Strategies for Genetically Engineering Hypoimmunogenic Universal Pluripotent Stem Cells

Abstract

Despite progress in developing cell therapies, such as T cell or stem cell therapies to treat diseases, immunoincompatibility remains a major barrier to clinical application. Given the fact that a host's immune system may reject allogeneic transplanted cells, methods have been developed to genetically modify patients' primary cells. To advance beyond this time-consuming and costly approach, recent research efforts focus on generating universal pluripotent stem cells to benefit a broader spectrum of patients. In this review, we first summarize current achievements to harness immunosuppressive mechanisms in cells to reduce immunogenicity. Then, we discuss several recent studies demonstrating the feasibility of genetically modifying pluripotent stem cells to escape immune attack and summarize the methods to evaluate hypoimmunogenicity. Although challenges remain, progress to develop genetically engineered universal pluripotent stem cells holds the promise of expediting their use in future gene and cell therapeutics and regenerative medicine.

Keywords: Biological Sciences; Cell Biology; Genetic Engineering; Stem Cells Research.

Figure 1

Comprehensive Strategies to Develop Hypoimmunogenic Pluripotent Stem Cells in Recent Studies

Figure 2

Overview of Genetic Modification Strategies Employed to Generate Hypoimmunogenic Cells Allogeneic cells can be rejected through innate immunity (NK cells and macrophages) as well as adaptive immunity (T cells). (A) Allogeneic cells activate T cells by HLA-I and HLA-II molecules. HLA-I molecules suppress NK cells. (B) Elimination of HLA-I by knocking out B2M and elimination of HLA-II by knocking out CIITA can evade T cell surveillance but trigger NK cell activity. (C) Elimination of HLA-I and HLA-II to evade T cell attack combined with expression of the CD47 molecules to inactivate NK cells and macrophages (Deuse et al., 2019). (D) Simultaneous deletion of HLA-A/B and HLA-II genes, and not HLA-C, allows HLA-C/G and HLA-E to suppress NK cells (Xu et al., 2019). (E) Simultaneous knockout of HLA-A/B/C and HLA-II combined with expression of the immunomodulatory factors PD-L1, HLA-G, and CD47 (Han et al., 2019). These strategies combined can make the engineered cells to evade immune surveillance by T cells, NK cells, and macrophages. Orange lines and blue lines depict pathway activation and suppression, respectively. NK, natural killer; Th, T helper; MAC, macrophage; CTL, cytotoxic lymphocyte.

Figure 3

Evolution of In Vivo Models to Evaluate Hypoimmunogenicity of Genetically Engineered Pluripotent Stem Cells (A) Size of teratomas measured following injection of engineered cells into immunocompetent mice to evaluate mouse cell engraftment. (B) NSG mice injected with engineered cells plus T cells before measurement of teratoma size. (C) To recapitulate the human immune system, human HSCs transplanted into NSG mice and injected with the engineered cells. (D) To promote T cell reconstitution, maturation, and selection, human HSC cells, fetal liver, and fetal thymus tissue transplanted into NSG mice before injection with the engineered cells to evaluate immunogenicity.