PRF1 and COVID-19: Suggestive of driving reduced anti-viral immune responses, significant negative associations were reported between MDSC frequency and the percentage of perforin+ CD3+T cells and perforin+ NK cells (235), whilst in ex vivo cultures, depletion of G-MDSC's from PBMC samples of severe COVID-19 patients restored the proliferative capacity and cytokine production of T cells (236).