genomewide characterization of host-pathogen interactions by ...

Maren Depke
Introduction
extracellular fibrinogen-binding protein Efb. Efb most probably inhibits the binding of C3b, the
bigger fragment of C3 after cleavage during complement activation, to the bacterial surface and
therefore reduces opsonization. Efb is able to bind both C3 and fibrinogen at the same time,
because the C3-binding region is located C-terminal and the fibrinogen-binding region N-terminal
in the Efb protein (Lee et al. 2004a, 2004b). More recently, a homologous and 44 % identical
protein to Efb was identified. The protein called “Efb homologous protein”, Ehp, features the
same C3b-deposition inhibitory function as Efb does, but in an even more potent manner. An
additional C3-binding domain is proposed to cause this stronger inhibitory effect (Hammel et al.
2007).
S. aureus does not only use its own proteins for immune-modulatory purposes, but also
engages and manipulates host factors to perform a protective effect for the bacterium. Host
plasminogen is the inactive precursor of the serine protease plasmin, which is the main enzyme
involved in fibrinolysis, the degradation of fibrin clots after injury and blood coagulation. Bacterial
staphylokinase binds plasminogen to the staphylococcal cell surface and mediates the activation
of the precursor into active plasmin. The plasminogen activation mechanism is not mediated via
cleavage by staphylokinase as the bacterial protein does not possess enzymatic activity. More
precisely, staphylokinase binds plasminogen in a 1:1 stoichiometric ratio and renders the enzyme
precursor by formation changes more susceptible for activation. Activation finally occurs by other
already active proteases, e. g. trace amounts of plasmin, which are able to start a reinforcing
activation cascade (Silence et al. 1995). In a simple mode, active plasmin will help the pathogen
to spread from site of infection, either by enzymatic fibrinolysis of blood coagulation clots or by
degradation of extracellular matrix molecules. More sophisticated, the activated plasmin
protease molecules degrade IgG and C3b from the bacterial surface and thus reverse
opsonization (Rooijakkers et al. 2005a).
When S. aureus is finally phagocytosed it has to deal with reactive oxygen species (ROS).
Superoxide dismutase enzymes and non-enzymatic superoxide dismutase Mn 2+ inactivate
superoxide anion O 2 – . Deletion mutants in superoxide dismutase or manganese cation uptake
systems are less virulent than wild type strains. When ROS react with protein residues to
methionine sulfoxide, these residues are reduced by methionine sulphoxide reductases, which
also have impact on in vivo virulence (Foster 2005, Horsburgh et al. 2002a, Karavolos et al. 2003,
Singh/Moskovitz 2003).
Modulation of complement, opsonization and phagocytosis is not the only mechanism
S. aureus applies for immune evasion. It also developed resistance mechanisms against killing by
antimicrobial peptides, especially after phagocytosis, when the four groups of antimicrobial
substances interfere with the bacterial integrity: small anionic peptides (e. g. dermicidin in airway
surfactant), small cationic peptides (e. g. cathelicidins in neutrophils), cationic disulphide bond
forming peptides (e. g. α- and β-defensins), and anionic and cationic peptide fragments derived
from larger proteins (Foster, 2005). In this context, staphylokinase has a further function. It binds
α-defensins, which are secreted by neutrophils, and inhibits most of the defensins’ bactericidal
potency (Jin et al. 2004). Vice versa, defensins inhibit the staphylokinase’s ability of activating
plasminogen. This effect can be relevant in clinical settings when recombinant staphylokinase
(sakSTAR) is planned to be developed into a thrombolytic pharmaceutical and might be applied in
inflammation paralleled disorders like vascular occlusive diseases (Bokarewa/Tarkowski 2004).
S. aureus secretes different proteases. One of them, aureolysin, cleaves the bactericidal peptide
26