Thymic Epithelial Cell Alterations and Defective Thymopoiesis Lead to Central and Peripheral Tolerance Perturbation in MHCII Deficiency.

Francesca Ferrua,Elena Fontana,Annarosa Soresina,Rosita Rigoni,Chaim M Roifman,Andrew R Gennery,Maria Carmina Castiello,Blachy J Dávila Saldaña,Despina Moshous,Pietro Luigi Poliani,Francesca Brambilla,Tom Taghon,Victoria Bordon,Dario Di Silvestre,Paolo Uva,Silvia Giliani,Ottavia M Delmonte,Ileana Bortolomai,Sara Signa,Catharina Schuetz,Gloria Delfanti,Genni Enza Marcovecchio,Marita Bosticardo,Anna Villa,Luigi D Notarangelo,Pierluigi Mauri ,Ansgar Schulz ,Elena Drăghici ,Capucine Pïcard

doi:10.3389/fimmu.2021.669943

Abstract

Major Histocompatibility Complex (MHC) class II (MHCII) deficiency (MHCII-D), also known as Bare Lymphocyte Syndrome (BLS), is a rare combined immunodeficiency due to mutations in genes regulating expression of MHCII molecules. MHCII deficiency results in impaired cellular and humoral immune responses, leading to severe infections and autoimmunity. Abnormal cross-talk with developing T cells due to the absence of MHCII expression likely leads to defects in thymic epithelial cells (TEC). However, the contribution of TEC alterations to the pathogenesis of this primary immunodeficiency has not been well characterized to date, in particular in regard to immune dysregulation. To this aim, we have performed an in-depth cellular and molecular characterization of TEC in this disease. We observed an overall perturbation of thymic structure and function in both MHCII−/− mice and patients. Transcriptomic and proteomic profiling of murine TEC revealed several alterations. In particular, we demonstrated that impairment of lymphostromal cross-talk in the thymus of MHCII−/− mice affects mTEC maturation and promiscuous gene expression and causes defects of central tolerance. Furthermore, we observed peripheral tolerance impairment, likely due to defective Treg cell generation and/or function and B cell tolerance breakdown. Overall, our findings reveal disease-specific TEC defects resulting in perturbation of central tolerance and limiting the potential benefits of hematopoietic stem cell transplantation in MHCII deficiency.

Highlights

Major Histocompatibility Complex (MHC) class II (MHCII) deficiency (MHCII-D), known as Bare Lymphocyte Syndrome (BLS), is a rare autosomal recessive combined immunodeficiency (CID) due to mutations in genes regulating expression of MHCII molecules [OMIM#209920] (CIITA, RFXANK, RFX5, RFXAP) [1,2,3,4]
Our findings in the MHCII deficient mouse model and in human samples from MHCII-D patients suggest that lack of MHCII molecules leads to altered thymic structure and function, resulting in a tolerance impairment broader than previously recognized, involving both central and peripheral mechanisms
A significantly reduced expression of Aire and Aire-dependent and -independent tissue restricted antigens (TRA) mRNA by qRTPCR was previously reported in another MHCII ko mouse model (Aa−/−) [24], suggesting a functional impairment

Summary

Introduction

Major Histocompatibility Complex (MHC) class II (MHCII) deficiency (MHCII-D), known as Bare Lymphocyte Syndrome (BLS), is a rare autosomal recessive combined immunodeficiency (CID) due to mutations in genes regulating expression of MHCII molecules [OMIM#209920] (CIITA, RFXANK, RFX5, RFXAP) [1,2,3,4]. Study of thymic function in 8 MHCII-deficient patients [7] showed that, despite normal TCR-Vb repertoire of total CD3+ T cells, clonal abnormalities emerged at flow cytometric evaluation of TCR-Vb repertoire on CD4+ T cells and at spectratyping evaluation of TCR-Vg repertoire on total CD3+ lymphocytes [7]. These findings suggest a reduced global thymic activity in MHCII deficiency and emphasize the key role of MHCII molecules in the process of normal thymic maturation of T lymphocytes. TCR excision circles (TREC) were detectable in patients’ total lymphocytes and sorted CD4+ cells, reflecting normal early T-cell development [4, 7]

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Results

Conclusion