Amino acid sequence requirements in the human IgA1 hinge for cleavage by streptococcal IgA1 proteases

Pathogenicity and virulence of Streptococcus pneumoniae: Cutting to the chase on proteases

Bacterial proteases and peptidases are integral to cell physiology and stability, and their necessity in Streptococcus pneumoniae is no exception. Protein cleavage and processing mechanisms within the bacterial cell serve to ensure that the cell lives and functions in its commensal habitat and can respond to new environments presenting stressful conditions. For S. pneumoniae, the human nasopharynx is its natural habitat. In the context of virulence, movement of S. pneumoniae to the lungs, blood, or other sites can instigate responses by the bacteria that result in their proteases serving dual roles of self-protein processors and virulence factors of host protein targets.

Small-Molecule Inhibitors of Haemophilus influenzae IgA1 Protease

Newly identified, nontypable Haemophilus influenzae (H. influenza) strains represent a serious threat to global health. Due to the increasing prevalence of antibiotic resistance, virulence factors have emerged as potential therapeutic targets that would be less likely to promote resistance. IgA1 proteases are secreted virulence factors of many Gram-negative human pathogens. These enzymes play important roles in tissue invasion as well as evasion of the immune response, yet there has been limited work on pharmacological inhibitors. Here, we report the discovery of the first small molecule, nonpeptidic inhibitors of H. influenzae IgA1 proteases. We screened over 47 000 compounds in a biochemical assay using recombinant protease and identified a hit compound with micromolar potency. Preliminary structure-activity relationships produced additional inhibitors, two of which showed improved inhibition and selectivity for IgA protease over other serine proteases. We further showed dose-dependent inhibition against four different IgA1 protease variants collected from clinical isolates. These data support further development of IgA protease inhibitors as potential therapeutics for antibiotic-resistant H. influenza strains. The newly discovered inhibitors also represent valuable probes for exploring the roles of these proteases in bacterial colonization, invasion, and infection of mucosal tissues.

Dermatitis herpetiformis sera or goat anti-transglutaminase-3 transferred to human skin-grafted mice mimics dermatitis herpetiformis immunopathology

Dermatitis herpetiformis (DH) is characterized by deposition of IgA in the papillary dermis. However, indirect immunofluorescence is routinely negative, raising the question of the mechanism of formation of these immune deposits. Sárdy et al. (2002. J. Exp. Med. 195: 747-757) reported that transglutaminase-3 (TG3) colocalizes with the IgA. We sought to create such deposits using passive transfer of Ab to SCID mice bearing human skin grafts. IgG fraction of goat anti-TG3 or control IgG were administered i.p. to 20 mice. Separately, sera from seven DH patients and seven controls were injected intradermally. Biopsies were removed and processed for routine histology as well as direct immunofluorescence. All mice that received goat anti-TG3 produced papillary dermal immune deposits, and these deposits reacted with both rabbit anti-TG3 and DH patient sera. Three DH sera high in IgA anti-TG3 also produced deposits of granular IgA and TG3. We hypothesize that the IgA class anti-TG3 Abs are directly responsible for the immune deposits and that the TG3 is from human epidermis, as this is its only source in our model. These deposits seem to form over weeks in a process similar to an Ouchterlony immunodiffusion precipitate. This process of deposition explains the negative indirect immunofluorescence results with DH serum.

Cleavage of the human immunoglobulin A1 (IgA1) hinge region by IgA1 proteases requires structures in the Fc region of IgA

Secretory immunoglobulin A (IgA) protects the mucosal surfaces against inhaled and ingested pathogens. Many pathogenic bacteria produce IgA1 proteases that cleave in the hinge of IgA1, thus separating the Fab region from the Fc region and making IgA ineffective. Here, we show that Haemophilus influenzae type 1 and Neisseria gonorrhoeae type 2 IgA1 proteases cleave the IgA1 hinge in the context of the constant region of IgA1 or IgA2m(1) but not in the context of IgG2. Both C(alpha)2 and C(alpha)3 but not C(alpha)1 are required for the cleavage of the IgA1 hinge by H. influenzae and N. gonorrhoeae proteases. While there was no difference in the cleavage kinetics between wild-type IgA1 and IgA1 containing only the first GalNAc residue of the O-linked glycans, the absence of N-linked glycans in the Fc increased the ability of the N. gonorrhoeae protease to cleave the IgA1 hinge. Taken together, these results suggest that, in addition to the IgA1 hinge, structures in the Fc region of IgA are required for the recognition and cleavage of IgA1 by the H. influenzae and N. gonorrhoeae proteases.