Humanized Mice Reveal New Insights Into the Thymic Selection of Human Autoreactive CD8+ T Cells

Abstract

Thymic selection constitutes the first checkpoint in T-cell development to purge autoreactive T cells. Most of our understanding of this process comes from animal models because of the challenges of studying thymopoiesis and how T cell receptor (TCR) specificity impacts thymocyte phenotype in humans. We developed a humanized mouse model involving the introduction of autoreactive TCRs and cognate autoantigens that enables the analysis of selection of human T cells in human thymic tissue in vivo. Here, we describe the thymic development of MART1-specific autoreactive CD8+ T cells that normally escape deletion and how their phenotype and survival are affected by introduction of the missing epitope in the hematopoietic lineage. Expression of the epitope in a fraction of hematopoietic cells, including all major types of antigen-presenting cells (APCs), led to profound yet incomplete deletion of these T cells. Upregulation of PD-1 upon antigen encounter occurred through the different stages of thymocyte development. PD-1 and CCR7 expression were mutually exclusive in both transgenic and non-transgenic thymocytes, challenging the view that CCR7 is necessary for negative selection in humans. In the presence of antigen, MART1-reactive T cells down-regulated TCR, CD3, CD8, and CD4 in the thymus and periphery. Moreover, expression of secondary TCRs influences MHC class I-restricted T cells to develop as CD4+, particularly regulatory T cells. This new model constitutes a valuable tool to better understand the development of autoreactive T cells identified in different human autoimmune diseases and the role of different APC subsets in their selection.

Reduced positive selection of a human TCR in a swine thymus using a humanized mouse model for xenotolerance induction

“…We detected a MART1 TCR dim population in the periphery of mice that was similar between mice regardless of transplanted thymus. Such populations have been observed previously in TCR transgenic mouse and humanized mouse models, and have been shown by antibody staining or sequencing to express multiple TCRα chains. Our observation that MART1 TCR dim cells have similar CD3 expression as MART1 TCR bright cells, indicating similar expression of surface TCR but reduced expression of tetramer‐stainable MART1 TCRs, suggests a similar phenomenon is taking place in our system.…”

“…We examined MART1 TCR dim and MART1 TCR bright cells and found that they had similar CD3 expression, as shown in Figure S5a‐c. Because TCR and CD3 are expressed on the surface of T cells with a well‐described stoichiometry, the fact that MART1 TCR dim cells showed reduced MART1 TCR expression with unchanged CD3 expression suggests that these T cells may express endogenously rearranged TCRα chains, which has been previously observed in murine and humanized mouse transgenic TCR systems, and in ordinary humanized mouse models, where 10% of normal human thymocytes selected in a human thymus graft expressed multiple TCRα chains . MART1 TCR dim cells were present in similar proportions among both GFP + CD4 + and CD8 + cells (Figure S5e) and as a proportion of splenocytes (Figure S5f) regardless of selecting thymus.…”

“…This model could also be used to understand mechanisms of tolerization of xenoreactive TCRs in xenogeneic thymus transplantation and other methods for xenotolerance induction. Beyond xenotransplantation, our laboratory and others have used this and similar model systems to characterize the biology of human TCR deletion in central tolerance and are studying how cells expressing patient‐derived autoreactive TCRs might escape tolerance in conditions of autoimmunity. Humanized mouse models such as this will likely be essential for assessing tolerance of human xenoreactive TCRs and testing strategies for induction of xenotolerance, with the ultimate goal of inducing patient tolerance to pig organs without immunosuppression or immune compromise to allow optimal pig‐to‐human solid organ xenotransplantation.…”

The development of human immune system mice and their use to study tolerance and autoimmunity

“…A recent study specifically investigated the ability of human thymic APCs to promote negative selection of CD8 T cells. To do so, BLT hu-mice were transplanted with a mix of human CD34 + HSCs: some were transduced with vectors encoding either a MART1 or a control antigen, while others were transduced with a TCR specific for a MART1 peptide in the context of HLA-A2 [ 98 ]. In these animals, T cells expressing the MART1-specific TCR developed among other T cells and in the presence of APCs that either did or did not express the MART1 peptide.…”

Human Immune System Mice With Autologous Tumor for Modeling Cancer Immunotherapies

“…Human immune system (HIS) mice have been highly instrumental for in vivo studies of human immune function and immune disorders. The HIS mouse was the first model that made it possible to study human immune responses in real time in vivo under physiologic or pathogenic conditions, such as HIV pathogenesis ( 1 ), human xeno-immune responses ( 2 ), complex interplay between hypercholesterolemia and human adaptive immunity ( 3 ), and intrathymic selections of human T cells ( 4 ). HIS mice, conjunct with tissue chimeras (i.e., with organ repopulation by human parenchymal cells), were found highly valuable in elucidating immunopathology of human-tropic viral infections, such as hepatitis B and hepatitis C viruses ( 5 , 6 ) and respiratory viruses ( 7 ).…”