Flow Cytometric Analysis of Depletion and Recovery Kinetics of T Cell Subsets in Rats After Total-BodyIrradiation: Footprints of Treg-Augmented Immunosuppression

Abstract

OBJECTIVETotal-body-irradiation (TBI) causes significant immunosuppression, but different lymphocyte subsets havevarious radiosensitivities. Regulatory T (Treg) cells, which are crucial for self-tolerance and are potent suppressors of antitumor immunity are found to be resistant to radiotherapy (RT) compared with T helper (Th)cells and Cytotoxic-T lymphocytes (CTL) in both in-vivo and in-vitro studies, but the data on this subjectis relatively scarce. Besides, recent developments in the context of combination of immunotherapy with RTcompelled us to revisit the concept of radiation-induced quantitative and functional changes in lymphocytesubsets by flow cytometry using animal models with the aim of transitioning the findings to clinical studies.METHODSTwenty-three Swiss albino rats were exposed to TBI at a single fraction of 5 Gy. Immediately prior to irradiation, at time points of 1 day and 7 and 14 days post-TBI, flow cytometric analyses were performed.RESULTSThere has been statistically significant decrease in all T lymphocyte subsets at 1, 7 and 14 days post-TBI. Thedecrease in Th subset was more pronounced compared to CTL. Baseline CD4+/CD8+ ratio was 0.85 whichsignificantly decreased to 0.29 1 day post-TBI, then increased steadily in subsequent measurements andreached near normal. The number of Treg cells markedly declined to 6.5% of baseline value one day after TBI,and then steadily increased during the follow-up. By the end of 14 days, it reached half of its baseline value.CONCLUSIONRadiation-induced immunosuppression may be explained not only by the decrease in lymphocyte cell number but also by the relative increase in Treg cell number because the higher regenerative capacity may presentan additional role.