Nanoparticles and their Current Potential Use to Detect Monocyte Subsets in Vivo

Abstract

Background Monocyte subsets: A growing unresolved complexity Circulating human monocytes are bone marrowderived leukocytes that can differentiate into macrophages and dendritic cells. They participate in the induction and regulation of the inflammatory processes as well as in the adaptive immune responses [1]. The monocyte population is heterogeneous, exhibits high plasticity, and includes cellular subsets with different morphological features and functions. The monocyte subpopulations are conventionally defined according to the relative surface expression of CD14 (co-receptor, along with the toll-like receptor 4/TLR4, of the bacterial lipopolysaccharide/LPS), and CD16 (FcRIIIa). Although three different monocyte subsets were initially defined: classical (CD14++CD16−), intermediate (CD14+CD16+), and non-classical (CD14+ CD16++) [2], the advances in molecular immunology have demonstrated that this cellular lineage is more complex than previously observed as it is described later. Classical monocytes constitute about 80–95% of the circulating monocytes and have prominent phagocytic capabilities [3]. Classical monocytes are important scavenger cells [4] that remove apoptotic bodies in a non-inflammatory fashion. They are rapidly recruited to sites of infection [5] and injury [6-8], where they exhibit considerable functional plasticity [9]. The intermediate subset comprises about 2–8% of the circulating monocytes, increase in inflammatory or infectious conditions, and produce TNF-α, IL-1β, IL-6, and IL-10 [10, 11]. These cells participate in the proliferation and stimulation of T-cells, the inflammatory responses, and the angiogenesis. It is proposed that intermediate monocytes make up a transitional population bridging the classical and non-classical ones [12].