Peripheral regulation of graft-versus-host alloreactivity in mixed chimeric miniature swine

Leveraging the lymphohematopoietic graft-versus-host reaction (LGVHR) to achieve allograft tolerance and restore self tolerance with minimal toxicity

Mixed allogeneic chimerism has considerable potential to advance the achievement of immune tolerance to alloantigens for transplantation and the restoration of self-tolerance in patients with autoimmune disease. In this article, I review evidence that graft-versus-host (GVH) alloreactivity without graft-vs-host disease (GVHD), termed a lymphohematopoietic graft-vs-host reaction (LGVHR), can promote the induction of mixed chimerism with minimal toxicity. LGVHR was originally shown to occur in an animal model when non-tolerant donor lymphocytes were administered to mixed chimeras in the absence of inflammatory stimuli and was found to mediate powerful graft-vs-leukemia/lymphoma effects without GVHD. Recent large animal studies suggest a role for LGVHR in promoting durable mixed chimerism and the demonstration that LGVHR promotes chimerism in human intestinal allograft recipients has led to a pilot study aiming to achieve durable mixed chimerism.

Immunomodulatory effects of mixed hematopoietic chimerism: immune tolerance in canine model of lung transplantation

Long-term survival after lung transplantation is limited by acute and chronic graft rejection. Induction of immune tolerance by first establishing mixed hematopoietic chimerism (MC) is a promising strategy to improve outcomes. In a preclinical canine model, stable MC was established in recipients after reduced-intensity conditioning and hematopoietic cell transplantation from a DLA-identical donor. Delayed lung transplantation was performed from the stem cell donor without pharmacological immunosuppression. Lung graft survival without loss of function was prolonged in chimeric (n = 5) vs. nonchimeric (n = 7) recipients (p < or = 0.05, Fisher's test). There were histological changes consistent with low-grade rejection in 3/5 of the lung grafts in chimeric recipients at > or =1 year. Chimeric recipients after lung transplantation had a normal immune response to a T-dependent antigen. Compared to normal dogs, there were significant increases of CD4+INFgamma+, CD4+IL-4+ and CD8+ INFgamma+ T-cell subsets in the blood (p < 0.0001 for each of the three T-cell subsets). Markers for regulatory T-cell subsets including foxP3, IL10 and TGFbeta were also increased in CD3+ T cells from the blood and peripheral tissues of chimeric recipients after lung transplantation. Establishing MC is immunomodulatory and observed changes were consistent with activation of both the effector and regulatory immune response.

Partial donor-specific tolerance to delayed skin grafts after rejection of hematopoietic cell graft

BACKGROUND

Donor-specific tolerance (DST) is induced after allogeneic hematopoietic cell transplantation (HCT) and is a potential strategy for prolonging survival of solid organ grafts. DST may persist in recipients with transient mixed hematopoietic chimerism (MC) when solid organ transplantation and HCT are done concomitantly.

METHODS

In a canine model of allogeneic HCT after nonmyeloablative conditioning, DST to skin grafts was evaluated in dog leukocyte antigen (DLA)-identical recipients with stable MC (n=11), or after rejection of the hematopoietic cell (HC) graft (n=19).

RESULTS

There was significant improvement in the survival of DLA-identical HC donor-derived skin grafts in recipients with MC compared to normal recipients (n=7; P<0.0001). However, HC donor-derived skin grafts in four recipients with MC developed an inflammatory reaction without skin graft loss. This may represent partial DST. Survival of DLA-identical HC donor-derived skin grafts was also significantly prolonged compared to normal recipients even when skin grafting was delayed until after rejection of the HC graft (P=0.002). An inflammatory reaction developed in all nine of the surviving HC donor-derived skin grafts in this group, but there was no graft loss at last follow-up (median, 30 [range, 9-84] weeks). An increased time to rejection of the hematopoietic graft was associated with prolonged survival of the subsequent skin graft (P=0.02).

CONCLUSION

In a model of stable MC, DST to skin grafts may be complete or partial. Partial DST can persist after HC graft rejection even if solid organ transplantation is delayed. Further investigations are required to understand the mechanisms responsible for DST after allogeneic HCT.

Stable multilineage chimerism without graft versus host disease following nonmyeloablative haploidentical hematopoietic cell transplantation

BACKGROUND

Hematopoietic cell transplantation may offer the only cure for patients with hematological diseases. The clinical application of this therapy has been limited by toxic conditioning and lack of matched donors. Haploidentical transplantation would serve to extend the potential donor pool; however, transplantation across major histocompatibility complex barriers is often associated with severe graft-versus-host disease. Here we evaluate a novel protocol to achieve engraftment across mismatch barriers without toxic conditioning or significant posttransplant complications.

METHODS

Nine major histocompatibility complex (MHC)-defined miniature swine received haploidentical hematopoietic cell transplantation following standard myeloablative conditioning. Nine additional animals received haploidentical hematopoietic cell transplantation following a minimally myelosuppressive regimen, consisting of 100 cGy total body irradiation, immunotoxin mediated T-cell depletion, and a short course of cyclosporine. Donor cell engraftment and peripheral chimerism was assessed by polymerase chain reaction and flow cytometry. Graft-versus-host disease was monitored by clinical grading and histology of skin biopsy specimens.

RESULTS

All animals conditioned for haploidentical hematopoietic cell transplantation using myeloablative conditioning were euthanized within 2 weeks due to engraftment failure or graft-versus-host disease. All animals conditioned with the nonmyeloablative regimen developed multilineage peripheral blood chimerism during the first 2 months following transplantation. Six animals evaluated beyond 100 days maintained multilineage chimerism in the peripheral blood and lymphoid tissues, showed evidence of progenitor cell engraftment in the bone marrow, and had minimal treatment-related complications.

CONCLUSIONS

Here we report that stable multilineage chimerism and engraftment can be established across haploidentical major histocompatibility complex barriers with minimal treatment-related toxicity and without significant risk of graft-versus-host disease.

Immunological Unresponsiveness in Chimeric Miniature Swine following MHC-Mismatched Spleen Transplantation

BACKGROUND

In rodents, spleen allotransplantation (SpTx) induces tolerance. We investigated the induction of chimerism and donor-specific unresponsiveness following pig SpTx.

METHODS

Thirteen pigs underwent splenectomy (day 0); all received a blood transfusion. In 11/13 pigs, SpTx was performed across a MHC class I (n=1) or full (n=10) barrier; two control pigs received no SpTx. All pigs were monitored for chimerism, and anti-donor immune responses, including suppressor assays. Four pigs (two asplenic controls and two with SpTx) underwent delayed donor-matched kidney transplantation without immunosuppression.

RESULTS

Six of the 11 spleen grafts were lost from rejection (n=5) or splenic vein thrombosis (n=1), and five remained viable. All 11 SpTx recipients developed multilineage chimerism, but chimerism was rapidly lost if the graft failed. Two control pigs showed <6% blood chimerism for 4 and 11 days only. Pigs with functioning spleen grafts had multilineage chimerism in blood, thymus and bone marrow for at least 2-6 months, without graft-versus-host disease. These pigs developed in vitro donor-specific hyporesponsiveness and suppression. In 2 pigs tolerant to the spleen graft, donor MHC-matched kidney grafts survived for >4 and >7 months in the absence of exogenous immunosuppression; in two asplenic pigs, kidney grafts were rejected on days 4 and 15.

CONCLUSIONS

Successful SpTx can result in hematopoietic cell engraftment and in vitro donor-specific unresponsiveness, enabling prolonged survival of subsequent donor-matched kidney grafts without immunosuppression.

Stem cell plasticity and blood and marrow transplantation: a clinical strategy

The newly described phenomenon of stem cell plasticity raises interesting biological questions and offers exciting opportunities in clinical application. This review uses the well-established practice of blood and marrow transplantation as a paradigm to explore the clinical consequences of this finding. Recently proposed non-myeloablative conditioning regimens have shown that mixed donor-host hematolymphoid chimerism can be established with relatively low toxicity in both animal studies and human trials. Hematopoietic growth factor treatment of transplanted patients can mobilize a large number of donor stem cells to migrate from marrow to non-hematopoietic organs. We propose that these advances, in conjunction with the developmental plasticity of stem cells, can constitute components of a clinical strategy to use blood and marrow transplantation as a platform to treat systemic diseases involving non-hematopoietic tissues.