TL was measured by Flow-FISH, currently considered as the gold standard technique [27 (link)]. The severe lymphopenia of most COVID-19 patients, together with the possible acute infection-driven shortening of telomeres in T lymphocytes, support the measurement of TL in the granulocytes rather than the lymphocytes. To confirm that granulocyte TL properly reflects lymphocyte TL, we compared their respective percentile ranges for individuals of the cohort. We found a good consistency between the two cell populations as, for nearly 60% of the individuals of the cohort, the percentile range was the same while, for 33% of them, only one category difference was observed (Supplementary Figure 1 ). Hence, for the large majority of individuals, granulocyte TL properly reflects lymphocyte TL.
All TL measurements were performed in duplicate in the Cliniques Universitaires Saint-Luc through Flow-FISH as described previously [15 (link)]. After red blood cell lysis with ammonium chloride as described in [15 (link)], white blood cells were frozen at -80° C until analysis. Aliquots of calf thymus were mixed to each sample for internal control. Telomeres were stained with FITC-labelled (CCCTAA)3 PNA probe (Panagene) and fluorescence was measured with a Navios EX flow cytometer (Beckman Coulter).
All TL measurements were performed in duplicate in the Cliniques Universitaires Saint-Luc through Flow-FISH as described previously [15 (link)]. After red blood cell lysis with ammonium chloride as described in [15 (link)], white blood cells were frozen at -80° C until analysis. Aliquots of calf thymus were mixed to each sample for internal control. Telomeres were stained with FITC-labelled (CCCTAA)3 PNA probe (Panagene) and fluorescence was measured with a Navios EX flow cytometer (Beckman Coulter).