Protective Role of Mincle in Bacterial Pneumonia by Regulation of Neutrophil Mediated Phagocytosis and Extracellular Trap Formation

Lower respiratory tract infection with bacteria can lead to sepsis development, which is a complex immune disorder characterized by a systemic hyperinflammation. There are currently no effective therapies for sepsis, which results in 750 000 hospitalizations annually in the United States and has a mortality rate of 20%–50% [1, 2]. Nosocomial infections with the opportunistic pathogen Klebsiella pneumoniae account for 5%–20% of cases of gram-negative bacterial sepsis [3–5]. Additionally, emergence of multidrug-resistant isolates of K. pneumoniae in clinical settings is a serious health concern. Because innate mucosal immunity plays a direct role in bacterial killing and immunomodulation in this acute infection [6–10], an understanding of functioning of host innate immune components might provide targets for modulation of the host immune system in a beneficial manner. Mincle is a C-type lectin receptor (CLR) belonging to the dectin-2 subfamily of innate immune receptors that can function as an activating receptor for host-associated molecular patterns (alarmins) and pathogen-associated molecular patterns (PAMPs) [11, 12]. It is an inducible receptor, expressed mainly by myeloid cells such as macrophages, neutrophils, myeloid dendritic cells, and some B-cell subsets [11, 13, 14]. Functional analysis of this receptor in macrophages has received the most attention, where its association with FcRγ activates downstream signaling cascades involving Syk kinases, resulting in induction of protective inflammatory response [15–17]. While the function of Mincle in chronic bacterial infections such as tuberculosis and fungal infections was examined in these studies, its role in acute pneumonic infections leading to sepsis development has not been explored. Furthermore, its functions other than as an inflammatory pattern-recognition receptor (PRR) have received little, if any, attention. Neutrophil-mediated responses are essential for combating pneumonic bacterial infection, and their protective role in sepsis and K. pneumoniae infection in particular has been described [18, 19]. The professional antimicrobial program of neutrophils mainly constitutes phagocytosis of infectious agents, followed by production of noxious agents, such as reactive oxygen species, that kill the internalized microbes. Another recently established mechanism of microbial killing by neutrophils is by their formation of extracellular traps (NETs), which are DNA fibrils expelled by neutrophils and are decorated with granular contents, such as various proteases, that can ensnare and kill microbes without phagocytosis [20–22]. Mincle has been shown to be expressed by neutrophils, and although it plays a role in neutrophil-mediated protective responses against Candida species and mycobacteria [23, 24], its direct role in bacterial phagocytosis and NET formation is not known. Because neutrophils are a key cell type in controlling K. pneumoniae infection, Mincle signaling in neutrophils may be a key event in control of K. pneumoniae infection and sepsis. In this study, we examined the role of Mincle in acute K. pneumoniae infection causing pneumonic sepsis. Our results suggest a novel protective function of Mincle as a nonopsonic phagocytic receptor for bacteria and in regulation of NET formation, indicating the importance of this CLR in neutrophil-specific bacterial clearance mechanisms in pneumonic infections.