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.
Highlights
When infectious agents pass the peripheral defenses and invade sterile body territories they face innate antimicrobial mechanisms
Fibroblasts, and other cells can phagocytose, in this text the term phagocytes is used for cells whose main function is phagocytosis and that have been classically called professional or dedicated phagocytes, namely neutrophils, inflammatory monocytes, macrophages, and immature dendritic cells (DCs)
Neutrophils, monocytes/macrophages and immature myeloid DCs have a common origin: The initial view that neutrophils and macrophages arise from a common late bone marrow precursor (Metcalf, 1989; Inaba et al, 1993a) has been confirmed by results showing that these phagocytes originate from hematopoietic stem cells (HSC) which differentiate through common pathways that lead to immature myeloid DCs (Akashi et al, 2000; Iwasaki and Akashi, 2007; Figure 1)
Summary
When infectious agents pass the peripheral defenses and invade sterile body territories they face innate antimicrobial mechanisms. This proposal was formally published (van Furth et al, 1972) and described the MPS as a system of dedicated phagocytic cells with similar morphology, function, origin and kinetics, grouping monocytes/macrophages, and their precursors but again excluding neutrophils. Neutrophils have been acquiring a progressive status of fundamental phagocytic immune cells (Cassatella, 1995, 1999; Rabinovitch, 1995; Cassatella et al, 2009; Borregaard, 2010; Silva, 2010b; Costantini and Cassatella, 2011; Mantovani et al, 2011) with phagocytic abilities superior to those of macrophages (see Section Neutrophils: archetypes of phagocyte cells) Comprehensive reviews on these leukocytes include recent contributions (Nathan, 2006; Nauseef, 2007; Dale et al, 2008; Borregaard, 2010). NEUTROPHILS AS ESSENTIAL MEMBERS OF PHAGOCYTE CELL SYSTEMS Several arguments justify the inclusion of neutrophils in any comprehensive phagocyte cell system, as follows
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