CD6 attenuates early and late signaling events, setting thresholds for T-cell activation
Oliveira MI, Gonçalves CM, Pinto M, Fabre S, Santos AM, Lee SF, Castro MA, Nunes RJ, Barbosa RR, Parnes JR, Yu C, Davis SJ, Moreira A, Bismuth G, Carmo AM. (2012), Eur J Immunol. 42, 195-205
The T lineage glycoprotein CD6 is generally considered to be a costimulator of T-cell activation. Here, we demonstrate that CD6 significantly reduces early and late T-cell responses upon superantigen stimulation or TCR triggering by Abs. Measuring calcium mobilization in single cells responding to superantigen, we found that human T cells expressing rat CD6 react significantly less well compared with T cells not expressing the exogenous receptor. When the cytoplasmic domain of rat CD6 was removed, calcium responses were recovered, indicating that the inhibitory properties of CD6 are attributable to its cytoplasmic domain. Calcium responses, and also late indicators of T-cell activation such as IL-2 release, were also diminished in TCR-activated Jurkat cells expressing human CD6, compared with CD6-deficient cells or cells expressing a cytoplasmic deletion mutant of human CD6. Similarly, calcium signals triggered by anti-CD3 were enhanced in human T lymphocytes following morpholino-mediated suppression of CD6 expression. Finally, the proliferation of T lymphocytes was increased when the CD6-CD166 interaction was blocked with anti-CD166 Abs, but inhibited when anti-CD6 Abs were used. Our data suggest that CD6 is a signaling attenuator whose expression alone, i.e. in the absence of ligand engagement, is sufficient to restrain signaling in T cells.
Key figure: Expression of hCD6 downregulates calcium signals
(A) Purified human primary T lymphocytes were treated with specific hCD6 MOs to suppress endogenous hCD6 expression (hCD6 kd), or with control scramble morpholinos (Control). Knockdown of hCD6 was evaluated by Western blot of cell lysates, using β-actin as a loading control. Numbers on the left indicate molecular mass markers (MM) in kilodaltons (kDa). (B) Cells were incubated with calcium indicators and after stimulation with CD3 mAb (OKT3), cells expressing hCD6+ (grey line) and cells where hCD6 was knocked down (hCD6kd, black line) were evaluated for calcium mobilization by cytometry. Data are shown from one out of the three experiments with similar results.