Neutrophils and macrophages: the main partners of phagocyte cell systems

Abstract

Biological cellular systems are groups of cells sharing a set of characteristics, mainly key function and origin. Phagocytes are crucial in the host defense against microbial infection. The previously proposed phagocyte cell systems including the most recent and presently prevailing one, the mononuclear phagocyte system (MPS), grouped mononuclear cells but excluded neutrophils, creating an unacceptable situation. As neutrophils are archetypical phagocytes that must be members of comprehensive phagocyte systems, Silva recently proposed the creation of a myeloid phagocyte system (MYPS) that adds neutrophils to the MPS. The phagocytes grouped in the MYPS include the leukocytes neutrophils, inflammatory monocytes, macrophages, and immature myeloid DCs. Here the justifications behind the inclusion of neutrophils in a phagocyte system is expanded and the MYPS are further characterized as a group of dedicated phagocytic cells that function in an interacting and cooperative way in the host defense against microbial infection. Neutrophils and macrophages are considered the main arms of this system.

Keywords: DCs; macrophages; microbial infections; monocytes; neutrophils; phagocyte systems.

FIGURE 1

Main steps and players of myelopoiesis in the context of the MYPS. Graph depicting one interpretation of the still evolving view of the myelopoiesis pathways (blue arrows) associated with the poiesis of the members of the MYPS in mice and humans. This interpretation takes into consideration recent publications whose references are given below (see also the reviews Friedman, 2002; Iwasaki and Akashi, 2007; Varol et al., 2009; Geissmann et al., 2010b). HSCs give rise to CMPs (1) (Akashi et al., 2000; Manz et al., 2002). CMPs originate GMPs (2) and MEPs (3) (Akashi et al., 2000; Manz et al., 2002). GMPs differentiate into Monocytes (4), Eosinophils, Basophils, Mast cells, and Neutrophils (5) (Akashi et al., 2000; Iwasaki and Akashi, 2007) and MDPs (7) (Iwasaki and Akashi, 2007). An alternative to the view that MDPs originate from GMPs considered that MDPs would directly derive from CMPs; Fogg et al., 2006). Monocytes mature into Macrophages (6) (van Furth et al., 1973; Akashi et al., 2000; Sunderkotter et al., 2004; Fogg et al., 2006). The pathways to myeloid DCs include MDPs [originated from GMPs (7); Iwasaki and Akashi, 2007] that differentiate into CDPs (8) (Onai et al., 2007; Liu et al., 2009) and these give rise to Immature myeloid DCs (9) [that originate cDCs (10)], and pDCs (11) (Naik et al., 2006, 2007). The members of the MYPS are highlighted in bold. MYPS, myeloid phagocyte system; HSCs, hematopoietic stem cells; CMPs, common myeloid precursors; GMPs, granulocyte/macrophage precursors; MEPs, megakaryocyte/erythrocyte progenitors; MDPs, macrophage/DC progenitors; CDPs, common DC progenitors; cDCs, conventional (or classic) myeloid DCs; pDCs, plasmacytoid DCs.