Roles of Capsule and Lipopolysaccharide O Antigen in Interactions of Human Monocyte-Derived Dendritic Cells and Klebsiella pneumoniae

Abstract

In humans, Klebsiella pneumoniae is a saprophytic bacterium of the nasopharyngeal and intestinal mucosae that is also frequently responsible for severe nosocomial infections. Two major factors of virulence, capsular polysaccharide (CPS) and lipopolysaccharide (LPS) O antigen, are involved in mucosal colonization and the development of infections. These bacterial surface structures are likely to play major roles in interactions with the mucosal immune system, which are orchestrated by a network of surveillance based on dendritic cells (DCs). To determine the roles of K. pneumoniae CPS and LPS in the DC response, we investigated the response of immature human monocyte-derived DCs to bacterial challenge with a wild-type strain and its isogenic mutants deficient in CPS or LPS O-antigen production. As observed by flow cytometry and confocal laser microscopy, the rate of phagocytosis was inversely proportional to the amount of CPS on the bacterial cell surface, with LPS playing little or no role. The K. pneumoniae wild-type strain induced DC maturation with upregulation of CD83, CD86, and TLR4 and downregulation of CD14 and DC-SIGN. With CPS mutants, we observed a greater decrease in DC-SIGN, suggesting a superior maturation of DCs. In addition, incubation of DCs with CPS mutants, and to a lesser extent with LPS mutants, resulted in significantly higher Th1 cytokine production. Combined, our findings suggest that K. pneumoniae CPS, by hampering bacterial binding and internalization, induces a defective immunological host response, including maturation of DCs and pro-Th1 cytokine production, whereas the LPS O antigen seems to be involved essentially in DC activation.

Cryptococcus gattii Is Killed by Dendritic Cells, but Evades Adaptive Immunity by Failing To Induce Dendritic Cell Maturation

“…Organisms such as Klebsiella pneumoniae evade DC binding through the production of an external polysaccharide capsule that masks the inherent pathogen-associated molecular patterns recognized by PRRs, resulting in decreased binding and internalization of the organism (77). In our studies, however, it is evident that DCs were able to bind the encapsulated C. gattii and internalize the organism, which was a process dependent on actin polymerization (49), indicating that the polysaccharide capsule of C. gattii was not inhibiting recognition or activation of these DC processes.…”

Klebsiella pneumoniae: Going on the Offense with a Strong Defense

“…Capsular K. pneumoniae strains are significantly less likely than acapsular strains to be phagocytosed by innate immune cells in both the presence and absence of opsonins (196). Capsule-mediated prevention of bacterial binding and internationalization by immune cells helps limit early inflammatory signals, resulting in a less robust induction of the immune response (242). Specifically, studies performed with K. pneumoniae-infected lungs found that capsular strains induced lower levels of the proinflammatory cytokines TNF-␣ and IL-6 and higher levels of the anti-inflammatory cytokine IL-10 than did acapsular strains (198,243).…”

Klebsiella pneumoniae O antigen loss alters the outer membrane protein composition and the selective packaging of proteins into secreted outer membrane vesicles

“…Hospital acquired K. pneumoniae infections have a 50% mortality rate if left untreated, largely due to the rapid acquisition of antibiotic resistance (Nordmann et al, 2009;Evrard et al, 2010). Given the clinical significance of this pathogen, knowledge of virulence factors other than the capsule and mechanisms of virulence are lacking.…”

“…We focused on the membrane and vesicles from K. pneumoniae, a nosocomial pathogen associated with urinary tract infections, bacteremia, pneumonia, and pyogenic liver abscess (Podschun and Ullmann, 1998;Cancino-Diaz et al, 2000;Evrard et al, 2010;Hsieh et al, 2012). Hospital acquired K. pneumoniae infections have a 50% mortality rate if left untreated, largely due to the rapid acquisition of antibiotic resistance (Nordmann et al, 2009;Evrard et al, 2010).…”