Can Humanized Immune System Mouse and Rat Models Accelerate the Development of Cytomegalovirus-Based Vaccines Against Infectious Diseases and Cancers?

Abstract

Over the past three decades, immunodeficient mouse models carrying human immune cells, with or without human lymphoid tissues, termed humanized immune system (HIS) rodent models, have been developed to recapitulate the human immune system and associated immune responses. HIS mouse models have successfully modeled many human-restricted viral infections, including those caused by human cytomegalovirus (HCMV) and human immunodeficiency virus (HIV). HIS mouse models have also been used to model human cancer immunobiology, which exhibits differences from murine cancers in traditional mouse models. Variants of HIS mouse models that carry human liver cells, lung tissue, skin tissue, or human patient-derived tumor xenografts and human hematopoietic stem cells-derived-human immune cells with or without lymphoid tissue xenografts have been developed to probe human immune responses to infections and human tumors. HCMV-based vaccines are human-restricted, which poses limitations for mechanistic and efficacy studies using traditional animal models. The HCMV-based vaccine approach is a promising vaccine strategy as it induces robust effector memory T cell responses that may be critical in preventing and rapidly controlling persistent viral infections and cancers. Here, we review novel HIS mouse models with robust human immune cell development and primary and secondary lymphoid tissues that could address many of the limitations of HIS mice in their use as animal models for HCMV-based vaccine research. We also reviewed novel HIS rat models, which could allow long-term (greater than one year) vaccinology studies and better recapitulate human pathophysiology. Translating laboratory research findings to clinical application is a significant bottleneck in vaccine development; HIS rodents and related variants that more accurately model human immunology and diseases could increase the translatability of research findings.

Keywords: HCMV-based vaccines; HIV vaccines; HIV/AIDS-animal models; human cancer xenograft models; humanized immune system and rats.

Figure 1

The generations of HIS rodent models. HIS-mouse models are broadly divided into three generations. First-generation HIS mouse models are reconstituted with PBMCs (or T cells from PBMCs), resulting in predominately T cell (CD4+ T cells) reconstitution in severely immunodeficient mice and rats. The major advantage of first-generation HIS mouse models is their low cost and ease of construction; however, they develop graft versus host diseases (GVHD) in a few weeks, resulting in a short experimental window. For second-generation HIS mouse models, immune reconstitution is mediated via transplantation of CD34+ hematopoietic stem cells. Various immune cell types are developed from CD34+ cells in immunodeficient mice and rats, albeit T and B cell reconstitution dominates, with limited reconstitution of myeloid and NK cells. The major advantage of second-generation HIS mouse models is their reconstitution with a myriad of human immune cells, albeit they lack human lymphoid tissues and associated myeloid cells and may develop graft versus host diseases (GVHD) in a few months. Third-generation HIS rodent models, including Bone Marrow-Liver-Thymus-Spleen (BLTS) and Bone Marrow-Liver-Thymus (BLT)-mice, are reconstituted with a myriad of immune cells via CD34+ stem cell transplantation and lymphoid tissues, and exhibit robust immune response. The robust development of the human thymus plus human spleen in third-generation HIS mice results in viremic control of live-attenuated HIV, which indicates a functional immune system. Advancements in immunodeficient rat models suggest that third-generation HIS rats could be generated. The major advantage of third-generation HIS mouse models is their reconstitution with a myriad of human immune cells and human lymphoid tissues, which can generate robust antigen-specific immune responses. However, third-generation HIS-mouse models may develop graft versus host diseases (GVHD) in a few months. Furthermore, third-generation HIS mouse models are difficult to construct, and ethical concerns exist regarding the use of fetal tissues.

Figure 2

Schematic of third-generation HIS rodent models and their application in vaccinology research. (A) Third-generation HIS rodent models, including Bone Marrow-Liver-Thymus-Spleen (BLTS) and Bone Marrow-Liver-Thymus (BLT)-humanized mice, as well as tissue-specific variants (lung, skin, etc.) are reconstituted with a myriad of human immune cells and lymphoid tissues, and most accurately recapitulate the human immune system. The recent development of the Rat Rag−/− Gamma chain−/− human signal regulatory protein alpha-positive (RRGS) immunodeficient rat model will likely enable the development of third-generation HIS-rat models. (B–G,I) Third-generation HIS rodent models provide in vivo platforms for studying human-specific infectious diseases (such as HIV/AIDS and skin infections) and human tumor biology (including breast cancer), along with the associated immune responses to vaccines. HIS-rodent models allow comprehensive sampling of lymphoid tissues (spleen, thymus, lymph nodes), biological materials (blood, urine, stool), and major organs for investigating human diseases and vaccine-induced immune responses that cannot be addressed in clinical trials. Furthermore, these models enable the rapid evaluation of various vaccine modalities through multiple routes. A significant advantage of third-generation HIS-rodent models is their ability to replicate human-adaptive immune responses driven by human-specific molecular signaling. Rodent models also allow live monitoring of biological processes, enhancing mechanistic studies. Further ex vivo mechanistic studies can be conducted with tissues, cells, and biological materials from third-generation HIS rodent models to elucidate the mechanisms of human diseases and human immunity. Additional studies employing a systems biology approach in third-generation HIS rodent models could determine the efficacy, safety, and human immune correlates of novel vaccines against infectious diseases and cancers with a high degree of certainty. (J) The limited ethical, legal, and safety concerns involving rodent models make this technology ideal for biomedical research. (K) Studies in HIS rodent models are inherently collaborative and interdisciplinary, involving the application of concepts from molecular biologists and human samples (tissues and cells) and insights from clinicians and public health experts. Future efforts should include computational biologists to improve rigor in mechanistic studies in HIS rodent models.

Figure 3

Schematic of a framework for accelerating HCMV-based vaccine development using HIS-rodent models. Advances in the molecular virology of HCMV-viral vector could spur novel immunobiology hypothesis that requires animal models with human tissues and cells. HIS rodent models (rats and mice) provide in vivo platforms for investigating the immunobiology of HCMV-based vaccines, which could enable rapid and rational implementation of clinical trials to determine efficacy and safety and subsequent approval.