Acquisition of Regulators of Complement Activation byStreptococcus pyogenesSerotype M1

Abstract

Opsonization of bacteria by complement proteins is an important component of the immune response. The pathogenic bacterium Streptococcus pyogenes has evolved multiple mechanisms for the evasion of complementmediated opsonization. One mechanism involves the binding of human regulators of complement activation such as factor H (FH) and FH-like protein 1 (FHL-1). Acquisition of these regulatory proteins can limit deposition of the opsonin C3b on bacteria, thus decreasing the pathogen's susceptibility to phagocytosis. Binding of complement regulatory proteins by S. pyogenes has previously been attributed to the streptococcal M and M-like proteins. Here, we report that the S. pyogenes cell surface protein Fba can mediate binding of FH and FHL-1. We constructed mutant derivatives of S. pyogenes that lack Fba, M1 protein, or both proteins and assayed the strains for FH binding, susceptibility to phagocytosis, and C3 deposition. Fba expression was found to be sufficient for binding of purified FH as well as for binding of FH and FHL-1 from human plasma. Plasma adsorption experiments also revealed that M1 ؉ Fba ؉ streptococci preferentially bind FHL-1, whereas M1 ؊ Fba ؉ streptococci have similar affinities for FH and FHL-1. Fba was found to contribute to the survival of streptococci incubated with human blood and to inhibit C3 deposition on bacterial cells. Streptococci harvested from log-phase cultures readily bound FH, but binding was greatly reduced for bacteria obtained from stationary-phase cultures. Bacteria cultured in the presence of the protease inhibitor E64 maintained FH binding activity in stationary phase, suggesting that Fba is removed from the cell surface via proteolysis. Western analyses confirmed that E64 stabilizes cell surface expression of Fba. These data indicate that Fba is an antiopsonic, antiphagocytic protein that may be regulated by cell surface proteolysis.The gram-positive bacterium Streptococcus pyogenes, or group A streptococcus (GAS), is an important human pathogen. Most streptococcal infections (e.g., pharyngitis, impetigo) are relatively mild and readily treated with antibiotics. However, GAS cause a variety of severe and invasive diseases as well (12). For example, rheumatic fever, a poststreptococcal sequela, is endemic in the third world, where it is a major cause of cardiovascular disease and mortality in persons under 50 years of age (36). Also, since the mid-1980s, Western countries have reported significant increases in severe GAS infections (e.g., sepsis, necrotizing fasciitis, streptococcal toxic shock syndrome) with high mortality rates (7,17,35).GAS express an array of cell surface molecules that contribute to pathogenesis. Among the best studied of these are the M proteins (20). M proteins are multifunctional proteins that contribute to GAS pathogenesis in a number of ways, including by adherence to host tissues, intracellular invasion, and autoaggregation of bacterial cells. Perhaps the most important function of M protein, however, is to confer bacterial resistance to phag...

Cleavage of Complement C3b to iC3b on the Surface ofStaphylococcus aureusIs Mediated by Serum Complement Factor I

“…This result suggests that serum proteins other than complement compo- nents are needed to cleave C3b to iC3b. We hypothesized that the serum complement regulatory proteins factor H and factor I could contribute to the cleavage of C3b to iC3b on the S. aureus surface because other gram-positive bacteria, Streptococcus pyogenes and Streptococcus pneumoniae, bind factor H (8,14,15,21,22), a cofactor for factor I-mediated cleavage of C3b to iC3b.…”

“…Factor H can also accelerate the decay of the C3Bb convertase of the alternative complement pathway, resulting in decreased activation of this pathway (reviewed in reference 16). Notably, two other gram-positive bacteria, Streptococcus pyogenes and Streptococcus pneumoniae, are both able to bind complement factor H (8,14,15,21,22). We hypothesized that S. aureus might convert bound C3b to iC3b via similar interactions with factor H and factor I.…”

Induction of a Regulatory Phenotype in Human CD4+ T Cells by Streptococcal M Protein

“…Heat-inactivated bacteria were prepared from overnight cultures and grown in Todd-Hewitt broth without shaking in 5% CO 2 at 37°C. The bacteria were harvested, washed, and resuspended in PBS containing 10 g/ml protease inhibitor E64 to 10 10 bacteria/ml (23). The bacterial suspension was incubated at 90°C for 14 min, washed, and resuspended in PBS supplemented with E64.…”

Complement Resistance of Borrelia burgdorferi Correlates with the Expression of BbCRASP-1, a Novel Linear Plasmid-encoded Surface Protein That Interacts with Human Factor H and FHL-1 and Is Unrelated to Erp Proteins

“…Several pathogenic microorganisms express surface proteins capable of interacting with factor H and FHL-1, such as the Mand the Fba protein of S. pyogenes (16,27,28), PspC, Hic proteins of pneumococci (20, 29 -31), Por1A protein of nonsialylated N. gonorrhoeae (32), and envelope proteins gp41 and gp120 of the human immunodeficiency virus (33). Our previous studies indicated that moderate or fully complement-resistant B. burgdorferi and Borrelia afzelii strains, but not complementsensitive Borrelia garinii strains, express up to five factor H-and FHL-1-binding surface molecules termed complement regulator-acquiring surface proteins (CRASPs) (13,34,35).…”