A sensitive enzyme-linked immunosorbent assay for IgA protease activity

Development of a high-throughput screen and its use in the discovery of Streptococcus pneumoniae immunoglobulin A1 protease inhibitors

Streptococcus pneumoniae relies on a number of virulence factors, including immunoglobulin A1 protease (IgA1P), a Zn(2+) metalloprotease produced on the extracellular surface of the bacteria, to promote pathogenic colonization. IgA1P exhibits a unique function, in that it catalyzes the proteolysis of human IgA1 at its hinge region to leave the bacterial cell surface masked by IgA1 Fab, enabling the bacteria to evade the host's immune system and adhere to host epithelial cells to promote colonization. Thus, S. pneumoniae IgA1P has emerged as a promising antibacterial target; however, the lack of an appropriate screening assay has limited the investigation of this metalloprotease virulence factor. Relying on electrostatics-mediated AuNP aggregation, we have designed a promising high-throughput colorimetric assay for IgA1P. By using this assay, we have uncovered inhibitors of the enzyme that should be useful in deciphering its role in pneumococcal colonization and virulence.

Relaxed cleavage specificity of an immunoglobulin A1 protease from Neisseria meningitidis

Respiratory pathogens, such as Neisseria meningitidis, secrete site-specific proteases able to cleave human immunoglobulin A1 (IgA1), the first line of defense at mucosal membranes. Bacterial isolates show wide variability in IgA1 protease activity, and those isolated from patients with clinical infection possess the highest levels of activity. A feature of this enzyme is the self-cleavage required for secretion of the mature extracellular form. Known cleavage targets contain a proline-rich consensus recognition sequence, Pro-Pro-Ser-Pro, residing in the variable linker region that connects the protease and translocator domains. Here, we report the sequence of the NMB IgA1 protease and the unexpected self-cleavage and subsequent extracellular release of mature IgA1 protease from mutants lacking the previously defined consensus cleavage site. We investigated the possible link between enzyme secretion and variability in the linker sequence segment using site-directed mutagenesis and linker domain swapping to construct mutated and chimeric forms of the IgA1 protease from N. meningitidis strain NMB. The observed change in secreted activity levels compared to the wild-type clone indicated that the precise amino acid sequence of the intervening region, between mature IgA1 protease and the beta-core translocator domain, influences the efficacy of autoproteolytic processing. The broader specificity uncovered for the NMB IgA1 protease suggests that it could cleave a far wider range of human proteins than previously appreciated.

Physiological and serological variation in Streptococcus mitis biovar 1 from the human oral cavity during the first year of life

OBJECTIVE

The purpose of the study was to explore the physiological and antigenic diversity of a large number of Streptococcus mitis biovar 1 isolates in order to begin to determine whether these properties contribute to species persistence.

DESIGN

S. mitis biovar 1 was collected from four infants from birth to the first year of age. At each of eight to nine visits, 60 isolates each were obtained from the cheeks, tongue and incisors (once erupted) yielding 4440 in total. These were tested for production of neuraminidase, beta1-N-acetylglucosaminidase, beta1-N-acetylgalactosaminidase, IgA1 protease and amylase-binding. Antigenic diversity was examined by ELISA and Western immunoblotting using antisera raised against S. mitis biovar 1 NCTC 12261(T) and SK145.

RESULTS

Three thousand three hundred and thirty (75%) of the isolates were identified as S. mitis biovar 1 and 3144 (94.4%) could be divided into four large phenotypic groups based on glycosidase production. Fifty-four percent of the isolates produced IgA1 protease, but production was disproportionate among the phenotypes. Between one-third and one-half of the strains of each phenotype bound salivary alpha-amylase. Antisera against strains NCTC 12261(T) and SK145 displayed different patterns of reactivity with randomly selected representatives of the four phenotypes.

CONCLUSIONS

S. mitis biovar 1 is physiologically and antigenically diverse, properties which could aid strains in avoiding host immunity and promote re-colonization of a habitat or transfer to a new habitat.

Invasive isolates of Neisseria meningitidis possess enhanced immunoglobulin A1 protease activity compared to colonizing strains

Neisseria meningitidis, Haemophilus influenzae, and Streptococcus pneumoniae possess the ability to cleave human IgA1 antibodies, and all successfully colonize and occasionally invade the human upper respiratory tract. N. meningitidis invades the bloodstream after a period of nasopharyngeal colonization. We directly compared levels of IgA1 protease activity in strains (n=52) derived from the cerebrospinal fluid or blood of patients with meningococcal disease with strains of N. meningitidis obtained from asymptomatic carriers (n=25). IgA1 protease activity was determined by a sensitive semiquantitative ELISA assay. Levels of IgA1 protease activity were significantly higher (P<0.0001) in strains associated with invasive meningococcal disease (98% with detectable activity, mean = 580 mU) than with those obtained from asymptomatic carriers (76% with detectable activity, mean = 280 mU). Despite marked variation in enzyme activity, almost all strains (96%) possessed the gene for IgA1 protease. Given the panmictic population structure of the bacterial isolates investigated, these data, obtained from two groups infected with N. meningitidis, but with markedly different clinical outcomes, provide the first quantitative evidence that IgA1 protease activity is a virulence determinant that contributes to the pathogenic phenotype, and suggest IgA1 protease as a potential target for prophylaxis.

A method for titration of inhibiting antibodies to bacterial immunoglobulin A1 proteases in human serum and secretions

Bacterial IgA1 proteases specifically cleave IgA1, including S-IgA1, molecules into Fab alpha and Fc alpha fragments. Hereby these enzymes interfere with the protective functions of antibodies belonging to this isotype. Antibodies inhibiting IgA1 proteases have been detected in humans, but the titration of such antibodies is a matter of methodological concern. Because human serum and secretions contain IgA1 substrate, it is impossible to provide uniform substrate conditions for samples of IgA1 protease incubated with inhibitors differing in their origin and state of dilution. This study demonstrates that such variations in substrate are not prohibitive for a reliable titration of inhibiting antibodies. This was evident from experiments demonstrating that the variations do not interfere with the quantification of residual IgA1 protease activity provided the activity is measured in terms of the proportion of IgA1 substrate cleaved during incubation. Proportions of cleaved IgA1 were measured by exploiting the differential reactivity of cleaved and intact IgA1 molecules in an ELISA using anti-Fc alpha and enzyme-conjugated anti-light chain antibodies for catching and development, respectively. A protocol for the titration of IgA1 protease-inhibiting antibodies based on this ELISA is described. By application of the protocol to chromatographic fractions of saliva, IgA1 protease-inhibiting activity was found to co-purify with salivary S-IgA.

Carbohydrate depletion of immunoglobulin A1 by oral species of gram-positive rods

Bacterial deglycosylation of immunoglobulin Al (IgA1), the dominant isotype of antibody in the oral cavity, probably provides both nutrition as well as protection to the oral bacterial community. Representative strains of oral gram-positive rods were tested for their ability to remove carbohydrates from IgA1. Detection of sialic acids was performed by means of high-performance liquid chromatography (HPLC) separation (Aminex HPX-87H) and ultraviolet light absorption at 190 nm, and neutral carbohydrates were measured by HPLC separation (Capcell Pak C-18 SG 120) and ultraviolet light absorption at 245 nm after derivatization. Four strains of Actinomyces naeslundii, two strains of Corynebacterium matruchotii and one of two strains of Actinomyces odontolyticus partially or totally removed sialic acid, while two strains of Propionibacterium propionicus and the other strain of A. odontolyticus did not. Complete correlation was observed between sialic acid removal, neuraminidase activity measured with fluorogenic substrate and with one exception, altered immunoelectrophoretic mobility of IgA1. Only limited removal of other carbohydrates was observed with poor correlation to exoglycosidase activities measured with chromogenic substrates. Desialylation increases the susceptibility of glycoproteins, including IgA1, to proteolysis. Therefore, the desialylation of IgA1 by oral gram-positive rods may facilitate the proteolytic activities of other oral bacteria, and the concerted action may positively influence the survival of the bacteria in the oral community.

Inhibition of Streptococcus mutans adherence to saliva-coated hydroxyapatite by human secretory immunoglobulin A (S-IgA) antibodies to cell surface protein antigen I/II: reversal by IgA1 protease cleavage

The effect of human secretory immunoglobulin A (S-IgA) and serum antibodies to surface protein antigen (Ag) I/II on the adherence of Ag I/II-bearing Streptococcus mutans and of free Ag I/II to saliva-coated hydroxyapatite (SHA) was investigated. The inhibition by S-IgA of binding of both S. mutans and free Ag I/II to SHA was dependent on antibody to Ag I/II. Essentially no difference was found between S-IgA1 and S-IgA2 with respect to antibody-dependent inhibition of Ag I/II binding to SHA, but S-IgA1 inhibited S. mutans adherence more effectively than did either serum immunoglobulin A1 (IgA1) or IgG antibodies. The antiadherence effect of S-IgA was abrogated after cleavage by IgA1 protease. Purified Fab alpha fragments containing Ag I/II-binding activity enhanced the binding of free Ag I/II to SHA and showed greater binding to SHA than did intact S-IgA1. Despite its relative inability to interact with precoated SHA, S-IgA1 containing antibody to Ag I/II was readily incorporated into the salivary pellicle during coating, but this did not promote Ag I/II binding. These data suggest that S-IgA antibodies can inhibit the initial adherence of S. mutans to salivary pellicle-coated tooth surfaces in an adhesin-specific fashion, but the presence in the oral cavity of bacterial IgA1 proteases would potentially interfere with this antiadherence mechanism.

A comparative genetic study of serologically distinct Haemophilus influenzae type 1 immunoglobulin A1 proteases

The bacterial immunoglobulin A1 (IgA1) proteases are putative virulence factors secreted by a number of human pathogens capable of penetrating the mucosal barrier. Among Haemophilus influenzae strains, the IgA1 protease is found in several allelic forms with different serological neutralizing properties. A comparison of the primary structures of four serologically distinct H. influenzae IgA1 proteases suggests that this variation is caused by epitopes of the discontinuous conformational type. Analysis of the homologies among the four iga genes indicates that the variation results from transformation and subsequent homologous recombination in the iga gene region among H. influenzae strains. We find evidence for gene rearrangements, including transpositions in the iga gene region encoding the secretory part of the IgA1 preprotease. The amino acid sequence of the C terminus of the preprotease (the beta-core), which is assumed to be involved in secretion of the protease by forming a pore in the outer membrane, is highly conserved. In contrast to conserved areas in the protease domain, the nucleotide sequence encoding the beta-core showed a striking paucity of synonymous site variation.

Immunoglobulin-degrading enzymes in localized juvenile periodontitis

Previous reports have indicated the association of periodontal diseases with elevated levels of serum immunoglobulin G (IgG) antibodies to periodontally relevant bacteria. Recent results from this laboratory suggest that enzymes proteolytic for immunoglobulins are important virulence factors of several periodontal bacteria. Specifically, enzymes from Porphyromonas (Bacteroides) gingivalis culture supernatant fluid (SF) cleaved human IgG (4 subclasses), IgA1 and IgA2, IgM, IgD and IgE. Proteolytic enzymes from Actinobacillus actinomycetemcomitans culture SF cleaved IgG, IgA and IgM. An enriched Ig proteolytic preparation from Capnocytophaga ochracea culture SF was shown to extensively cleave all 4 subclasses of human IgG. Extensive degradation of IgG and IgA in crevicular fluid samples on SDS-PAGE from periodontal disease sites of localized juvenile periodontitis (LJP) patients in comparison to little degradation in healthy sites indicated the potential role the proteolytic enzymes from periodontopathogenic bacteria may play in situ. Treatment of IgG with P. gingivalis, A. actinomycetemcomitans and C. ochracea SF resulted in similar patterns of degradation. LJP patients had significantly higher levels of IgG and IgA proteolytic activity in whole saliva than age-, sex-, and race-matched periodontal disease-free controls. However, not all of the proteolytic activity could be ascribed to bacterial proteases since neutrophils are also present in large numbers at diseased sites. Using similar techniques, lysates of neutrophils from healthy controls cleaved IgG, IgA and IgM. The observation of enhanced Ig cleavage activity in crevicular fluid and saliva in LJP patients suggest a role for Ig proteolytic enzymes in LJP.

Molecular aspects of immunoglobulin A1 degradation by oral streptococci

Using a panel of 143 strains classified according to a novel taxonomic system for oral viridans-type streptococci, we reexamined the ability of oral streptococci to attack human immunoglobulin A1 (IgA1) molecules with IgA1 protease or glycosidases. IgA1 protease production was an exclusive property of all strains belonging to Streptococcus sanguis and Streptococcus oralis (previously S. mitior) and of some strains of Streptococcus mitis biovar 1. These are all dominant initiators of dental plaque formation. Degradation of the carbohydrate moiety of IgA1 molecules accompanied IgA1 protease activity in S. oralis and protease-producing strains of S. mitis biovar 1. Neuraminidase and beta-galactosidase were identified as extracellular enzymes in organisms of these taxa. By examination with enzyme-neutralizing antisera, four distinct IgA1 proteases were detected in S. sanguis biovars 1 to 3, S. sanguis biovar 4, S. oralis, and strains of S. mitis, respectively. The cleavage of IgA1 molecules by streptococcal IgA proteases was found to be influenced by their state of glycosylation. Treatment of IgA1 with bacterial (including streptococcal) neuraminidase increased susceptibility to protease, suggesting a cooperative activity of streptococcal IgA1 protease and neuraminidase. In contrast, a decrease in susceptibility was observed after extensive deglycosylation of the hinge region with endo-alpha-N acetylgalactosaminidase. The effector functions of IgA antibodies depend on the carbohydrate-containing Fc portion. Hence, the observation that oral streptococci may cleave not only the alpha 1 chains but also the carbohydrate moiety of IgA1 molecules suggests that the ability to evade secretory immune mechanisms may contribute to the successful establishment of these bacteria in the oral cavity.

Estimation of immunoglobulin protease activity by quantitative rocket immunoelectrophoresis

Previous methods for estimating immunoglobulin protease activity have involved the use of enzyme-linked immunosorbent assays (ELISA) or sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by autoradiography or Western blotting techniques. An alternative method has been developed to estimate proteolytic activity on human IgA1 and IgG using quantitative rocket immunoelectrophoresis. The method uses agarose containing anti-human IgA or anti-human IgG heavy chain-specific reagent to which protease-digested human immunoglobulin samples are applied to wells and electrophoresed overnight. Because proteolytic activity of immunoglobulins results in many smaller fragments, the optimal antigen-antibody ratio for precipitation changes and migration in an electric field results in a larger rocket. Consequently, the area of the rocket will be larger in a protease-treated immunoglobulin sample than a saline-treated immunoglobulin control. These increased rocket areas are correlated with our ELISA protease results (r greater than or equal to 0.90), as well as with our immunoblot results. The method is sensitive to increasing exposure to proteolysis, as well as to increasing amounts of protease. This technique can be used to quickly estimate the ability of a sample to cleave immunoglobulins.

Interference of IgA protease with the effect of secretory IgA on adherence of oral streptococci to saliva-coated hydroxyapatite

It has previously been shown that secretory immunoglobulin A (S-IgA) influences the sorption of oral streptococci to hydroxyapatite as well as to cell surfaces. The present experiments demonstrate that bacterial IgA proteases, which cleave S-IgA in the hinge region, are capable of interfering with this mechanism. This result was obtained with an IgA1 specific protease from Haemophilus influenzae and with a protease from Clostridium ramosum that cleaves IgA1 as well as IgA2 of A2m(1) allotype. The modulation of S-IgA-mediated effects by IgA proteases were studied by means of an in vitro method which permits quantitative determination of the sorption of radiolabeled oral bacteria to hydroxyapatite beads. Other authors have suggested that IgA protease-mediated effects may be explained by a strongly reduced antigen-binding capacity of released Fab alpha fragments. Here we present evidence that streptococci, after exposure to specific S-IgA and IgA protease, are coated with Fab alpha fragments.

Bacterial immunoglobulin A proteases monitored by continuous spectrophotometry

IgA proteases were estimated in a turbid aqueous two-phase system with 10% polyethylene glycol-Tris buffer, where IgA spontaneously concentrates in microscopic spherical particles (less than 1 micron). After enzymatic cleavage of IgA into Fab alpha and Fc alpha fragments, these fragments are soluble and decreasing turbidity is observed. The reaction may be followed by conventional spectrophotometry. In this manner, IgA proteases may be estimated in 10 min. Examples of the utility of the method are given with results from inhibitor studies, estimation of Km and purification of IgA protease from Haemophilus influenzae.

Purification and characterization of an immunoglobulin A1 protease from Bacteroides melaninogenicus

Attention has recently been focused on bacterial proteases with the capacity to cleave immunoglobulin A (IgA proteases) as possible pathogenic factors in bacterial meningitis, gonorrhoea, and destructive periodontal disease. Here, we describe a method for the rapid purification of a specific IgA1 protease from Bacteroides melaninogenicus. The IgA1 protease was purified 6,172-fold with a yield of 9% by ammonium sulfate precipitation, DEAE-ion exchange chromatography, and separation on a preparative TSK-G 3000SWG high-pressure gel permeation chromatography column. The enzyme was specific for human IgA1 and cleaved a prolyl-seryl peptide bond in the hinge region of the alpha 1 chain between residues 223 and 224. The molecular weight of the enzyme was 62,000, the isoelectric point was 5.0, and the Km was 3.4 X 10(-6). The enzyme was active over a broad pH range and had maximal activity at pH 5.0. B. melaninogenicus IgA1 protease was classified as a thiol protease on the basis of its inhibition by traditional protease inhibitors and the fact that it was active only under reducing conditions.

A rapid method for the detection and quantitation of IgA protease activity by macrobore gel-permeation chromatography

A rapid assay to detect and quantitate immunoglobulin A1 (IgA1) protease activity was developed by the use of a high-performance gel-permeation chromatography column. The assay measured the disappearance of intact substrate and the emergence of cleavage fragments and the results could be expressed in absolute units. The utility of the assay was demonstrated in the partial purification of an IgA1 protease from a strain of Haemophilus influenzae.