In Vitro Experiments Demonstrate that Monocytes and Dendritic Cells are Rendered Apoptotic by Extracorporeal Photochemotherapy, but Exhibit Unaffected Surviving and Maturing Capacity after 30 Gy Gamma Irradiation

Abstract

Extracorporeal photochemotherapy (ECP) has been shown to induce apoptosis in lymphocytes. Until recently the prevailing opinion has been that the monocytes were mainly not affected by this treatment. This study has investigated the effect of ECP and gamma irradiation on monocytes and immature dendritic cells (DC) in vitro and followed the ability of the cells to differentiate and survive post treatment. ECP induced apoptosis in lymphocytes, monocytes and immature DC within 72 h following treatment, in contrast to 30 Gy gamma irradiation, which seemed mainly to affect lymphocytes. The minority of the surviving ECP‐treated monocytes presented a reduced ability to differentiate into immature DC within this time frame. We also demonstrated that immature DC after ECP‐treatment lost their normal ability to mature on stimulation with lipopolysaccharide. As monocytes and immature DC seem to have a reduced ability to differentiate after ECP‐treatment, it is suggested that the therapeutic effect of ECP is caused by in vivo effects of reinfused apoptotic cells, rather than by infusion of monocytes induced to differentiate into immature DC.

Dendritic Cell Apoptosis: Regulation of Tolerance versus Immunity

“…In addition to lymphocyte apoptosis, ECP also results in modulation of APC function and induction of human DC apoptosis, which plays a key role in ECP-induced immunosuppression (54). Immature human DCs have a higher susceptibility to ECP-induced apoptosis, with up to 50% of DCs undergoing apoptosis within 24 h of treatment and .90% undergoing apoptosis within 72 h of ECP treatment (55). Thereafter, the remaining viable DCs become tolerogenic with a severely diminished ability to undergo maturation in response to LPS, with a defect in inducing T cell proliferation (56).…”

Apoptotic cell-based therapies against transplant rejection: role of recipient’s dendritic cells

“…Among the cells irradiated during ECP, circulating lymphocytes, monocytes and DC are sensitive to apoptosis [100–105], generating apoptotic bodies and blebs with immunoregulatory properties [106]. Even before dying, human DC treated with 8-methoxypsoralen and UVA, promote Th2-biased responses and decrease drastically their ability to drive Th1 polarization [107, 108].…”

Biological quality control for extracorporeal photochemotherapy: Assessing mononuclear cell apoptosis levels in ECP bags of chronic GvHD patients

“…The test needs to be reproducible, reliable, and (in future) standardizable from one center to another. Since we were interested in finding a versatile test, and since the literature on the mechanism of action of ECP suggests that ECP‐induced DNA damage is closely related to the apoptotic process, we considered assessing MNC apoptosis as a test for the biological validation of ECP. In this study, we used FITC‐labeled annexin V binding to phosphatidylserine (PS) in association with propidium iodide (PI) staining to analyze the kinetics of apoptosis induced by the ECP procedure and to quantify early and late apoptosis in ECP‐treated and untreated MNCs.…”