Monocyte-mediated defense against bacteria, fungi, and parasites

• Monocytes are essential for host defense against bacteria, fungi, and parasites.
• Monocytes can sense, engulf and kill these pathogens.
• Monocytes regulate innate and adaptive immune responses.
• Monocyte responses are specific to pathogens, bacterial commensals, and tissue context.

Circulating blood monocytes are a heterogeneous leukocyte population that contributes critical antimicrobial and regulatory functions during systemic and tissue-specific infections. These include patrolling vascular tissue for evidence of microbial invasion, infiltrating peripheral tissues and directly killing microbial invaders, conditioning the inflammatory milieu at sites of microbial tissue invasion, and orchestrating the activation of innate and adaptive immune effector cells. The central focus of this review is the in vivo mechanisms by which monocytes and their derivative cells promote microbial clearance and immune regulation. We include an overview of murine models to examine monocyte functions during microbial challenges and review our understanding of the functional roles of monocytes and their derivative cells in host defense against bacteria, fungi, and parasites.