Role of C-type lectins in mycobacterial infections

Worldwide clinical cases due to multi drug- and extensively drug-resistant strains of Mycobacterium tuberculosis (M.tb) are increasing making the need for new therapies more critical than ever. A major obstacle for designing new drugs to treat mycobacterial infections is our limited knowledge of the interface between the bacillus (especially M.tb) and its host. The pulmonary innate immune system plays a key role in the recognition of microbes entering via the respiratory route. Although the specificity of this system is broad and based on the recognition of pathogen-associated molecular patterns (PAMPs), it is uniquely regulated to limit inflammation and thereby prevent damage to the gas-exchanging alveoli. Pulmonary surfactant proteins A and D (SP-A and SP-D) are collagenous, soluble, C-type (Ca(2+)-dependent) lectins (named collectins) of the lung innate immune system that are secreted into the alveoli by resident type II alveolar epithelial cells and distal bronchiolar Clara cells. The related collectin in serum, mannose-binding lectin/protein (MBL or MBP), provides first-line defense against several microbes. Phagocytes represent the first cellular defense in the alveoli and their surface is rich in C-type lectin pattern recognition receptors (PRRs), including the mannose receptor (MR), dendritic cell-specific ICAM-3-grabbing nonintegrin (DC-SIGN) and DC-associated C-type lectin-1 (Dectin-1). This review will discuss the important roles of the cell-associated C-type lectin PRRs and soluble collectins in the innate immune response to mycobacterial infections, and will present the current state of knowledge regarding the potential uses of these C-type lectins in therapy against infections, focusing on M.tb.