Streptococci are highly intertwined with specific host species, despite their ability to cause zoonosis or anthroponosis in other uncommon hosts. Only few factors linking streptococci to their natural host have so far been described.
We have identified and characterized a novel family of IgG degrading enzymes designated IgdE. IgdE of the endemic pig pathogen S. suis is the founding member of the new C113 cysteine protease family in the MEROPS peptidase database. Interestingly, IgdE proteases exhibit pronounced substrate specificities that were found to mirror adaption of the bacteria to their hosts.
Through a computational approach we identified putative IgdE family proteases in nine different streptococcal species. The proteolytic capacities of IgdE proteases of S. agalactiae, S. porcinus, S. pseudoporcinus and S. equi were assessed. Proteolytic activity was found to be restricted towards IgG of the pathogens main hosts. Moreover these proteases also showed pronounced specificity towards IgG subtypes, as IgdE from S. agalactiae and S. pseudoporcinus only cleaved human IgG1, while IgdE from S. equi was subtype specific for equine IgG7.
An igdE in-frame deletion strain in S. suis was found to be attenuated in piglet blood survival assays ex vivo, suggesting an important role during infection. Prolonged and reoccurring infections require streptococcal immune evasion mechanisms to circumvent detection and eradication by the host’s immune responses. Several streptococcal species evade antibody mediated immune defences by secretion of Ig-degrading enzymes. However, the importance for streptococci to neutralise certain IgG subtypes indicates decisive roles of these subtypes in counteracting infection or colonization.