Development of Severe Combined Immunodeficient (SCID) Pig Models for Translational Cancer Modeling: Future Insights on How Humanized SCID Pigs Can Improve Preclinical Cancer Research

Abstract

Within the last decade there have been several severe combined immunodeficient (SCID) pig models discovered or genetically engineered. The animals have mutations in ARTEMIS, IL2RG, or RAG1/2 genes, or combinations thereof, providing SCID pigs with NK cells, but deficient in T and B cells, or deficient in NK, T, and B cells for research studies. Biocontainment facilities and positive pressure isolators are developed to limit pathogen exposure and prolong the life of SCID pigs. Raising SCID pigs in such facilities allows for completion of long-term studies such as xenotransplantation of human cells. Ectopically injected human cancer cell lines develop into tumors in SCID pigs, thus providing a human-sized in vivo model for evaluating imaging methods to improve cancer detection and therapeutic research and development. Immunocompromised pigs have the potential to be immunologically humanized by xenotransplantation with human hematopoietic stem cells, peripheral blood leukocytes, or fetal tissue. These cells can be introduced through various routes including injection into fetal liver or the intraperitoneal (IP) space, or into piglets by intravenous, IP, and intraosseous administration. The development and maintenance of transplanted human immune cells would be initially (at least) dependent on immune signaling from swine cells. Compared to mice, swine share higher homology in immune related genes with humans. We hypothesize that the SCID pig may be able to support improved engraftment and differentiation of a wide range of human immune cells as compared to equivalent mouse models. Humanization of SCID pigs would thus provide a valuable model system for researchers to study interactions between human tumor and human immune cells. Additionally, as the SCID pig model is further developed, it may be possible to develop patient-derived xenograft models for individualized therapy and drug testing. We thus theorize that the individualized therapeutic approach would be significantly improved with a humanized SCID pig due to similarities in size, metabolism, and physiology. In all, porcine SCID models have significant potential as an excellent preclinical animal model for therapeutic testing.

Humanization of Immunodeficient Animals for the Modeling of Transplantation, Graft Versus Host Disease, and Regenerative Medicine

“…Several lines of immunodeficient pigs carrying spontaneous or induced inactivation of RAG1 , RAG2 , ARTEMIS , or IL2RG alone or combined ( RAG2 and IL2RG ) have been described. 216 However, in pigs that carry mutations of the IL2RG , 184 - 186 RAG1 , 188 RAG2 , 191 RAG2 , and IL2RG 192 genes, there are no descriptions of transplantation of human cells or tissues. In T (−) B (−) NK (+) SCID pigs carrying spontaneous point mutations in the ARTEMIS gene, 193 human cryopreserved deceased skin was accepted for at least 28 d. 194 Since the successful in utero engraftment and differentiation of human CD34 + progenitors in immunocompetent pigs has been reported, 217 it has been proposed that immunodeficient pigs could be used in this setting to generate large numbers of human cells.…”

“…In T (−) B (−) NK (+) SCID pigs carrying spontaneous point mutations in the ARTEMIS gene, 193 human cryopreserved deceased skin was accepted for at least 28 d. 194 Since the successful in utero engraftment and differentiation of human CD34 + progenitors in immunocompetent pigs has been reported, 217 it has been proposed that immunodeficient pigs could be used in this setting to generate large numbers of human cells. 216 Human lymphocytes can recognize porcine MHC molecules and respond at similar levels to human allogenic MHC proteins. 218 , 219 The homology between human and porcine MHC molecules means that antibodies against HLA class II antigens have a propensity to also bind swine leukocyte antigen class II antigens.…”

Controllable Immunosuppression in Pigs as a Basis for Preclinical Studies on Human Cell Therapy

“…Finally, we would like to refer briefly to the availability of SCID pigs in preclinical study on human cell therapy. According to a well-constructed review [23], there are 11 SCID pig strains so far; one was naturally found and others were genetically modified. The mutated genes in these strains are ARTEMIS (a gene encoding a nuclear protein that is involved in V(D)J recombination and DNA repair), interleukin 2 receptor gamma chain (IL2RG), recombination-activating genes (RAG)1, and RAG2.…”

“…SCID pigs need the highest antibacterial care because of their vulnerability to infection. Indeed, they have been reported to survive for only 6 months at the longest [23]. In addition, as general features of mutant pigs, there are diversities in phenotypic severity and small litter size.…”

Novel Engraftment and T Cell Differentiation of Human Hematopoietic Cells in ART−/−IL2RG−/Y SCID Pigs

“…An important step in further developing the SCID pig model is to immunologically humanize these animals through the introduction of human CD34 + hematopoietic stem cells. Similarities between human and porcine immune genes (16) suggest that human immune development would be supported in vivo within the pig (17). Development of such a model could provide researchers with a larger humanized animal for use in cancer (13,17), HIV, and vaccine development research.…”