Activation of human monocyte-derived dendritic cells by Burkholderia pseudomallei does not require binding to the C-type lectin DC-SIGN

Plasmacytoid dendritic cell bactericidal activity against Burkholderia pseudomallei

Melioidosis sepsis, caused by Burkholderia pseudomallei, is associated with high mortality due to an overwhelming inflammatory response. Plasmacytoid dendritic cells (pDC) are potent producers of type I interferons (IFN). This study investigated whether pDC and type I IFN play a role during the early stages of B. pseudomallei infection. Human and murine pDC internalised and killed B. pseudomallei as efficiently as murine conventional DC (cDC). pDC derived from B. pseudomallei-susceptible (BALB/c) mice demonstrated poor intracellular killing and increased IFN-alpha compared to pDC derived from B. pseudomallei-resistant (C57BL/6) mice. This is the first evidence of pDC bactericidal activity against B. pseudomallei infection.

Migration of dendritic cells facilitates systemic dissemination of Burkholderia pseudomallei

Burkholderia pseudomallei, the etiological agent for melioidosis, is an important cause of community-acquired sepsis in northern Australia and northeast Thailand. Due to the rapid dissemination of disease in acute melioidosis, we hypothesized that dendritic cells (DC) could act as a vehicle for dissemination of B. pseudomallei. Therefore, this study investigated the effect of B. pseudomallei infection on DC migration capacity and whether migration of DC enabled transportation of B. pseudomallei from the site of infection. B. pseudomallei stimulated significantly increased migration of bone marrow-derived DC (BMDC), both in vitro and in vivo, compared to uninfected BMDC. Furthermore, migration of BMDC enabled significantly increased in vitro trafficking of B. pseudomallei and in vivo dissemination of B. pseudomallei to secondary lymphoid organs and lungs of C57BL/6 mice. DC within the footpad infection site of C57BL/6 mice also internalized B. pseudomallei and facilitated dissemination. Although DC have previously been shown to kill intracellular B. pseudomallei in vitro, the findings of this study demonstrate that B. pseudomallei-infected DC facilitate the systemic spread of this pathogen.

Interaction of Burkholderia pseudomallei and Burkholderia thailandensis with human monocyte-derived dendritic cells

Burkholderia pseudomallei is the causative agent of melioidosis, a disease endemic in areas of South-East Asia and northern Australia, and is classed as a category B select agent by the Centers for Disease Control and Prevention (CDC). Factors that determine whether host infection is achieved or if disease is chronic or acute are unknown but the type of host immune response that is mounted is important. B. pseudomallei can replicate within macrophages, causing them to multinucleate. In light of the common lineage of macrophages with dendritic cells (DCs), and the role played by DCs in orchestration of the immune response, we investigated the interactions of a variety of B. pseudomallei and B. thailandensis strains with DCs. This study demonstrates that, in the majority of cases, infection of human monocyte-derived dendritic cells is dramatically decreased or cleared by 12 h post-infection, showing a lack of ability to replicate and survive within DCs. Additionally we have shown that B. pseudomallei activates DCs, as measured by cytokine secretion, and live bacteria are not required for activation.

DC-SIGN Family of Receptors

In the immune system, C-type lectins and CTLDs have been shown to act both as adhesion and as pathogen recognition receptors. The Dendritic cell-specific ICAM-3 grabbing non-integrin (DC-SIGN) and its homologs in human and mouse represent an important C-type lectin family. DC-SIGN contains a lectin domain that recognizes in a Ca²⁺-dependent manner carbohydrates such as mannose-containing structures present on glycoproteins such as ICAM-2 and ICAM-3. DC-SIGN is a prototype C-type lectin organized in microdomains, which have their role as pathogen recognition receptors in sensing microbes. Although the integrin LFA-1 is a counter-receptor for both ICAM-2 and ICAM-3 on DC, DC-SIGN is the high affinity adhesion receptor for ICAM-2/-3. While cell–cell contact is a primary function of selectins, collectins are specialized in recognition of pathogens. Interestingly, DC-SIGN is a cell adhesion receptor as well as a pathogen recognition receptor. As adhesion receptor, DC-SIGN mediates the contact between dendritic cells (DCs) and T lymphocytes, by binding to ICAM-3, and mediates rolling of DCs on endothelium, by interacting with ICAM-2. As pathogen receptor, DC-SIGN recognizes a variety of microorganisms, including viruses, bacteria, fungi and several parasites (Cambi et al. 2005). The natural ligands of DC-SIGN consist of mannose oligosaccharides or fucose-containing Lewis-type determinants. In this chapter, we shall focus on the structure and functions of DC-SIGN and related CTLDs in the recognition of pathogens, the molecular and structural determinants that regulate the interaction with pathogen-associated molecular patterns. The heterogeneity of carbohydrate residues exposed on cellular proteins and pathogens regulates specific binding of DC-expressed C-type lectins that contribute to the diversity of immune responses created by DCs (van Kooyk et al. 2003a; Cambi et al. 2005).

Impact of streptozotocin-induced diabetes on functional responses of dendritic cells and macrophages towards Burkholderia pseudomallei

Diabetes mellitus is a documented risk factor for melioidosis, a tropical infection caused by Burkholderia pseudomallei. The increased susceptibility of diabetic individuals to infections with other pathogens has been associated with immune dysregulation. However, the impact of diabetes on the functional responses of dendritic cells (DC) and macrophages during B. pseudomallei infection has not been investigated. This study compared the responses of macrophages and DC towards B. pseudomallei using bone marrow-derived DC (BMDC) and peritoneal elicited macrophages (PEM) isolated from streptozotocin-induced diabetic C57BL/6 mice exhibiting hyperglycaemia for 9 days (acute) or 70 days (chronic) and age-matched nondiabetic C57BL/6 mice. Following coincubation of BMDC and PEM with a highly virulent B. pseudomallei isolate, maturation, bacterial internalization plus intracellular survival and cytokine gene expression profiles were assessed. No significant differences in functional responses of BMDC or PEM isolated from acute diabetic and nondiabetic mice were observed. However, significant differences in BMDC and PEM function were observed when chronic diabetic and nondiabetic mice were compared. This study demonstrates that diabetic mice with extended periods of uncontrolled hyperglycaemia have impaired DC and macrophage function towards B. pseudomallei, which may contribute to the high susceptibility observed in clinical practice.