Abstract
The Gram-positive bacterium Streptococcus pneumoniae is a major human pathogen that causes infections ranging from acute otitis media to life-threatening invasive disease. Pneumococci have evolved several strategies to circumvent the host immune response, in particular the complement attack. The pneumococcal glycolytic enzyme phosphoglycerate kinase (PGK) is both secreted and bound to the bacterial surface and simultaneously binds plasminogen and its tissue plasminogen activator tPA. In the present study we demonstrate that PGK has an additional role in modulating the complement attack. PGK interacted with the membrane attack complex (MAC) components C5, C7, and C9, thereby blocking the assembly and membrane insertion of MAC resulting in significant inhibition of the hemolytic activity of human serum. Recombinant PGK interacted in a dose-dependent manner with these terminal pathway proteins, and the interactions were ionic in nature. In addition, PGK inhibited C9 polymerization both in the fluid phase and on the surface of sheep erythrocytes. Interestingly, PGK bound several MAC proteins simultaneously. Although C5 and C7 had partially overlapping binding sites on PGK, C9 did not compete with either one for PGK binding. Moreover, PGK significantly inhibited MAC deposition via both the classical and alternative pathway at the pneumococcal surface. Additionally, upon activation plasmin(ogen) bound to PGK cleaved the central complement protein C3b thereby further modifying the complement attack. In conclusion, our data demonstrate for the first time to our knowledge a novel pneumococcal inhibitor of the terminal complement cascade aiding complement evasion by this important pathogen.
Highlights
Pneumococci employ multiple strategies to escape host complement attack
phosphoglycerate kinase (PGK) interacted with the membrane attack complex (MAC) components C5, C7, and C9, thereby blocking the assembly and membrane insertion of MAC resulting in significant inhibition of the hemolytic activity of human serum
Microtiter plates coated with PGK were incubated with several dilutions of human serum, and binding of plasminogen was detected with specific Abs
Summary
Pneumococci employ multiple strategies to escape host complement attack. Results: PGK directly inhibits MAC formation and aids degradation of C3b by plasminogen. Upon contact with bacterial or non-self surfaces, the complement components initiate a tightly regulated proteolytic cascade resulting in the opsonization of bacteria with C3b for phagocytosis, generation of anaphylatoxins C3a and C5a, and formation of the cytolytic membrane attack complex (MAC) or C5b-9 for direct bacterial killing [1,2,3]. Depending on the mode of recognition the complement system can be activated by three different routes, the classical, the lectin, and the alternative pathway all of which results in the formation of C3 convertases This enzyme complex cleaves the central complement protein C3 into C3a and C3b (opsonin). Considering the destructive potential of complement and to restrict its action on non-self surfaces, the cascade is under tight regulation of both soluble and surface-bound inhibitors such as soluble vitronectin and clusterin or the membrane-bound glycoprotein CD59 that all inhibit MAC formation (6 –9). We provide the first data presenting S. pneumoniae PGK as a terminal pathway inhibitor
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