When two is better than one: macrophages and neutrophils work in concert in innate immunity as complementary and cooperative partners of a myeloid phagocyte system

Abstract The presence of two dedicated phagocytic cells working cooperatively represents an advantageous innate immune strategy that allows the efficient and safe utilization of powerful but dangerous microbicidal molecules. The antimicrobial effector activity of phagocytes is crucial in the host innate defense against infection, and the classic view is that the phagocytes operating against intracellular and extracellular microbial pathogens are, respectively, macrophages and neutrophils. As a result of the common origin of the two phagocytes, they share several functionalities, including avid phagocytosis, similar kinetic behavior under inflammatory/infectious conditions, and antimicrobial and immunomodulatory activities. However, consequent to specialization during their differentiation, macrophages and neutrophils acquire distinctive, complementary features that originate different levels of antimicrobial capacities and cytotoxicity and different tissue localization and lifespan. This review highlights data suggesting the perspective that the combination of overlapping and complementary characteristics of the two professional phagocytes promotes their cooperative participation as effectors and modulators in innate immunity against infection and as orchestrators of adaptive immunity. In the concerted activities operating in antimicrobial innate immunity, macrophages and neutrophils are not able to replace each other. The common and complementary developmental, kinetic, and functional properties of neutrophils and macrophages make them the effector arms of a myeloid phagocyte system that groups neutrophils with members of the old mononuclear phagocyte system. The use by mammals of a system with two dedicated phagocytic cells working cooperatively represents an advantageous innate immune attack strategy that allows the efficient and safe use of powerful but dangerous microbicidal molecules. This crucial strategy is a target of key virulence mechanisms of successful pathogens.