Enhanced skin allograft survival after photodynamic therapy. Association with lymphocyte inactivation and macrophage stimulation.

ABSTRACT

It has been found previously that peritoneal exposure to hematoporphyrin derivative (HpD) photodynamic therapy (PDT) can induce systemic immunosuppression of contact hypersensitivity. We have now found that HpD-PDT also significantly prolongs survival of murine skin allografts. Normal A/J mice transplanted with BALB/c skin rejected the grafts within 10 +/- 0.9 days. Recipient mice treated 24 hr previously with HpD-PDT rejected skin allografts at 16 +/- 1.2 days. HpD alone or irradiation alone had no effect on skin graft survival, nor did HpD-PDT administered shortly after grafting. Flow cytometric analyses showed a nearly complete depletion of peritoneal lymphocytes 3 days after HpD-PDT. Lymphocyte levels were normal in the spleen, an organ not directly targeted by the PDT treatment, but the cells were totally unresponsive to Con A and LPS mitogens. Conversely, peritoneal HpD-PDT caused a striking enhancement in macrophage function as measured by phagocytosis of antibody-coated sheep erythrocytes. Humoral immunity to hen egg-white lysozyme was not significantly changed by HpD-PDT. These results demonstrate that HpD-PDT causes systemic immunosuppression of cellular immunity which, in turn, allows prolonged survival of allografts. Humoral immunity appears to remain largely unaffected by HpD-PDT and macrophages become activated, suggesting that this therapy might be more effective in specifically targeting T cell-mediated immunity than current immunosuppressive treatments.