The role of the different CD3γ domains in TCR expression and signaling

The CD3 subunits of the T-cell antigen receptor (TCR) play a central role in regulation of surface TCR expression levels. Humans who lack CD3 gamma (gamma(-)) show reduced surface TCR expression levels and abolished phorbol ester (PMA)-induced TCR down-regulation. The response to PMA is mediated by a double leucine motif in the intracellular (IC) domain of CD3 gamma. However, the molecular cause of the reduced TCR surface expression in gamma(-) lymphocytes is still not known. We used retroviral vectors carrying wild type CD3 gamma or CD3 delta or the following chimeras (EC-extracellular, TM-transmembrane and IC): delta(EC)gamma(TM)gamma(IC) (delta gamma gamma for short), gamma gamma delta, gamma delta delta and gamma gamma-. Expression of gamma gamma gamma, gamma gamma delta, gamma delta delta or gamma gamma- in the gamma(-) T cell line JGN, which lacks surface TCR, demonstrated that cell surface TCR levels in JGN were dependent on the EC domain of CD3 gamma and could not be replaced by the one of CD3 delta. In JGN and primary gamma(-) patient T cells, the tested chimeras confirmed that the response to PMA maps to the IC domain of CD3 gamma. Since protein homology explains these results better than domain structure, we conclude that CD3 gamma contributes conformational cues that improve surface TCR expression, likely at the assembly or membrane transport steps. In JGN cells all chimeric TCRs were signalling competent. However, an IC domain at CD3 gamma was required for TCR-induced IL-2 and TNF-alpha production and CD69 expression, indicating that a TCR without a CD3 gamma IC domain has altered signalling capabilities.

Automated summary

CD3 subunits, especially CD3γ, play a crucial role in regulating surface TCR expression levels. Studies have shown that the EC domain of CD3γ is essential for surface TCR levels, while the IC domain is important for responding to PMA.