Two different forms of inherited human TCRα chain deficiency

Genetic defects that result in the absence of all T cells, including both αβ and γδ T cells, are classified as severe combined immunodeficiency (SCID), a life-threatening condition requiring immediate hematopoietic stem cell transplantation (HSCT) in affected newborns. Previously, patients with a homozygous c.*+1G>A splice variant in the constant chain (TRAC) of the T cell receptor (TCR) α were found to lack only αβ T cells and demonstrated longer survival compared to SCID patients lacking both αβ and γδ T cells. This observation suggested that γδ T cells might partially compensate for the absence of αβ T cells. Here, we describe two children with biallelic premature stop codons in TRAC. These mutations result in a complete loss of TCRαβ expression on the cell surface and an absence of αβ T cells, leading to severe immunodeficiency and early death. Additionally, we demonstrate that the previously reported c.*+1G>A TRAC variant retains partial activity in vitro, enabling low-level expression of functional TCRαβ. This residual expression likely explains the milder phenotype and extended survival observed in patients carrying this variant. In conclusion, we clarify the non-redundant role of αβ T cells in humans. Our findings show that complete TCRα deficiency causes a SCID-like clinical presentation, whereas partial TCRα deficiency is associated with milder clinical outcomes and longer survival.