CD28 ligation in the absence of TCR promotes RelA/NF‐κB recruitment and trans‐activation of the HIV‐1 LTR

Abstract

CD28 is one of the most important co-stimulatory receptors necessary for full T lymphocyte activation. CD28 can act as a TCR-independent signalling unit by delivering specific signals which may induce HIV transcription and replication. However, the mechanisms by which CD28 regulates HIV expression remain largely unknown. Here we show that the TCRindependent CD28 signals lead to the trans-activation of HIV-1 LTR in an NF-jB-dependent manner. In particular, we found that CD28 engagement by B7 induces the specific recruitment of RelA/NF-jB subunit to the HIV-1 LTR promoter both in vitro and in ex vivo infected cells. The results obtained by mutating specific tyrosine residues within the CD28 cytoplasmic tail as well as by using LY294002 inhibitory drug evidenced that the recruitment and activation of the phosphatidylinositol 3-kinase/Akt signalling pathway is crucial in mediating CD28-induced HIV transcription through RelA/NF-jB.

CD28 co‐signaling in the adaptive immune response

“…Several labs including our own have also reported that CD28 ligation in the absence of TCR engagement can activate NFAT and NFkB activity as well as protein arginine methylation. 88,89,92,118,119 CD28 has long been found to preferentially activate JNK and NFkB, an event that may be accompanied by a unique outcome such as TH2 differentiation. Similarly, P3K activation of numerous PH-domain carrying proteins would cooperate with aspects of TCR signaling.…”

“…Annibaldi et al revealed that the mutation of Y in the YMNM motif (i.e., abolished PI3K and Grb2 binding) significantly reduced NFkB activation in comparison to wild-type CD28. 92 Further, they showed that the inhibition of PI3K by LY294002 or presence of a kinase-dead mutant of PKB/ AKT reduced NFkB translocation as well. 92 Takeda and coworkers further reported that CD28-mediated NFkB activation was dependent on PKC-u and CARMA1 and required Grb2, but not the PI3K interaction with CD28.…”

“…92 Further, they showed that the inhibition of PI3K by LY294002 or presence of a kinase-dead mutant of PKB/ AKT reduced NFkB translocation as well. 92 Takeda and coworkers further reported that CD28-mediated NFkB activation was dependent on PKC-u and CARMA1 and required Grb2, but not the PI3K interaction with CD28. 93 Mutation of the M in the motif for PI3K binding had no effect on activation, whereas loss of the Grb2 binding (i.e., N) had a marked inhibitory effect (Fig.…”

“…Stimulation of T-cells with APCs or cross-linking of TCR/CD3 plus CD28 by antibodies activates the NFkB pathway, where a TCR signal alone is insufficient. 92,93 There are several possible theoretical connections between CD28 and NFkB (Fig. 6).…”

Intracellular Signals of T Cell Costimulation

“…CD28 can cooperate with the guanine nucleotide exchange factor VAV/SLP-76 adaptors to upregulate IL-2/4 transcriptions independently of TCR ligation, and CD28 signaling is dependent on the formation of VAV/SH2-domain-containing leukocyte protein of 76 kDa (SLP-76) complex and induction of these complexes to locate on membrane localization (44). These complexes cooperate with ribosomes assembly chaperone (Rac) signaling pathway downstream effector of Vav and the mitogen-activated kinase kinase 1 (MEKK1), a kinase known to regulate the c-jun N-terminal kinase (JNK) pathway, regulates both NF-κB and the activator protein 1 (AP-1), which controls cytokine secretion (43,45,46). Moreover, Vav-1 plays a key role in the control of NF-κB pathway by targeting IKKα in the T cell membrane and favoring its activation in response to CD28 stimulation (47), which increases protein arginine methyltransferase activity and subsequently promotes the methylation on arginine residues of Vav1 (48).…”