Reason for peripheral blood natural killer cell depletion in severe COVID-19 patients.

In a paper by Garcia-Muñoz et al., the authors evaluated the peripheral blood natural killer (NK) cells in patients with Coronavirus disease 2019 (COVID-19) and found that the number of these cells was 75 cells/mcL (range 10–424) in severe patients compared to 150 cells/mcL (range 49–618) in the non-severe group that showed a statistically significant (p = 0.0374) difference [1].

Infiltration of NK cells into the lungs is suggested to be due to Very Late Antigen-4 (VLA-4)/Vascular cell adhesion molecule 1 (VCAM-1) adhesion [2]. VLA-4 is one of the typical signs of tissue-resident cells and is usually expressed on the surface of NK cells and can bind to VCAM-1 [3]. It has been shown that Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) infection induces leukocyte adhesions including VCAM-1 [4].

The VLA-4/VCAM-1 pathway plays a role in inflammation [5]. Thereby, the usage of anti-trafficking agents such as natalizumab that block VLA-4 should be studied in order to find a route to avoid lung inflammation in patients with COVID-19.