Streptococcus Suis Serotype 2 SntA Inhibit Classical Complement Pathway

Complement system is a vital part of innate immunity of host the first immune defense, and it acts as a bridge between innate immunity and acquired immunity. Complement system can quickly identify and eliminate pathogenic microorganisms, but pathogenic microorganisms have evolved a variety of mechanisms to escape the complement system. Streptococcus suis serotype 2 is a Amphixenosis, and is seriously harmful to pig industry and human healthy. Preliminary studies in our lab have found that SS2 surface protein SntA is a Heme binding protein, and also is one of virulence factors, can interact with complement C1qA of pig. This paper aims to find the influence of the activation of complement classical pathway when SntA interacts with C1qA, we obtain the following results:SntA and pig IgG can competitively bind to C1qA:First, SntA and C1qA were expressed. Coating the protein C1qA, and then add pig IgG and/or SntA. After washing, HRP-anti-pig IgG antibody was added and was chromogenic. Instead of SntA, BSA was added. It shows SntA can competitively bind to ClqA rather than pig IgG. With the increasing of SntA, the inhibition of pig IgG grows. When the concentration of SntA is 4.9mg/m, the inhibition is up to 99.55%. So, the protein SntA can competitively bind to C1qA with pig IgG.SntA can prevent the interaction of Clq and antigen-antibody complex:In this study, the first step of classical pathway was repeated in vitro. After the formation of antigen-antibody complex, the binding site of Clq of the Fc fragment of IgG is exposed, and then combining with C1q. In the case of the presence of both C1q and SntA, C1q binding rate was measured. With the increasing of SntA, C1q binding rate is apparently decreased. So, it suggested SntA can prevent the interaction of C1q and antigen-antibody complex through combining with C1q.The influence of SntA to the C3 deposition:In theory, SntA can interact with Clq so that it prevents the activation of Clr, and C3 deposition was interrupted. In the experiment, after the formation of antigen-antibody complex, C1q and then C1q-depelet serum are added in turn. At last, the anti-C3 antibody is used to test C3. At present, C3 was not tested, the experimental conditions are needed to be optimized.The influence of Heme in protein SntA to complement system activation: Streptococcus Suis type 2 SntA is a Heme binding protein, and a paper reported that Heme can interact with Clq so that complement system was inhibited. In order to clarify wether Heme can mediate the inhibition of complement classical pathway, C1qA was coated, the Heme, Heme-SntA, SntA was simultaneouly added to it. The study suggest that Heme is not directly interacted with C1qA, and don’t attend the inhibition of classical pathway.So, Streptococcus suis type 2 SntA inhibit the activation of classical complement pathway, and it binds to the A chain of q submit of complement 1, the interaction of antigen-antibody complex and C1 is interrupted, the serine proteinase of C1r is not activated, and finally it inhibits the classical complement pathway. And Heme does not attend this process. This may be a way that Streptococcus Suis type 2 escapes from the clearance of host, this mechanism can protect itself. As well, this may be a pathogenic mechanism of the host against Streptococcus Suis. So, Streptococcus Suis type 2 surface protein SntA is an inhibiting factor of complement system.