Immunologic Effects Of Peritoneal Photodynamic Treatment

Abstract

One of the side effects of peritoneal photodynamic treatment (PDT) of mice is a systemic suppression of contact hypersensitivity (CH) responses. Treatment with either laser alone or the photosensitizer, Photofrin II (PFII), alone does not cause suppression of CH responses. Immunosuppression of CH responses is an active process that is adoptively transferable using viable cells, but not serum, from PDT-treated mice. The induction of adoptively transferable suppressor cells in PDT-treated mice requires exposure to an antigenic stimulus, yet the suppressor cells are antigen non-specific in their function. T cell function in PDT-treated mice, as measured by the ability of splenic lymphoid cells to generate allogeneic cytotoxic T lymphocyte responses, is comparable to that detected in normal mice. However, the ability of spleen cells from PDT-treated mice to act as stimulators in a mixed lymhocyte reaction is dramatically impaired, suggesting that the major cell type affected by peritoneal PDT is of the macrophage lineage. Support for this concept is provided by experiments in which spleen cells from PDT-treated mice were chromatographically separated into populations of T cells, B cells and macrophages prior to adoptive transfer into naive recipients. The results indicate that the cell type mediating adoptively transferable suppression of CH responsiveness is of the macrophage lineage. Analysis of hematologic parameters revealed that induction of suppression by PDT-treatment was associated with a marked neutrophilia and lymphocytosis, and was also accompanied by a 5-fold increase in concentration of the acute phase protein, Serum Amyloid P. Finally, attempts to ameliorate PDT-induced immunosuppression by pharmacologic intervention have proved successful using implants of pellets that release indomethacin at a rate of 1.25µg/day. Thus, the data suggest that PDT-treatment induces macrophages to produce factors (e.g., prostaglandins) that are known to be potently immunosuppressive.