Asthma: Is It Time for Monocytes to Share the Spotlight?

The evolving understanding of the regulation of allergic airway inflammation has offered new approaches for asthma therapy. For example, our understanding of the role of alarmins, Th2 cytokines and eosinophils has allowed the development of biologics that have revolutionized therapy for severe asthma. However, many questions remain and several of our patients are still not controlled with the current available therapies. Many immune cells have been implicated in asthma pathophysiology, but one cell that is missing from these studies is the monocyte. Monocytes (Mos) are bone marrow-derived cells that circulate in the blood and develop into macrophages and/or dendritic cells following migration to peripheral tissues. Macrophages (Møs) and dendritic cells have been implicated in the development and progression, of allergic airway inflammation, but also in tissue repair after inflammation. Recent studies in animal models suggest a major role for Mos in allergic airway inflammation, primarily through their ability to mediate recruitment of eosinophils and/or neutrophils to the airways. However, there is little information regarding the role of monocytes in human asthma. Here we review the literature regarding the presence and functions of peripheral blood and airway Mos in human asthma and suggest further work that needs to be done to consolidate the information on Mo functions. Studies show changes in Mo numbers and activation status in the peripheral blood of patients with asthma, changes that in many cases correlate with disease severity and/or activity. Studies also show altered phenotype of Mos present in the airways of patients with asthma. Detailed human studies need to be performed, if possible, studies that include therapeutic interventions, to allow for a full understanding of the role of Mos in asthma.