Differences between CEM and human peripheral blood T lymphocytes in cAMP-dependent HIV viral fusion and CXCR4...

CXCR4, a G-protein-coupled chemokine receptor and HIV coreceptor, has been shown to play a central role in both chemotaxis and HIV-1-entry into T lymphocytes. Recent efforts have focused on identifying the signaling pathways that modulate CXCR4 expression in order to modulate HIV infectivity. Toward this effort, we previously demonstrated cAMP-dependent up-regulation of CXCR4 mRNA and protein in human peripheral blood T cells (PBL), resulting in increased HIV infectivity. Regulation of CXCR4 mRNA was mediated, in part, by a CRE element within the CXCR4 promoter. In order to develop a model system to examine cAMP regulation, the responses of the T lymphoblastoid cell line CEM were compared to those of human PBL. In sharp contrast to that of human PBL, HIV-1 entry into CEM cells was dramatically reduced in response to dibutyryl cAMP (DcAMP). Furthermore, while total cellular and cell surface CXCR4 protein levels were up-regulated in human PBL and in Jurkat T cells in response to DcAMP or forskolin stimulation, CXCR4 levels were unchanged by stimulation in CEM cells. Surprisingly, the CXCR4 promoter (nucleotides -1098 to +59) fused to luciferase was found to be activated similarly in CEM and Jurkat cells in response to DcAMP in a concentration-dependent manner. RT-PCR analyses confirmed that CXCR4 mRNA levels were increased by cAMP agonists. Taken together, our findings suggest that total and cell surface CXCR4 protein expression is regulated differently in human PBL than in CEM cells, a finding that correlates with the differential HIV-1 fusion in response to cAMP signaling. Moreover, our results suggest that, for CXCR4 expression and HIV viral infectivity, CEM cells may not be a faithful model of primary human lymphocytes.