Immune regulation by monocytes.
Murray Peter J
Seminars in immunology
Monocytes emerging from the bone marrow are the progenitors of monocyte-derived macrophages. An essential function of monocytes is to seed tissues with sufficient macrophages to replace loss from infection and tissue damage. Recent work from diverse inflammatory and homeostatic settings has shown monocytes also possess direct protective and pathogenic activities. Thus, monocytes are not simply needed to generate macrophages, but instead contribute to the overall orchestration of immunity. Some recently described properties of monocytes are both surprising and mechanistically specific; for example, inflammatory monocytes are required for the efficacy of transferred activated cytotoxic T cells, but can have potent tissue damaging effects while patrolling monocytes are required for anti-tumor immunity in some cases, but in another example provokes resistance to chemotherapy and thereby aid tumor growth. This summary will therefore focus on new findings about the regulatory activities of monocytes themselves.
Contributions of monocytes to nervous system disorders.
Garré Juan Mauricio,Yang Guang
Journal of molecular medicine (Berlin, Germany)
Monocytes are a class of leukocytes derived from progenitors in the bone marrow and are prevalent in the blood stream. Although the main function of monocytes is to provide innate immune defenses against infection and injury, their contributions to the central nervous system (CNS) disorders are increasingly recognized. In this review article, we summarize the molecular and physiological properties of monocytes in relation to other myeloid cells. Primarily, we discuss how monocytes (or leukocytes) may affect neuronal function in diseases that are characterized by dysregulated innate immunity and cognitive dysfunction. Under these pathological conditions, monocytes and monocyte-derived cells (1) fail to remove neurotoxic products from CNS, (2) interact with astrocytes at the periphery-brain interfaces to alter synapse development and plasticity, or (3) infiltrate into the CNS to exacerbate neuroinflammation. Through these cellular mechanisms, we speculate that monocytes and other peripheral immune cells may affect brain functioning and contribute to behavioral and cognitive deficits. Better understanding of neuroimmune interactions will help the development of strategies to ameliorate neuronal and cognitive dysfunction associated with dysregulated innate immunity.
Capturing the Fantastic Voyage of Monocytes Through Time and Space.
Teh Ye Chean,Ding Jeak Ling,Ng Lai Guan,Chong Shu Zhen
Frontiers in immunology
Monocytes are a subset of cells that are categorized together with dendritic cells (DCs) and macrophages in the mononuclear phagocyte system (MPS). Despite sharing several phenotypic and functional characteristics with MPS cells, monocytes are unique cells with the ability to function as both precursor and effector cells in their own right. Before the development of hematopoietic stem cells (HSCs) , monocytes are derived from erythro-myeloid precursors (EMPs) in the fetal liver that are important for populating the majority of tissue resident macrophages. After birth, monocytes arise from bone marrow (BM)-derived HSCs and are released into the circulation upon their maturation, where they survey peripheral tissues and maintain endothelial integrity. Upon sensing of microbial breaches or inflammatory stimuli, monocytes migrate into tissues where their plasticity allows them to differentiate into cells that resemble macrophages or DCs according to the environmental niche. Alternatively, they may also migrate into tissues in the absence of inflammation and remain in an undifferentiated state where they perform homeostatic roles. As monocytes are typically on the move, the availability of intravital imaging approaches has provided further insights into their trafficking patterns in distinct tissue compartments. In this review, we outline the importance of understanding their functional behavior in the context of tissue compartments, and how these studies may contribute towards improved vaccine and future therapeutic strategies.
Nonclassical Monocytes in Health and Disease.
Narasimhan Prakash Babu,Marcovecchio Paola,Hamers Anouk A J,Hedrick Catherine C
Annual review of immunology
Monocytes are innate blood cells that maintain vascular homeostasis and are early responders to pathogens in acute infections. There are three well-characterized classes of monocytes: classical (CD14CD16 in humans and Ly6C in mice), intermediate (CD14CD16 in humans and Ly6CTreml4 in mice), and nonclassical (CD14CD16 in humans and Ly6C in mice). Classical monocytes are critical for the initial inflammatory response. Classical monocytes can differentiate into macrophages in tissue and can contribute to chronic disease. Nonclassical monocytes have been widely viewed as anti-inflammatory, as they maintain vascular homeostasis. They are a first line of defense in recognition and clearance of pathogens. However, their roles in chronic disease are less clear. They have been shown to be protective as well as positively associated with disease burden. This review focuses on the state of the monocyte biology field and the functions of monocytes, particularly nonclassical monocytes, in health and disease.
Developmental and Functional Heterogeneity of Monocytes.
Guilliams Martin,Mildner Alexander,Yona Simon
Novel experimental approaches such as fate-mapping and single-cell analysis have brought fresh insight into monocyte development and function over the past decade and helped redefine the monocyte field. Monocytes are now known to consist of multiple subsets generated through distinct developmental pathways with diverse functional specializations. Their fates under homeostatic conditions include the accumulation in peripheral reservoirs and the engraftment into certain resident macrophage pools. Under pathological conditions, monocytes acquire inflammatory effector functions, but can also develop regulatory properties essential for tissue repair. Importantly, monocytes recruited during inflammation are often functionally distinct from resident macrophages or conventional dendritic cells. Here we outline emerging concepts in monocyte heterogeneity, emergency monopoiesis, and trained immunity and discuss how these bring new perspectives to monocyte research.