Mucocutaneous fungal infections are typically found in patients who have no known immune defects. We describe a family in which four women who were affected by either recurrent vulvovaginal candidiasis or onychomycosis had the early-stop-codon mutation Tyr238X in the β-glucan receptor dectin-1. The mutated form of dectin-1 was poorly expressed, did not mediate β-glucan binding, and led to defective production of cytokines (interleukin-17, tumor necrosis factor, and interleukin-6) after stimulation with β-glucan or Candida albicans. In contrast, fungal phagocytosis and fungal killing were normal in the patients, explaining why dectin-1 deficiency was not associated with invasive fungal infections and highlighting the specific role of dectin-1 in human mucosal antifungal defense.

Glycosylation of proteins has proven extremely important in a variety of cellular processes, including enzyme trafficking, tissue homing and immune functions. In the past decade, increasing interest in carbohydrate‐mediated mechanisms has led to the identification of novel carbohydrate‐recognizing receptors expressed on cells of the immune system. These non‐enzymatic lectins contain one or more carbohydrate recognition domains (CRDs) that determine their specificity. In addition to their cell adhesion functions, lectins now also appear to play a major role in pathogen recognition. Depending on their structure and mode of action, lectins are subdivided in several groups. In this review, we focus on the calcium (Ca²⁺)‐dependent lectin group, known as C‐type lectins, with the dendritic cell‐specific ICAM‐3 grabbing non‐integrin (DC‐SIGN) as a prototype type II C‐type lectin organized in microdomains, and their role as pathogen recognition receptors in sensing microbes. Moreover, the cross‐talk of C‐type lectins with other receptors, such as Toll‐like receptors, will be discussed, highlighting the emerging model that microbial recognition is based on a complex network of interacting receptors.

Dendritic cells (DC) that express the type II C‐type lectin DC‐SIGN (CD209) are located in the submucosa of tissues, where they mediate HIV‐1 entry. Interestingly, the pathogen Candida albicans,the major cause of hospital‐acquired fungal infections, penetrates at similar submucosal sites. Here we demonstrate that DC‐SIGN is able to bind C. albicans both in DC‐SIGN‐transfected cell lines and in human monocyte‐derived DC. The binding was shown to be time‐ as well as concentration‐dependent, and live as well as heat‐inactivated C. albicans were bound to the same extent. Moreover, in immature DC, DC‐SIGN was able to internalize C. albicans in specific DC‐SIGN‐enriched vesicles, distinct from those containing the mannose receptor, the other known C. albicans receptor expressed by DC. Together, these results demonstrate that DC‐SIGN is an exquisite pathogen‐uptake receptor that captures not only viruses but also fungi.

Early studies have revealed that some mammalian plasma membrane proteins exist in small nanoclusters. The advent of super-resolution microscopy has corroborated and extended this picture, and led to the suggestion that many, if not most, membrane proteins are clustered at the plasma membrane at nanoscale lengths. In this Commentary, we present selected examples of glycosylphosphatidyl-anchored proteins, Ras family members and several immune receptors that provide evidence for nanoclustering. We advocate the view that nanoclustering is an important part of the hierarchical organization of proteins in the plasma membrane. According to this emerging picture, nanoclusters can be organized on the mesoscale to form microdomains that are capable of supporting cell adhesion, pathogen binding and immune cell-cell recognition amongst other functions. Yet, a number of outstanding issues concerning nanoclusters remain open, including the details of their molecular composition, biogenesis, size, stability, function and regulation. Notions about these details are put forth and suggestions are made about nanocluster function and why this general feature of protein nanoclustering appears to be so prevalent.

Mature dendritic cells (mDCs) can trigger the effector functions of natural killer (NK) cells. Knock-out, small-interfering RNA or neutralizing antibodies targeting interleukin 12 (IL-12) subunits revealed a critical role for IL-12 in NK cell interferon γ (IFN-γ) secretion promoted by mDCs. However, NK cell activation by DCs also required direct cell-to-cell contacts. DC-mediated NK cell activation involved the formation of stimulatory synapses between DCs and NK cells. The formation of DC/NK cell conjugates depended on cytoskeleton remodeling and lipid raft mobilization in DCs. Moreover, the disruption of the DC cytoskeleton using pharmacologic agents or the loss-of-function mutation of the Wiskott-Aldrich syndrome protein abolished the DC-mediated NK cell activation. Synapse formation promoted the polarized secretion of preassembled stores of IL-12 by DCs toward the NK cell. The synaptic delivery of IL-12 by DCs was required for IFN-γ secretion by NK cells, as assessed using inhibitors of cytoskeleton rearrangements and transwell experiments. Therefore, the cross-talk between DCs and NK cells is dictated by functional synapses. (Blood. 2004;104:3267-3275)

Recruitment of receptor proteins to lipid rafts has been proposed as an important mechanism to regulate their cellular function. In particular, rafts have been implicated in regulation of integrin-mediated cell adhesion, although the underlying mechanism remains elusive. We used single-molecule near-field optical microscopy (NSOM) with localization accuracy of approximately 3 nm, to capture the spatio-functional relationship between the integrin LFA-1 and raft components (GPI-APs) on immune cells. Dual color nanoscale imaging revealed the existence of a nanodomain GPI-AP subpopulation that further concentrated in regions smaller than 250 nm, suggesting a hierarchical prearrangement of GPI-APs on resting monocytes. We previously demonstrated that in quiescent monocytes, LFA-1 preorganizes in nanoclusters. We now show that integrin nanoclusters are spatially different but reside proximal to GPI-AP nanodomains, forming hotspots on the cell surface. Ligand-mediated integrin activation resulted in an interconversion from monomers to nanodomains of GPI-APs and the generation of nascent adhesion sites where integrin and GPI-APs colocalized at the nanoscale. Cholesterol depletion significantly affected the reciprocal distribution pattern of LFA-1 and GPI-APs in the resting state, and LFA-1 adhesion to its ligand. As such, our data demonstrate the existence of nanoplatforms as essential intermediates in nascent cell adhesion. Since raft association with a variety of membrane proteins other than LFA-1 has been documented, we propose that hotspots regions enriched with raft components and functional receptors may constitute a prototype of nanoscale inter-receptor assembly and correspond to a generic mechanism to offer cells with privileged areas for rapid cellular function and responses to the outside world.