The effect of age on monocyte phenotype and function

Inflammageing, the process of chronic systemic low-grade inflammation, characterised by elevated circulating IL-6, IL-8 and C Reactive protein (CRP), occurs with increasing age.

Inflammageing is known to be detrimental to the health of older adults, as elevated circulating inflammatory markers are associated with frailty, cognitive decline and all-cause morbidity and mortality.

In addition, increased inflammation is damaging to a functioning immune system with reduced vaccine efficacy and antigen-specific immunity. The origins of inflammageing in older adults is believed to be multifactorial.

To date the majority of the research on inflammageing has focussed on the role of senescent cells, however what is increasingly clear is that mononuclear phagocytes and in particular monocytes may drive the inflammageing process and warrant further investigation.

Monocytes historically were presumed to be precursor cells for macrophages and dendritic cells - although this can be true, monocytes are recognised as established immune effector cells in their own right. Human monocytes are defined by their expression of the cell surface markers CD14 and CD16.

The classical monocytes are defined as CD14+CD16-, the intermediate monocytes are CD14+CD16+ and then the non-classical monocytes are defined as CD16+CD14-/lo.

Studies, have shown that there is an increased frequency of CD16+ monocytes in the peripheral blood of older adults (≥65-years), data from my lab has expanded this observation and shown that there is a significant increase in both intermediate and non-classical monocytes in older adults as compared to young (