Extracorporeal photochemotherapy (ECP) has been shown to be an effective therapy for patients with acute and chronic graft-versus-host disease (GVHD) following allogeneic bone marrow transplantation, but its biologic mechanism is not understood. We reported that clinical response to ECP was associated not only with normalization of skewed CD4/CD8 ratios but also with an increase in CD3-/CD56+ natural killer cells and a decrease in the number of CD80+ and CD123+ circulating dendritic cells (DCs). To further elucidate the effects of ECP on activated lymphocyte subpopulations and the interaction between effector lymphocytes and antigen-presenting DCs, we isolated and characterized DC populations from patients with chronic GVHD undergoing ECP therapy. Antigen-presenting activity of DCs was measured as proliferation of antigen-stimulated autologous and allogeneic T cells by mixed-lymphocyte reaction (MLR). In MLR assays the proliferation of T cells was decreased in all 10 patients by a mean of 84% (range, 75%-95%; P ≤ .002) after a 2-day cycle of ECP and longitudinally over the 12-month course of therapy. Immunophenotypic analysis of DC populations revealed a preponderance of DC1 monocytic dendritic cells in all patients before the initiation of ECP. Nine of 10 patients demonstrated a shift from DC1 to DC2 and as a concordant shift from a predominantly Th1 (interleukin-2 [IL-2], interferon-γ) to Th2 (IL-4, IL-10) cytokine profile after ECP, and 8 of 10 had a clinical response to ECP. Our results suggest that ECP alters alloreactivity by affecting allo-targeted effector T cells and antigen-presenting DCs.
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