Campylobacter infections illustrates the long-term problems sometimes encountered. Because it is unlikely that all infectious agents will ever be removed from food and water in any country, sound medical intervention tailored to the extent of illness will be the mainstay of handling such illnesses.

Role of Hpn and NixA of Helicobacter pylori in susceptibility and resistance to bismuth and other metal ions

BACKGROUND

Helicobacter pylori produces Hpn, a 60-amino acid, histidine-rich protein that avidly binds nickel and zinc ions, and NixA, a high-affinity nickel transporter in the cytoplasmic membrane. We tested the hypothesis that Hpn and NixA govern susceptibility to metal ions in H. pylori.

MATERIALS AND METHODS

Hpn-negative mutants of four H. pylori strains were constructed by standard allelic exchange techniques to yield isogenic Hpn+/Hpn-deficient pairs. A metal concentration that inhibited growth by 50% (IC50) was calculated for Ni2+, Zn2+, Cu2+, and Co2+ by comparing OD600 of cultures in metal-supplemented and control media.

RESULTS

Among all four pairs of isogenic strains, the tolerance for Ni2+ was reduced significantly (p <.001) in the Hpn mutants; the mean IC50 value for wild-type strains was 1.9 mM; for the mutant, it was 0.8 mM. In contrast, growth inhibition by Zn2+ was identical within the fours pairs, as was Cu2+ and Co2+ tolerance in one pair tested. We also found that deletion of the hpn gene increases susceptibility to therapeutic forms of bismuth by testing a mutant and wild-type pair with ranitidine bismuth citrate, bismuth citrate, and four antibiotics. Minimal inhibitory concentrations of ranitidine bismuth citrate dropped from 9.2 to 2.3 microg/ml, and those of bismuth citrate dropped from 7.4 to 3.2 microg/ml (p <.05 for both comparisons), while susceptibility to the antibiotics was unaffected. Disruption of the nixA gene encoding the specific Ni2+ transport protein of H. pylori did not change susceptibility to bismuth.

CONCLUSION

We concluded that bacteria lacking Hpn, cultured in vitro, are more susceptible than is the wild type to bismuth and Ni2+.

Human milk lactoferrin inactivates two putative colonization factors expressed by Haemophilus influenzae

Haemophilus influenzae is a major cause of otitis media and other respiratory tract disease in children. The pathogenesis of disease begins with colonization of the upper respiratory mucosa, a process that involves evasion of local immune mechanisms and adherence to epithelial cells. Several studies have demonstrated that human milk is protective against H. influenzae colonization and disease. In the present study, we examined the effect of human milk on the H. influenzae IgA1 protease and Hap adhesin, two autotransported proteins that are presumed to facilitate colonization. Our results demonstrated that human milk lactoferrin efficiently extracted the IgA1 protease preprotein from the bacterial outer membrane. In addition, lactoferrin specifically degraded the Hap adhesin and abolished Hap-mediated adherence. Extraction of IgA1 protease and degradation of Hap were localized to the N-lobe of the bilobed lactoferrin molecule and were inhibited by serine protease inhibitors, suggesting that the lactoferrin N-lobe may contain serine protease activity. Additional experiments revealed no effect of lactoferrin on the H. influenzae P2, P5, and P6 outer-membrane proteins, which are distinguished from IgA1 protease and Hap by the lack of an N-terminal passenger domain or an extracellular linker region. These results suggest that human milk lactoferrin may attenuate the pathogenic potential of H. influenzae by selectively inactivating IgA1 protease and Hap, thereby interfering with colonization. Future studies should examine the therapeutic potential of lactoferrin, perhaps as a supplement in infant formulas.

Identification, cloning, and sequencing of the immunoglobulin A1 protease gene of Streptococcus pneumoniae

The pneumococcus expresses a protease that hydrolyzes human immunoglobulin A1 (IgA1). A gene for IgA1 protease was identified from a plasmid library of pneumococcal DNA because of the effect of its overexpression on the colony morphology of Streptococcus pneumoniae. The deduced 1,964-amino-acid sequence is highly homologous to that of the IgA1 protease from Streptococcus sanguis. The similarity to the S. sanguis enzyme and the presence of a putative zinc-binding site suggest that the pneumococcal enzyme is a metalloprotease. The two streptococcal sequences differ in a hydrophilic region with 10 tandem repeats of a 20-mer in S. sanguis, which is replaced by a similar but less repetitive sequence in S. pneumoniae. Antiserum reactive with the pneumococcal IgA1 protease was used to demonstrate that the majority of the protein is cell associated. The expression and function of this gene were confirmed by insertional mutagenesis. Interruption of the chromosomal gene resulted in loss of expression of an approximately 200-kDa protein and complete elimination of detectable IgA1 protease activity.

Hydrogen uptake hydrogenase in Helicobacter pylori

The peptic ulcer-causing bacterium Helicobacter pylori was found to contain an H2-uptake hydrogenase activity coupled to whole cell (aerobic) respiration. The activity was localized to membranes which functioned in the H2-oxidizing direction with a variety of artificial and physiological electron acceptors of positive redox potential. Immunoblotting of H. pylori membrane components with anti (B. japonicum) hydrogenase large and small subunit-specific antisera identified H. pylori hydrogenase peptides of approximately 65 and 26 kDa respectively, and H. pylori genomic DNA fragments hybridizing to the (B. japonicum) hydrogenase structural genes were identified. The membrane-bound activity was subject to anaerobic activation, like many NiFe hydrogenases. Difference absorption spectral studies revealed absorption peaks characteristic of b and c-type cytochromes, as well as of a bd-type terminal oxidase in the H. pylori H2-oxidizing membrane-associated respiratory chain.

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.

Unique susceptibility of Helicobacter pylori to simethicone emulsifiers in alimentary therapeutic agents

Helicobacter pylori is killed in vitro by polyoxyethylene acyl esters and ethers similar to simethicone emulsifiers in therapeutic antifoams. The MBC of these compounds for Helicobacter pylori was less than 20 micrograms/ml, while other gram-negative bacteria were unaffected by much higher concentrations of up to 50 mg/ml.

Lack of interference between IgA-binding proteins and IgA proteases of human pathogenic bacteria

Some human bacterial pathogens produce specific immunoglobulin A1 (IgA1) proteases that cleave the heavy chain of human IgA1, generating intact Fab and Fc fragments. Other pathogenic bacterial species express surface proteins which bind to the Fc part of human IgA in a non-immune manner. To analyse whether IgA-binding proteins affect the activity of IgA1 proteases, the ability of seven different IgA1 proteases to hydrolyse IgA1 in the presence of either of two different bacterial IgA-binding proteins was tested. Data obtained in two different types of experiment suggest that IgA1 bound to IgA-binding proteins still functions as a substrate for IgA1 proteases. As Fc fragments produced by cleaving IgA1 with IgA1 proteases still bind to IgA-binding proteins, we conclude that these two types of bacterial protein act independently of each other.

Protein Hpn: cloning and characterization of a histidine-rich metal-binding polypeptide in Helicobacter pylori and Helicobacter mustelae

Helicobacter pylori is a human gastrointestinal pathogen involved in gastritis, duodenal ulcers, and gastric neoplasia. This microorganism produces large amounts of a urease which, like all known ureases, has nickel in the active site. We have identified a protein in clinical isolates of H. pylori and an identical protein in the ferret pathogen Helicobacter mustelae that strongly binds Ni2+ and Zn2+. This protein has been named Hpn to emphasize its origins in H. pylori and its affinity for nickel. The encoding hpn gene, cloned and expressed in Escherichia coli ER1793, has an open reading frame (180 bp) that specifies a protein with a calculated molecular mass of 7,077 Da and with the same amino-terminal sequence as that of wild-type Hpn. The deduced sequence of Hpn consists of 60 amino acids, of which 28 (47%) are histidines. The hpn gene does not map with the urease gene cluster on the H. pylori chromosome. An Hpn-negative, isogenic H. pylori strain, generated by hpn gene deletion and grown on blood agar, had the same urease activity that wild-type cells did. Thus, the role of Hpn in helicobacters is unknown.

Post-infectious human serum antibodies inhibit IgA1 proteinases by interaction with the cleavage site specificity determinant

Bacterial pathogens of the genera Neisseria and Haemophilus secrete IgA1 proteinases which cleave human IgA1 in the heavy chain hinge region. The exact peptide bond cleaved is strain-dependent, but remains invariant despite repeated subculture. Haemophilus influenzae and Neisseria meningitidis produce proteinases of two cleavage site specificities (type 1 and type 2). We examined serial acute and convalescent sera from patients recovering from meningitis due to N. meningitidis or H. influenzae, and found a significant rise in serum titer of inhibitory antibodies against these enzymes. In each case the proteinase from the infecting organism was more susceptible to inhibition than were proteinases from that genus that had different cleavage specificity. Inhibition of sixteen type 1-type 2 hybrid H. influenzae IgA1 proteinases revealed complete concordance between inhibitory titer and cleavage site specificity. Inhibition of hybrid proteinases differing in a 123 amino acid segment known to determine cleavage site specificity (termed the CSD) further localized the site of antibody action to this site. These results from a limited number of patients with natural infections suggest that inhibiting antibody recognizes epitopes within the CSD. Alternatively, antibody may bind to epitopes outside the CSD and inhibit via steric hindrance.

Growth of Haemophilus influenzae in human milk: synthesis, distribution, and activity of IgA protease as determined by study of iga+ and mutant iga- cells

The nonencapsulated, IgA protease-positive Haemophilus influenzae strain Rd and serogroup b clinical isolates were found to proliferate in human milk. Growth did not require supplemental X and V factors. In milk, strain Rd synthesized IgA protease, but it was completely inhibited by antibody, so secretory IgA in milk cultures remained intact. Inhibition was largely attributable to IgA1 antibodies. Rd cells also aggregated during growth in milk and showed colony size variation, whereas a protease-negative mutant of Rd (Rd225DK) aggregated less and had uniform colony size. Like differences in protease inhibition, these differences in growth pattern were mediated by secretory IgA1. Thus, milk antibody not only inhibited the extracellular protease but also interacted directly with the enzyme precursor or related antigens on growing bacterial cells. This self-protective property of milk secretory IgA may be an important immunologic attribute for the upper respiratory mucosa of the infant.

Endoglucanase A from Cellulomonas fimi in which the hinge sequence of human IgA1 is substituted for the linker connecting its two domains is hydrolyzed by IgA proteases from Neisseria gonorrhoeae

The hinge in IgA1 and the linker in endoglucanase A (CenA) are quite similar. The IgA1 hinge is 18 amino acids long and contains only proline, threonine and serine. The linker in CenA is 27 amino acids long and contains only proline, threonine and a single serine. IgA proteases from Neisseria gonorrhoeae cleave Pro-Ser and Pro-Thr bonds within the IgA1 hinge sequence, but they do not attack CenA. When the linker sequence of CenA is replaced with the hinge sequence of IgA1, the hybrid polypeptide is susceptible to the N. gonorrhoeae proteases. It is cleaved within the hinge sequence at the same sites as IgA1.

Inhibition of dipeptidyl aminopeptidase IV (DP-IV) by Xaa-boroPro dipeptides and use of these inhibitors to examine the role of DP-IV in T-cell function

Dipeptidyl peptidase IV (DP-IV; dipeptidyl-peptide hydrolase, EC 3.4.14.5) is a serine protease with a specificity for cleaving Xaa-Pro dipeptides from polypeptides and proteins. It is found in a variety of mammalian cells and tissues, including those of lymphoid origin where it is found specifically on the surface of CD4+ T cells. Although the functional significance of this enzyme has not been established, a role in T-cell activation and immune regulation has been proposed. Here we report that Ala-boroPro and Pro-boroPro, where boroPro is the alpha-amino boronic acid analog of proline, are potent and specific inhibitors of DP-IV, having Ki values in the nanomolar range. Blocking the N terminus of Ala-boroPro abolishes the affinity of this inhibitor for DP-IV, while removal of the N-terminal residue, to give boroPro, reduces the affinity for DP-IV by 5 orders of magnitude. The dipeptide boronic acids exhibit slow-binding kinetics, while boroPro does not. We also report here that low concentrations of Pro-boroPro inhibit antigen-induced proliferation and interleukin 2 production in murine T-cell lines but do not inhibit the response of these T cells to the mitogen concanavalin A. These results indicate that DP-IV plays a role in antigen-induced, but not mitogen-induced, activation of T lymphocytes.

Inhibition of IgA1 proteinases from Neisseria gonorrhoeae and Hemophilus influenzae by peptide prolyl boronic acids

The alpha-aminoboronic acid analog of proline has been synthesized and incorporated into a number of peptides as the COOH-terminal residue. These peptide prolyl boronic acids are potent inhibitors of both the type 1 and type 2 IgA proteinases from Neisseria gonorrhoeae and Hemophilus influenzae, but not of the functionally similar IgA proteinase from Streptococcus sanguis. The best inhibitors synthesized thus far have Ki values in the nanomolar range (4.0 to 60 nM). These results indicate that the N. gonorrhoeae and the H. influenzae enzymes belong to the serine protease family of proteolytic enzymes while that from S. sanguis does not. As a group, the IgA proteinases have been noted for their remarkable specificity; thus, the peptide prolyl boronic acids reported here are the first small synthetic molecules to exhibit a relatively high affinity for the active site of an IgA proteinase and are therefore the first to yield some insight into the active site structure and specificity requirements of these enzymes.

Localization of the cleavage site specificity determinant of Haemophilus influenzae immunoglobulin A1 protease genes

Immunoglobulin A1 (IgA1) proteases are produced by a number of different species of bacteria which cause infection at human mucosal surfaces. The sole substrate of these proteases is human IgA1. Cleavage is within the hinge region of IgA1, although there is variability in the exact peptide bond within the hinge region that is cut by a particular protease. The cleavage site of the Haemophilus influenzae type 1 protease is located four amino acids from the cleavage site of the type 2 enzyme. In this study, the region of the H. influenzae IgA1 protease gene (iga) that determines the cleavage site specificity was localized through the comparison of the type 1 and type 2 genes and the construction and analysis of type 1-type 2 hybrid genes. The hybrid genes were generated by in vivo and in vitro techniques which facilitated the selection and screening of randomly generated hybrids. The cleavage site determinant was found to be within a 370-base-pair region near the amino-terminal coding region, in one of two large areas of nonhomology between the two types of H. influenzae iga genes. DNA sequence analysis of the cleavage site determinant and surrounding regions did not reveal a simple mechanism whereby one enzyme type could be converted to the other type. Comparison of the type 2 gonococcal IgA1 protease gene to the two Haemophilus genes revealed a significant amount of homology around the cleavage site determinant, with the two type 2 genes showing greater homology.

Tetrapeptide inhibitors of the IgA1 proteinases from type I Neisseria gonorrhoeae

The six series of unique tetrapeptides and their blocked N-acetyl, C-amide, and N-acetyl-C-amide analogues which comprise the hinge region of human IgA1 (Ser224 to Ser240) have been synthesized and tested as inhibitors of the type 1 IgA1 proteinase elaborated by Neisseria gonorrhoeae (EC 3.4.24.13). Most series had at least one member with an IC50 value less than 1 mM. The most effective inhibitors came from the series Ser-Thr-Pro-Pro (P4-P1) and Pro-Thr-Pro-Ser (P1-P3'). One member from each series had an IC50 value in the low microM range. Magnetic resonance studies (Siemion, I. Z.; et al. Biophys. Chem. 1988, 31, 35) indicate that the various tetrapeptide series appear to have different preferred solution conformations. However, these do not appear to be correlated with affinity for the neisserial proteinase. The most effective inhibitors tend to have a threonine residue adjacent to the N-terminus and the P1 or P1' residues at either the N- or the C-terminus. These relationships are not exclusive however, as other inhibitors, which do not meet these criteria, bind reasonably well. The most effective substrate analogues outlined here are about one-half the size and bind to the neisserial proteinase 2 orders of magnitude more tightly than previously reported inhibitors.

Cloning of the gene encoding streptococcal immunoglobulin A protease and its expression in Escherichia coli

We have identified and cloned a 6-kilobase-pair segment of chromosomal DNA from Streptococcus sanguis ATCC 10556 that encodes immunoglobulin A (IgA) protease activity when cloned into Escherichia coli. The enzyme specified by the iga gene in plasmid pJG1 accumulates in the periplasm of E. coli MM294 cells and has a substrate specificity for human IgA1 identical to that of native S. sanguis protease. Hybridization experiments with probes from within the encoding DNA showed no detectable homology at the nucleotide sequence level with chromosomal DNA of gram-negative bacteria that excrete IgA protease. Moreover, the S. sanguis iga gene probes showed no detectable hybridization with chromosomal DNA of S. pneumoniae, although the IgA proteases of these two streptococcal species cleaved the identical peptide bond in the human IgA1 heavy-chain hinge region.

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.