The novel subset of CD14+/CD16+ blood monocytes exhibits features of tissue macrophages.

The CD14+/CD16+ cells account for about 10% of all blood monocytes. They are characterized by a low level expression of the CD14 molecule and a high level expression of the CD16 (Fc gamma R III) molecule. Polymerase chain reaction analysis of mRNA prevalence in CD14+/CD16+ cells (compared to the regular CD14++ blood monocytes) demonstrates low levels of CD14 transcripts and high levels of CD16 transcripts, suggestive of a transcriptional control for both of these proteins. Analysis of additional cell surface molecules in three-color immunofluorescence reveals that CD14+/CD16+ cells express the Fc gamma R II in all, and Fc gamma R I and ICAM-1 in some donors. Furthermore, class II antigens are expressed at fourfold higher levels, while both, CD11b and CD33 cell surface proteins, are decreased by a factor of two. Transcript levels were reduced in CD14+/CD16+ cells for all three cell surface molecules. Since these phenotypic markers of the CD14+/CD16+ blood monocytes are reminiscent of tissue macrophages, we performed a comparative analysis with alveolar macrophages (AM). These cells are similar to the CD14+/CD16+ monocytes in that they show low levels of CD14 and strong expression of CD16. Furthermore, similar to the CD14+/CD16+ cells, the AM also exhibit higher levels of class II and lower levels of CD11b and CD33 when compared to the regular CD14++ blood monocytes. In vitro induction of maturation of blood monocytes by 5 day culture of peripheral blood mononuclear cells in 10% human serum will result in decreased CD14 and increased CD16 cell surface expression on the monocyte derived macrophages. At the same time, these cells acquire increased levels of class II and decreased levels of CD11b and CD33. Taken together, these data show that CD14+/CD16+ monocytes, while still in circulation, have acquired features in common with mature tissue macrophages.