"The role of CD14-NFAT pathway in the regulation of innate immune functions"

Innate and adaptive defense mechanisms participate to immunological protection. Innate immune cells like macrophages, neutrophils and dendritic cells (DCs) regulate the first part of the response, which is rapidly activated at the site of infection. At this stage, phagocytic cells eliminate pathogens from the infected tissue and additionally establish a local inflammatory state by releasing proinflammatory cytokines (e.g. TNF-alpha, IL-1beta) and lipid mediators (PGE2). Subsequently, activated tissue-resident DCs acquire a migratory phenotype and reach the draining lymph node to prime antigen-specific T cells for the initiation of adaptive immunity. Innate immune cells detect invading pathogens through a set of receptors recognizing conserved patterns that are unique to microbes. Specifically, Gram-negative bacteria are perceived by means of TLR4 and CD14, which act as a receptor complex for lipopolysaccharide (LPS), the main constituent of the bacterial cell wall. Upon LPS recognition, TLR4 and CD14 cooperate to trigger signal transduction cascades leading to activation of the transcription factors NF-kappaB, AP-1 and IRF3, which in turn promote expression of proinflammatory genes. Despite the signaling pathways induced by LPS have been mainly described in macrophages, it is widely believed that they do not differ significantly across innate immune cell types, including DCs. However, DCs do show very specific biological responses to LPS stimulation such as the ability to release IL-2, a key factor for Natural Killer (NK) cell activation. We found that CD14 triggers a pathway involving Ca++ influx, Src family kinases (SFKs) and PLC-gamma2 that leads to NFAT activation in LPS-stimulated DCs. Notably, activation of this pathway was DC-specific and TLR4-independent. We also showed that CD14-NFAT pathway promotes the expression of pro-apoptotic genes that induce apoptosis of activated DCs as a strategy to prevent excessive immune responses. Our work revealed several novel aspects as: (i) the unexpected capability of CD14 to function independently of TLR4 as a transducing receptor; (ii) the previously unrecognized biological role of NFAT in DCs; (iii) a molecular explanation for the differential life cycle of DCs and macrophages after activation, consistent with their specific physiological role in innate immunity. Additional work revealed that LPS-induced NFAT activation in DCs is also necessary for the efficient synthesis of PGE2, a crucial lipid mediator regulating many proinflammatory processes such as swelling and pain. We also showed that tissue edema formation induced by subcutaneous administration of LPS is CD14-NFAT-dependent, and that DCs play a major role in this process. Not only these results highlight another crucial biological function of this signal transduction pathway in DCs, but they also point to a key direct role of this cell type in the proinflammatory response to LPS. These data suggest that CD14 signaling in DCs may constitute a valuable target for the development of a novel class of anti-inflammatory compounds devoid of the secondary effects observed with COX-2 inhibitors.