Cellular Based Immunodepletion and Tomotherapy for Induction of Immunosuppression in a Rhesus Macaque Renal Allotransplant Model.

Abstract

PURPOSE: To determine the safety and efficacy of inducing an immunosuppressive environment for mixed chimerism in a nonhuman primate renal transplant model. METHODS: Eight rhesus macaques were divided into three treatment groups: Group 1 (n=1) received only anti-thymocyte globulin (ATG)(5 days at 4 mg/kg), Group 2 (n=3) received both ATG and helical tomotherapy based total lymphoid irradiation (TLI) (10 fractions of 1.2 Gy), and Group 3 (n=4) received ATG, TLI and a renal allograft. Animals received maintenance tacrolimus and mycophenolate mofetil. Weekly blood differential counts, serum chemistries, and flow cytometry was performed in all animals. Cytomegalovirus was measured by quantitative PCR. These studies differed from TLI/ATG protocols used in humans, since the latter include anti-viral prophylaxis to prevent CMV reactivation, and vertebral bodies are shielded to prevent severe neutropenia. RESULTS: All animals did well with no fever or major illness. Animal weight temporarily decreased an average of 19.1% following treatment (6.3% Group 1; 14.7% Group 2; 25.7% Group 3), with resolution at a mean of 90 days. Serum chemistries remained normal other than elevated glucose (>200) in 3 animals in groups 2 and 3. ATG immunodepletive therapy alone (Group 1) resulted in lymphocyte depletion that recovered after day 7. Use of ATG and TLI in Groups 2 and 3 demonstrated adequate lymphocyte depletion, lasting beyond 14 days; however, severe neutropenia (ANC<500) was noted in 5 animals between days 9 and 21 in the absence of vertebral body shielding. Two animals in Group 3 developed wound infections (day 7 and 56 respectively), treated successfully with antibiotics. All animals in group 3 without anti-viral prophylaxis developed CMV reactivation requiring antiviral therapy. CONCLUSION: A combination of ATG and helical tomotherapy for TLI can be safely administered to the rhesus macaque, providing effective immunosuppression beyond 2 weeks without toxicity. This is ideal to allow for engraftment of donor HSC and T cell infusion on treatment day 11. CMV reactivation should be treated prophylactically rather than preemptively. Severe neutropenia was well tolerated, however, further adjustments to TLI dose and distribution may result in less neutropenia and less CMV reactivation.