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.

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.