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Prevention of skin allograft rejection by photodynamic therapy (PDT) using benzoporphyrin derivative monoacid ring A (BPD) verteporfin Obochi, Modestus Onuora Kay


The effect of photodynamic therapy (PDT) using the photosensitizer, benzoporphyrin derivative monoacid ring A (BPD) verteporfin which absorbs strongly at 690 nm, on murine skin allograft rejection was tested using a unique approach. Skin sections (C57BL/6)were exposed in vitro to varying doses of BPD (0.125 - 1.0 μg/mL) and 10 J/cm² light at a wavelength of 690 nm prior to implantation onto recipients (BALB/c). We found that pretreatment of skin to be grafted with PDT can significantly prolong the survival of allografts from 9.3 (n = 42) ± 2.2 days (control group) to 16.9 (n = 20) ± 1.7 days (treated group). Higher doses of BPD did not result in longer survival of the skin allografts. Rather, the most beneficial effects of the treatment were observed at lower doses of BPD (0.25 - 0.5 /μg/mL) as opposed to 1.0 /μg/mL and light (10 J/cm², 690 + 10 nm wavelength). Our strategy was, therefore, termed low-dose PDT to reflect the fact that we used low doses of BPD and visible light, doses where, even at the highest level, the histological features of pre-treated donor skins revealed no obvious tissue damage. In order to identify mechanisms by which pretreatment of donor skin could effect anti-allograft immune responses, we investigated the effect of low-dose PDT on the antigen presenting cells of the epidermis, the Langerhans cells (LC). Using flow cytometry, we found that the levels of expression of the major histocompatibility complex (class I and II) and the costimulatory (B7-1 and B7-2) molecules on LC isolated from treated skin was substantially reduced (60 - 90 % reduction) in comparison to the control preparations. On the contrary, the levels of the leukocyte common antigen (CD45), the adhesion molecule (ICAM-1), the endocytic receptor (DEC-205), the ectophosphatase intensities on L C , as well as LC viabilities, were unchanged. Furthermore, the ability of LC to stimulate the proliferation of naive or pre-sensitized alloreactive T cells was impaired. Finally, the histology and immunohistology of graft tissues from graft recipients revealed that this treatment kept the levels of inflammatory cellular infiltration into the graft low compared to the control grafts. Our findings suggest that the immunomodulatory effects of low-dose PDT of tissue grafts associated with extended engraftment may depend on a selective effect upon epidermal cells, especially LC and may not require cell depletion in order to permit the acceptance of skin allograft. Since the engagement of T cell receptors (TCR) in the absence of costimulation results in suboptimal activation of T cells and ultimately anergy, it appears that the immunomodulatory effects of low-dose PDT depends, in part, upon decreased expression of MHC and costimulatory molecules.

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