![]() These are found at low frequency, but they have unique features and expand with cytokine treatment and in inflammation. 9, 10 In addition, monocytes with an intermediate phenotype between classical and CD14 lowCD16 + monocyte subsets have been described. The more recent approach of expression profiling and hierarchical clustering has substantiated the close relationship of the 2 types of cells. 7, 8 The classical CD16 − monocytes and these CD16 + cells were shown to share morphology, cytochemistry, and many cell-surface markers. 4-6 In addition, these cells were shown to expand in inflammatory diseases. This technology enabled the identification of a CD16 + subpopulation, 3 which is characterized by higher major histocompatibility complex (MHC) class II expression and after stimulation by Toll-like receptor (TLR) ligands by higher tumor necrosis factor (TNF) production. 2 In humans, monocytes were initially defined on the basis of morphology and cytochemistry (monocyte-specific esterase) and later by flow cytometry that was based on light scatter properties and on cell-surface markers such as CD14. They are multifunctional with roles in homeostasis, immune defense, and tissue repair, and they were shown to express an extremely diverse transcriptome. 1 Cells of this lineage are collectively referred to as mononuclear phagocytes or monocytes/macrophages. Early studies have shown that bone marrow precursors give rise to monocytes in blood, which circulate for a few days before they migrate into tissue where they develop into different types of macrophages.
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