Identification of checkpoints in human T-cell development using severe combined immunodeficiency stem cells

Area of Science:

• Immunology
• Developmental Biology
• Genetics

Background:

• Severe combined immunodeficiency (SCID) is a group of congenital disorders characterized by a lack of T-cells due to arrested thymic development.
• The precise nature of these developmental blocks in SCID has been difficult to ascertain due to challenges in obtaining thymic biopsy samples from affected children.

Purpose of the Study:

• To identify the specific stages of T-cell development arrest in humans caused by major types of SCID.
• To utilize a xenograft mouse model for studying human T-cell development in the context of SCID.

Main Methods:

• Transplantation of CD34(+) bone marrow stem/progenitor cells from SCID patients into an optimized NSG xenograft mouse model.
• Detailed phenotypic and molecular characterization using flow cytometry, spectratyping of immunoglobulin and T-cell receptor loci, and deep sequencing of IGH and TRD loci.

Main Results:

• Mutations in IL-7 receptor alpha (IL7RA) and IL-2 receptor gamma (IL2RG) cause T-cell development arrests at very early thymocyte stages, earlier than previously predicted.
• T-cell receptor (TCR) rearrangements were found to be functionally essential at the CD4(-)CD8(-)CD7(+)CD5(+) stage for development, as evidenced by developmental blocks and rearrangement extents in mice with Artemis-SCID cells.
• The xenograft model was not informative for adenosine deaminase-SCID, though hypomorphic mutations resulted in less severe developmental arrests.

Conclusions:

• Transplantation of SCID patient CD34(+) stem cells into a xenograft mouse model offers unprecedented insights into human T-cell development.
• This approach functionally identifies the specific thymic developmental arrest points caused by various SCID-associated mutations.