Streptococcus suis AdcA interacts with factor H and inhibits C3b deposition on the bacteria to participate in complement evasion

Abstract

Streptococcus suis is an important zoonotic pathogen. It can escape from the host complement attack through various strategies. In this study, the possible complement C3b interacted proteins of S. suis were screened by co-immunoprecipitation (Co-IP) using C3b antibody in human serum. A bacterial Zn²⁺ transporter AdcA detected to be a cell wall protein was identified. The interaction of AdcA with C3b was verified in the same Co-IP setup using AdcA antibody by western-blot, but after far-western blot analysis, AdcA was found to not interact with C3b directly, but interacted directly with factor H (FH), the complement regulatory factor inhibiting the cleavage of C3 and the production of C3b. Thereafter, the interaction sites of AdcA and FH were predicted using molecular docking. Then, an adcA gene deletion mutant ΔadcA, a complementary strain CΔadcA and point mutant strains containing AdcA-FH interaction sites ΔadcA_FH(P-G) (N-terminal) and ΔadcA_FH(Y-P) (C-terminus) were constructed. The deposition of C3b on the surface of ΔadcA, ΔadcA_FH(P-G) and ΔadcA_FH(Y-P) was significantly increased compared to the wild-type (WT) or CΔadcA. The resistance to opsonophagocytosis and survival rates in serum of ΔadcA and ΔadcA_FH(P-G) were significantly reduced compared to WT or CΔadcA. Additionally, deletion of adcA decreased bacterial loads of S. suis in the blood, brain and lung of mice. Taken together, AdcA inhibited C3b deposition on the surface of S. suis by binding to FH, further inhibiting the C3b-mediated opsonophagocytosis and serum survivability of S. suis. Both the complement evasion and the known Zn²⁺ transport roles of AdcA contributed to pathogenicity of S.suis.