Signaling C-type lectin receptors in antimycobacterial immunity

The mammalian innate immune system is composed of phagocytes such as macrophages and dendritic cells that serve as the first line of defense against microbial infections. These cells express various pattern recognition receptors (PRRs) that recognize specific pathogen-associated molecular patterns (PAMPs) on the surface of or inside microorganisms [1]. PRRs such as Toll-like receptors (TLRs), C-type lectin receptors (CLRs), and Nucleotide-binding Oligomerization Domain (NOD)-like receptors (NLRs) have been widely studied in antimicrobial immunity and homeostasis. These PRRs have also been implicated in antimycobacterial immunity, with CLRs recently receiving considerable attention. CLRs are a large family of proteins containing at least 1 carbohydrate-recognition domain (CRD) that in most cases binds a range of carbohydrate-based PAMPs, including trehalose 6,6’ dimycolate (TDM), lipoarabinomannan (LAM), lipomannan (LM), and phosphatidylinositol mannosides (PIMs) [2–4]. Interactions of CLRs with mycobacterial PAMPs induce intracellular signaling that triggers responses ranging from cytokine production to induction of adaptive immunity (Table 1). Here, we discuss signaling CLRs that recognize mycobacterial PAMPs and contribute to antimycobacterial immunity. We focus on the receptors that signal through the Spleen tyrosine kinase (Syk)/Caspase recruitment domain family member 9 (CARD9) pathway, including Dectin-1, Dectin-2, macrophage-inducible C-type lectin (Mincle), C-type lectin superfamily member 8 (Clecsf8) also called macrophage C-type lectin (MCL), and dendritic cell immunoactivating receptor (DCAR) (Fig 1).