SUMMARY 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.

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IntroductionAntibodies have been studied extensively for their use in immunotherapy of cancer. [1][2][3][4] It is evident that receptors for the Fc portion of immunoglobulins (FcRs) on myeloid cells are critical in triggering antitumor cytotoxicity in vivo. 5,6 Antibody-dependent cellular cytotoxicity (ADCC), considered crucial for antibody-mediated tumor cell degradation, can be mediated by polymorphonuclear leukocytes (PMNs), monocytes/macrophages, eosinophils, and natural killer (NK) cells. 7,8 These effector cells use different cytotoxic mechanisms, depending on their activation state and the nature of the target. 7,9-12 PMNs, representing the most populous type of white blood cell, exhibit fast recruitment activity in vivo. Potent and very rapid (within 30 minutes) PMN cytotoxicity toward various tumor targets has been documented. 10,[13][14][15] Two classes of immunoglobulin (Ig)G receptors (Fc␥RIIa, CD32, and Fc␥RIIIb, CD16) and one class of IgA receptor (Fc␣RI, CD89) have been identified on human PMNs, whereas Fc␥RI (CD64) expression is inducible on PMNs on stimulation with granulocyte colony-stimulating factor. 16 PMNs can trigger ADCC by engagement of Fc␥RI, Fc␥RIIa, and Fc␣RI. Fc␣RI, however, has been observed to be the most effective FcR for PMN-mediated tumor cell killing. 14,17 Mac-1 (CR3, CD11b/CD18) is a member of the ␤ 2 integrin family, which includes Mac-1, LFA-1 (CD11a/CD18), and gp150/95 (CR4, CD11c/CD18). 18,19 These receptors, sharing a common ␤-chain (CD18), can bind multiple ligands and can regulate various leukocyte functions. Mac-1 represents the predominant ␤ 2 integrin on PMNs and is furthermore expressed on monocytes/macrophages and NK cells. Several PMN functions are regulated by Mac-1, including adhesion, migration, chemotaxis, phagocytosis, respiratory burst activity, and degranulation. 18 On activation, Mac-1 is able to initiate signaling by its linkage to the actin cytoskeleton and associated signaling proteins. 20,21 A number of studies described Mac-1 cooperation with different receptors on PMNs, indicating Mac-1 to be a signaling partner for other receptors. 22 These include FMLP receptors, LPS/LBP receptors (CD14), urokinase plasminogen activator receptor (CD87), and Fc receptors. [22][23][24][25][26] Mac-1 was found to trigger Ab-dependent phagocytosis by Fc␥RIIIb in fibroblasts transfected with both Mac-1 and Fc␥RIIIb, whereas cells expressing only Mac-1 or Fc␥RIIIb were unable to ingest Ab-opsonized particles. 27 Mac-1 cooperation with Fc␥RIIIb in the generation of PMN respiratory burst has been described as well. 28 Furthermore, Mac-1 restored IgG-dependent phagocytosis of transfectants with Fc␥RIIa tail-minus mutants. 29 Importantly, PMNs from patients with leukocyte adhesion deficiency, who lack CD18, were shown to be severely impaired in mediating phagocytosis and ADCC. 18,[30][31][32] Although the relative contribution of individual ␤ 2 integrins remains to be determined, various studies point to an important role of Mac-1 in FcR-mediated cytotoxicity. Targets st...

Background. Intensive treatment of hematological malignancies with hematopoietic stem cell transplantation (HSCT) is accompanied by a high incidence of opportunistic invasive fungal infection, but individual risk varies significantly. Dectin-1, a C-type lectin that recognizes 1,3-beta-glucans from fungal pathogens, including Candida species, is involved in the initiation of the immune response against fungi. Methods. Screening for the DECTIN-1 Y238X polymorphism within a group of 142 patients undergoing HSCT was correlated with Candida colonization and candidemia. Furthermore, functional studies were performed on the consequences of the polymorphism. Results. Patients bearing the Y238X polymorphism in the DECTIN-1 gene were more likely to be colonized with Candida species, compared with patients bearing wild-type DECTIN-1, necessitating more frequent use of fluconazole in the prevention of systemic Candida infection. Functional assays demonstrated a loss-of-function phenotype of the polymorphism, as shown by the decreased cytokine production by immune cells bearing this polymorphism. Conclusions. The Y238X polymorphism is associated with increased oral and gastrointestinal colonization with Candida species. This suggests a crucial role played by dectin-1 in the mucosal antifungal mechanisms in immunocompromised hosts. The finding that DECTIN-1 polymorphisms rendered HSCT recipients at increased risk for fungal complications may contribute to the selection of high-risk patients who should be considered for antifungal prophylaxis to prevent systemic candidiasis.

A contemporary goal of researchers in leucocyte signalling has been to uncover how cells physically organize and compartmentalize signalling molecules into efficient, regulated signalling networks. This work has revealed important roles of membrane microdomains that are characterized by their distinctive protein and lipid compositions. Recent studies have demonstrated that besides typical cholesterol- and glycosphingolipid-enriched 'rafts', leucocyte membranes are equipped with a different type of microdomain, made up of tetraspanin proteins. Tetraspanin proteins are involved in the organization of tetraspanin-enriched microdomains by virtue of their capacity to specifically associate with key molecules, including integrins, leucocyte receptors and signalling proteins. The aspects of leucocyte function influenced by tetraspanin microdomains include adhesion, proliferation and antigen presentation. However, the mechanisms by which tetraspanin complexes link to intracellular signalling pathways, are still largely unknown. This review discusses how tetraspanin microdomains might function to regulate signalling in lymphoid and myeloid cells, and how they relate to lipid rafts. In addition, we discuss new insights into the role of tetraspanins in malignant disease.

The spatial organization of membrane proteins in the plasma membrane is critical for signal transduction, cell communication and membrane trafficking. Tetraspanins organize functional higher-order protein complexes called ‘tetraspanin-enriched microdomains (TEMs)’ via interactions with partner molecules and other tetraspanins. Still, the nanoscale organization of TEMs in native plasma membranes has not been resolved. Here, we elucidated the size, density and distribution of TEMs in the plasma membrane of human B cells and dendritic cells using dual color stimulated emission depletion (STED) microscopy. We demonstrate that tetraspanins form individual nanoclusters smaller than 120 nm and quantified that a single tetraspanin CD53 cluster contains less than ten CD53 molecules. CD53 and CD37 domains were adjacent to and displayed only minor overlap with clusters containing tetraspanins CD81 or CD82. Moreover, CD53 and CD81 were found in closer proximity to their partners MHC class II and CD19 than to other tetraspanins. Although these results indicate that tetraspanin domains are adjacently positioned in the plasma membrane, they challenge the current view of the tetraspanin web of multiple tetraspanin species organized into a single domain. This study increases the molecular understanding of TEMs at the nanoscale level which is essential for comprehending tetraspanin function in cell biology.

To facilitate the myriad of different (signaling) processes that take place at the plasma membrane, cells depend on a high degree of membrane protein organization. Important mediators of this organization are tetraspanin proteins. Tetraspanins interact laterally among themselves and with partner proteins to control the spatial organization of membrane proteins in large networks called the tetraspanin web. The molecular interactions underlying the formation of the tetraspanin web were hitherto mainly described based on their resistance to different detergents, a classification which does not necessarily correlate with functionality in the living cell. To look at these interactions from a more physiological point of view, this review discusses tetraspanin interactions based on their function in the tetraspanin web: (1) intramolecular interactions supporting tetraspanin structure, (2) tetraspanin-tetraspanin interactions supporting web formation, (3) tetraspanin-partner interactions adding functional partners to the web and (4) cytosolic tetraspanin interactions regulating intracellular signaling. The recent publication of the first full-length tetraspanin crystal structure sheds new light on both the intra- and intermolecular tetraspanin interactions that shape the tetraspanin web. Furthermore, recent molecular dynamic modeling studies indicate that the binding strength between tetraspanins and between tetraspanins and their partners is the complex sum of both promiscuous and specific interactions. A deeper insight into this complex mixture of interactions is essential to our fundamental understanding of the tetraspanin web and its dynamics which constitute a basic building block of the cell surface.