*Purpose: Extracorporeal photopheresis (ECP) treatment following heart transplantation (HTx) may induce transplant tolerance. A monitoring tool for successful ECP treatment would allow determination of patient-specific duration and frequency of ECP therapy. We evaluated the immunological effects of ECP long-term therapy.

*Methods: ECP therapy started 3 months after HTx (n=19). ECP therapy included a total of 5 cycles, each on 2 consecutive days every 4-8 wks over a mean period of 8.4±0.4 months. Blood samples were drawn prior to ECP (baseline), before each cycle and 4 months after the last cycle. Flow cytometry analysis of all samples specific subsets of both dendritic cells (DCs) and regulatory T cells (T_regs), which are involved in the immune tolerance response.

*Results: Compared to baseline, ECP reduced CD4⁺ T cells (baseline: 23.2±9.2%, cycle 5: 23.6±3.4%, after ECP: 16.3±7.4%, p<0.01) and increased total T_regs (baseline: 9.8±2.4%, cycle 5: 12.5±1.4%, after ECP: 17.5±4.0%,p<0.01). While CD120b⁺, CD147⁺ and CD39⁺ T_regs were unaffected by ECP, CD62L⁺ T_regs were downregulated (baseline: 78.9±12.9%, cycle 5: 55.9±13.6%, after ECP: 81.0±11.7%, p>0.01). DC subsets expressing blood dendritic cell antigen (BDCA) 1, 2, 3 or 4 increased during ECP but returned to baseline levels 4 months after ECP (BDCA1_{prior ECP}: 42.1±13.4%, BDCA1_{cycle 5}: 53.4±3.7%, BDCA1_{after ECP}: 43.6±8.7%, BDCA2_{prior ECP}: 20.2±9.1%, BDCA2_{cycle 5}: 32.6±3.2%, BDCA2_{after ECP}: 24.0±6.0%, BDCA3_{prior ECP}: 73.2±17.8%, BDCA3_{cycle 5}: 90.4±5.0%, BDCA3_{after ECP}: 75.4±12.4%, BDCA4_{prior ECP}: 21.6±10.0%, BDCA4_{cycle 5}: 31.7±3.9%, BDCA4_{after ECP}: 21.4±5.9%, all p≤0.01).

*Conclusions: Our results suggest that a combined increase of BDCA1⁺, 2⁺, 3⁺ and 4⁺ DCs, total T_regs and the highly suppressive CD39⁺ T_reg subset could be useful to monitor ECP therapy after HTx, and, to establish an individual ECP treatment schedule. Future studies need to explore ECP effects on immune cells in the context with clinical events like rejection or infection.

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