Phenotypic and functional heterogeneity of human intermediate monocytes based on HLA‐DR expression

Abstract Human blood monocytes are subclassified as classical, intermediate and nonclassical. In this study, it was shown that conventionally defined human intermediate monocytes can be divided into two distinct subpopulations with mid‐ and high‐level surface expression of HLA ‐ DR (referred to as DR mid and DR hi intermediate monocytes). These IM subpopulations were phenotypically and functionally characterized in healthy adult blood by flow cytometry, migration assays and lipoprotein uptake assays. Their absolute numbers and proportions were then compared in blood samples from obese and nonobese adults. DR mid and DR hi intermediate monocytes differentially expressed several proteins including CD 62L, CD 11a, CX 3 CR 1 and CCR 2. Overall, the DR mid intermediate monocytes surface profile more closely resembled that of classical monocytes while DR hi intermediate monocytes were more similar to nonclassical. However, in contrast to classical monocytes, DR mid intermediate monocytes migrated weakly to CCL 2, had reduced intracellular calcium flux following CCR 2 ligation and favored adherence to TNF α‐activated endothelium over transmigration. In lipid uptake assays, DR mid intermediate monocytes demonstrated greater internalization of oxidized and acetylated low‐density lipoprotein than DR hi intermediate monocytes. In obese compared to nonobese adults, proportions and absolute numbers of DR mid , but not DR hi intermediate monocytes, were increased in blood. The results are consistent with phenotypic and functional heterogeneity within the intermediate monocytes subset that may be of specific relevance to lipoprotein scavenging and metabolic health.