Potential large animal models for gene therapy of human genetic diseases of immune and blood cell systems

Severe combined immunodeficiency pig as an emerging animal model for human diseases and regenerative medicines

Severe combined immunodeficiency (SCID) is a group of inherited disorders characterized by compromised T lymphocyte differentiation related to abnormal development of other lymphocytes [i.e., B and/or natural killer (NK) cells], leading to death early in life unless treated immediately with hematopoietic stem cell transplant. Functional NK cells may impact engraftment success of life-saving procedures such as bone marrow transplantation in human SCID patients. Therefore, in animal models, a T cell−/B cell−/NK cell+ environment provides a valuable tool for understanding the function of the innate immune system and for developing targeted NK therapies against human immune diseases. In this review, we focus on underlying mechanisms of human SCID, recent progress in the development of SCID animal models, and utilization of SCID pig model in biomedical sciences. Numerous physiologies in pig are comparable to those in human such as immune system, X-linked heritability, typical T−B+NK− cellular phenotype, and anatomy. Due to analogous features of pig to those of human, studies have found that immunodeficient pig is the most appropriate model for human SCID.

Pigs with Severe Combined Immunodeficiency Are Impaired in Controlling Influenza A Virus Infection

Influenza A viruses (IAV) infect many host species, including humans and pigs. Severe combined immunodeficiency (SCID) is a condition characterized by a deficiency of T, B, and/or natural killer (NK) cells. Animal models of SCID have great value for biomedical research. Here, we evaluated the pathogenesis and the innate immune response to the 2009 H1N1 pandemic IAV (H1N1pdm09) using a recently identified line of naturally occurring SCID pigs deficient in T and B lymphocytes that still have functional NK cells. SCID pigs challenged with H1N1pdm09 showed milder lung pathology compared to the non-SCID heterozygous carrier pigs. Viral titers in the lungs and nasal swabs of challenged SCID pigs were significantly higher than in carrier pigs 7 days postinfection, despite higher levels of IL-1β and IFN-α in the lungs of SCID pigs. The lower levels of pulmonary pathology were associated with the T and B cell absence in response to infection. The higher viral titers, prolonged shedding, and delayed viral clearance indicated that innate immunity was insufficient for controlling IAV in pigs. This recently identified line of SCID pigs provides a valuable model to understand the immune mechanisms associated with influenza protection and recovery in a natural host.

Contemporary Animal Models For Human Gene Therapy Applications

Over the past three decades, gene therapy has been making considerable progress as an alternative strategy in the treatment of many diseases. Since 2009, several studies have been reported in humans on the successful treatment of various diseases. Animal models mimicking human disease conditions are very essential at the preclinical stage before embarking on a clinical trial. In gene therapy, for instance, they are useful in the assessment of variables related to the use of viral vectors such as safety, efficacy, dosage and localization of transgene expression. However, choosing a suitable disease-specific model is of paramount importance for successful clinical translation. This review focuses on the animal models that are most commonly used in gene therapy studies, such as murine, canine, non-human primates, rabbits, porcine, and a more recently developed humanized mice. Though small and large animals both have their own pros and cons as disease-specific models, the choice is made largely based on the type and length of study performed. While small animals with a shorter life span could be well-suited for degenerative/aging studies, large animals with longer life span could suit longitudinal studies and also help with dosage adjustments to maximize therapeutic benefit. Recently, humanized mice or mouse-human chimaeras have gained interest in the study of human tissues or cells, thereby providing a more reliable understanding of therapeutic interventions. Thus, animal models are of great importance with regard to testing new vector technologies in vivo for assessing safety and efficacy prior to a gene therapy clinical trial.

Lentiviral Vector-Mediated Correction of a Mouse Model of Leukocyte Adhesion Deficiency Type I

Leukocyte adhesion deficiency type I (LAD-I) is a primary immunodeficiency caused by mutations in the ITGB2 gene and is characterized by recurrent and life-threatening bacterial infections. These mutations lead to defective or absent expression of β₂ integrins on the leukocyte surface, compromising adhesion and extravasation at sites of infection. Three different lentiviral vectors (LVs) conferring ubiquitous or preferential expression of CD18 in myeloid cells were constructed and tested in human and mouse LAD-I cells. All three hCD18-LVs restored CD18 and CD11a membrane expression in LAD-I patient-derived lymphoblastoid cells. Corrected cells recovered the ability to aggregate and bind to sICAM-1 after stimulation. All vectors induced stable hCD18 expression in hematopoietic cells from mice with a hypomorphic Itgb2 mutation (CD18^HYP), both in vitro and in vivo after transplantation of corrected cells into primary and secondary CD18^HYP recipients. hCD18⁺ hematopoietic cells from transplanted CD18^HYP mice also showed restoration of mCD11a surface co-expression. The analysis of in vivo neutrophil migration in CD18^HYP mice subjected to two different inflammation models demonstrated that the LV-mediated gene therapy completely restored neutrophil extravasation in response to inflammatory stimuli. Finally, these vectors were able to correct the phenotype of human myeloid cells derived from CD34⁺ progenitors defective in ITGB2 expression. These results support for the first time the use of hCD18-LVs for the treatment of LAD-I patients in clinical trials.

Analysis of blood leukocytes in a naturally occurring immunodeficiency of pigs shows the defect is localized to B and T cells

Severe combined immunodeficiency (SCID) is the result of a set of inherited genetic defects which render components of the immune response nonfunctional. In Arabian horses, Jack Russell terriers, and mice, the disorder is a consequence of the absence of T and B lymphocytes, while natural killer (NK) cell and other leukocyte populations remain intact. Preliminary analysis of a naturally acquired form of inherited SCID in a line of pigs showed several defects in the architecture and composition of secondary lymphoid organs. In this study, a quantitative assessment of lymphocyte populations in affected and normal littermates showed depleted T or B lymphocyte populations in affected pigs; however, NK cells and neutrophils were present in numbers comparable to unaffected littermates. The results indicate that the immune defect in pigs shares the same features as other SCID-affected species.

Repeated lentivirus-mediated granulocyte colony-stimulating factor administration to treat canine cyclic neutropenia

Cyclic neutropenia occurs in humans and gray collie dogs, is characterized by recurrent neutropenia, and is treated by repeated injections of recombinant granulocyte colony-stimulating factor (rG-CSF). As dose escalation of lentivirus may be clinically necessary, we monitored the outcome of four sequential intramuscular injections of G-CSF-lentivirus (3 × 10(7) IU/kg body weight) to a normal dog and a gray collie. In the normal dog absolute neutrophil counts were significantly increased after each dose of virus, with mean levels of 27.75 ± 3.00, 31.50 ± 1.40, 35.05 ± 1.68, and 43.88 ± 2.94 × 10(3) cells/μl, respectively (p<0.001), and elevated neutrophil counts of 31.18 ± 7.81 × 10(3) cells/μl were maintained for more than 6 years with no adverse effects. A gray collie dog with a mean count of 1.94 ± 1.48 × 10(3) cells/μl received G-CSF-lentivirus and we observed sustained elevations in neutrophil levels for more than 5 months with a mean of 26.00 ± 11.00 × 10(3) cells/μl, significantly increased over the pretreatment level (p<0.001). After the second and third virus administrations mean neutrophil counts of 15.80 ± 6.14 and 11.52 ± 4.90 × 10(3) cells/μl were significantly reduced compared with cell counts after the first virus administration (p<0.001). However, after the fourth virus administration mean neutrophil counts of 15.21 ± 4.50 × 10(3) cells/μl were significantly increased compared with the previous administration (p<0.05). Throughout the nearly 3 years of virus administrations the dog gained weight, was healthy, and showed neutrophil counts significantly higher than pretreatment levels (p<0.001). These studies suggest that patients with cyclic and other neutropenias may be treated with escalating doses of G-CSF-lentivirus to obtain a desired therapeutic neutrophil count.

Optimizing culture conditions of a porcine epithelial cell line IPEC-J2 through a histological and physiological characterization

The high similarity between pigs and humans makes pigs a good gastrointestinal (GI) model for humans. Recently an epithelial cell line originating from the jejunum of pig (IPEC-J2) became available. Once validated, this model can be used to investigate the complex interactions occurring in the intestine. The advantages of using IPEC-J2 as in vitro model of the GI tract are the high resemblance between humans and pigs, and the ease of extrapolating in vitro to in vivo characteristics. In this study, the IPEC-J2 cells were functionally characterized by measuring the trans-epithelial electrical resistance (TEER), and by histological and ultrastructural studies. IPEC-J2 cells grown on six different permeable support systems, were investigated. The Transwell(®)-COL collagen-coated membrane (1.12 cm(2)) showed the best results concerning time efficiency and TEER values. The optimum seeding density of 12 × 10(5) cells/mL ensured that after 9 days of differentiation a confluent monolayer was formed. The decrease in TEER values after a maximum had been reached, coincided with the ultrastructural development of apical microvilli. We conclude that IPEC-J2 cells grown on collagen-coated membranes represent a valuable in vitro model system for the small intestinal epithelium which can be of great interest for intestinal research.

Phenotypic and functional characterization of a CD4(+) CD25(high) FOXP3(high) regulatory T-cell population in the dog

Relatively little is known about regulatory T (Treg) cells and their functional responses in dogs. We have used the cross-reactive anti-mouse/rat Foxp3 antibody clone FJK-16s to identify a population of canine CD4(+) FOXP3(high) T cells in both the peripheral blood (PB) and popliteal lymph node (LN). FOXP3(+) cells in both PB and LN yielded positive staining with the newly developed anti-murine/human Helios antibody clone 22F6, consistent with the notion that they were naturally occurring Treg cells. Stimulation of mononuclear cells of LN origin with concanavalin A (Con A) in vitro yielded increased proportions and median fluorescence intensity of FOXP3 expression by both CD4(+) and CD8(+) T cells. Removal of the Con A and continued culture disclosed a CD4(+)  FOXP3(high) population, distinct from the CD4(+) FOXP3(intermediate) T cells; very few CD8(+) FOXP3(high) T cells were observed, though CD8(+) FOXP3(intermediate) cells were present in equal abundance to CD4(+) FOXP3(intermediate) cells. The CD4(+)  FOXP3(high) T cells were thought to represent activated Treg cells, in contrast to the FOXP3(intermediate) cells, which were thought to be a more heterogeneous population comprising predominantly activated conventional T cells. Co-staining with interferon-γ (IFN-γ) supported this notion, because the FOXP3(high) T cells were almost exclusively IFN-γ(-) , whereas the FOXP3(intermediate) cells expressed a more heterogeneous IFN-γ phenotype. Following activation of mononuclear cells with Con A and interleukin-2, the 5% of CD4(+) T cells showing the highest CD25 expression (CD4(+) CD25(high) ) were enriched in cells expressing FOXP3. These cells were anergic in vitro, in contrast to the 20% of CD4(+) T cells with the lowest CD25 expression (CD4(+) CD25(-) ), which proliferated readily. The CD4(+) CD25(high) FOXP3(high) T cells were able to suppress the proliferation of responder CD4(+) T cells in vitro, in contrast to the CD4(+) CD25(-) cells, which showed no regulatory properties.

Protein replacement therapy and gene transfer in canine models of hemophilia A, hemophilia B, von willebrand disease, and factor VII deficiency

Dogs with hemophilia A, hemophilia B, von Willebrand disease (VWD), and factor VII deficiency faithfully recapitulate the severe bleeding phenotype that occurs in humans with these disorders. The first rational approach to diagnosing these bleeding disorders became possible with the development of reliable assays in the 1940s through research that used these dogs. For the next 60 years, treatment consisted of replacement of the associated missing or dysfunctional protein, first with plasma-derived products and subsequently with recombinant products. Research has consistently shown that replacement products that are safe and efficacious in these dogs prove to be safe and efficacious in humans. But these highly effective products require repeated administration and are limited in supply and expensive; in addition, plasma-derived products have transmitted bloodborne pathogens. Recombinant proteins have all but eliminated inadvertent transmission of bloodborne pathogens, but the other limitations persist. Thus, gene therapy is an attractive alternative strategy in these monogenic disorders and has been actively pursued since the early 1990s. To date, several modalities of gene transfer in canine hemophilia have proven to be safe, produced easily detectable levels of transgene products in plasma that have persisted for years in association with reduced bleeding, and correctly predicted the vector dose required in a human hemophilia B liver-based trial. Very recently, however, researchers have identified an immune response to adeno-associated viral gene transfer vector capsid proteins in a human liver-based trial that was not present in preclinical testing in rodents, dogs, or nonhuman primates. This article provides a review of the strengths and limitations of canine hemophilia, VWD, and factor VII deficiency models and of their historical and current role in the development of improved therapy for humans with these inherited bleeding disorders.