NF-κB family of transcription factors: Biochemical players of CD28 co-stimulation

The signalling pathways that lead from antigen-receptor and/or co-receptor triggering to the activation of transcription factors of the NF-κB family have a crucial role in the regulation of immune responses. The understanding of the molecular mechanisms that control NF-κB activation in lymphocytes has, therefore, important implications for the therapy of immune diseases.CD28 is one of the most important co-stimulatory receptors necessary for full T lymphocyte activation. CD28-mediated signals lower T cell receptor (TCR) activation threshold, thus leading to the enhancement of several T cell functions, including cytokine production, cell cycle progression, survival and regulation of both cytotoxic and humoral T cell responses. However, most of these pathways are under the tight control of TCR and may be bypassed by repeated antigen stimulation. On the contrary, the activation of the NF-κB pathway and NF-κB-regulated genes is a unique feature of CD28. The ultimate nature of CD28 signalling relies on its cytoplasmic tail and on its ability to recruit several signalling mediators involved in coupling CD28 to distinct NF-κB cascades. This review will focus on the current knowledge of the molecular mechanisms whereby CD28 co-operates with the TCR in activating NF-κB. We also describe recent finding on the existence of autonomous signals emanating from CD28, which through a non-conventional NF-κB cascade may account for the critical role of CD28 in regulating cytokine/chemokine production and T cell survival.