Interaction of soluble CD4 with the chemokine receptor CCR5

Control of CCR5 Cell-Surface Targeting by the PRAF2 Gatekeeper

The cell-surface targeting of neo-synthesized G protein-coupled receptors (GPCRs) involves the recruitment of receptors into COPII vesicles budding at endoplasmic reticulum exit sites (ERESs). This process is regulated for some GPCRs by escort proteins, which facilitate their export, or by gatekeepers that retain the receptors in the ER. PRAF2, an ER-resident four trans- membrane domain protein with cytoplasmic extremities, operates as a gatekeeper for the GB1 protomer of the heterodimeric GABAB receptor, interacting with a tandem di-leucine/RXR retention motif in the carboxyterminal tail of GB1. PRAF2 was also reported to interact in a two-hybrid screen with a peptide corresponding to the carboxyterminal tail of the chemokine receptor CCR5 despite the absence of RXR motifs in its sequence. Using a bioluminescence resonance energy transfer (BRET)-based subcellular localization system, we found that PRAF2 inhibits, in a concentration-dependent manner, the plasma membrane export of CCR5. BRET-based proximity assays and Co-IP experiments demonstrated that PRAF2/CCR5 interaction does not require the presence of a receptor carboxyterminal tail and involves instead the transmembrane domains of both proteins. The mutation of the potential di-leucine/RXR motif contained in the third intracellular loop of CCR5 does not affect PRAF2-mediated retention. It instead impairs the cell-surface export of CCR5 by inhibiting CCR5's interaction with its private escort protein, CD4. PRAF2 and CD4 thus display opposite roles on the cell-surface export of CCR5, with PRAF2 inhibiting and CD4 promoting this process, likely operating at the level of CCR5 recruitment into COPII vesicles, which leave the ER.

Experimental Andes virus infection in deer mice: characteristics of infection and clearance in a heterologous rodent host

New World hantaviruses can cause hantavirus cardiopulmonary syndrome with high mortality in humans. Distinct virus species are hosted by specific rodent reservoirs, which also serve as the vectors. Although regional spillover has been documented, it is unknown whether rodent reservoirs are competent for infection by hantaviruses that are geographically separated, and known to have related, but distinct rodent reservoir hosts. We show that Andes virus (ANDV) of South America, carried by the long tailed pygmy rice rat (Oligoryzomys longicaudatus), infects and replicates in vitro and in vivo in the deer mouse (Peromyscus maniculatus), the reservoir host of Sin Nombre virus (SNV), found in North America. In experimentally infected deer mice, viral RNA was detected in the blood, lung, heart and spleen, but virus was cleared by 56 days post inoculation (dpi). All of the inoculated deer mice mounted a humoral immune response by 14 dpi, and produced measurable amounts of neutralizing antibodies by 21 dpi. An up-regulation of Ccl3, Ccl4, Ccl5, and Tgfb, a strong CD4⁺ T-cell response, and down-regulation of Il17, Il21 and Il23 occurred during infection. Infection was transient with an absence of clinical signs or histopathological changes. This is the first evidence that ANDV asymptomatically infects, and is immunogenic in deer mice, a non-natural host species of ANDV. Comparing the immune response in this model to that of the immune response in the natural hosts upon infection with their co-adapted hantaviruses may help clarify the mechanisms governing persistent infection in the natural hosts of hantaviruses.

CD4-CCR5 interaction in intracellular compartments contributes to receptor expression at the cell surface

The association of CD4, a glycoprotein involved in T-cell development and antigen recognition, and CC chemokine receptor 5 (CCR5), a chemotactic G protein-coupled receptor, which regulates trafficking and effector functions of immune cells, forms the main receptor for HIV. We observed that the majority of CCR5 is maintained within the intracellular compartments of primary T lymphocytes and in a monocytic cell line, contrasting with its relatively low density at the cell surface. The CCR5-CD4 association, which occurs in the endoplasmic reticulum, enhanced CCR5 export to the plasma membrane in a concentration-dependent manner, whereas inhibition of endogenous CD4 with small interfering RNAs decreased cell-surface expression of endogenous CCR5. This effect was specific for CCR5, as CD4 did not affect cellular distribution of CXCR4, the other HIV coreceptor. These results reveal a previously unappreciated role of CD4, which contributes to regulating CCR5 export to the plasma membrane.

CD4 and CCR5 constitutively interact at the plasma membrane of living cells: a confocal fluorescence resonance energy transfer-based approach

Human immunodeficiency virus entry into target cells requires sequential interactions of the viral glycoprotein envelope gp120 with CD4 and chemokine receptors CCR5 or CXCR4. CD4 interaction with the chemokine receptor is suggested to play a critical role in this process but to what extent such a mechanism takes place at the surface of target cells remains elusive. To address this issue, we used a confocal microspectrofluorimetric approach to monitor fluorescence resonance energy transfer at the cell plasma membrane between enhanced blue and green fluorescent proteins fused to CD4 and CCR5 receptors. We developed an efficient fluorescence resonance energy transfer analysis from experiments carried out on individual cells, revealing that receptors constitutively interact at the plasma membrane. Binding of R5-tropic HIV gp120 stabilizes these associations thus highlighting that ternary complexes between CD4, gp120, and CCR5 occur before the fusion process starts. Furthermore, the ability of CD4 truncated mutants and CCR5 ligands to prevent association of CD4 with CCR5 reveals that this interaction notably engages extracellular parts of receptors. Finally, we provide evidence that this interaction takes place outside raft domains of the plasma membrane.

Interleukin-16 and peptide derivatives as immunomodulatory therapy in allergic lung disease

The therapeutic potential of interleukin (IL)-16 and derived peptides in allergic asthma is considered, focusing on key interactions with CD4 and associated chemokine receptors. IL-16 is a pleiotropic cytokine that has multiple effector functions with putative roles in varied T cell-mediated inflammatory diseases, such as asthma, inflammatory bowel disease and atopic dermatitis. Both in vitro and in vivo, IL-16 downregulates antigen-driven T cell activation, T helper 2 cytokine production and allergic airway inflammation. Peptides derived from the C-terminal bioactive portion of IL-16 offer advantages related to their retained immunomodulatory properties and absence of signalling in and chemoattraction to T cells.

Construction of human CD81 eukaryotic expression vector and expression of the gene segment in COS-7 cell line

AIM: To construct a human CD81 eukaryotic expression vector and to analyze the expression of CD81 in COS-7 cells.

METHODS: CD81 gene from the pMD18-T-CD81 vector with double-enzyme digestion was cloned into the pVAX1 eukaryotic expression vector, named pVAX1-CD81. The recombinant vector pVAX1-CD81 and pVAX1 as controls were transfected into COS-7 cells by lipofectamine, and the transient expression product on the transfected cells was analyzed with anti-CD81 monoclonal antibody by indirect immunofluorescence assay (IFA).

RESULTS: The identification of the eukaryotic expression vector pVAX1-CD81 by PCR and restriction enzyme analysis showed that CD81 gene was rightly inserted into the vector; and the product of the CD81 gene was successfully expressed on surface of COS-7 cells.

CONCLUSION: The eukaryotic expression vector with CD81 gene is constructed and efficiently expressed in COS-7 cells. The results indicate that the transfected CD81 cells will need to further studies on the roles of CD81 in the process of HCV infection and entrance to cells.

Regulation of CXCR4 receptor dimerization by the chemokine SDF-1alpha and the HIV-1 coat protein gp120: a fluorescence resonance energy transfer (FRET) study

Both the chemokine SDF-1alpha and the human immunodeficiency virus-1 (HIV-1) coat protein gp120 can bind to CXCR4 chemokine receptors but with different signaling consequences. To understand the molecular basis for these differences, we tagged the rat CXCR4 receptor with enhanced cyan (ECFP) and yellow (EYFP) derivatives of the green fluorescent protein and investigated CXCR4 receptor dimerization in human embryonic kidney (HEK)-tsA201 cells using fluorescence resonance energy transfer (FRET). Elevated FRET was detected under basal conditions from EYFP-CXCR4 and ECFP-CXCR4 receptor-transfected cells indicating a high level of CXCR4 receptor dimerization. In comparison, EYFP-CXCR4 and ECFP-mu-opioid receptor-cotransfected cells displayed a much lower FRET signal. The FRET signal resulting from EYFP-CXCR4- and ECFP-CXCR4-expressing cells could be attenuated by coexpressing nontagged CXCR4 receptors suggesting competition with fluorophore-tagged receptors in the membrane. Nontagged mu-opioid, kappa-opioid, and muscarinic receptors also decreased the FRET between the tagged CXCR4 receptor pairs but to a lesser extent. Application of the CXCR4 receptor agonist SDF-1alpha (50 nM) further increased the FRET signal from tagged CXCR4 receptors, an effect that was inhibited by the CXCR4 antagonist AMD3100. SDF-1alpha had no effect when EYFP-CXCR4 and ECFP-mu-opioid receptors were coexpressed. The effect of gp120IIIB on CXCR4 FRET was dependent on the coexpression of human CD4 (hCD4) when it increased the FRET signal, and this was decreased by AMD3100 pretreatment. FRET analysis of tagged hCD4 constructs demonstrated that there was significant association of hCD4 and CXCR4, as well as hCD4 dimerization. These data suggest that CXCR4 dimerization is involved in SDF-1alpha- and gp120-induced signaling events.