Seasonal variations of main lymphocyte subsets are unexpectedly mild

Lymphocyte subsets is one of the basic parameters of immune status; beside clinical conditions, subset counts are influenced by regional and environmental factors. Only fragmentary data exist on seasonal variations, particularly in children, still, the issue may be important for diagnosis and for monitoring patients with immune abnormalities. Continuous cohort of 5964 lymphocyte subset tests in 1-17 years old children was retrospectively studied, proven cases of primary immunodeficiencies and leukemias were not included. The tests were performed in Clinical Laboratory of Children’s Clinical University Hospital, Riga in 2012-2019, on IVD 6-color single tube platform - 6-TBNK test, BD FACSCanto II flow cytometer, FACSCanto software (Becton Dickinson Co, BD Biosciences, San Jose, USA). Lymphocytes were defined as CD45+/FSlow/SSlow cells and further split into CD3+ T-cells, CD3+/CD4+ cells, CD3+/CD8+ cells, CD3-/CD19+ B-cells and CD3-/CD16+CD56+ NK cells. Anonymized data were retrieved from the Hospital LIS, split by seasons and age groups and analyzed by IBM SPSS v.25 software; Kruskal-Wallis H test was used to evaluate differences. Lymphocyte count followed leukocyte trend with peak in winter and drop in summer, the same tendency was seen in all subsets, the pattern was roughly repeated in all age groups. Variations, though consistent, were unexpectedly mild: maximal deviation of lymphocytes was 2.8%, T-cells 1.0%, T4 cells 1.7%, T8 cells 2.8%, NK cells 3.1%, B-cells 3.6% and T4/T8 ratio 1.8% and statistically nonsignificant, in contrast to significant seasonal dynamics of leukocytes (deviation 5.4%, p=1.5E-10). Overall, lymphocytes and lymphocyte subsets demonstrated remarkable seasonal stability; that, considering numerous stimuli, would ask for a robust stabilizing mechanism. The findings suggest that seasonal factor may be disregarded when defining normal reference ranges for subsets. Seasonal oscillation of lymphocytes and lymphocyte subsets described in a large set of pediatric tests may be important for understanding populational dynamics of immune response.