The Wiskott-Aldrich Syndrome protein is required for positive selection during T cell lineage differentiation

The Wiskott-Aldrich syndrome (WAS) is an X-linked primary immune deficiency caused by a mutation in the WAS gene. This leads to altered or absent WAS protein (WASp) expression and function resulting in thrombocytopenia, eczema, recurrent infections, and auto-immunity. In T cells, WASp is required for immune synapse formation. WAS patients have reduced numbers of peripheral blood T lymphocytes and an altered T-cell receptor repertoire. We studied the role of WASp during T-cell differentiation in artificial thymic organoid (ATO)-cultures and in the thymus of humanized mice. CRISPR/Cas9 knocked-out HSPCs rearranged the T-cell receptor and differentiated to TCR+ double positive (DP) cells similar to wildtype HSPCs. We consistently observed a partial defect in the generation of CD8 single positive (SP) cells suggesting that WASp is involved in positive selection. TCR repertoire analysis of the DP and CD8 SP population, however, showed a polyclonal repertoire with no bias towards autoimmunity.