HylS’, a fragment of truncated hyaluronidase of <i>streptococcus suis</i>, contributes to immune evasion by interaction with host complement factor C3b

Pathogenic bacteria have evolved many strategies to evade surveillance and attack by complements. <i>Streptococcus suis</i> is an important zoonotic pathogen that infects humans and pigs. Hyaluronidase (HylA) has been reported to be a potential virulence factor of <i>S. suis</i>. However, in this study, it was discovered that the genomic region encoding HylA of the virulent <i>S. suis</i> strain SC19 and other ST1 strains was truncated into four fragments when aligned with a strain containing intact HylA and possessing hyaluronidase activity. As a result, SC19 had no hyaluronidase activity, but one truncated HylA fragment, designated as HylS,’ directly interacted with complement C3b, as confirmed by western ligand blotting, pull-down, and ELISA assays. The deposition of C3b and membrane attack complex (MAC) formation on the surface of a HylS’-deleted mutant (Δ<i>hylS’</i>) was significantly increased compared to wild-type SC19. In human sera and whole blood, Δ<i>hylS’</i> survival was significantly reduced compared to that in SC19. The resistance of Δ<i>hylS’</i> to macrophages and human polymorphonuclear neutrophil PMNs also decreased. In a mouse infection model, Δ<i>hylS’</i> showed reduced lethality and lower bacterial load in the organs compared to that of SC19. We conclude that the truncated hyaluronidase HylS’ fragment contributes to complement evasion and the pathogenesis of <i>S. suis</i>.