Macrophages in diabetes mellitus (DM) and COVID-19: do they trigger DM?

Diabetes mellitus (DM) augments the risk of hospitalization and mortality resulting from viral, bacterial, or fungal pathogen infection. This has been also true for the past SARS and MERS, and current SARS-CoV-2 coronavirus epidemics. Clinical data indicate that SARS-CoV-2 infection triggers a severe course of COVID-19 more frequently in diabetic than non-diabetic patients. Here we overview the cellular and molecular mechanisms associated with this phenomenon. We focus on alterations in the immune cells, especially monocytes and macrophages, involved in innate immune response and inflammatory processes, which differ in type 1 diabetes mellitus (T1DM) and type 2 diabetes mellitus (T2DM). We also describe the DM-related changes in the monocyte/macrophages functions, how they could lead to the severe outcome of SARS-CoV-2 infection, and importantly, if and how they could initiate DM in DM-susceptible patients.