Monocytes originate from the bone marrow (BM), are distributed in the bloodstream, and can differentiate in the tissue into skin macrophages or intestinal dendritic cells (DCs). 1 They play an essential role in the defense against pathogens2 and are implicated in a range of diseases. 3

Labeling experiments with 6, 6-2H2-glucose4 and 3H-thymidine5, 6 have suggested a monocyte residence time in blood of 2 to 4 days. Although these experiments considered monocytes as a single population, the currently held view is that at least 3 monocyte subsets exist: classical CD14 11 CD16–monocytes (CMs), intermediate CD14 11 CD16 1 monocytes (IMs), and nonclassical CD14 1 CD16 11 monocytes (NCMs). The residence times of these subsets in blood and their interrelationship remain to be determined. The 3 subsets are characterized by the gradually changing expression of surface markers, 7 differential gene enhancer profiles8 expression profiles, 9, 10 and differences in functionality. 3, 11 Their maturation kinetics also differ, because human CMs repopulate the bloodstream first after hematopoietic stem cell transplantation, followed by IMs and later by NCMs. 12 In rhesus macaques, similar results were found by using in vivo bromodeoxyuridine labeling. 13 The gradually changing expression patterns, combined with their consecutive repopulation/labeling kinetics has led to the prevailing idea that monocytes differentiate from CMs via IMs to NCMs. 9, 13 In mice, there is more direct evidence for such a linear differentiation pattern. 14 Adoptively transferred CM homologs were shown to differentiate into NCMs, 15, 16 and in vivo imaged monocytes were shown to lose CM marker CCR2 while acquiring NCM marker CXC3CR1 at sites of sterile inflammation. 17 However, adoptively transferred CM preparations might contain small numbers of monocyte progenitors15 and thus, even in mice, there remains discussion on the developmental relation of these monocyte subsets under homeostatic