CBAP promotes thymocyte negative selection by facilitating T-cell receptor proximal signaling.

T-cell receptor (TCR)-transduced signaling is critical to thymocyte development at the CD4/CD8 double-positive stage, but the molecules involved in this process are not yet fully characterized. We previously demonstrated that GM-CSF/IL-3/IL-5 receptor common ß-chain-associated protein (CBAP) modulates ZAP70-mediated T-cell migration and adhesion. On the basis of the high expression of CBAP during thymocyte development, we investigated the function of CBAP in thymocyte development using a CBAP knockout mouse. CBAP-deficient mice showed normal early thymocyte development and positive selection. In contrast, several negative selection models (including TCR transgene, superantigen staphylococcal enterotoxin B, and anti-CD3 antibody treatment) revealed an attenuation of TCR-induced thymocyte deletion in CBAP knockout mice. This phenotype correlated with a reduced accumulation of BIM upon TCR crosslinking in CBAP-deficient thymocytes. Loss of CBAP led to reduced TCR-induced phosphorylation of proteins involved in both proximal and distal signaling events, including ZAP70, LAT, PLCγ1, and JNK1/2. Moreover, TCR-induced association of LAT signalosome components was reduced in CBAP-deficient thymocytes. Our data demonstrate that CBAP is a novel component in the TCR signaling pathway and modulates thymocyte apoptosis during negative selection.

CBAP interacts with the un-liganded common beta-subunit of the GM-CSF/IL-3/IL-5 receptor and induces apoptosis via mitochondrial dysfunction.

The cytoplasmic domain of the common beta-chain (betac) of the granulocyte-macrophage-colony-stimulating factor (GM-CSF)/interleukin-3 (IL-3)/IL-5 receptor contains a membrane proximal region that is sufficient to mediate ligand-dependent mitogenic activity. Within this region two motifs, designated as box 1 and box 2, are highly conserved among members of the cytokine receptor superfamily. Whereas box 1 is required for the recruitment and phosphorylation of Janus kinase-2, the function of box 2 remains largely unknown. Here, we report the identification of a novel transmembrane protein (common beta-chain associated protein (CBAP)) which directly associated with betac via the box 2 motif. Interestingly, such an association only occurred in the absence of GM-CSF in vivo. Ectopic overexpression of CBAP triggered apoptosis of factor-dependent cells via mitochondrial dysfunction, which could be inhibited by Bcl-2 overexpression. Reduced expression of endogenous CBAP by small interfering RNA did not interfere GM-CSF-activated signaling molecules, but such treatment significantly inhibited apoptosis induced by GM-CSF deprivation, but not other death stimuli. Domain mapping studies indicated that one apoptogenic domain of CBAP correlated with its ability to interact with betac. Taken together, these results suggest that CBAP modulates GM-CSF-deprivation-induced apoptosis possibly via a novel mechanism involving interaction with an un-liganded betac molecule.