Elevated serum interleukin-10 at time of hospital admission is predictive of mortality in patients with Staphylococcus aureus bacteremia

BACKGROUND

Staphylococcus aureus bacteremia (SaB) carries considerable morbidity and mortality. We examined the predictive value of serum concentrations of interleukin (IL)-10, proinflammatory cytokines, and terminal complement on patient survival and SaB duration.

METHODS

Clinical information on consecutive patients with SaB at a tertiary medical center were collected prospectively. Patient serum samples obtained at the day of clinical presentation were assayed for tumor necrosis factor-α, IL-1β, IL-10, and complement membrane attack complex C5b-9 concentrations using enzyme-linked immunoassay. Logistic regression identified predictors of mortality and duration of bacteremia.

RESULTS

In 59 patients with SaB, 14% died and 17% had prolonged bacteremia (>4 days). Elevated IL-10 serum concentrations (>7.8 pg/mL) identified all 8 patients who died, whereas there were no deaths in patients with normal IL-10 (P = .016). The lack of an IL-1β response (≤0.45 pg/mL) defined all patients with SaB >4 days. In multivariate analysis, patient age (odds ratio [OR], 1.16; P = .022), duration of bacteremia (OR, 1.16; P = .031), and serum IL-10 (OR, 1.05; P = .014) were identified as independent predictors of patient mortality.

CONCLUSIONS

SaB mortality was confined strictly to patients with elevated IL-10 concentrations. We recommend that future clinical trials of SaB stratify patients according to IL-10 and IL-1β serum concentrations in order to better evaluate the impact of therapeutic interventions on patient outcome.

Complement inhibition by gram-positive pathogens: molecular mechanisms and therapeutic implications

The plasma proteins of the complement system are essential in the innate immune response against bacteria. Complement labels bacteria with opsonins to support phagocytosis and generates chemoattractants to attract phagocytes to the site of infection. In turn, bacterial human pathogens have evolved different strategies to specifically impair the complement response. Here, we review the large arsenal of complement inhibitors produced by the gram-positive pathogens Staphylococcus aureus and Group A Streptococcus. We discuss how these bacterial molecules provide us with new tools to treat both infectious and inflammatory disease conditions in humans.

Clarithromycin-susceptible and -resistant Helicobacter pylori isolates with identical randomly amplified polymorphic DNA-PCR genotypes cultured from single gastric biopsy specimens prior to antibiotic therapy

Of the Helicobacter pylori populations from 976 patients, six contained clarithromycin-resistant as well as -susceptible colonies. In each heterogeneous H. pylori population, resistant H. pylori colonies harbored identical 23S ribosomal DNA (rDNA) mutations associated with clarithromycin resistance, while the susceptible H. pylori colonies all had wild-type 23S rDNA. The resistant and susceptible colonies of each of the heterogeneous H. pylori populations had identical randomly amplified polymorphic DNA-PCR genotypes. In conclusion, evaluation of antimicrobial susceptibility can be misinterpreted if only a single colony from the primary H. pylori population is used to test for clarithromycin susceptibility.

Staphylococcal protein Ecb impairs complement receptor-1 mediated recognition of opsonized bacteria

Staphyloccus aureus is a major human pathogen leading frequently to sepsis and soft tissue infections with abscesses. Multiple virulence factors including several immune modulating molecules contribute to its survival in the host. When S. aureus invades the human body, one of the first line defenses is the complement system, which opsonizes the bacteria with C3b and attract neutrophils by release of chemotactic peptides. Neutrophils express Complement receptor-1 [CR1, CD35) that interacts with the C3b-opsonized particles and thereby plays an important role in pathogen recognition by phagocytic cells. In this study we observed that a fraction of S. aureus culture supernatant prevented binding of C3b to neutrophils. This fraction consisted of S. aureus leukocidins and Efb. The C-terminus of Efb is known to bind C3b and shares significant sequence homology to the extracellular complement binding protein [Ecb). Here we show that S. aureus Ecb displays various mechanisms to block bacterial recognition by neutrophils. The presence of Ecb blocked direct interaction between soluble CR1 and C3b and reduced the cofactor activity of CR1 in proteolytic inactivation of C3b. Furthermore, Ecb could dose-dependently prevent recognition of C3b by cell-bound CR1 that lead to impaired phagocytosis of NHS-opsonized S. aureus. Phagocytosis was furthermore reduced in the presence of soluble CR1 [sCR1). These data indicate that the staphylococcal protein Ecb prevents recognition of C3b opsonized bacteria by neutrophil CR1 leading to impaired killing by phagocytosis and thereby contribute to immune evasion of S. aureus.

Bacterial complement escape

Complement activation is a crucial step in our innate immune defense against invading bacteria. Complement proteins can quickly recognize invading bacteria and subsequently label them for phagocytosis or kill them by direct lysis. In order to survive in the human host, bacterial pathogens have evolved a number of excreted and membrane-bound proteins that interfere with several steps of the complement cascade. In this chapter we summarize the most successful complement-modulating strategies by human bacterial pathogens.

Staphylococcus aureus Depends on Eap Proteins for Preventing Degradation of Its Phenol-Soluble Modulin Toxins by Neutrophil Serine Proteases

Neutrophil granulocytes act as a first line of defense against pathogenic staphylococci. However, Staphylococcus aureus has a remarkable capacity to survive neutrophil killing, which distinguishes it from the less-pathogenic Staphylococcus epidermidis. Both species release phenol-soluble modulin (PSM) toxins, which activate the neutrophil formyl-peptide receptor 2 (FPR2) to promote neutrophil influx and phagocytosis, and which disrupt neutrophils or their phagosomal membranes at high concentrations. We show here that the neutrophil serine proteases (NSPs) neutrophil elastase, cathepsin G and proteinase 3, which are released into the extracellular space or the phagosome upon neutrophil FPR2 stimulation, effectively degrade PSMs thereby preventing their capacity to activate and destroy neutrophils. Notably, S. aureus, but not S. epidermidis, secretes potent NSP-inhibitory proteins, Eap, EapH1, EapH2, which prevented the degradation of PSMs by NSPs. Accordingly, a S. aureus mutant lacking all three NSP inhibitory proteins was less effective in activating and destroying neutrophils and it survived less well in the presence of neutrophils than the parental strain. We show that Eap proteins promote pathology via PSM-mediated FPR2 activation since murine intraperitoneal infection with the S. aureus parental but not with the NSP inhibitors mutant strain, led to a significantly higher bacterial load in the peritoneum and kidneys of mFpr2^-/- compared to wild-type mice. These data demonstrate that NSPs can very effectively detoxify some of the most potent staphylococcal toxins and that the prominent human pathogen S. aureus has developed efficient inhibitors to preserve PSM functions. Preventing PSM degradation during infection represents an important survival strategy to ensure FPR2 activation.

The extracellular adherence protein from Staphylococcus aureus inhibits the classical and lectin pathways of complement by blocking formation of the C3 proconvertase (INM8P.361)

The pathogenic bacterium Staphylococcus aureus actively evades many aspects of human innate immunity by expressing a series of small inhibitory proteins. A number of these proteins inhibit the complement system, with the majority of S. aureus acting on the alternative pathway, blocking the amplification loop. Only a few proteins act on the initial recognition cascades that constitute the classical pathway (CP) and lectin pathway (LP). By screening a collection of recombinant, secreted staphylococcal proteins, we identified the extracellular adherence protein (Eap) as a potent, specific inhibitor of both the CP and LP. We found that Eap blocked CP/LP-dependent activation of C3, but not C4, and that Eap likewise inhibited deposition of C3b on the surface of S. aureus cells. In turn, this significantly diminished the extent of S. aureus opsonophagocytosis and killing by neutrophils. This combination of functional properties suggested that Eap acts specifically at the level of the CP/LP C3 convertase (C4b2a). Indeed, we demonstrated a direct, nanomolar-affinity interaction of Eap with C4b. Eap binding to C4b inhibited binding of both full-length C2 and its C2b fragment, which indicated that Eap disrupts formation of the CP/LP C3 proconvertase (C4b2). As a whole, our results demonstrate that S. aureus inhibits two initiation routes of complement by expression of the Eap protein, and thereby define a novel mechanism of immune evasion.