Extracorporeal photopheresis-New insights into an old procedure.

Background: 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.

Aims: 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.

Methods: 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.

Results: 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.

Conclusion: 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.