# Extracorporeal photopheresis—New insights into an old procedure

## TL;DR

This study explores how extracorporeal photopheresis affects T-lymphocyte proliferation in blood samples from healthy donors.

## Contribution

The study provides new insights into the immunomodulatory effects of extracorporeal photopheresis on T-lymphocyte proliferation.

## Key findings

- ECP treatment significantly reduced T-cell receptor-induced proliferation of viable T-lymphocytes in leukocyte concentrates and peripheral blood.
- No significant differences in apoptotic cell proportions were observed 48 hours post-ECP treatment.
- The findings suggest T-lymphocyte proliferation inhibition as a key mode of action of ECP.

## Abstract

Extracorporeal photopheresis (ECP) is a safe immunomodulatory strategy that induces cell‐type selective apoptosis through photodynamic processes. Despite decades of use, the mechanisms underlying ECP remain largely unexplored, particularly in studies examining specific immune cell subsets in ex vivo setups.

This proof‐of‐concept pilot study presents data on apoptosis and proliferation of T‐lymphocytes following ex vivo ECP application to leukocyte concentrates (LC) and peripheral blood (PB) samples from healthy donors.

LC and PB were diluted to a haematocrit of 2% and treated with 8‐methoxypsoralen, followed by ECP (ECP+) or no ECP (ECP−) in a discontinued system. Apoptosis of mononuclear cells was assessed 48 h post‐ECP using annexin V and 7 Aminoactinomycin D (7‐AAD) staining with flow‐cytometric quantification. The proliferative capacity of non‐apoptotic T‐lymphocytes was measured after 72 h of post‐ECP stimulation with anti‐CD3/CD28 cross‐linking, using Violet Proliferation Dye 450.

ECP exposure significantly reduced the median T‐cell receptor‐induced proliferation of viable T‐lymphocytes from both LC (4.6%, p = 0.02) and PB (4.2%, p = 0.03). However, 7‐AAD staining 48 h post‐ECP showed no significant differences in the proportions of apoptotic cells in this experimental model.

Ex vivo ECP treatment inhibited T‐lymphocyte proliferation in both LC and PB from healthy individuals, suggesting this as a key mode of action. Our findings highlight ECP's potential applications, including its implications for modern immune therapies' adverse effects. Further analyses of functional characteristics of remaining vital cells are necessary.