InvivoGen (France)

Dectin-2 is a C-type lectin involved in the recognition of several pathogens such as Aspergillus fumigatus, Candida albicans, Schistosoma mansonii, and Mycobacterium tuberculosis that triggers Th17 immune responses. Identifying pathogen ligands and understanding the molecular basis of their recognition is one of the current challenges. Purified M. tuberculosis mannose-capped lipoarabinomannan (ManLAM) was shown to induce signaling via Dectin-2, an activity that requires the (α1 → 2)-linked mannosides forming the caps. Here, using isogenic M. tuberculosis mutant strains, we demonstrate that ManLAM is a bona fide and actually the sole ligand mediating bacilli recognition by Dectin-2, although M. tuberculosis produces a variety of cell envelope mannoconjugates, such as phosphatidyl-myo-inositol hexamannosides, lipomannan or manno(lipo)proteins, that bear (α1 → 2)-linked mannosides. In addition, we found that Dectin-2 can recognize lipoglycans from other bacterial species, such as Saccharotrix aerocolonigenes or the human opportunistic pathogen Tsukamurella paurometabola, suggesting that lipoglycans are prototypical Dectin-2 ligands. Finally, from a structure/function relationship perspective, we show, using lipoglycan variants and synthetic mannodendrimers, that dimannoside caps and multivalent interaction are required for ligand binding to and signaling via Dectin-2. Better understanding of the molecular basis of ligand recognition by Dectin-2 will pave the way for the rational design of potent adjuvants targeting this receptor.

We describe novel STING-activating cyclic dinucleotides whose constituent nucleosides are adenosine and inosine and that vary by ribose substitution, internucleotide linkage position, and phosphate modification. In mammalian cells in vitro, some of these cAIMP analogs induce greater STING-dependent IRF and NF-κB pathway signaling than do the reference agonists for murine (DMXAA) or human (2',3'-cGAMP) STING. In human blood ex vivo, they induce type I interferons (IFNs) and proinflammatory cytokines: for the former, 3',3'-cAIMP (9; EC50 of 6.4 μM) and analogs 52-56 (EC50 of 0.4-4.7 μM), which contain one or two 2'-fluoro-2'-deoxyriboses and/or bis-phosphorothioate linkages, are more potent than 2',3'-cGAMP (EC50 of 19.6 μM). Interestingly, 9 induces type I IFNs more strongly than do its linkage isomers 2',3'-cAIMP (10), 3',2'-cAIMP (23), and 2',2'-cAIMP (27). Lastly, some of the cAIMP analogs are more resistant than 2',3'-cGAMP to enzymatic cleavage in vitro. We hope to exploit our findings to develop STING-targeted immunotherapies.

Author SummarySecretory IgA (SIgA) antibodies are secreted into the gut lumen and are considered to be a first line of defense in protecting the intestinal epithelium from gut pathogens. SIgA patrol the mucus and are usually known to help immune tolerance via entrapping dietary antigens and microorganisms and other mechanisms. SIgA, in complex with its antigens, can also be taken back up by the intestinal epithelium in a process known as reverse transcytosis. SIgA can thereby promote the uptake and delivery of antigens from the intestinal lumen to the Gut-Associated Lymphoid Tissues (GALT), influencing inflammatory responses. This reverse transcytosis of SIgA is mediated by specialized epithelial M cells. Because M cells possess the ability to take up antigens and are therefore important to the local immune system, they are a key target for the specific delivery of novel mucosal vaccines against various diseases. M cell receptors that take up the SIgA-antigen complexes, which serve as mucosal vaccine vehicles, represent an important aspect of this vaccine strategy. The identification of SIgA receptor(s) on the surface of M cells has, however, remained elusive for more than a decade. In this study, we now identify Dectin-1 and Siglec-5 as the key receptors for M cell–mediated reverse transcytosis of SIgA complexes. We further find that the glycosylation modification, and particularly sialylation, of SIgA is required for its uptake by M cells. We show that, when administered orally in complex with SIgA, the HIV p24 antigen is taken up in a strictly Dectin-1-dependent manner to stimulate a mucosal and systemic antibody response. These findings are considered important for understanding gut immunity.