A novel role for constitutively expressed epithelial-derived chemokines as antibacterial peptides in the intestinal mucosa

Intestinal-derived chemokines have a central role in orchestrating immune cell influx into the normal and inflamed intestine. Here, we identify the chemokine CCL6 as one of the most abundant chemokines constitutively expressed by both murine small intestinal and colonic epithelial cells. CCL6 protein localized to crypt epithelial cells, was detected in the gut lumen and reached high concentrations at the mucosal surface. Its expression was further enhanced in the small intestine following in vivo administration of LPS or after stimulation of the small intestinal epithelial cell line, mIC(c12), with IFNgamma, IL-4 or TNFalpha. Recombinant- and intestinal-derived CCL6 bound to a subset of the intestinal microflora and displayed antibacterial activity. Finally, the human homologs to CCL6, CCL14 and CCL15 were also constitutively expressed at high levels in human intestinal epithelium, were further enhanced in inflammatory bowel disease and displayed similar antibacterial activity. These findings identify a novel role for constitutively expressed, epithelial-derived chemokines as antimicrobial peptides in the intestinal mucosa.

Group A streptococcal phagocytosis resistance is independent of complement factor H and factor H-like protein 1 binding

Factor H (FH) and factor H-like protein 1 (FHL-1) regulate complement activation through the alternative pathway. Several extracellular bacterial pathogens, prime targets for the complement system, bind FH and FHL-1, thereby acquiring a potential mechanism for minimizing complement deposition on their surface. For group A streptococci (GAS), surface-bound antiphagocytic M proteins mediate the interaction. To study the role of the FH-FHL-1 interaction for complement deposition and opsonophagocytosis of GAS, we first constructed a set of truncated M5 protein variants and expressed them on the surface of a homologous M-negative GAS strain. Binding experiments with the resulting strains demonstrated that the major FH-FHL-1 binding is located in a 42-amino-acid region within the N-terminal third of M5. Measurement of bacteria-bound complement factor C3 after incubation in plasma showed that the presence of this region had little impact upon complement deposition through the alternative pathway. Moreover, streptococci expressing M5 proteins lacking the major FH and FHL-1 binding sequence resisted phagocytosis in human blood as efficiently as bacteria expressing the wild-type protein. Consequently, the data suggest that the binding of the regulators of the alternative pathway is of limited importance for GAS phagocytosis resistance.

A role for the fibrinogen-binding regions of streptococcal M proteins in phagocytosis resistance

All virulent group A streptococcal isolates bind fibrinogen, a property that is closely linked to expression of type-specific antiphagocytic surface molecules designated M proteins. Here we show that although the M proteins from two different strains, M1 and M5, both bind fibrinogen with high affinity, they interact with different regions in the ligand. Moreover, mapping experiments demonstrated that the fibrinogen-binding regions in the M1 and M5 proteins are quite dissimilar at the amino acid sequence level and that they bind to different regions in the plasma protein. In spite of these differences, the fibrinogen-binding regions of M1 and M5 could both be shown to contribute to streptococcal survival in human blood, providing evidence for the distinct function of a plasma protein interaction in bacterial pathogenesis.

Strain-specific restriction of the antiphagocytic property of group A streptococcal M proteins

Group A streptococcal M proteins are type-specific virulence factors that inhibit phagocytosis. We used two M proteins, M5 and Emm22, to analyze the influence of genetic background on the properties of M proteins. Mutant strains, engineered to lack these M proteins, were complemented with genes encoding the homologous or heterologous M protein, and the complemented strains were analyzed for phagocytosis resistance. Neither the M5 nor the Emm22 protein conferred phagocytosis resistance in the heterologous background, but they did do so in the homologous background. This was not due to lack of surface expression in the heterologous background. Moreover, the M5 and Emm22 proteins expressed in heterologous background appeared to have normal structure, since they were not affected in their ability to bind different human plasma proteins. In particular, M5 or Emm22 had normal ability to bind human complement inhibitors, a property that has been implicated in phagocytosis resistance. Results similar to those obtained with M5 and Emm22 were obtained in experiments with the M6 and Emm4 proteins. Together, these data suggest that the surface expression of M protein alone may not be sufficient to confer phagocytosis resistance and consequently that strain-specific factors other than M and Emm proteins may contribute to the ability of group A streptococci to resist phagocytosis.

Role of the hypervariable region in streptococcal M proteins: binding of a human complement inhibitor

Antigenic variation allows pathogenic microorganisms to evade the immune system of the infected host. The variable structure must play an important role in pathogenesis, but its function is in most cases unknown. Here, we identify a function for the surface-exposed hypervariable region of streptococcal M5 protein, a virulence factor that inhibits phagocytosis. The hypervariable region of M5 was found to bind the human complement inhibitor FHL-1 (factor H-like protein 1), a 42-kDa plasma protein. Plasma absorption experiments with M5-expressing bacteria showed that the interaction with FHL-1 occurs also under physiologic conditions. Studies of another extensively characterized M protein, M6, indicated that this protein also has a binding site for FHL-1 in the hypervariable region. The complement-inhibitory function of FHL-1 was retained after binding to streptococci, suggesting that bound FHL-1 protects bacteria against complement attack. All available data now indicate that FHL-1, or another human complement inhibitor, binds to the hypervariable region of M proteins. These findings provide insights into the forces that drive antigenic variation and may explain why the hypervariable region of M protein is essential for phagocytosis resistance. Moreover, these data add to a growing body of evidence that human complement inhibitors are major targets for pathogenic microorganisms.

Kringle 2 mediates high affinity binding of plasminogen to an internal sequence in streptococcal surface protein PAM

Many cells express receptors for plasminogen (Pg), although the responsible molecules in most cases are poorly defined. In contrast, the group A streptococcal surface protein PAM contains a domain with two 13-amino acid residue long repeated sequences (a1 and a2) responsible for Pg binding. Here we identify the region in Pg that interacts with PAM. A radiolabeled proteolytic plasminogen fragment containing the first three kringles (K1-K3) interacted with streptococci expressing PAM or a chimeric surface protein harboring the a1a2 sequence. In contrast, plasminogen fragments containing kringle 4 or kringle 5 and the activable serine proteinase domain failed to bind to PAM-expressing group A streptococci. A synthetic and a recombinant polypeptide containing the a1a2 sequence both bound to immobilized recombinant K2 (rK2) but not to rK1 or rK3. The interaction between the a repeat region and rK2 was reversible, and rK2 completely blocked the binding of Pg to the a1a2 region. The binding of the a repeat containing polypeptide to K2 occurred with an equilibrium association constant of 4.5 x 10(7) M-1, as determined by surface plasmon resonance, a value close to that (1.6 x 10(7) M-1) calculated for the a1a2-Pg interaction. Inhibition experiments suggested involvement of the lysine-binding site of K2 in the interaction. These data demonstrate that K2 contains the major Pg-binding site for PAM, providing the first well defined example of an interaction between an internal Pg-binding region in a protein and a single kringle domain.

Identification of a domain in human factor H and factor H-like protein-1 required for the interaction with streptococcal M proteins

The plasma protein factor H (FH) inhibits the alternative pathway of complement activation. Previous work has shown that FH binds to group A streptococci and that the interaction does not interfere with the complement-inhibitory capacity of FH. In this work, we report a molecular analysis of this interaction. In absorption experiments with human plasma, M protein-expressing group A streptococci bound both FH and FH-like protein-1 (FHL-1), an active 42-kDa splice product of the FH-gene transcript comprising the first 7 of its 20 short consensus repeat (SCR) domains. rFHL-1 also bound to M protein-expressing streptococci, but rFH fragments containing SCR 1-5 or SCR 1-6 did not. rFHL-1 bound to purified M5 protein with an affinity that was higher than the value calculated for the interaction between FH and M5 protein. The binding of radiolabeled rFHL-1 to immobilized M5 was blocked completely by unlabeled rFHL-1, but was inhibited only partially by SCR 1-6, emphasizing the importance of SCR 7 for the interaction. In experiments with the FH-related proteins FHR-3 and FHR-4, only the former bound to M protein-expressing streptococci, again pointing to an involvement of SCR 7, since FHR-3, but not FHR-4, contains a domain that is similar to SCR 7. Finally, the interaction between rFHL-1 and purified M5 protein was inhibited by heparin, which binds FH via SCR 7. Together, these data indicate that the interaction between streptococcal M proteins and FH or FHL-1 requires SCR 7.