CD28-dependent HIV-1 transcription is associated with Vav, Rac, and NF-κB activation

Activation of HIV-1-infected T cells through the T cell receptor and costimulatory molecule CD28 induces proviral transcription; however, the mechanism behind this enhanced virus expression is unknown. Jurkat T cells and primary CD4⁺ T cells expressing a CD8α/CD28 chimeric receptor containing a mutation at tyrosine 200 in the cytoplasmic tail were unable to fully induce HIV-1 proviral transcription in response to CD8α/28 receptor cross-linking in comparison to CD28 costimulation. The loss of transactivation seen with the mutant chimeric receptor correlated with a decrease in Vav tyrosine phosphorylation. CD28-dependent activation of HIV-1 transcription also required the GTPase activity of Rac1, which was not activated during costimulation with the mutated receptor. Furthermore, the mutated receptor was unable to induce NF-κB DNA binding or transactivation, as demonstrated by electromobility shift assays and HIV-1 long terminal repeat and NF-κB-dependent reporter constructs. These studies show that signaling events initiated by tyrosine 200 of CD28 are required for efficient expression of HIV-1 transcription in activated T cells.