A new immunosuppressant, FTY720, prolongs limb allograft survival in rats

Pharmacologic approaches to composite tissue allograft

This article discusses the pharmacologic approaches and the most promising new compounds for composite tissue allograft tolerance. Although some approaches rely on a combination of immunosuppressive agents that act synergistically against rejection, other strategies use immunologic manipulation, including major histocompatibility complex matching, induction of chimerism, and use of monoclonal antibodies to abrogate the immune response. There is still a need, however, to reproduce these findings in species phylogenetically closer to humans. This may be the target of future research efforts, which may overcome the challenge of limb and face transplant rejection.

Elimination of cell-cycle regulators during caspase-3-dependent apoptosis caused by an immunosuppressant, FTY720

The immunosuppressant, FTY720 causes apoptosis of lymphocytes, reduces numbers of lymphocytes in peripheral blood, and prevents infiltration of lymphocytes into allografts, which may be one of the mechanisms involved in its effects. Here we compared caspase activation and expression of cell-cycle regulators during apoptosis caused by FTY720, and Fas-stimulation in a mouse lymphoma transfected with human Fas antigen. FTY720 activated caspases-3, -8, and -9 as rapidly as did Fas-mediated apoptosis. The activation was blocked by a peptide inhibitor for caspase-3, DEVD-CHO. Fas-induced activation of caspases-8 and -9 was unaffected by the inhibitor. FTY720 eliminated proliferating cell nuclear antigen, retinoblastoma family members, differentiation regulated transcription factor polypetide-1, and cyclin H. These cell-cycle regulators were not eliminated when the peptide inhibitor was used. Dysfunction of cell-cycle regulators may play a critical role in the signal transduction pathway for activation of FTY720-mediated apoptosis.

Immunosuppressant FTY720 induces apoptosis by direct induction of permeability transition and release of cytochrome c from mitochondria

FTY720 has immunosuppressive activity in experimental organ transplantation and shows a prompt and protracted decrease of blood T lymphocytes upon oral administration. The blood lymphocyte decrease in vivo was mainly a result of FTY720-induced apoptosis. However, this apoptotic mechanism is not well understood. We examined the mechanism of FTY720-induced apoptosis in lymphoma. Western blotting and fluorescent caspase-specific substrate revealed that caspase-3 is involved in FTY720-induced apoptosis, whereas caspase-1 is not. Apoptotic cell death was inhibited by the pan-caspase inhibitor, Z-VAD-FMK, suggesting that caspase activation is essential for FTY720-induced apoptosis. FTY720 reduced mitochondrial transmembrane potential and released cytochrome c from the mitochondria of intact cells as well as in a cell-free system even in the presence of Z-VAD-FMK. As these mitochondrial reactions occurred before caspase activation, we concluded that FTY720 directly influences mitochondrial functions. The inhibition of mitochondrial permeability transition by Bcl-2 overexpression or by chemical inhibitors prevented all apoptotic events occurring in intact cells and in a cell-free system. Moreover, using a cell-free system, FTY720 did not directly affect isolated nuclei or cytosol. These results indicate that FTY720 directly affects mitochondria and triggers permeability transition to induce further apoptotic events.

Evidence that FTY720 induces T cell apoptosis in vivo

The immunosuppressant FTY720 induces a drastic decrease in blood lymphocytes, especially T cells; a decrease which is assumed to be the immunosuppressive mechanism of this drug. FTY720 causes cell death in vitro in lymphocytes and leukemia cells. However, the deletion mechanism of blood lymphocytes in vivo remains unclear. We investigated whether administration of FTY720 induced lymphocyte apoptosis in blood circulation. A marked decrease in the number of blood lymphocytes was observed within an hour after a single oral administration of FTY720 at doses of 5-10 mg/kg in rats and mice. Experiments using fluorescein isothiocyanate (FITC)-Annexin V and APO-BRDU methods revealed that FTY720 induced blood lymphocyte apoptosis in a dose-dependent manner. On the other hand, lymphocyte homing to Peyer's patches was proposed as the mechanism underlying the blood lymphocyte decrease at these doses. However, similar results were obtained when using aly/aly mice, which lack Peyer's patches and lymph nodes. Thus, we concluded that apoptosis of blood lymphocytes was induced immediately after administration of FTY720, and the cells could be immediately scavenged by phagocytes or the reticuloendothelial system in addition to Peyer's patches homing. We also concluded that T cells were highly sensitive to FTY720, which induced apoptosis in vivo.

Scoring of skin rejection in a swine composite tissue allograft model

BACKGROUND

For the first time, we define and correlate visual and histologic grading systems of composite tissue allograft (CTA) skin rejection in a large-animal model and determine the utility of these grading systems for early diagnosis and monitoring of rejection.

MATERIALS AND METHODS

Sixteen pairs of outbred swine underwent transplant of a forelimb osteomyocutaneous free flap. Group I (n = 6) did not receive immunosuppressive therapy. Group II (n = 10) received oral cyclosporin A, mycophenolate mofetil, and prednisone. The flap was visually inspected and protocol skin biopsies were taken at frequent intervals over a 90-day period. Visual Grades 0 (no rejection) to 4 (severe rejection) were assigned based on skin color, bleeding from biopsy site, and blister formation. Histologic Grades 0 to 4 were assigned based on the degree of vasculitis, folliculitis, dermal inflammation, and epidermal degeneration present.

RESULTS

All Group I animals progressively rejected their graft by Day 7. Group II grafts survived from 19 and 90 days; 93% of 115 biopsy specimens were read to be within +/-1 histologic score of their assigned flap visual grade. Visual assessment carried an 8% false positive and 39% false negative rate with regard to biopsy-proven rejection. However, 81% of missed rejection specimens were histologic Grade 1. Biopsy, when visually indicated, would detect all rejection episodes when histologically Grade 1 or 2 and still potentially reversible.

CONCLUSIONS

Visual scoring of CTA skin serves as a useful tool for initially detecting rejection, but repeated histologic evaluation is necessary for monitoring the subsequent course of the graft.