Circulating Natural Killer Cells in Patients With Type 2 Diabetes and Peripheral Neuropathy Are Altered in Their Frequency and Exhibit Impaired Functions and Angiogenic Activities.

Patients with type 2 diabetes mellitus (T2DM) and critical limb-threatening ischemia (CLTI), a severe vascular complication associated with amputation and high mortality, exhibit chronic inflammation and endothelial dysfunction. We previously reported an increased frequency of natural killer (NK) cells in advanced CLTI; however, whether NK cell alterations are already present in patients with peripheral neuropathy, which often precedes diabetic foot lesions with or without distal macroangiopathy, remains unclear. We conducted an observational pilot study to assess circulating NK cell frequency by flow cytometry and inflammatory plasma cytokines profiling by ELISA in 177 subjects stratified into five groups: healthy controls (HC, n = 25); T2DM without neurovascular complications (D, n = 26); non-macrovascular neuropathic T2DM without foot lesions (N, n = 40); non-macrovascular neuropathic T2DM with lower-limb ulcers/lesions/osteomyelitis (N1, n = 42); and macrovascular revascularized neuropathic T2DM with ulcers/lesions/osteomyelitis (NV, n = 44). NK cell subset frequencies were evaluated in an independent cohort of patients with non-macrovascular neuropathic T2DM (n = 22) and healthy controls (n = 23). NKs were further characterized for degranulation capacity, secretome composition, and pro-angiogenic activity in vitro and in vivo (Hirudo verbana). NK cell frequency was significantly increased in patients with non-macrovascular neuropathic T2DM compared with controls and was accompanied by elevated plasma levels of soluble mediators associated with NK cell activation and function, including TNF-α, IL-12p70, MCP-1, and IL-4. NKs from these patients exhibited impaired degranulation and reduced angiogenic capacity in vitro and in vivo. Their secretome is characterized by decreased release of pro-angiogenic factors and increased secretion of mediators involved in monocyte/macrophage recruitment, resulting in enhanced macrophage recruitment in vivo. If confirmed in larger cohorts, higher frequency and functional impairment of circulating NKs may act as novel therapeutic targets and prognostic biomarkers in peripheral vascular complications of T2DM.