Titration of inhibiting antibodies to bacterial IgA1 proteases in human sera and secretions

Induction of Susceptibility to Disseminated Infection with IgA1 Protease-Producing Encapsulated Pathogens Streptococcus pneumoniae, Haemophilus influenzae Type b, and Neisseria meningitidis

Streptococcus pneumoniae, Neisseria meningitidis, and Haemophilus influenzae are the principal causes of bacterial meningitis. It is unexplained why only occasional individuals develop invasive infection, while the vast majority remain healthy and develop immunity when encountering these pathogens. A capsular polysaccharide and an IgA1 protease are common to these pathogens. We tested the hypothesis that patients are primed to susceptibility to invasive infection by other bacteria that express the same capsular polysaccharide but no IgA1 protease. Thereby, the subsequently colonizing pathogen may protect its surface with IgA1 protease-generated Fab fragments of IgA1 devoid of Fc-mediated effector functions. Military recruits who remained healthy when acquiring meningococci showed a significant response of inhibitory antibodies against the IgA1 protease of the colonizing clone concurrent with serum antibodies against its capsular polysaccharide. At hospitalization, 70.8% of meningitis patients carried fecal bacteria cross-reactive with the capsule of the actual pathogen, in contrast to 6% of controls (P < 0.0001). These were Escherichia coli K100, K1, and K92 in patients with infection caused by H. influenzae type b and N. meningitidis groups B and C, respectively. This concurred with a significant IgA1 response to the capsule but not to the IgA1 protease of the pathogen. The demonstrated multitude of relationships between capsular types and distinct IgA1 proteases in pneumococci suggests an alternative route of immunological priming associated with recombining bacteria. The findings support the model and offer an explanation for the rare occurrence of invasive diseases in spite of the comprehensive occurrence of the pathogens.

IgA nephropathy enigma

IgA nephropathy (IgAN) is the leading cause of primary glomerulonephritis in the world. The disease is characterized by the presence of IgA-containing immune complexes in the circulation and in mesangial deposits with ensuing glomerular injury. Although in humans there are two IgA subclasses, only IgA1 molecules are involved. The exclusivity of participation of IgA1 in IgAN prompted extensive structural and immunological studies of the unique hinge region (HR) of IgA1, which is absent in otherwise highly homologous IgA2. HR of IgA1 with altered O-glycans serves as an antigen recognized by autoantibodies specific for aberrant HR glycans leading to the generation of nephritogenic immune complexes. However, there are several unresolved questions concerning the phylogenetic origin of human IgA1 HR, the structural basis of its antigenicity, the origin of antibodies specific for HR with altered glycan moieties, the regulatory defects in IgA1 glycosylation pathways, and the potential approaches applicable to the disease-specific interventions in the formation of nephritogenic immune complexes. This review focuses on the gaps in our knowledge of molecular and cellular events that are involved in the immunopathogenesis of IgAN.

Levels and complexity of IgA antibody against oral bacteria in samples of human colostrum

Streptococcus mutans (SM) have three main virulence antigens: glucan binding protein B (gbpB), glucosyltransferase (Gtf) and antigens I/II (Ag I/II) envolved in the capacity of those bacteria to adhere and accumulate in the dental biofilm. Also, the glycosyltransferases 153 kDa of Streptococcus gordonii (SGO) and 170kDa of Streptococcus sanguinis (SSA) were important antigens associated with the accumulation of those bacterias. Streptococcus mitis (SMI) present IgA1 protease of 202 kDa. We investigated the specificity and levels IgA against those antigens of virulence in samples of human colostrum. This study involved 77 samples of colostrum that were analyzed for levels of immunoglobulian A, M and G by Elisa. The specificity of IgA against extracts of SM and initials colonizators (SSA, SMI, SGO) were analyzed by the Western blot. The mean concentration of IgA was 2850.2 (±2567.2) mg/100 mL followed by IgM and IgG (respectively 321.8±90.3 and 88.3±51.5), statistically different (p<0.05). Results showed that the majority of samples had detectable levels of IgA antibodies to extracts of bacteria antigens and theirs virulence antigens. To SM, the GbpB was significantly lower detected than others antigens of SM (p<0.05). High complexities of response to Ags were identified in the samples. There were no significant differences in the mean number of IgA-reactive Ags between the antigens (p>0.4). So, the breast milk from first hours after birth presented significant levels of IgA specific against important virulence of antigens those oral streptococci, which can disrupt the installation and accumulation process of these microorganisms in the oral cavity.

Biological Functions of IgA

Immunoglobulin A (IgA) is the most enigmatic of immunoglobulins. It is by far the most abundant of human Igs, being present in the blood plasma at concentrations approximating 2–3mg/mL, as well as the dominant isotype in most secretions where its output amounts to some 5–8g/day in adults. Furthermore, its evolutionary origins appear to precede the synapsid– diapsid divergence in tetrapod phylogeny (>300 million years ago) because it is present in both mammals and birds and therefore possibly also in reptiles (reviewed in Peppard et al., 2005); an IgA-like molecule has now been identified in a lizard (Deza et al., 2007).

The atypical amino-terminal LPNTG-containing domain of the pneumococcal human IgA1-specific protease is required for proper enzyme localization and function

Streptococcus pneumoniae produces a zinc metalloproteinase, Iga, which cleaves human immunoglobulin A1 (IgA1), and whose activity is predominantly localized to the bacterial surface. However, proper surface localization is not predicted using current models, as the LPNTG sorting motif is located atypically near the amino- rather than the carboxy-terminus. The cell-associated form of Iga was confirmed to be external to the bacterial membrane, and while bound tightly, its attachment to the cell wall is non-covalent, but dependent on both a complete LPNTG sequence and sortase activity. Disruption of the region between the signal peptidase cleavage site and the LPNTG domain resulted in a localization defect, premature degradation, and an alteration of the ability of the enzyme to act on a monoclonal human IgA1 substrate and to enhance bacterial adherence, linking localization to enzyme function. Edman sequencing of cell-associated Iga determined that the enzyme is processed at an unexpected site downstream of the sorting signal yet still associates with the bacterial surface. Our results indicate a non-covalent re-association between the carboxy-terminal enzymatic domain and the cleaved, sorted amino-terminal localization domain. This amino-terminal motif is shared among the other zinc metalloproteinases in streptococci and suggests a novel conserved mechanism for the surface localization of protease activity.

Structural integrity of infant salivary immunoglobulin A (IgA) in IgA1 protease-rich environments

IgA1 protease-secreting Streptococcus mitis often dominate the oral flora of the neonate and young infant at a time when salivary IgA concentrations are low and usually enriched in the secretory IgA1 subclass. To study the possible influence of these degradative enzymes on emerging host immunity, the presence of IgA1 protease-secreting streptococci was related to the structural integrity of salivary IgA in 24 infants who were between 3 and 18 weeks of age. At least one IgA1 protease-secreting strain could be isolated from the oral mucosa of 79% of the infants and comprised a mean of 38% of the total streptococcal flora of these infants. Chromatographic analyses of resting whole saliva from 16 infants revealed, however, that 95% of the secretory IgA (range 88-100%) remained intact, indicating that minimal immediate IgA proteolysis occurred in the bulk salivary phase. Proteolysis of infant salivary IgA, presumably by indigenous IgA1 protease, could be observed after extended (more than 7 h) in situ incubation of whole saliva at 37 degrees C. Salivary IgA antibody activities to S. mitis components were demonstrated by Western blot in infants colonized with an IgA1 protease-secreting flora. Preliminary evidence suggested that salivary antibody activity in some infants may be directed to IgA1 protease. Thus, the infant's antibody defenses not only appear very early in life but are substantively intact in the bulk salivary phase, even when the oral cavity is colonized with IgA1 protease-secreting streptococcal flora.

Comparative analysis of immunoglobulin A1 protease activity among bacteria representing different genera, species, and strains

Immunoglobulin A1 (IgA1) proteases cleaving human IgA1 in the hinge region are produced constitutively by a number of pathogens, including Haemophilus influenzae, Neisseria meningitidis, Neisseria gonorrhoeae, and Streptococcus pneumoniae, as well as by some members of the resident oropharyngeal flora. Whereas IgA1 proteases have been shown to interfere with the functions of IgA antibodies in vitro, the exact role of these enzymes in the relationship of bacteria to a human host capable of responding with enzyme-neutralizing antibodies is not clear. Conceivably, the role of IgA1 proteases may depend on the quantity of IgA1 protease generated as well as on the balance between secreted and cell-associated forms of the enzyme. Therefore, we have compared levels of IgA1 protease activity in cultures of 38 bacterial strains representing different genera and species as well as strains of different pathogenic potential. Wide variation in activity generation rate was found overall and within some species. High activity was not an exclusive property of bacteria with documented pathogenicity. Almost all activity of H. influenzae, N. meningitidis, and N. gonorrhoeae strains was present in the supernatant. In contrast, large proportions of the activity in Streptococcus, Prevotella, and Capnocytophaga species was cell associated at early stationary phase, suggesting that the enzyme may play the role of a surface antigen. Partial release of cell-associated activity occurred during stationary phase. Within some taxa, the degree of activity variation correlated with degree of antigenic diversity of the enzyme as determined previously. This finding may indicate that the variation observed is of biological significance.

Biological significance of IgA1 proteases in bacterial colonization and pathogenesis: critical evaluation of experimental evidence

IgA1 protease activity, which allows bacteria to cleave human IgA1 in the hinge region, represents a striking example of convergent evolution of a specific property in bacteria. Although it has been known since 1979 that IgA1 protease is produced by the three leading causes of bacterial meningitis in addition to important urogenital pathogens and some members of the oropharyngeal flora, the exact role of this enzyme in bacterial pathogenesis is still incompletely understood owing to lack of a satisfactory animal model. Cleavage of IgA1 by these post-proline endopeptidases efficiently separates the monomeric antigen-binding fragments from the secondary effector functions of the IgA1 antibody molecule. Several in vivo and in vitro observations indicate that the enzymes are important for the ability of bacteria to colonize mucosal membranes in the presence of S-IgA antibodies. Furthermore, the extensive cleavage of IgA sometimes observed in vivo, suggests that IgA1 protease activity results in a local functional IgA deficiency that may facilitate colonization of other microorganisms and the penetration of potential allergens. It has been hypothesized that IgA1 protease activity of Haemophilus influenzae, Neisseria meningitidis, and Streptococcus pneumoniae, under special immunological circumstances, allows these bacteria to take advantage of specific IgA1 antibodies in a strategy to evade other immune factors of the human body. The decisive factor is the balance between IgA antibodies against surface antigens of the respective bacteria and their IgA1 protease. Recent studies have shown that serine-type IgA1 proteases of H. influenzae, meningococci, and gonococci belong to a family of proteins used by a diverse group of Gram-negative bacteria for colonization and invasion.

Neisseria gonorrhoeae IgA1 proteases share epitopes recognized by neutralizing antibodies

The antigenic diversity among IgA1 proteases of 61 Neisseria gonorrhoeae strains isolated during a period of 23 years and on four continents was examined in enzyme neutralization assays employing rabbit antisera raised against selected IgA1 proteases. The antigenic analyses were compared with results of iga gene-region RFLP patterns and enzyme cleavage specificity for substrate IgA1. Type 1 IgA1 proteases were antigenically uniform while six different antigenic types were detected among type 2 enzymes. Extensive cross-reactions of antibodies against the different antigenic types suggested only minor differences in relevant epitopes. Epitopes previously found to be common to all Neisseria meningitidis IgA1 proteases were also shared by all N. gonorrhoeae IgA1 proteases in the collection. Human sera from patients with gonorrhoea showed broadly cross-reactive neutralizing activity at titers comparable to those of sera from immunized rabbits. In conclusion, N. gonorrhoeae IgA1 proteases show a remarkable lack of diversity of epitopes recognized by enzyme-neutralizing antibodies. If future studies confirm that cleavage of IgA1 is an important step in gonococcal infections, Neisseria IgA1 proteases may be attractive vaccine candidates.