The Mononuclear Phagocyte System: The Relationship between Monocytes and Macrophages

Trends Immunol. 2019 Feb;40(2):98-112. doi: 10.1016/ Epub 2018 Dec 19.


The mononuclear phagocyte system (MPS) is defined as a cell lineage in which committed marrow progenitors give rise to blood monocytes and tissue macrophages. Here, we discuss the concept of self-proscribed macrophage territories and homeostatic regulation of tissue macrophage abundance through growth factor availability. Recent studies have questioned the validity of the MPS model and argued that tissue-resident macrophages are a separate lineage seeded during development and maintained by self-renewal. We address this issue; discuss the limitations of inbred mouse models of monocyte-macrophage homeostasis; and summarize the evidence suggesting that during postnatal life, monocytes can replace resident macrophages in all major organs and adopt their tissue-specific gene expression. We conclude that the MPS remains a valid and accurate framework for understanding macrophage development and homeostasis.

Keywords: CSF1R; heterogeneity; homeostasis; macrophage; monocyte; ontogeny.

Publication types

  • Research Support, Non-U.S. Gov't
  • Review

MeSH terms

  • Animals
  • Homeostasis
  • Humans
  • Macrophages / immunology*
  • Mice
  • Monocytes / immunology*
  • Mononuclear Phagocyte System / immunology*