Modulation of monocyte subsets in infectious diseases

Monocytes are effector immune cells but a precise analysis of their role in immune response has been precluded by their heterogeneity. Indeed,human monocytes are composed of at least three different subsets with different phenotypic characteristics and functional properties,the so-called classical,intermediate and nonclassical monocytes. A review of the literature shows that these monocyte subsets are differently affected during viral,bacterial,parasitic and fungal infections.The expansion of the CD16+ compartment(intermediate and non-classical monocytes) is typically observed in the majority of infectious diseases and the increased proportion of CD16+ monocytes is likely related to their activation through their direct interaction with the pathogen or the inflammatory context. In contrast,the number of non-classical and intermediate monocytes is decreased in Q fever endocarditis,suggesting that complex mechanisms govern the equilibrium among monocyte subsets. The measurement of monocyte subsets would be useful in better understanding of the role of monocyte activation in the pathophysiology of infectious diseases.