Multifaceted role of CD14 in innate immunity and tissue homeostasis

CD14 is a co-receptor of Toll-like receptor (TLR)-4, with a critical role in innate immune responses. CD14 recognizes bacterial lipopolysaccharides, pathogen-, and damage-associated molecular patterns, thereby facilitating inflammatory immune responses. In addition to its well-established association with TLR4, CD14 is also implicated in TLR4-independent signaling, which leads to the apoptotic death of differentiated dendritic cells and activation of the noncanonical inflammasome pathway. CD14 also has a role beyond that of the immune responses. It contributes to tissue homeostasis by promoting the clearance of various apoptotic cells via recognizing externalized phosphatidylinositol phosphates. CD14 also has context-dependent roles, particularly in barrier tissues that include the skin and gastrointestinal tract. For example, CD14+ dendritic cells in the skin can induce immunostimulatory or immunosuppressive responses. In the gastrointestinal system, CD14 is involved in producing inflammatory cytokines in inflammatory bowel disease and maintaining of intestinal integrity. This review focuses on the multifaceted roles of CD14 in innate immunity and its potential regulatory functions in barrier tissues characterized by rapid cell renewal. By providing insights into the diverse functions of CD14, this review offers potential therapeutic implications for this versatile molecule in immune modulation and tissue homeostasis.