Strategies for induction of clinical tolerance

Composite Vascularized Skin/Bone Transplantation Models for Bone Marrow-Based Tolerance Studies

There is an ongoing need to understand the mechanisms of bone marrow-based allograft tolerance. This is important in clarifying the diverse variables influencing the ultimate outcome of the solid organ and composite tissue transplants. To establish bone marrow transplantation as a routine clinical application, further experimental studies should be conducted to overcome the obstacles related to the bone marrow transplantation. These obstacles include graft versus host disease, immunocompetence, and toxicity of the conditioning regimens. For these purposes, novel experimental models are needed. In an attempt to provide a reliable research tool for bone marrow-based tolerance induction studies, we introduced different experimental models of modified vascularized skin/bone marrow (VSBM) transplantation technique for tolerance induction, monitoring, and maintenance studies. In this skin/bone transplantation model, the technical feasibility of concurrent or consecutive transplantation of the combination of bilateral vascularized skin, vascularized bone marrow, or vascularized skin/bone marrow transplants was investigated. Isograft transplantations were performed between genetically identical Lewis (LEW, RT1) rats. Five different experimental designs in 5 groups of 5 animals each were studied. Group I: Bilateral vascularized skin (VS) transplantation; group II: bilateral vascularized skin/bone transplantation; group III: vascularized skin transplantation on one side and vascularized skin/bone transplantation on the contralateral side; group IV: vascularized bone transplantation on one side and vascularized skin/bone transplantation on the contralateral side; group V: vascularized bone transplantation on one side and vascularized skin transplantation on the contralateral side. Successful transplantations were performed in all groups. The survival of the isograft transplants was evaluated clinically and histologically. All skin flaps remained pink and pliable and grew new hair. The viability of the compact bone, bone marrow and skin at 100 days posttransplant was confirmed by histologic evaluation, and bone marrow revealed active hematopoiesis. Bilateral skin/bone transplantation model may serve as an experimental tool to study new strategies in tolerance induction by altering the amount of the immunogenic load in the form of skin transplant and bone marrow delivery in the vascularized form, allowing for expedited engraftment of stem and progenitor cells.

Potent Skin Allograft Survival Prolongation Using a Committed Progenitor Fraction of Bone Marrow in Mice

BACKGROUND

The infusion of donor bone marrow (BM) into mice conditioned with antilymphocyte serum (ALS) and sirolimus (Sir) prolongs skin allograft survival and produces chimerism. This study identifies the BM cell(s) responsible for this effect and determines whether enrichment for these cells will improve efficacy.

METHODS

Skin grafts from BALB/C mice were transplanted into C57BL/6 or C57BL/10 recipients by using ALS, Sir, and BM (or fractions). BM was fractionated by using immunomagnetic beads. Flow cytometry was used for phenotyping and detecting chimerism.

RESULTS

The median graft survival in mice receiving 25 million BM cells was 61 days. Infusion of BM depleted of cells expressing CD19, CD3, CD11c, and c-kit had no effect on median graft survival, whereas infusion of fractions enriched for those cells resulted in median graft survival of 38, 48, 28, and 83 days, respectively. The administration of higher doses (4 x 10(6) and 8x10(6)) of fractions enriched for c-kit resulted in median graft survival of 124 and 197 days, respectively, without chimerism. This favorably compared with mice receiving 150 million BM cells that demonstrated transient mixed chimerism and a median graft survival of 190 days. The majority of cells in the c-kit+-enriched fraction expressed lineage markers. Removal of lineage positive cells from BM before infusion shortened median graft survival (90 days), indicating that the c-kit+ lin+ population is largely responsible for prolongation of graft survival.

CONCLUSIONS

Cells enriched for C-kit+lin+ constitute approximately 5% of murine BM cells and are more potent than whole BM at prolonging skin allograft survival in mice treated with ALS and Sir.

Identification of gene expression profile in tolerizing murine cardiac allograft by costimulatory blockade

The induction of specific tolerance would be the ultimate achievement in transplant immunology, but the precise mechanisms of immunologic tolerance remain largely unknown. Here, we investigated global gene expression analysis in tolerizing murine cardiac allografts by means of oligonucleotide microarrays. Tolerance induction was achieved in cardiac allografts from BALB/c to C57BL/6 mice by daily intraperitoneal injection of anti-CD80 and anti-CD86 monoclonal antibodies (mAbs). Comparative analysis revealed 64 genes to be induced more extensively in the tolerizing than in the syngeneic isografts, and 16 genes than in the rejecting allografts. Two genes were specifically upregulated in the tolerizing allografts. In the tolerizing allografts there were induced marked expressions of a number of genes for pro-inflammatory factors, including interferon-gamma-inducible cytokines and chemokines, as well as apoptosis-related genes, which were also upregulated in the rejecting allografts. Moreover, these gene expression patterns continued to be upregulated more than 70 days posttransplant. These results provide evidence that immunologic tolerance can be induced and maintained in the presence of prominent pro-inflammatory gene expression in vivo.

Intraportal donor bone marrow transplantation improves intestinal allograft survival in rats under FK506-based immunosuppression

Donor-specific immunosuppression is important in transplant surgery. We examined the effect of intraportal donor-specific bone marrow transplantation on heterotopic small bowel transplantation in the high responder rat combination, ACI to Lewis. The study comprised five treatment groups: untreated controls (group 1); FK506 alone (group 2); low-dose predonine + FK506 (group 3); high-dose predonine + FK506 (group 4); and intraportal donor-specific bone marrow transplantation + FK506 (group 5). Intraportal transplantation was performed pre-operatively and FK506 and predonine given post-operatively. Intestinal allograft survival and changes of intragraft cytokine expression were analysed using the reverse transcription polymerase chain reaction. Allograft survival (mean +/- SD) was lowest in group 1 and greatest in group 5. The group 5 treatment regimen also down-regulated interferon-gamma and interleukin-2 transcription in the transplanted intestine. Intraportal donor bone marrow transplant combined with FK506 immunosuppression was found therefore to be the most beneficial treatment regimen.

Tailoring of immunosuppression in renal and liver allograft recipients displaying donor specific T-suppressor cells

Although transplantation tolerance cannot be yet reliably achieved in humans, there is evidence that active immunosuppression contributes to the maintenance of quiescence. However, the mechanism that underlies quiescence and the precise identity of regulatory cells are not completely understood. We have demonstrated that allograft recipients who remain rejection-free display allospecific T-suppressor cells (Ts). Ts express the CD8(+) CD28(-) phenotype, recognize major histocompatibility complex (MHC) class I antigens, and suppress the up-regulation of costimulatory molecules induced by CD40 ligation of donor antigen presenting cells. The presence of Ts is inversely correlated with T cell alloreactivity to donor MHC peptides, alloantibody production, and rejection. Monitoring of Ts has been successfully used in our studies for tailoring immunosuppression in kidney and liver allograft recipients.