Abstract
Myeloid cells such as monocytes, dendritic cells (DC) and macrophages (MΦ) are key components of the innate immune system contributing to the maintenance of tissue homeostasis and the development/resolution of immune responses to pathogens. Monocytes and DC, circulating in the blood or infiltrating various lymphoid and non-lymphoid tissues, are derived from distinct bone marrow precursors and are typically short lived. Conversely, recent studies revealed that subsets of tissue resident MΦ are long-lived as they originate from embryonic/fetal precursors that have the ability to self-renew during the life of an individual. Pathogens such as the human immunodeficiency virus type 1 (HIV-1) highjack the functions of myeloid cells for viral replication (e.g., MΦ) or distal dissemination and cell-to-cell transmission (e.g., DC). Although the long-term persistence of HIV reservoirs in CD4+ T-cells during viral suppressive antiretroviral therapy (ART) is well documented, the ability of myeloid cells to harbor replication competent viral reservoirs is still a matter of debate. This review summarizes the current knowledge on the biology of monocytes and DC during homeostasis and in the context of HIV-1 infection and highlights the importance of future studies on long-lived resident MΦ to HIV persistence in ART-treated patients.
Highlights
IntroductionCells of the innate immune system, such as myeloid cells (e.g., monocytes, macrophages (MΦ)
Cells of the innate immune system, such as myeloid cells (e.g., monocytes, macrophages (MΦ)and dendritic cells (DC)), contribute to the first line of defense against pathogens
This is in contrast to previous studies by Schakel et al reporting that IL-12 production is associated with M-DC8+ monocytes [69], which are mainly non-classical monocytes [51]
Summary
Cells of the innate immune system, such as myeloid cells (e.g., monocytes, macrophages (MΦ). Monocytes belong to the innate arm of the immune system providing responses against viral, bacterial, fungal or parasitic infections [10,13] Their functions include the killing of pathogens via phagocytosis, the production of reactive oxygen species (ROS), nitric oxide (NO), myeloperoxidase and inflammatory cytokines [13]. Monocytes can stimulate or inhibit T-cell responses during cancer as well as infectious and autoimmune diseases They are involved in tissue repair and neovascularization [14]. In the intestine, the majority of MΦ and DC derive from blood monocytes that are continuously replenished [11] These new insights in the ontogeny of myeloid cells may have profound implication for understanding disease pathogenesis and optimizing treatment. This may explain the transcriptional and functional heterogeneity of monocyte subsets in the blood, subsets that represent distinct stages of differentiation [8,30,31,32]
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