The Interaction Between Plasma Lipoproteins And Pathogenic Bacteria (Ⅰ)

Plasma lipoproteins function primarily in lipid transportation. The cumulative evidence suggests a correlation between plasma lipoproteins and pathogenic infection, and plasma lipoproteins usually play an important role in preventing some infections.It is well documented that high-density lipoprotein (HDL) as part of innate immunity serves a protective role against infection and inflammation. We unexpectedly found that HDL bound to a recombinant Streptococcal collagen-like protein 1 (rScl1), which is a virulence factor on the surface of Group A Streptococcus (GAS). Interestingly, Scl1 expressed by some M-type GAS strains was recently demonstrated to bind low-density lipoprotein (LDL).To confirm the interaction of Scl1 with HDL, DNA fragment encoding the extracellular portions of Scl1 ,which were derived from two serotype M41 GAS strains (MGAS6183, CMCC32198) , were coloned into the plasmid vector pASK-IBA2.Full-length or different truncted variant of Scl1 were expressed in E. coli BL21. Purified rScl1s were immobilized onto affinity-chromatography columns and tested for the direct binding to purified HDL or HDL in plasma. SDS-PAGE ,Western blot and enzyme-linked immunosorbent assay (ELISA) revealed that both of rScl1s derived from above two strains via their variable region (V region) of 84 amino acids (84aa) could bind to HDL. However, neither N-terminal 42 aa- truncated variant nor C-terminal 42 aa-deleted variant at V region of rScl1 was not able to bind to HDL. This might imply that the entire structure of V region might be responsible for the binding. Furthermore, the rScl1-HDL binding might be mediated by hydrophobic interaction since Tween 20 inhibited it. Additionally, rScl1 might have a potential application in the production of lipid-free serum due to the dual HDL and LDL binding capabilities. To our knowledge, it is the first time to identify the binding of rScl1 to HDL. However, the pathogenic or anti-infective role of the HDL-GAS interaction remains elusive.The biological function of lipoprotein (a) [Lp(a)] is yet unclear. The apolipoprotein (a) [Apo (a)] of Lp(a) has a high similarity to plasminogen (Plg). We hypothesized that Lp(a) may bind to Plg receptor on bacterial surface, thereby inhibiting the binding of Plg to bacteria, and ultimately inhibiting the activation of bacterium-absorbed Plg by Plg activator from either host or some bacteria, as well as impeding the bacteria from passing the barriers of human tissues. To test our hypothesis, the binding between Lp (a) and S. aureus were detected by enzyme-linked immunosorbent assay , and the preliminary data showed that Lp (a) bound to the cell of S. aureus CMCC26003 strain, and the lysine analogues EACA (20mM) inhibited the binding. The results indicated that Lp (a) might through its lysine binding sites bind to S. aureus. Incubation of S. aureus with plasminogen and Lp(a) followed by uPA treatment showed the decreased plasmin activity on the surface of S.aureus when compared with the similar activiation experiment in the absence of Lp(a). These findings suggested that Lp(a) inhibited plg from recruiting by S. aureus.To our knowledge, this is the first time to demonstrate that Lp (a) might bind to S. aureus, and inhibited the recruitment of Plg by S. aureus. These findings primarily confirm our initial hypothesis and might shed light on the role of Lp(a) in preventing infections. Moreover, a potential technique in the treatment and prevention of infection in humans might be developed based on our results.In addition, these results provide the novel evidence to further support that plasma lipoproteins may be important components of the host defense system.