Primate Monocytes - CD14, CD16 - Ziegler-Heitbrock

The secret life of nonclassical monocytes.

Abstract

CURRENT nomenclature divides monocytes into three subpopulations in humans and two subsets in mice. Human monocyte subpopulations are defined by their differential expression of CD14 and CD16, while mouse monocytes are classified according to differential expression of Ly6C. Classical monocytes (CD14++CD16– in humans, Ly6Chigh in mouse) express high levels of CCR2 and migrate to sites of injury where they differentiate into inflammatory macrophages. Conversely, nonclassical monocytes (CD14+CD16++ in humans, Ly6Clow in mouse) express high levels of the fractalkine adhesion receptor CX3CR1 and exhibit unique patrolling behavior along the resting vasculature (1) (see Fig. 1). A third subset of intermediate monocytes has also been described in humans (CD14++ CD16+), which express multiple surface markers at levels between the classical and nonclassical subsets (2), and generally exhibit inflammatory functions. Previous studies grouped intermediate monocytes with the nonclassical subset due to their high expression of CX3CR1; however, increasing evidence suggests that intermediate monocytes are a third, distinct monocyte population in humans