Immunomodulation as one of the key hallmarks of PUVA therapy: results from a mouse lymphoma skin model
Saptaswa Dey1, Pablo Vieyra-Garcia1, Theresa Benezeder1 and Peter Wolf1
1Medical University of Graz, Austria

Lesional skin of cutaneous T-cell lymphoma (CTCL) patients is colonized by T-cells with aberrant phenotype and non-malignant immune cells, both driven by soluble inflammatory mediators. The combination of psoralen and UVA irradiation (PUVA) is used to treat these patients with more than 70% of clinical response. DNA damage in malignant cells triggered by PUVA is thought the main mechanism of action of this therapy, however, the contribution to clearance of CTCL lesions by PUVA-induced immunomodulation and cell-membrane damage remains elusive.
C57BL/6 mice were injected with EL-4 cells subcutaneously into the back skin and treated with PUVA (20µg psoralen, 1.5J/cm2 2x/week). Tumor tissue, blood, spleen and lymph nodes (LN) were collected for flow cytometry and histological analyses.
The area under the curve of tumor growth was reduced from 22.811.0 mm2 in PUVA-treated mice compared to untreated animals. Flow cytometry analyses after 48h of a single PUVA treatment showed that the total percentage of CD4+ T-cells was reduced by 53.4% in LN. CD8+ T-cells were reduced by 57.2% in blood and 63.7% in LN (13.25.6 and 28.710.3). In contrast, the percentage of Tregs increased in blood of PUVA-treated mice from 2.74.7%. The expression of LAG3, an immunomodulatory molecule that inhibits MHC-II engagement increased in CD4+ T-cells in blood and LN (206521 and 51.782.7). Histological analysis of tumorous skin after 2 weeks of cell injection showed clearance of malignant cells in a band-like zone of the superficial dermis adjacent to the epidermis and a decrease of overall epidermal thickness in PUVA-treated mice.
The therapeutic effect hallmarks of PUVA are recapitulated in our model by reducing tumor growth and partially clearing malignant cells from tissue. PUVA triggered a reduction of systemic levels of CD4+ and CD8+ T-cells and increased Treg. The upregulation of LAG3 in PUVA-treated mice suggests an important role of this molecule in the immunomodulatory effect of this therapy. Together these results highlight the involvement of immunomodulation in the mechanisms of PUVA by easing the tumor burden in lesional skin, possibly by affecting LAG3 and Tregs.