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

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 CD62L, CD11a, CX3CR1 and CCR2. 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 CCL2, had reduced intracellular calcium flux following CCR2 ligation and favored adherence to TNFα-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.