Specific Elimination of Alloreactive T Lymphocytes Using Photodynamic Therapy Prevents GVHD and Enables Rapid Immune Reconstitution.

Graft-versus-host disease (GVHD) and impaired immune reconstitution are the primary obstacles limiting the efficacy of allogeneic stem cell transplantation (SCT).

Graft-versus-host disease (GVHD) and impaired immune reconstitution are the primary obstacles limiting the efficacy of allogeneic stem cell transplantation (SCT). The purpose of this study was to determine whether selective depletion of donor alloantigen-specific T lymphocytes using photodynamic therapy (PDT) would prevent GVHD and enable immune reconstitution in the context of MHC-mismatched SCT. This question was addressed in an MHC-incompatible mouse model of GVHD. The donor (C57BL/6; H-2b) derived spleen cells were first activated against C3H/HeJ (H-2k) host spleen cells in a one-way mixed lymphocyte culture and then exposed to photodynamic treatment, using dibromorhodamine methyl ester (TH9402) as a photosensitizer. Activated T cells showed preferential retention of this photosensitizer compared to resting lymphocytes. In addition, in vitro experiments revealed that PDT eradicated a significantly higher proportion of activated than resting T cells. When lethally irradiated H-2k mice (C3H/HeJ and B10BR) were transplanted with C57BL/6 derived T cell-depleted bone marrow cells supplemented with C57BL/6 derived spleen cells activated with C3H/HeJ targets, they rapidly succumbed to acute GVHD (within 10–47 days). In contrast, both mouse strains receiving histoincompatible C57BL/6 T cells previously exposed to PDT after activation against C3H/HeJ survived until the end of the observation period (>100 days)(p<0.0001). Additionally, transplantation of treated T cells induced lethal GVHD in C57BL/6 histoincompatible strains of mice (third party), suggesting PDT specifically eradicated activated T cells while sparing most resting T lymphocytes. Analysis of immune recovery, evaluating T and B cell populations in thymus and spleen, activated T cells components (CD44, CD62L, CD69), proliferative responses (anti-CD3 and conA), and Vβ repertoire indicated that T and B cell reconstitution in MHC-mismatched mice transplanted with treated primed cells was similar to that of mice transplanted with treated or control autologous cells indicating that immune cells were preserved and functional. These results demonstrate that PDT can selectively eliminate alloreactive T cells and prevent the development of GVHD, while sparing T cells reactive against non-host antigens, thus offering protection against infection and disease relapse.