Resistance of Capnocytophaga canimorsus to killing by human complement and polymorphonuclear leukocytes

Capnocytophaga canimorsus are bacteria of the canine oral flora, known since 1976 to cause rare but severe septicemia and peripheral gangrene in patients that have been in contact with a dog. It was recently shown that these bacteria do not elicit an inflammatory response (Shin et al., J Infect Dis 195:375-86, 2007). Here, we analyze their sensitivity toward the innate immune system. Bacteria from the archetype strain Cc5 were highly resistant to killing by complement. There was little membrane attack complex (MAC) deposition in spite of C3b deposition. Cc5 bacteria were as resistant to phagocytosis by human polymorphonuclear leukocytes as Yersinia enterocolitica MRS40, endowed with an antiphagocytic type-III secretion system. We isolated Y1C12, a transposon mutant hypersensitive to killing by complement via the Ab-dependent classical pathway. The mutation inactivated a putative glycosyltransferase gene, suggesting that the Y1C12 mutant was affected at the level of a capsular or lipopolysaccharide (LPS) structure. Cc5 appeared to have several polysaccharidic structures, one being altered in Y1C12. The structure missing in Y1C12 could be purified by classical LPS purification procedures and labeled by tritiated palmitate, indicating that it is more likely a LPS structure than a capsule. Y1C12 bacteria were also more sensitive to phagocytosis by PMNs than wildtype bacteria. In conclusion, a polysaccharide structure, likely a LPS, protects C. canimorsus from deposition of the complement MAC and from efficient phagocytosis by PMNs.