Groundbreaking humanized mouse model offers recent insight into immunotherapy development

A breakthrough in biomedical research promises recent insights into developing immunotherapy and disease modeling. Researchers at the University of Texas Health Science Center at San Antonio have created a humanized mouse model with a human immune system and a gut microbiome similar to that of humans that is capable of generating specific antibody responses.

The researchers were led by Paolo Casali, MD, Ashbel Smith Professor at the University of Texas and Distinguished Research Professor, Department of Microbiology, Immunology and Molecular Genetics at the Joe R. and Teresa Lozano Long School of Medicine. Casali has five decades of experience in biomedical research in immunology and microbiology and is a leading investigator in the molecular genetics and epigenetics of antibody responses.

The aim of the multi-year project, which will be published in the August 2024 issue of the magazine, is to: Nature’s Immunologyaimed to overcome the limitations of currently available human in vivo models by creating a humanized mouse with a fully developed and functional human immune system.

Mice are widely used in biological and biomedical research because they are miniature, effortless to handle, share many immune components and biological properties with humans, and are effortless to genetically engineer. However, many of the more than 1,600 immune response genes of mice are incompatible with their human counterparts, resulting in discrepancies or deficiencies in mice as predictors of human immune responses. This has made the availability of a “humanized” mouse model that faithfully replicates human immune responses a priority.

The first humanized mice were created in the 1980s to model human HIV infection and the human immune response to HIV. Humanized mice were and have been created ever since by injecting immunodeficient mice with human peripheral lymphocytes, hematopoietic stem cells, or other human cells. However, previous and current models do not develop a fully functional human immune system, have a low lifespan, and do not mount effective immune responses. This makes them unsuitable for developing human immunotherapies in vivo, modeling human disease, or developing human vaccines.

Casali’s team began by injecting human stem cells that they had purified from cord blood into the heart (left ventricle) of immunodeficient NSG W41 mutant mice. Several weeks later, after the grafts had formed, the mice were hormone-conditioned with 17β-estradiol (E2), the most potent and plentiful form of estrogen in the body. The estrogen-based hormone conditioning was prompted by earlier studies by Casali and others suggesting that estrogen increases human stem cell survival, increases B-cell differentiation, and increases the production of antibodies against viruses and bacteria.

The resulting humanized mice, called TruHuX (for “true human” or THX), possess a fully developed and fully functional human immune system, including lymph nodes, germinal centers, thymic epithelial cells, human T and B lymphocytes, memory B lymphocytes, and plasma cells that produce highly specific antibodies and autoantibodies identical to those in humans.

THX mice produce mature neutralizing antibodies Salmonella Typhimurium virus and SARS-CoV-2 Spike S1 RBD after vaccination Salmonella flagellin and Pfizer’s COVID-19 mRNA vaccine. THX mice are also susceptible to developing full-fledged systemic lupus autoimmunity when injected with pristane, an oil that triggers an inflammatory response.

Casali said the discovery of the THX mouse opens up possibilities for in vivo experiments in humans, the development of immunotherapeutics such as cancer checkpoint inhibitors, the development of human bacterial and viral vaccines, and the modeling of many human diseases. He also hopes that the recent approach could render obsolete the exploit of nonhuman primates for biomedical research in immunity and microbiology.

Since research on the effects of estrogen on the immune system is limited to date, Casali hopes this finding will inspire further research on the topic.

By critically utilizing estrogen activity to support human stem cell and human immune cell differentiation and antibody responses, THX mice provide a platform for human immune research, human vaccine development and drug testing.

Paolo Casali, MD, Ashbel Smith Professor at the University of Texas and Distinguished Research Professor, Department of Microbiology, Immunology, and Molecular Genetics at the Joe R. and Teresa Lozano Long School of Medicine

With the THX model, the Casali laboratory is currently studying the in vivo human immune response to SARS-CoV-2 (COVID-19) at the systemic and local levels, and human memory B cells, the dependence on the nuclear receptor RORα for their generation, and the events leading to RORα expression and dysregulation. They are also investigating the epigenetic factors and mechanisms that mediate the generation of human plasma cells, the cell factories that produce antibodies—literally thousands per second—to bacteria, viruses, or cancer cells.