T cell repertoire development in XSCID dogs following nonconditioned allogeneic bone marrow transplantation

Evolution of haematopoietic cell transplantation for canine blood disorders and a platform for solid organ transplantation

Pre-clinical haematopoietic cell transplantation (HCT) studies in canines have proven to be invaluable for establishing HCT as a highly successful clinical option for the treatment of malignant and non-malignant haematological diseases in humans. Additionally, studies in canines have shown that immune tolerance, established following HCT, enabled transplantation of solid organs without the need of lifelong immunosuppression. This progress has been possible due to multiple biological similarities between dog and mankind. In this review, the hurdles that were overcome and the methods that were developed in the dog HCT model which made HCT clinically possible are examined. The results of these studies justify the question whether HCT can be used in the veterinary clinical practice for more wide-spread successful treatment of canine haematologic and non-haematologic disorders and whether it is prudent to do so.

In Vivo Gene Therapy for Canine SCID-X1 Using Cocal-Pseudotyped Lentiviral Vector

Hematopoietic stem and progenitor cell (HSPC)-based ex vivo gene therapy has demonstrated clinical success for X-linked severe combined immunodeficiency (SCID-X1) patients who lack a suitable donor for HSPC transplantation. Nevertheless, this form of treatment is associated with an increased risk of infectious disease complications and genotoxicity mainly due to the conditioning regimen. In addition, ex vivo gene therapy approaches require sophisticated facilities to manufacture gene-modified cells and to care for the patients after chemotherapy. Considering these impediments, we have developed an in vivo gene therapy approach to treat canine SCID-X1 after HSPC mobilization and systemic delivery of the therapeutic vector. Here, we investigated the use of the cocal envelope to pseudotype a lentiviral (LV) vector expressing a functional gammaC gene. The cocal envelope is resistant to serum inactivation compared with the commonly used vesicular stomatitis virus envelope glycoprotein (VSV-G) envelope and thus well suited for systemic delivery. Two SCID-X1 neonatal canines treated with this approach achieved long-term therapeutic immune reconstitution with no prior conditioning. Therapeutic levels of gene-corrected CD3⁺ T cells were demonstrated for at least 16 months, and all other correlates of T cell functionality were within normal range. Retroviral integration-site analysis demonstrated polyclonal T cell reconstitution. Comparative analysis of integration profiles of foamy viral (FV) vector and cocal LV vector after in vivo gene therapy found distinct integration-site patterns. These data demonstrate that clinically relevant and durable correction of canine SCID-X1 can be achieved with in vivo delivery of cocal LV. Since manufacturing of cocal LV is similar to VSV-G LV, this approach is easily translatable to a clinical setting, thus providing for a highly portable and accessible gene therapy platform for SCID-X1.

Rapid immune reconstitution of SCID-X1 canines after G-CSF/AMD3100 mobilization and in vivo gene therapy

Hematopoietic stem-cell gene therapy is a promising treatment of X-linked severe combined immunodeficiency disease (SCID-X1), but currently, it requires recipient conditioning, extensive cell manipulation, and sophisticated facilities. With these limitations in mind, we explored a simpler therapeutic approach to SCID-X1 treatment by direct IV administration of foamy virus (FV) vectors in the canine model. FV vectors were used because they have a favorable integration site profile and are resistant to serum inactivation. Here, we show improved efficacy of our in vivo gene therapy platform by mobilization with granulocyte colony-stimulating factor (G-CSF) and AMD3100 before injection of an optimized FV vector incorporating the human phosphoglycerate kinase enhancerless promoter. G-CSF/AMD3100 mobilization before FV vector delivery accelerated kinetics of CD3+ lymphocyte recovery, promoted thymopoiesis, and increased immune clonal diversity. Gene-corrected T lymphocytes exhibited a normal CD4:CD8 ratio and a broad T-cell receptor repertoire and showed restored γC-dependent signaling function. Treated animals showed normal primary and secondary antibody responses to bacteriophage immunization and evidence for immunoglobulin class switching. These results demonstrate safety and efficacy of an accessible, portable, and translatable platform with no conditioning regimen for the treatment of SCID-X1 and other genetic diseases.

Perturbation of the T cell receptor repertoire occurs with increasing age in dogs

Immunosenescence is the gradual deterioration in immune system function associated with ageing. This decline is partly due to involution of the thymus, which leads to a reduction in the output of naive T cells into the circulating lymphocyte pool. Expansion of existing naive and memory T cell populations, to compensate for the reduction in thymic output, can lead to reduced diversity in the T cell repertoire with increasing age, resulting in impairment of immune responses to novel antigenic challenges, such as during infection and vaccination. Since associations between T cell repertoire and age have only been examined in a limited number of species, to gain further insights into this relationship, we have investigated age-related changes in the canine T cell receptor (TCR) repertoire. Blood samples were obtained from Labrador retriever dogs of varying ages and variation in the complementary determining region 3 (CDR3) of the T cell receptor beta (TCRB) chain was investigated. CDR3 size spectratyping was employed to evaluate clonal expansion/deletion in the T cell repertoire, allowing identification of profiles within individual variable (V) region families that skewed away from a Gaussian distribution. Older dogs (10-13 years) were found to have an increased number of TCRB V gene spectratypes that demonstrated a skewed distribution, compared with young dogs (≤3 years). Additionally, there was a reduction in the number of clonal peaks present in the spectratypes of old dogs, compared with those of young dogs. The study findings suggest that there is an age-associated disturbance in the diversity of the T cell receptor repertoire in dogs.

Canine and Feline Models of Human Genetic Diseases and Their Contributions to Advancing Clinical Therapies

For many lethal or debilitating genetic disorders in patients there are no satisfactory therapies. Several barriers exist that hinder the developments of effective therapies including the limited availability of clinically relevant animal models that faithfully recapitulate human genetic disease. In 1974, the Referral Center for Animal Models of Human Genetic Disease (RCAM) was established by Dr. Donald F. Patterson and continued by Dr. Mark E. Haskins at the University of Pennsylvania with the mission to discover, understand, treat, and maintain breeding colonies of naturally occurring hereditary disorders in dogs and cats that are orthologous to those found in human patients. Although non-human primates, sheep, and pig models are also available within the medical community, naturally occurring diseases are rarely identified in non-human primates, and the vast behavioral, clinicopathological, physiological, and anatomical knowledge available regarding dogs and cats far surpasses what is available in ovine and porcine species. The canine and feline models that are maintained at RCAM are presented here with a focus on preclinical therapy data. Clinical studies that have been generated from preclinical work in these models are also presented.

Gene therapy studies in a canine model of X-linked severe combined immunodeficiency

Since the occurrence of T cell leukemias in the original human γ-retroviral gene therapy trials for X-linked severe combined immunodeficiency (XSCID), considerable effort has been devoted to developing safer vectors. This review summarizes gene therapy studies performed in a canine model of XSCID to evaluate the efficacy of γ-retroviral, lentiviral, and foamy viral vectors for treating XSCID and a novel method of vector delivery. These studies demonstrate that durable T cell reconstitution and thymopoiesis with no evidence of any serious adverse events and, in contrast to the human XSCID patients, sustained marking in myeloid cells and B cells with reconstitution of normal humoral immune function can be achieved for up to 5 years without any pretreatment conditioning. The presence of sustained levels of gene-marked T cells, B cells, and more importantly myeloid cells for almost 5 years is highly suggestive of transduction of either multipotent hematopoietic stem cells or very primitive committed progenitors.

Intravenous injection of a foamy virus vector to correct canine SCID-X1

Current approaches to hematopoietic stem cell (HSC) gene therapy involve the collection and ex vivo manipulation of HSCs, a process associated with loss of stem cell multipotency and engraftment potential. An alternative approach for correcting blood-related diseases is the direct intravenous administration of viral vectors, so-called in vivo gene therapy. In this study, we evaluated the safety and efficacy of in vivo gene therapy using a foamy virus vector for the correction of canine X-linked severe combined immunodeficiency (SCID-X1). In newborn SCID-X1 dogs, injection of a foamy virus vector expressing the human IL2RG gene resulted in an expansion of lymphocytes expressing the common γ chain and the development of CD3(+) T lymphocytes. CD3(+) cells expressed CD4 and CD8 coreceptors, underwent antigen receptor gene rearrangement, and demonstrated functional maturity in response to T-cell mitogens. Retroviral integration site analysis in 4 animals revealed a polyclonal pattern of integration in all dogs with evidence for dominant clones. These results demonstrate that a foamy virus vector can be administered with therapeutic benefit in the SCID-X1 dog, a clinically relevant preclinical model for in vivo gene therapy.

Ex vivo γ-retroviral gene therapy of dogs with X-linked severe combined immunodeficiency and the development of a thymic T cell lymphoma

We have previously shown that in vivo γ-retroviral gene therapy of dogs with X-linked severe combined immunodeficiency (XSCID) results in sustained T cell reconstitution and sustained marking in myeloid and B cells for up to 4 years with no evidence of any serious adverse effects. The purpose of this study was to determine whether ex vivo γ-retroviral gene therapy of XSCID dogs results in a similar outcome. Eight of 12 XSCID dogs treated with an average of dose of 5.8 × 10(6) transduced CD34(+) cells/kg successfully engrafted producing normal numbers of gene-corrected CD45RA(+) (naïve) T cells. However, this was followed by a steady decrease in CD45RA(+) T cells, T cell diversity, and thymic output as measured by T cell receptor excision circles (TRECs) resulting in a T cell lymphopenia. None of the dogs survived past 11 months post treatment. At necropsy, few gene-corrected thymocytes were observed correlating with the TREC levels and one of the dogs was diagnosed with a thymic T cell lymphoma that was attributed to the gene therapy. This study highlights the outcome differences between the ex vivo and in vivo approach to γ-retroviral gene therapy and is the first to document a serious adverse event following gene therapy in a canine model of a human genetic disease.

Effect of ex vivo culture of CD34+ bone marrow cells on immune reconstitution of XSCID dogs following allogeneic bone marrow transplantation

Successful genetic treatment of most primary immunodeficiencies or hematological disorders will require the transduction of pluripotent, self-renewing hematopoietic stem cells (HSC) rather than their progeny to achieve enduring production of genetically corrected cells and durable immune reconstitution. Current ex vivo transduction protocols require manipulation of HSC by culture in cytokines for various lengths of time depending upon the retroviral vector that may force HSC to enter pathways of proliferation, and possibly differentiation, which could limit their engraftment potential, pluripotentiality and long-term repopulating capacity. We have compared the ability of normal CD34(+) cells cultured in a standard cytokine cocktail for 18hours or 4.5 days to reconstitute XSCID dogs following bone marrow transplantation in the absence of any pretransplant conditioning with that of freshly isolated CD34(+) cells. CD34(+) cells cultured under standard gamma-retroviral transduction conditions (4.5 days) showed decreased engraftment potential and ability to sustain long-term thymopoiesis. In contrast, XSCID dogs transplanted with CD34(+) cells cultured for 18hours showed a robust T cell immune reconstitution similar to dogs transplanted with freshly isolated CD34(+) cells, however, the ability to sustain long-term thymopoiesis was impaired. These results emphasize the need to determine ex vivo culture conditions that maintain both the engraftment potential and "stem cell" potential of the cultured cells.