A leaky mutation in CD3D differentially affects αß and γδ T cells and leads to a Tαß-Tγδ+B+NK+ human SCID.

T cells recognize antigens via their cell surface TCR and are classified as either αß or γδ depending on the variable chains in their TCR, α and ß or γ and δ, respectively. Both αß and γδ TCRs also contain several invariant chains, including CD3δ, which support surface TCR expression and transduce the TCR signal. Mutations in variable chains would be expected to affect a single T cell lineage, while mutations in the invariant chains would affect all T cells. Consistent with this, all CD3δ-deficient patients described to date showed a complete block in T cell development. However, CD3δ-KO mice have an αß T cell-specific defect. Here, we report 2 unrelated cases of SCID with a selective block in αß but not in γδ T cell development, associated with a new splicing mutation in the CD3D gene. The patients' T cells showed reduced CD3D transcripts, CD3δ proteins, surface TCR, and early TCR signaling. Their lymph nodes showed severe T cell depletion, recent thymus emigrants in peripheral blood were strongly decreased, and the scant αß T cells were oligoclonal. T cell-dependent B cell functions were also impaired, despite the presence of normal B cell numbers. Strikingly, despite the specific loss of αß T cells, surface TCR expression was more reduced in γδ than in αß T cells. Analysis of individuals with this CD3D mutation thus demonstrates the contrasting CD3δ requirements for αß versus γδ T cell development and TCR expression in humans and highlights the diagnostic and clinical relevance of studying both TCR isotypes when a T cell defect is suspected.

CD3γ-independent pathways in TCR-mediated signaling in mature T and iNKT lymphocytes.

Antigen recognition by T-lymphocytes through the T-cell antigen receptor, TCR-CD3, is a central event in the initiation of an immune response. CD3 proteins may have redundant as well as specific contributions to the intracellular propagation of TCR-mediated signals. However, to date, the relative role that each CD3 chain plays in signaling is controversial. In order to examine the roles of CD3γ chain in TCR signaling, we analyzed proximal and distal signaling events in human CD3γ(-/-) primary and Herpesvirus saimiri (HVS)-transformed T cells. Following TCR-CD3 engagement, certain early TCR signaling pathways (ZAP-70, ERK, p38 and mTORC2 phosphorylation, and actin polymerization) were comparable with control HVS-transformed T cells. However, other signaling pathways were affected, such TCRζ phosphorylation, indicating that the CD3γ chain contributes to improve TCR signaling efficiency and survival. On the other hand, CD3γ(-/-) primary invariant NKT cells (iNKT cells) showed a normal expansion in response to alpha-galactosylceramide (α-GalCer) and TCRVß11(bright) iNKT cells were preferentially selected in this in vitro culture system, perhaps as a consequence of selective events in the thymus. Our results collectively indicate that a TCR lacking CD3γ can propagate a number of signals through the remaining invariant chains, likely the homologous CD3δ chain, which replaces it at the mutant TCR.