Function of the lectin domain of Mac-1/complement receptor type 3 (CD11b/CD18) in regulating neutrophil adhesion

HIV-1 and its transmembrane protein gp41 bind to different Candida species modulating adhesion

Oral candidiasis in HIV-1-infected individuals is widely believed to be triggered by the acquired T-lymphocyte immunodeficiency. Recently, binding of the HIV-1 envelope protein gp160 and its subunit gp41, and also of the whole virus itself, to Candida albicans has been shown. The present study shows that, in addition to C. albicans, HIV-1 gp41 also binds to yeast and hyphal forms of Candida dubliniensis, a species which is closely related to C. albicans, and to Candida tropicalis but not to Candida krusei, Candida glabrata or Saccharomyces cerevisiae. The previous finding that gp41 binding to C. albicans augments fungal virulence in vitro is supported by the observation that the yeast showed an enhanced adhesion to HIV-infected H9 cells in comparison to uninfected cells. In line with these results soluble gp41 itself reduced binding of C. albicans to both endothelial and epithelial cell lines, confirming a dominant role of the gp41 binding moiety on the surface of Candida for adhesion. Surface-associated secreted aspartic proteinases (Saps) play an important role in candidial adhesion, but are not likely to be involved in the interaction as gp41 binding to the C. albicans parental wild-type strain was comparable to that of three different isogenic Sap deletion mutants. Furthermore, gp41 binding to the yeast killer toxin-susceptible C. albicans strain 10S was not inhibitable by an anti-YKT receptor antibody. In conclusion, HIV-1 interacts with different clinically important Candida spp., and may thereby affect the outcome of the respective fungal infection.

Coxiella burnetii avoids macrophage phagocytosis by interfering with spatial distribution of complement receptor 3

Phagocytosis is a highly localized event requiring the formation of spatially and temporally restricted signals. Numerous microorganisms have taken advantage of this property to invade host cells. Coxiella burnetii, the agent of Q fever, is an obligate intracellular bacterium that has developed a survival strategy in macrophages based on subversion of receptor-mediated phagocytosis. The uptake of C. burnetii is mediated by alpha(v)beta(3) integrin and is restricted by impaired cross-talk of alpha(v)beta(3) integrin and complement receptor 3 (CR3) (CD11b/CD18). In this study, we showed that CR3 molecules remained outside the pseudopodal extensions induced by C. burnetii in THP-1 monocytes, although alpha(v)beta(3) integrin was present in the pseudopods. Chemoattractants such as RANTES restored CR3 localization to the front of pseudopodal extensions and increased C. burnetii phagocytosis, demonstrating that the localization of CR3 is critical for bacterial uptake. In addition, monocyte activation due to the expression of HIV-1 Nef protein also restored CR3-mediated phagocytosis of C. burnetii by allowing CR3 redistribution toward bacterial-induced pseudopods. The redistribution of CR3 and increased C. burnetii phagocytosis in THP-1 cells stimulated by RANTES or expressing Nef were associated with the inhibition of intracellular replication of C. burnetii. Hence, the localization of CR3 is critical for bacterial phagocytosis and also for the control of bacterial replication. This study describes a nonpreviously reported strategy of phagocytosis subversion by intracellular pathogens based on altered localization of monocyte receptors.