Phenotypic Characterization of Human Intermediate Monocytes

A full understanding of the immune system requires full characterization of the cells, and its subsets, which play a role in the process of host defense. The expression pattern of cell surface molecules serves as a major criterion for assigning cells into discrete subsets. The availability of monoclonal reagents, specific fluorescent probes and compatible instrumentation over the past years has led to identification and characterization of a great number of leukocyte subsets, including monocyte subsets, recently reviewed by Ziegler-Heitbrock and Hofer (1). When using flow cytometry, human blood monocytes are characterized by forward and side scatter as well as the expression of the LPS coreceptor CD14 and the Fcgamma receptor III CD16 (2). Based on the relative expression of CD14 and CD16, monocytes used to be subdivided in classical monocytes and CD16 expressing monocytes, of which the latter population is expanded in various diseases (3) [reviewed by Ref. (4)]. In 2008, a group of experts proposed (a nomenclature for) three subsets of blood monocytes: classical (CD14++CD16−), intermediate (CD14++CD16+), and nonclassical (CD14+CD16++) (5). Since then, additional evidence from gene expression Phenotypic characterization of human intermediate monocytes