Tolerance induction by removal of alloreactive T cells: in-vivo and pruning strategies

Transforming growth factor beta (TGFβ) plays a crucial role in prolonging allograft survival in an allodepletion ("pruning") skin transplant model

Adoptive cell therapies involving cell manipulation to achieve tolerance are increasingly being studied in animal models and in human trials. We have demonstrated that the specific removal of allo-stimulated dividing cells (or "pruning") promotes long-term allograft survival across a major MHC mismatch in transplant models including skin, heart and islet transplants. In this study, we examine the role of transforming growth factor beta (TGFβ), an important regulatory cytokine, on allograft survival in our allodepletion or "pruning" skin transplant model. Increased proliferation of CD4(+) T cells was observed following allo-stimulation of BALB/c spleen cells (labeled with CFSE) in the presence of the regulatory cytokines TGFβ and (interleukin-2) IL-2 in a mixed lymphocyte culture (MLC). Expression of the regulatory gene forkhead box-3 (FoxP3) was increased in both the allo-stimulated non-dividing (ND) (CFSE(high)) and dividing (D) (CFSE(low)) CD4(+) T cell populations, with the highest expression found in the D CD4(+) T cell population. Mice reconstituted with allo-stimulated ND CD4(+) T cells following TGFβ/IL-2 stimulation showed prolonged allograft survival, similar to previous data. Significantly, TGFβ/IL-2 stimulation prevented acute rejection of allografts across a major MHC mismatch in the presence of highly activated allo-stimulated D CD4(+) T cells. Blockade of TGFβ promoted rejection of allografts even following depletion of allo-stimulated D CD4(+) T cells. These studies support a crucial role for TGFβ in the survival of allografts and shows that regulatory cytokines TGFβ/IL2 can delay the rejection of allografts, even in the presence of highly activated alloreactive T cells.

Lymphotoxin-beta receptor blockade induces inflammation and fibrosis in tolerized cardiac allografts

The lymphotoxin system (LT) regulates interactions between lymphocytes and stromal cells to maintain lymphoid microenvironmental homeostasis. Soluble LT beta-receptor-Ig (LTβRIg) blocks lymphocyte LTα1β2-stromal cell LTβR signaling. In a murine cardiac allograft model, LTbRIg treatment reversed the tolerance induced by anti-CD40L antibody leading to graft inflammation and fibrosis. LTβRIg treatment decreased PD-L1 expression by blood endothelial cells, and decreased VCAM-1 while increasing CXCL1, CXCL2, CXCL12, CCL5, CCL21 and IL-6 expression in fibroblastic reticular cells. In secondary lymphoid organs these effects caused T- and B cell zone disruption, loss of CD35(+) follicular dendritic cells and abnormal recruitment of CD11b(+) Ly6G(+) neutrophils. These disruptions correlated with increased numbers of CD8(+) T cells and CD11b(+) Ly6G(+) neutrophils, and decreased numbers of CD4(+) T cells and Foxp3(+) regulatory T cells in the grafts. Depleting neutrophils or blocking neutrophil-attracting chemokines restored normal histology in lymph node, spleen and grafts. Taken together, LTβRIg treatment altered stromal subset, particularly fibroblastic reticular cell, production of cytokines and chemokines, resulting in changes in neutrophil recruitment in spleen, lymph node and grafts, and inflammation and fibrosis associated with decreased Foxp3(+) regulatory T cells and increased CD8(+) T cell infiltration of grafts.

Ethylenecarbodiimide-fixed donor splenocyte infusions differentially target direct and indirect pathways of allorecognition for induction of transplant tolerance

Strategic exposure to donor Ags prior to transplantation can be an effective way for inducting donor-specific tolerance in allogeneic recipients. We have recently shown that pretransplant infusion of donor splenocytes treated with the chemical cross-linker ethylenecarbodiimide (ECDI-SPs) induces indefinite islet allograft survival in a full MHC-mismatched model without the need for any immunosuppression. Mechanisms of allograft protection by this strategy remain elusive. In this study, we show that the infused donor ECDI-SPs differentially target T cells with indirect versus direct allospecificities. To target indirect allospecific T cells, ECDI-SPs induce upregulation of negative, but not positive, costimulatory molecules on recipient splenic CD11c(+) dendritic cells phagocytosing the injected ECDI-SPs. Indirect allospecific T cells activated by such CD11c(+) dendritic cells undergo robust initial proliferation followed by rapid clonal depletion. The remaining T cells are sequestered in the spleen without homing to the graft site or the graft draining lymph node. In contrast, direct allospecific T cells interacting with intact donor ECDI-SPs not yet phagocytosed undergo limited proliferation and are subsequently anergized. Furthermore, CD4(+)CD25(+)Foxp3(+) T cells are induced in lymphoid organs and at the graft site by ECDI-SPs. We conclude that donor ECDI-SP infusions target host allogeneic responses via a multitude of mechanisms, including clonal depletion, anergy, and immunoregulation, which act in a synergistic fashion to induce robust transplant tolerance. This simple form of negative vaccination has significant potential for clinical translation in human transplantation.

Emerging concepts in haematopoietic cell transplantation

Haematopoietic cell transplantation (HCT) is the most widely used form of cellular therapy. It is the only known cure for some haematological malignancies and has recently been used in additional clinical settings, such as allograft tolerance induction and treatment of autoimmune diseases. Recent advances have enabled HCT in a wider range of patients with improved outcomes. This Review summarizes the latest developments in this therapy, focusing on issues that will affect future advancement.

Lymphodepletion and homeostatic proliferation: implications for transplantation

Control of the alloimmune response requires elimination and/or suppression of alloreactive immune cells. Lymphodepleting induction therapies are increasingly used to accomplish this goal, both as part of tolerance induction protocols or to reduce the requirements for maintenance immunosuppression in the peritransplant setting. However, it is well recognized that lymphopenia induces compensatory proliferation of immune cells, generally termed ``homeostatic proliferation,'' which favors the emergence of memory T cells. Paradoxically therefore, the result may be a situation that favors graft rejection and/or makes tolerance difficult to achieve or sustain. Yet all depletion is not alike, particularly with respect to the timing of reconstitution and the types of cells that repopulate the host. Thus, to design more effective induction strategies it is important to understand the homeostatic mechanisms, which exist to maintain a balanced repertoire of naïve and memory T and B cells in the periphery and how they respond to lymphodepletion. Here we will review the biology of homeostatic proliferation stimulated by lymphopenia, the effects of specific depleting agents on reconstitution of the T- and B-cell immune repertoire, drawing from both from animal models and human experience, and potential strategies to enhance allodepletion while minimizing the adverse effects of homeostatic proliferation.

In vitro-established alloantigen-specific CD8+ CTLs mediate graft-versus-tumor activity in the absence of graft-versus-host disease

Mature donor-derived T cells in allogeneic bone marrow (BM) transplants mediate the graft-versus-tumor (GVT) effect by recognizing alloantigens on leukemic cells. However, alloantigen reactivity towards non-malignant tissues also induces graft-versus-host disease (GVHD). Defining T-cell subpopulations that mediate the GVT effect in the absence of GVHD induction remains a major challenge in allogeneic BM transplantation. In this study, we show that in vitro-generated alloantigen-specific CD8(+) cytotoxic T cells (CTLs) established by weekly stimulation with alloantigen-expressing antigen-presenting cells did not induce GVHD in two major histocompatibility complex-mismatched BM transplantation models, where induction of lethal GVHD is dependent on the presence of either CD4(+) or CD8(+) T cells. Despite their strong alloantigen specificity, transplantation of CTLs did not induce the expression of GVHD-associated cytokines IFN-γ and TNF-α or clinical or histological signs of GVHD, and lead to a survival rate of above 90%. However, transplantation of unstimulated CD8(+) T cells, which were not primed by the alloantigen in vitro, induced GVHD in both the transplantation models. Although CTLs were impaired in GVHD induction, they efficiently eradicated Bcr-Abl-transformed B-cell leukemias or mastocytomas. Thus, in vitro-derived CTLs might be useful for optimizing anti-tumor therapy in the absence of GVHD induction.

Antileukemia and antitumor effects of the graft-versus-host disease: a new immunovirological approach

In leukemic mice, the native host's explicit and well-defined immune reactions to the leukemia virus (a strong exogenous antigen) and to leukemia cells (pretending in their native hosts to be protected "self" elements) are extinguished and replaced in GvHD (graft-versus-host disease) by those of the immunocompetent donor cells. In many cases, the GvHD-inducer donors display genetically encoded resistance to the leukemia virus. In human patients only antileukemia and anti-tumor cell immune reactions are mobilized; thus, patients are deprived of immune reactions to a strong exogenous antigen (the elusive human leukemia-sarcoma retroviruses). The innate and adaptive immune systems of mice have to sustain the immunosuppressive effects of leukemia-inducing retroviruses. Human patients due to the lack of leukemiainducing retroviral pathogens (if they exist, they have not as yet been discovered), escape such immunological downgrading. After studying leukemogenic retroviruses in murine and feline (and other mammalian) hosts, it is very difficult to dismiss retroviral etiology for human leukemias and sarcomas. Since no characterized and thus recognized leukemogenic-sarcomagenic retroviral agents are being isolated from the vast majority of human leukemias-sarcomas, the treatment for these conditions in mice and in human patients vastly differ. It is immunological and biological modalities (alpha interferons; vaccines; adoptive lymphocyte therapy) that dominate the treatment of murine leukemias, whereas combination chemotherapy remains the main remission-inducing agent in human leukemias-lymphomas and sarcomas (as humanized monoclonal antibodies and immunotoxins move in). Yet, in this apparently different backgrounds in Mus and Homo, GvHD, as a treatment modality, appears to work well in both hosts, by replacing the hosts' anti-leukemia and anti-tumor immune faculties with those of the donor. The clinical application of GvHD in the treatment of human leukemias-lymphomas and malignant solid tumors remains a force worthy of pursuit, refinement and strengthening. Graft engineering and modifications of the inner immunological environment of the recipient host by the activation or administration of tumor memory T cells, selected Treg cells and natural killer (NKT) cell classes and cytokines, and the improved pharmacotherapy of GvHD without reducing its antitumor efficacy, will raise the value of GvHD to the higher ranks of the effective antitumor immunotherapeutical measures. Clinical interventions of HCT/HSCT (hematopoietic cell/stem cell transplants) are now applicable to an extended spectrum of malignant diseases in human patients, being available to elderly patients, who receive non-myeloablative conditioning, are re-enforced by post-transplant donor lymphocyte (NK cell and immune T cell) infusions and post-transplant vaccinations, and the donor cells may derive from engineered grafts, or from cord blood with reduced GvHD, but increased GvL/GvT-inducing capabilities (graft-versus leukemia/tumor). Post-transplant T cell transfusions are possible only if selected leukemia antigen-specific T cell clones are available. In verbatim quotation: "Ultimately, advances in separation of GvT from GvHD will further enhance the potential of allogeneic HCT as a curative treatment for hematological malignancies" (Rezvani, A.R. and Storb, R.F., Journal of Autoimmunity 30:172-179, 2008 (see in the text)). It may be added: for cure, a combination of the GvL/T effects with new targeted therapeutic modalities, as elaborated on in this article, will be necessary.