Prolonged allograft survival through conditional and specific ablation of alloreactive T cells expressing a suicide gene

Antibody-mediated depletion of lymphocyte-activation gene-3 (LAG-3(+) )-activated T lymphocytes prevents delayed-type hypersensitivity in non-human primates

Lymphocyte-activation gene-3 (LAG-3, CD223) is a marker for recently activated effector T cells. Activated T lymphocytes are of major importance in many autoimmune diseases and organ transplant rejection. Therefore, specifically depleting LAG-3(+) T cells might lead to targeted immunosuppression that would spare resting T cells while eliminating pathogenic activated T cells. We have shown previously that anti-LAG-3 antibodies sharing depleting as well as modulating activities inhibit heart allograft rejection in rats. Here, we have developed and characterized a cytotoxic LAG-3 chimeric antibody (chimeric A9H12), and evaluated its potential as a selective therapeutic depleting agent in a non-human primate model of delayed-type hypersensitivity (DTH). Chimeric A9H12 showed a high affinity to its antigen and depleted both cytomegalovirus (CMV)-activated CD4(+) and CD8(+) human T lymphocytes in vitro. In vivo, a single intravenous injection at either 1 or 0·1 mg/kg was sufficient to deplete LAG-3(+) -activated T cells in lymph nodes and to prevent the T helper type 1 (Th1)-driven skin inflammation in a tuberculin-induced DTH model in baboons. T lymphocyte and macrophage infiltration into the skin was also reduced. The in vivo effect was long-lasting, as several weeks to months were required after injection to restore a positive reaction after antigen challenge. Our data confirm that LAG-3 is a promising therapeutic target for depleting antibodies that might lead to higher therapeutic indexes compared to traditional immunosuppressive agents in autoimmune diseases and transplantation.

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

PURPOSE OF REVIEW

Current depletion strategies used in clinical transplantation can prevent acute rejection of a transplanted organ; however, they are nonspecific and are limited by their efficacy or the side effects of wide ranging cellular depletion. This review will focus on strategies that prevent rejection of allografts using specific allodepletion of the T cells that mediate rejection.

RECENT FINDINGS

Strategies that use either in-vivo targeting of alloreactive T cells or ex-vivo manipulation to specifically reduce the alloreactive T-cell pool have been developed. The advantage of these approaches is that they are specific, by depleting cells that cause rejection while leaving the remaining immune system intact, thereby minimizing the detrimental complications associated with standard immunosuppression.

SUMMARY

Strategies to reduce the proportion of alloreactive T cells that initiate transplant rejection are emphasized. This factor has the specific advantage of leaving the remaining T-cell repertoire intact and may therefore be used in combination with other immunemodulating and tolerance strategies.

Depleting T-cell subpopulations in organ transplantation

T-cell depletion strategies are an efficient therapy for the treatment of acute rejection after organ transplantation and have been successfully used as induction regimens. Although eliminating whole T cells blocks alloreactivity, this therapy challenges the development of regulatory mechanisms because it depletes regulatory cells and modifies the profile of T cells after homeostatic repopulation. Targeting T-cell subpopulations or selectively activated T cells, without modifying Treg cells, could constitute a pro-tolerogenic approach. However, the perfect molecular target that would be totally specific probably still needs to be identified. In this study, we have reviewed the biological activities of broad or specific T-cell depletion strategies as these contribute to the induction of regulatory cells and tolerance in organ transplantation.

Transient depletion of dividing T lymphocytes in mice induces the emergence of regulatory T cells and dominant tolerance to islet allografts

We previously showed that transient depletion of dividing T cells at the time of an allogeneic transplantation induces long-term tolerance to the allograft. Here we investigated the role of homeostatic perturbation and regulatory T cells (Treg) in such tolerance. Transient depletion of dividing T cells was induced at the time of an allogeneic pancreatic islets graft, by administration of ganciclovir for 14 days, into diabetic transgenic mice expressing a thymidine kinase (TK) conditional suicide gene in T cells. Allograft tolerance was obtained in 63% of treated mice. It was not due to global immunosuppression, permanent deletion or anergy of donor-alloantigens specific T cells but to a dominant tolerance process since lymphocytes from tolerant mice could transfer tolerance to naïve allografted recipients. The transient depletion of dividing T cells induces a 2- to 3-fold increase in the proportion of CD4(+)CD25(+)Foxp3(+) Treg, within 3 weeks that persisted only in allograft-bearing mice but not in nongrafted mice. Tolerance with similar increased proportion of Treg cells was also obtained after a cytostatic hydroxyurea treatment in normal mice. Thus, the transient depletion of dividing T cells represents a novel means of immuno-intervention based on disturbance of T-cell homeostasis and subsequent increase in Treg proportion.

Comprehensive assessment and mathematical modeling of T cell population dynamics and homeostasis

Our current view of T cell differentiation and population dynamics is assembled from pieces of data obtained from separate experimental systems and is thus patchy. We reassessed homeostasis and dynamics of T cells 1) by generating a mathematical model describing the spatiotemporal features of T cell differentiation, and 2) by fitting this model to experimental data generated by disturbing T cell differentiation through transient depletion of dividing T cells in mice. This specific depletion was obtained by administration of ganciclovir to mice expressing the conditional thymidine kinase suicide gene in T cells. With this experimental approach, we could derive quantitative parameters describing the cell fluxes, residence times, and rates of import, export, proliferation, and death across cell compartments for thymocytes and recent thymic emigrants (RTEs). Among other parameters, we show that 93% of thymocytes produced before single-positive stages are eliminated through the selection process. Then, a postselection peripheral expansion of naive T cells contributes three times more to naive T cell production than the thymus, with half of the naive T cells consisting of dividing RTEs. Altogether, this work provides a quantitative population dynamical framework of thymocyte development, RTEs, and naive T cells.

Coexpression of the Uracil Phosphoribosyltransferase Gene with a Chimeric Human Nerve Growth Factor Receptor/Cytosine Deaminase Fusion Gene, Using a Single Retroviral Vector, Augments Cytotoxicity of Transduced Human T Cells Exposed to 5-Fluorocytosine

Donor T lymphocytes genetically engineered to express a "suicide gene" to facilitate negative selection represent a promising strategy for the management of graft-versus-host disease occurring after allogeneic hematopoietic cell transplantation (HCT). For this purpose, the herpes simplex virus thymidine kinase (HSV-tk) gene, although well studied, has limitations. Cytosine deaminase (CD), an alternative gene for negative selection, converts 5-fluorocytosine (5-FC) to the toxic metabolite 5-fluorouracil (5-FU). Sensitivity of cells to 5-FU can be further increased by expression of uracil phosphoribosyltransferase (UPRT), which catalyzes the conversion of 5-FU to 5-fluorouridine monophosphate. By using a chimeric gene (NG/CD) expressing the truncated human nerve growth factor receptor (NGFR) for positive selection fused to the Saccharomyces cerevisiae CD gene, we investigated strategies to achieve optimal T cell eradication by CD and UPRT expression, utilizing a single retroviral vector. Three vector strategies were compared on the basis of NGFR expression by flow cytometry, western analysis, and enzymatic activity. A construct (NG/CDiU) expressing UPRT and NG/CD, using a bicistronic message, provided the greatest UPRT activity and killing, reducing the lethal dose of 5-FC sufficient to eradicate 90% of cells from 38.7 microg/ml (300 microM) (NG/CD expression alone) to 0.13 microg/ml (1 microM). This approach provides an effective alternative to the HSV-tk system for eradication of donor T lymphocytes after allogeneic HCT.

The proatherogenic role of T cells requires cell division and is dependent on the stage of the disease

OBJECTIVE

The mechanism by which T cells exert a proatherogenic potential is unclear. In order to determine whether this potential requires their replication, we crossed atherosclerosis-prone apolipoprotein E knockout mice (ApoE degrees) with transgenic mice in which exclusive and conditional ablation of dividing T cells relies on their specific expression of the herpes simplex type 1 thymidine kinase (TK) suicide gene.

METHODS AND RESULTS

We first showed that conalbumin-immunized ApoE degrees TK mice mounted a significant immune response to the antigen that was fully and specifically blocked by an in vivo ganciclovir (GCV) treatment. Next, ApoE degrees TK mice and ApoE degrees mice were treated or not with GCV either during the first 4 weeks (GCV 1 to 4w), the last 4 weeks (GCV 5 to 8w), or during 8 weeks (GCV 1 to 8w). Strikingly, ApoE degrees TK mice displayed a dramatic decrease in lesion development in the GCV 1 to 8w and GCV 5 to 8w groups, whereas the GCV had no effect when administered during the first 4 weeks. In protected mice, the inflammatory parameters in lesions, the percentage of CD69+ CD3+ splenocytes, and the circulating natural killer T cells were reduced.

CONCLUSIONS

The present study, therefore, shows that the proatherogenic potential of T cells is crucial in the progression of fatty streaks to mature plaques and requires cell division.

Turning immunological memory into amnesia by depletion of dividing T cells

Immunological memory, defined as more efficient immune responses on antigen reexposure, can last for decades. The current paradigm is that memory is maintained by antigen-experienced "memory T cells" that can be long-lived quiescent or dividing. The contribution of T cell division to memory maintenance is poorly known and has important clinical implications. In this study, we directly addressed the role of dividing T cells in immunological memory maintenance by evaluating the consequences of their elimination. The specific ablation of dividing T cells was obtained by administration of ganciclovir to immune mice expressing the herpes simplex type 1 thymidine kinase suicide gene in T cells. We show that depletion of dividing T cells for 5 or 2 weeks suffices to abolish in vitro and in vivo memory responses against the male H-Y transplantation alloantigen or against lymphocytic choriomeningitis virus antigens, respectively. Similar results were obtained after the nonspecific elimination of all dividing cells by using hydroxyurea, a cytostatic toxic agent commonly used for cancer chemotherapy. This immune amnesia occurred in otherwise immunocompetent mice and despite the persistence of functional quiescent T cells displaying a "memory" phenotype. Thus, division of antigen-experienced T cells is an absolute requirement for immunological memory maintenance and the current concept of memory T cells is challenged.

Gene therapy in transplantation

Gene transfer and gene therapy represent a relatively new field that has grown and expanded enormously in the last 5-10 years. The application of gene transfer and gene medicines to transplantation is currently in its infancy. Consideration for gene medicines in transplantation requires delivery of vectors, either to the graft or to the immune system. Delivery of vectors to the graft provides a choice of potential immunologic targets including: costimulatory signals; inhibitory cytokines; adhesion molecules; and molecules relating to apoptosis. In addition, non-immunologic targets, that increase graft protective mechanisms by reducing ischemic and immunologic damage, represent significant targets for gene transfer. Delivery of vectors to the immune system includes potential targets to modify the immune system, and results in tolerance. Other considerations for gene therapy include the development of additional technologies, such as gene conversion or transgenesis coupled with xenotransplantation, which may provide genetically modified organs. Another important aspect of gene transfer relates to regulation of the transgene expression. A variety of issues concerning innate immunity, adaptive immunity, response to vector components, response to transgene products, and entry of vectors into the antigen presentation and processing pathway require further investigation and refinement of approaches. Lastly, regulatable promoters and the understanding of their interaction with individual cells, tissues and organs, and their interaction with innate and adaptive immunity, are of paramount importance to improving the efficacy and utility of gene transfer. There is no doubt that there is much exciting basic and translational science to be accomplished in the next decade in order to solve these potential barriers and advance gene medicines into the clinical realm in transplantation.

T-Cell suicide gene therapy for organ transplantation: induction of long-lasting tolerance to allogeneic heart without generalized immunosuppression

Standard immunosuppressive drugs used for allogeneic organ transplantation do not specifically target alloreactive T cells and must be given for the lifetime of the patient, resulting in significant morbidity and mortality. We aimed to induce experimental immune tolerance to vascularized heart allograft using a suicide gene allowing selective elimination of dividing T cells expressing Herpes simplex virus type 1 thymidine kinase upon ganciclovir administration. We show that without ganciclovir, transgenic mice selectively expressing thymidine kinase in T cells rejected a vascularized cardiac allograft in 7 days. In contrast, allograft was definitively accepted after a 7-day course of ganciclovir initiated at the time of allotransplantation. Interestingly, T cells from both rejecting and tolerant mice proliferated in response to donor or third-party allogeneic stimulation. This state of tolerance was challenged through a second vascularized cardiac allotransplantation. Third-party allografts were rejected while those syngeneic to the first allograft were accepted without any additional treatment. These results show that short-term pharmacogenetic immunosuppression can induce long-lasting, robust, and specific tolerance to solid vascularized allograft without generalized continuous immunosuppression.

Transient control of a virus-induced immunopathology by genetic immunosuppression

The ability to control T cell reactivity using suicide genes opens new perspectives for the treatment of T cell-mediated diseases. The therapeutic effect is achieved by the selective killing of thymidine kinase gene-modified activated T cells by ganciclovir (GCV). This strategy has been shown to control T cell alloreactivity efficiently after bone marrow or solid organ transplantation. Here, we aimed to determine whether an immunopathological process induced by a viral infection could be controlled by GCV when T cells express a thymidine kinase transgene. When transgenic mice were infected with the lymphocytic choriomeningitis virus, administration of GCV resulted in an efficient, but only transient, control of the immunopathological immune response. Further analysis revealed the existence of a minute population of GCV-insensitive T cells. These cells expand in response to the virus despite the presence of GCV and cause immunopathology before viral elimination is finally obtained. Thus, when confronted with a replicative virus, the efficacy of this genetic immunosuppression strategy is highly dependent on the presence of even small numbers of GCV-insensitive cells. These results emphasize the need for sufficient preclinical investigations with regard to the pathology and the nature of the immune response if suicide gene transfer is envisioned for new therapeutic indications.