Since CD28 provides cosignals in T cell responses, a key question is whether the coreceptor operates exclusively via TCRζ/CD3 or also operates as an independent signaling unit. In this study, we show that CD28 can cooperate with VAV/SLP-76 adaptors to upregulate interleukin 2/4 transcription independently of TCR ligation. CD28 signaling is dependent on VAV/SLP-76 complex formation and induces membrane localization of these complexes. CD28-VAV/SLP-76 also functions in nonlymphoid cells to promote nuclear entry of NFAT, indicating that these adaptors are the only lymphoid components needed for this pathway. Further downstream, CD28-VAV/SLP-76 synergizes with Rac1 and causes F-actin remodelling proximal to receptor. Autonomous CD28 signaling may account for the distinct nature of the second signal and in trans amplification of T cell responses.
