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

Rémi J. Creusot,Samhita Rao,Arnold Han,Nato Teteloshvili,Yang Li,Yong-Guang Yang,Shulian Tan

doi:10.3389/fimmu.2019.00063

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.

Highlights

T cells acquire a T cell receptor (TCR) during their development in the thymus, and become exposed to self-MHC for positive selection, and to MHC/self-peptide complexes for negative selection, leading to a purge of most autoreactive T cells before the remaining mature T cells can be released into the periphery [1, 2]
GFP expression in T cells increased over time as new hematopoietic stem cells (HSCs)-derived T cells replaced thymocytes that preexisted in the thymic tissue (Figure S1G)
There was no significant difference in the frequency or phenotype of MART1-TCR+ T cells between mice with or without thymectomy, or between NSG and HLA-A2 Tg NSG mice

Summary

Introduction

T cells acquire a T cell receptor (TCR) during their development in the thymus, and become exposed to self-MHC for positive selection, and to MHC/self-peptide complexes for negative selection, leading to a purge of most autoreactive T cells before the remaining mature T cells can be released into the periphery [1, 2]. As MART1 is a prominent melanoma antigen, T cells specific for this antigen have been cloned [10] and their TCRs transduced into mature polyclonal T cells for adoptive T cell immunotherapy of melanoma [11, 12] In this case, TCR transduction can engender unwanted pairings between transgenic (Tg) and endogenous TCR chains, decreasing the amount of desired TCR on surface and increasing the chance of off-target specificity. Such MART1reactive T cells have been produced in humanized mice from TCR-transduced HSCs developing in a HLA-A2 Tg mouse thymus [13] or a grafted HLA-A2 human thymus [14,15,16], which prevented expression of the endogenous TCRβ chain [13, 15]

Methods

Results

Conclusion