The mechanism of specific prolongation of class I-mismatched skin grafts induced by retroviral gene therapy

Cell-based therapy in allergy

IgE-mediated allergy is an immunological disorder occurring in response to otherwise harmless environmental antigens (i.e., allergens). Development of effective therapeutic or preventive approaches inducing robust tolerance toward allergens remains an unmet goal. Several experimental tolerance approaches have been described. The therapeutic use of regulatory T cells (Tregs) and the establishment of molecular chimerism are two cell-based strategies that are of particular interest. Treg therapy is close to clinical application, but its efficacy remains to be fully defined. Recent proof-of-concept studies demonstrated that transplantation of syngeneic hematopoietic stem cells modified in vitro to express a major allergen leads to molecular chimerism and robust allergen-specific tolerance. Here we review cell-based tolerance strategies in allergy, discussing their potentials and limitations.

Gene therapy in transplantation

Gene therapy is an exciting and novel technology that offers the prospect of improving transplant outcomes beyond those achievable with current clinical protocols. This review explores both the candidate genes and ways in which they have been deployed to overcome both immune and non-immune barriers to transplantation success in experimental models. Finally, the major obstacles to implementing gene therapy in the clinic are considered.

Optimal modes and targets of gene therapy in transplantation

Genetic modification strategies have the potential to improve outcome following cell/organ transplantation. A unique opportunity in transplantation is that gene therapies need not be restricted to in vivo approaches and that ex vivo genetic modification of cell and/or organs can be of value. Improvements in vector design, production, and delivery should enhance transfection efficiency and optimize gene expression. Herein, we discuss potential modes of gene therapy, focusing on viral, liposome, or naked DNA-based systems for gene delivery. We suggest gene therapy targets taking into consideration the essential constituents of anti-allograft repertory. In addition to strategies that may have salutary effects in mitigating the threat of acute rejection, we suggest genetic strategies for minimizing ischemia/reperfusion injury as well as for the perennial problem of progressive functional loss of the transplanted organ. Data from pre-clinical transplant models support the idea that gene therapy may improve allograft function and survival. We are optimistic that gene therapy will be of clinical value in the near future in the management of recipients of allografts; we believe that genetic strategies would be essential for successful breaching of the formidable challenge of xenotransplantation.

Expression of antigen on mature lymphocytes is required to induce T cell tolerance by gene therapy

Expression of a retrovirally encoded allogeneic MHC class I gene in bone marrow-derived cells can be used to induce tolerance to the product of the retrovirally transduced gene. In this work we examined whether expression of a retrovirally transduced allogeneic MHC class I gene in bone marrow-derived cells from recombinase-activating gene-1 (RAG-1)-deficient mice was sufficient to induce tolerance when transplanted into conditioned hosts together with bone marrow from MHC-matched wild-type mice. Reconstitution of mice with either MHC-matched RAG-1-deficient or wild-type bone marrow transduced with the allogeneic MHC class I gene H-2K(b) led to long-term expression of K(b) on the surface of bone marrow-derived hematopoietic lineages. T cells from mice reconstituted with H-2K(b)-transduced wild-type bone marrow were tolerant to K(b). In contrast, expression of K(b) in the periphery of mice reconstituted with a mixture of retrovirally transduced RAG-1-deficient bone marrow and mock-transduced wild-type bone marrow fell below detectable levels by 4 wk after transplantation. T cells that developed in these mice appeared to be hyporesponsive to K(b), demonstrating that expression of K(b) on bone marrow-derived APCs was not sufficient to induce tolerance. Our data suggest that induction of tolerance in molecular chimeras requires expression of the retrovirally transduced allogeneic MHC Ag on the surface of mature lymphocytes that populate the host thymus.

Induction of central deletional T cell tolerance by gene therapy

Transgenic mice expressing an alloreactive TCR specific for the MHC class I Ag K(b) were used to examine the mechanism by which genetic engineering of bone marrow induces T cell tolerance. Reconstitution of lethally irradiated mice with bone marrow infected with retroviruses carrying the MHC class I gene H-2K(b) resulted in lifelong expression of K(b) on bone marrow-derived cells. While CD8 T cells expressing the transgenic TCR developed in control mice reconstituted with mock-transduced bone marrow, CD8 T cells expressing the transgenic TCR failed to develop in mice reconstituted with H-2K(b) transduced bone marrow. Analysis of transgene-expressing CD8 T cells in the thymus and periphery of reconstituted mice revealed that CD8 T cells expressing the transgenic TCR underwent negative selection in the thymus of mice reconstituted with K(b) transduced bone marrow. Negative selection induced by gene therapy resulted in tolerance to K(b). Thus, genetic engineering of bone marrow can be used to alter T cell education in the thymus by inducing negative selection.

Induction of T-cell tolerance to an MHC class I alloantigen by gene therapy

Induction of immunologic tolerance to alloantigens is a major goal in the field of transplantation. Here, we demonstrate that efficient transduction and expression of a retrovirally transduced major histocompatibility complex (MHC) class I gene (H-2K(b)) in bone marrow (BM)-derived cells, resulting in a permanent state of hematopoietic molecular chimerism, induces stable tolerance to the transduced gene product. Reconstitution of lethally irradiated syngeneic recipients with BM transduced with virus encoding H-2K(b) resulted in life-long expression of the retroviral gene product on the surface of BM-derived hematopoietic lineages including Sca-1(+), lineage negative, hematopoietic progenitors. T cells from mice receiving MHC-transduced BM were unable to kill targets expressing H-2K(b) but were able to respond to third-party controls. Mice reconstituted with H-2K(b)-transduced BM exhibited long-term acceptance of H-2K(b) mismatched skin grafts but were able to rapidly reject third-party control grafts. Thus, gene therapy approaches may be used to induce T-cell tolerance.

Adenoviral transfer of a single donor-specific MHC class I gene to recipient bone marrow cells can induce specific immunological unresponsiveness in vivo

We investigated the delivery of a donor-specific MHC class I gene, H-2K(b), using a newly constructed replication-defective recombinant adenovirus (AdSV40K(b)) to recipient tissue before transplantation as a means of inducing donor-specific immunological unresponsiveness. AdSV40K(b) was able to transduce both a fibroblast cell line and freshly isolated bone marrow cells (BMCs) resulting in cell surface expression of H2-K(b) protein. Intravenous infusion of AdSV40K(b)-transduced syngeneic CBA/Ca (H-2(k)) BMCs into CBA recipient mice treated with an anti-CD4 monoclonal antibody 27 days before transplantation of a fully MHC-mismatched, C57BL/10 (H-2K(b+)), cardiac allograft resulted in significant long-term graft survival when compared with mice receiving the same dose of syngeneic BMCs transduced with a control adenovirus, AdRSVbetagal. Despite the induction of H-2K(b)-specific hyporesponsiveness following pretreatment with AdSV40K(b)-transduced CBA BMCs, persistence of H-2K(b) mRNA in central or peripheral tissues could not be demonstrated by RT-PCR. This result was in contrast to the observed persistence of K(b) mRNA both in the periphery and thymus following the infusion of transgenic CBK (H-2(k) + K(b)) BMCs. We conclude that ex vivo adenoviral gene transfer of a single donor MHC class I gene to recipient BMCs in combination with transient depletion of CD4(+) cells is sufficient to induce long-term graft survival of a fully allogeneic cardiac graft. In addition, detectable microchimerism is not a prerequisite for graft survival.

Establishment of gene-vaccinated skin grafting against Toxoplasma gondii infection in mice

Vaccine effects of in vivo gene-vaccinated skin graft were evaluated against Toxoplasma gondii (T. gondii) infection. By using a gene gun, cDNA coding T. gondii SAG1 molecule was intracutaneously vaccinated into C57BL/6 (B6; a susceptible strain), BALB/c (a resistant strain) and (C57BL/6 x BALB/c) F1 (CBF1) mice, and the gene-vaccinated skin of these strains was transplanted to CBF1 mice. Regarding the antibody production against SAG1, CBF1-recipient mice transplanted with the SAG1 gene-vaccinated B6 skin were high responders, whereas CBF1 mice skin grafted with vaccinated skin of both BALB/c and CBF1 mice were low responders. The donor-derived LC/DC migrated to the draining lymph nodes of the recipients from the skin graft within 3 days. The vaccine effect against T. gondii challenge infection was obtained in CBF1 mice which received the skin graft of the SAG1 gene-vaccinated BALB/c mice.

Defining the requirements for peptide recognition in gene therapy-induced T cell tolerance

Expression of a retrovirally transduced MHC class I Ag, H-2K(b) (K(b)), in bone marrow-derived cells leads to specific prolongation of K(b) disparate skin grafts. To examine the extent to which peptides derived from K(b) contribute to the induction of tolerance, retroviruses carrying mutant K(b) genes designed to enter separate pathways of Ag presentation were constructed. Thymectomized and CD8 T cell-depleted mice that had been irradiated and reconstituted with bone marrow cells expressing a secreted form of K(b) showed prolongation of K(b) disparate skin graft survival. Skin graft prolongation was not observed when similar experiments were performed using mice that were not CD8 T cell depleted. This suggests that hyporesponsiveness can be induced in CD4 T cells, but not CD8 T cells by Ags presented via the exogenous pathway of Ag processing. Modest prolongation of skin allografts was observed in mice reconstituted with bone marrow cells transduced with retroviruses carrying a gene encoding a mutant K(b) molecule expressed only in the cytoplasm. Prolongation was also observed in similar experiments in mice that were thymectomized and CD4 T cell depleted following complete reconstitution, but not in mice that were reconstituted and then thymectomized and CD8 T cell depleted. Thus, hyporesponsiveness can be induced in a subset of CD8 T cells by recognition of peptides derived from K(b) through both the direct and indirect pathways of Ag recognition, while CD4 T cell hyporesponsiveness to MHC class I disparate grafts occurs only through the indirect pathway of Ag recognition.

Gene therapy and transplantation

Advances in molecular biology and in techniques of gene transfer have resulted in the development of practical approaches to human gene therapy. Many applications are of relevance to manipulation of the immune system and have potential in organ and cell transplantation. For example, gene therapy approaches may facilitate the induction of immunological tolerance to a donor organ or protect it locally against the host's immune response. Based on a comprehensive review of the world literature, examples of current research efforts in both allogeneic and xenogeneic transplantation are presented and discussed.

Evidence for epitope spreading and active suppression in skin graft tolerance after donor-specific transfusion

BACKGROUND

To clarify the controversial results in the literature regarding the role of donor-specific transfusion (DST) on allograft survival, we have examined the influence of the following on DST-induced allograft survival in a 2C transgenic mouse model: varying the time between DST and transplantation; the role of MHC disparities between donor and recipient; whether tolerance induced by DST spreads to skin allografts expressing other alloantigens; and whether cyclosporine (CsA) treatment could further modulate skin allograft tolerance after DST.

METHODS AND RESULTS

The studies were performed in both 2C anti-Ld (MHC class I) transgenic and normal (nontransgenic) mice. Our data demonstrate that a single infusion of Ld-mismatched lymphocytes 7 days before transplantation leads to permanent acceptance of donor-specific skin allografts in both transgenic (58/58) and nontransgenic (8/8) mice in the absence of any other nonspecific immunosuppressive treatment. Pretransplantation DST from donors mismatched for more than one MHC antigen (Ag) has no beneficial effect on subsequent donor skin allograft survival. However, Ld plus multiple minor histocompatibility (mH) Ag-mismatched DST induced permanent acceptance of donor-specific skin allografts. Tolerance induced by one-locus Ld-mismatched DST spreads to skin allografts expressing either two-locus Ld or one-locus Ld plus multiple mH Ags. Administration of CsA after DST diminished skin allograft survival, rather than enhancing it, suggesting that tolerance in this model system is established by an active immunological process sensitive to CsA.

CONCLUSIONS

(1) Pretransplantation infusion of Ld-mismatched lymphocytes in the presence or absence of multiple mH mismatches induces permanent survival of donor-specific skin allografts. (2) CsA abrogates DST-induced transplantation tolerance.

Immunologic tolerance for immune system-mediated diseases

Induction of long-term, antigen-specific immunologic unresponsiveness holds great promise for the treatment of many immune system-mediated diseases, including asthma, allergies, autoimmune diseases, and transplant rejection. Unlike current immunosuppressive treatments, immunologic tolerance therapies would affect only the undesired immune responses, leaving protective immunity intact. A variety of approaches to immunologic tolerance induction are being taken, reflecting the molecular and cellular complexity of immune system activation and regulation. The presentations summarized in this report represent promising strategies, some of which are being evaluated in advanced animal models and human clinical trials. Approaches presented include the following: interference with costimulatory signals in T-cell induction, T-cell receptor antagonism by altered peptides, exploitation of antigen-induced apoptosis to eliminate undesired T cells, opposition of inflammation by the induction of regulatory cytokines, induction of transplant tolerance by mixed chimerism, and deviation from deleterious allergic antibody responses by use of immunostimulatory DNA sequences. These multifaceted approaches are strongly supported by knowledge of basic immune mechanisms, which should facilitate the rational development of these therapies for controlling immune-mediated diseases.

Transfer of porcine MHC DRalpha into IEalpha-deficient murine bone marrow results in reduced IE-restricted Vbeta usage

BACKGROUND

Allogeneic bone marrow transplantation has proven effective for inducing specific tolerance to subsequent solid organ allografts, although the clinical applicability of this approach is limited by the morbidity and mortality associated with this procedure. As an alternative, we are investigating the transfer of allogeneic MHC class II genes into recipient bone marrow cells (BMC), using the miniature swine as a model.

METHODS

To understand the mechanism of tolerance induction achieved through class II gene transfer, BMC from C57BL/10 mice, which lack expression of the MHC class II DRalpha equivalent (H-2 IEalpha), were transduced with a retrovirus vector for swine DRalpha.

RESULTS

Expression of the DRA-vector in bone marrow-derived cells was demonstrated by Northern analysis of colonies grown in vitro from transduced myeloid progenitors. Taking advantage of the fact that the introduced DRalpha chain was able to form heterodimers with endogenous IEbeta, surface expression of the transgene was demonstrated on splenocytes harvested 1, 17, and 28 weeks after bone marrow transplantation. Transgene expression was confirmed by reverse transcriptase-polymerase chain reaction in the thymus of those animals killed at weeks 17 and 28. Finally, the effects of bone marrow transduction on central tolerance induction was demonstrated by the progressive decrease of IE-reactive T-cell clones bearing Vbeta5 and Vbeta11 T cell receptors in the peripheral blood cells of engineered recipients.

CONCLUSIONS

Our results support the notion that transplantation tolerance, induced by class II gene transfer into syngeneic BMC, results in part from durable deletional unresponsiveness of graft-specific alloreactive T cells.

Long-term expression of the gene encoding green fluorescent protein in murine hematopoietic cells using retroviral gene transfer

BACKGROUND

A major goal in retroviral-based gene therapy is to establish methods that allow for selection and tracking of transduced cell populations. Green fluorescent protein (GFP) may be useful for gene therapy applications because it is a naturally fluorescent protein that can be detected using conventional flow cytometers facilitating rapid analysis and purification of transduced cell populations. However, it is unknown whether GFP can be stably expressed in vivo, particularly in multiple bone marrow-derived cell lineages.

METHODS

A murine retrovirus carrying the gene encoding GFP was used to infect murine bone marrow cells (BMCs). These studies were conducted to (1) directly determine whether GFP could be used as a marker of BMC transduction, (2) determine whether GFP is capable of being expressed in multiple bone marrow-derived hematopoietic cell lineages, and (3) determine whether GFP could be used to follow the fate of transduced cells in vivo.

RESULTS

Infection of BMCs with retroviruses carrying the gene encoding GFP resulted in a fluorescent signal in viable transduced cells that was detectable by flow cytometry. Expression of GFP was detected in multiple bone marrow-derived cell lineages after transduction, including stem cell antigen-positive (Sca-1+), lineage marker-negative (Lin-) cells. Using GFP as a selectable marker, we were able to enrich for transduced cells by cell sorting. Mice reconstituted with enriched populations of GFP+ cells showed a significant increase in the percentage of cells expressing GFP in the periphery when compared with mice reconstituted with unenriched transduced bone marrow.

CONCLUSIONS

These data indicate that GFP can be used to select for transduced BMCs in vitro, expressed in multiple bone marrow-derived cell lineages, used to select transduced cells, and follow the fate of transduced cells long-term in vivo.