Photopheresis in transplantation: future research and directions

Photopheresis in solid organ transplant rejection

Photopheresis has become a key component in the therapeutic armamentarium of cutaneous T-cell lymphoma, graft-versus-host disease following stem cell transplant, and allograft rejection of solid organs such as heart. Although it is considered a new treatment modality in its present form, the field of phototherapy dates back thousands of years. In this review, the reader will learn more about the history of photopheresis and how it became a therapeutic alternative for patients with solid organ transplants. An extensive literature search will highlight the evidence-based benefits of photopheresis (or lack thereof). A discussion of the mechanism of action of photopheresis and the technical aspects of the procedure will also be covered. Since photopheresis may be the best tolerated form of immunomodulation, current promising, albeit preliminary data on its efficacy warrant further investigation and understanding.

Intravenous Infusion of Syngeneic Apoptotic Cells by Photopheresis Induces Antigen-Specific Regulatory T Cells

The basis of extracorporeal photopheresis is the reinfusion of leukocytes previously exposed to 8-methoxypsoralen (8-MOP) and UVA radiation. It has been approved for the palliative treatment of cutaneous T cell lymphoma and has reported benefits in autoimmune diseases, transplant rejection, and graft-vs-host disease. However, the underlying mechanism of photopheresis remains unresolved. Because UVB radiation can cause immune tolerance via induction of regulatory T cells, we studied whether photopheresis exerts a similar effect extracorporeally. Therefore, we established a model of photopheresis using a murine model of contact hypersensitivity. Splenocytes and lymph node cells of mice that were sensitized with dinitrofluorobenzene were exposed to 8-MOP plus UVA in vitro. Intravenous injection of these cells into naive mice caused inhibition of a hapten immune response, which was lost upon depletion of CD11c(+) cells but not T cells. Mice that received untreated cells or cells exposed to UVA or 8-MOP alone were not affected. Inhibition was cell-mediated and Ag-specific as demonstrated by transfer of tolerance from the primary recipients into naive animals, which could, however, properly respond to the unrelated hapten oxazolone. Transfer activity was lost when cells were depleted of CD4(+) or CD25(+) subpopulations. These data suggest that photopheresis exerts its immunomodulatory effects via the induction of Ag-specific regulatory T cells.

Characteristics of photopheresis treatments for the management of rejection in heart and lung transplant recipients

Photopheresis has been used in the management of rejection of heart and/or lung transplants. Although its mechanism of action remains unknown, irradiated T-helper cell-induced immunosuppression is the main theory. Since transplant recipients are often lymphopenic and lymphocytes are the target cells in phototherapy, we performed this study to determine which factors affect the cellular yield to undergo irradiation. We reviewed the records of all photophereses performed in our institution between July 1998 and April 2000 using the UVAR (first generation) or XTS (second generation) instruments (Therakos, Exton, PA). Our data included patient's blood volume, absolute lymphocyte count and hematocrit, catheter type, flow rate of collection cycles and centrifuge bowl size, as well as volume, hematocrit, and lymphocyte count of the cell suspension. With a mixed model multivariate analysis we sought to determine which variables predicted the lymphocyte yield. A total of 406 procedures in 25 adult patients was analyzed. There was no significant difference between the lymphocyte yield among the procedures performed with the first- and the second-generation instruments. The patient's absolute lymphocyte count was the only parameter, which positively correlated with the total number of lymphocytes collected for irradiation (P < 0.0001). Indeed, based on the mixed model, the total number of lymphocytes for irradiation can be predicted from the pre-procedure lymphocyte count. Additional studies are necessary to correlate the number of treated cells with patient outcome.