Capnocytophaga canimorsus. interaction with the innate immune system

We show that Capnocytophaga canimorsus strain 5 (Cc5) is even more resistant to phagocytosis and killing by murine macrophages (J774.1) and human polymorphonuclear neutrophils (PMNs) than Yersinia enterocolitica, which is known as a model bacterium for resistance against phagocytosis due to its type 3 secretion system (Grosdent et al., 2002). We observed that Cc5 even becomes completely resistant to phagocytosis at high multiplicity of infection (moi of 50). In addition, we demonstrate that the Cc5 transposon mutant Y1C12, identified during a serum sensitivity screen, has an increased sensitivity to phagocytosis and killing by either murine macrophages or human PMNs even in the unopsonized state. This indicated that not an increased susceptibility for antibody binding or complement deposition led to an increased phagocytosis of the mutant, but that rather the outer surface was more readily recognized by the phagocytes.
Furthermore, we demonstrate that Cc5 induces the formation of neutrophil extracellular traps upon infection of human PMNs in vitro and that Cc5 is trapped and killed within neutrophil extracellular traps, indicating sensitivity of Cc5 towards antimicrobial peptides present in PMN granules.
Analysis of serum resistance in Cc5 revealed that serum resistance is probably linked to its lipopolysaccharide, which prevents deposition of the membrane attack complex on the bacterial surface.
Moreover, we have observed that upon growth in the presence of cells, Cc5 releases or modifies factor(s) in the medium, which interfere with the killing ability of macrophages. Investigating the underlying mechanism, we could show that Cc5 does not affect phagosome maturation, but blocks the oxidative burst. This capacity was shown to depend on the release of the zinc metallopeptidase pitrilysin by Cc5.
First analyses on the prevalence of the hypothetical virulence factors serum resistance and interference with the oxidative burst indicated that C. canimorsus strains might display strain variability. While 59% of the strains (50% of case strains, 61% of dog isolates) were able to block the killing ability of macrophages, 60% of the strains were highly serum resistant (100% of case strains, 54% of dog isolates). However, serum resistance could not be directly linked to a specific polysaccharide structure in C. canimorsus.
November 2009, Salome Casutt-Meyer