CD28 stimulation triggers NF-κB activation through the CARMA1–PKCθ–Grb2/Gads axis

CD28 stimulation contributes to activation of the IL-2 promoter by up-regulating the activity of several transcription factors, including nuclear factor KB (NF-KB)/Rel family members. However, the signal-transducing cascades linking the CD28 molecule and activation of NF-κB remain unclear. Protein kinase C (PKC) 0, CARMA1 and Bcl10 have recently been reported to integrate TCR-mediated NF-KB activation. However, since the data in these studies were drawn from experiments in which T cells were usually stimulated with both TCR and CD28, the relative contributions of TCR- and CD28-mediated signals to initiation of the NF-κB pathway remain elusive. To examine the role of these molecules in NF-κB activation through CD28-mediated stimulation, Bcl10 was over-expressed in Jurkat cells and their NF-κB activation by CD28- or TCR-cross-linking was evaluated. We found that CD28 stimulation alone can induce NF-κB activation in Bcl10-over-expressing Jurkat cells, whereas TCR stimulation alone has only little effect. In addition, we found that Bcl10-induced NF-κB activation through CD28-mediated stimulation could be blocked by the dominant-negative form of PKCθ or CARMA1. Furthermore, genetic studies revealed that Grb2/Gads binding, but not phosphatidylinositol 3-kinase binding, is important in CD28-mediated NF-κB activation. These findings indicate that the PKCθ-CARMA1-Bcl10 signaling pathway participates in the CD28 co-stimulatory signal independently of the TCR-signaling pathway, which leads us to propose that the activation of the NF-κB-signaling pathway via PKCθ-CARMA1-Bcl10 may be markedly dependent on CD28 stimulation rather than TCR stimulation.