Introduction. Monocytes are cells of the innate immunity system with high heterogeneity and plasticity and are involved in acute and chronic inflammatory states. Monocytes are traditionally distinguished in three subsets, based on CD14 (LPS co-receptor) and CD16 (FcγIII receptor with low IgG affinity) expression: classical, intermediate and non-classical. Monocyte subsets have a developmental relationship and differ in phenotypic and functional characteristics. Distribution of monocyte subsets has been shown to predict cardiovascular outcomes. Nevertheless, monocytes have now been redefined as a continuum of subsets with dynamic changes of their characteristics and classification into different subtypes may be an oversimplification. Monocytes have been studied in cardiovascular diseases because they are involved in inflammatory processes linked with these pathological states: they have a central role in the development of atherosclerotic plaques, that represent the major cause of cardiovascular events. Changes within different monocyte subsets are reported in several studies in relation with cardiovascular risk factors and cardiovascular diseases.
Aim of the study. The aim of this study is to establish whether distribution of monocytes based on CD14 and CD16 fluorescence intensity provides incremental and complementary information in relation to cardiovascular risk factors, prevalent cardiovascular diseases and cardiovascular outcomes beyond enumeration of traditional subsets.
Materials and methods. A cohort of 227 patients with high cardiovascular risk (patients with at least two classical cardiovascular risk factors or with establish cardiovascular disease) were recruited for this study and followed up for a median of 4 years. Monocyte subsets were quantified and characterized at baseline using polychromatic flow cytometry, based on the CD14 e CD16 expression; for each monocyte subset frequency and mean fluorescence intensity (MFI) of CD14 and CD16 were determined, evaluating the continuous distribution. These monocyte characteristics were studied in patients in relation to cardiovascular risk factors, prevalence of coronary artery disease (CAD) and occurrence of major adverse cardiovascular events (MACE) during follow-up.
Results. In relation to cardiovascular risk factors, every monocyte subset of patients with type 2 diabetes showed a consistent shift toward higher CD16 fluorescence intensity, despite no changes in their frequencies. Patients with coronary artery disease (CAD) at baseline displayed a doubled amount of CD14++ CD16+, intermediate monocytes, and a shift of non-classical and classical monocytes towards intermediates ones. During follow-up, cardiovascular death or cardiovascular events occurred in 26 patients, who showed monocyte displacement similar to those of patients with CAD at baseline. Using a Cox proportional hazard regression models, among monocytes parameters, only the higher CD16 expression on classical monocytes, independently predicted adverse cardiovascular outcomes, but not the level of intermediate monocytes or other subsets.
Discussion and conclusion. Changes within monocyte subsets in patients with CAD and in patients with incident MACE during follow-up suggested a shift of classical and non-classical monocytes towards intermediate monocytes, showing phenotypic changes within the monocyte continuum. The predictive role of CD16 MFI on classical monocytes highlights how the concept of monocyte continuum can be used to shape the cardiovascular risk more than frequencies of monocyte subsets can do.