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Regulation of CC chemokine receptor 5 in Hepatitis G virus infection
  AIDS 2003; 17(10):1457-1462
Jacob Nattermann; Hans-Dieter Nischalke; Bernd Kupfer a; Jurgen Rockstroh; Lothar Hess b; Tilman Sauerbruch; Ulrich Spengler
Department of General Internal Medicine, the aInstitute for Medical Microbiology and Immunology, and the bDepartment of Experimental Hematology and Transfusion Medicine, Rheinische Friedrich-Wilhelms-UniversitŠt, Bonn, Germany
Epidemiological data demonstrate an association between hepatitis G virus (HGV) co-infection and improved survival of HIV-positive individuals. However, the mechanism by which HGV affects progression of HIV disease remains unclear. As down-regulation of CC chemokine receptor 5 (CCR5) delays HIV progression, we investigated whether CCR5 expression is altered by exposure of lymphocytes to HGV proteins.
Methods: A cross-sectional analysis of CCR5 expression was carried out on CD4 and CD8 T lymphocytes of 11 HGV-positive and 12 HGV-negative persons, who were homozygous for the CCR5 wild-type gene. Binding of the HGV E2 protein to CD81 was analysed by flow cytometry. Lymphocytes were stimulated with immobilized HGV E2, anti-CD81 or serum proteins from HGV-infected subjects and changes in CCR5 expression and CC chemokine secretion were determined.
Results: We demonstrate that the HGV envelope protein E2 specifically binds to CD81 on T lymphocytes. This interaction induces a dose-dependent release of RANTES and down-regulation of CCR5 surface expression with concomitant intra-cellular accumulation of CCR5 proteins. This effect of HGV E2 on CCR5 expression was confirmed when lymphocytes were incubated with serum proteins from HGV-infected subjects. Finally, our cross-sectional analysis revealed CCR5 expression to be reduced by 53% and 36% on CD4 and CD8 lymphocytes of HGV-infected subjects, respectively (P < 0.01).
Conclusions: Our results demonstrate that an interaction of HGV E2 with CD81 leads to increased RANTES secretion and decreased CCR5 surface expression. This mechanism might contribute to the delayed progression of HIV-infection in HGV-coinfected patients.
Hepatitis G virus (HGV), a flavivirus closely related to hepatitis C virus (HCV), was discovered as a putative cause of parenterally transmitted viral hepatitis in 1995. However, the clinical role of HGV is still under debate. Recent reports propose an association between HGV infection in HIV-positive patients leading to delayed progression of HIV disease and improved survival. However, the mechanisms underlying this phenomenon remain unclear.
Down-regulated expression of CD4 or HIV co-receptors such as CC chemokine receptor (CCR) 5 blocks cellular entry and decreases cell-to-cell transmission of HIV within infected hosts. As reduced CCR5 expression on lymphocytes has recently been reported in chronic hepatitis C - a disease caused by another member of the family of flaviviruses - we investigated CCR5 expression in HGV-positive individuals and studied whether CCR5 expression is altered by exposure of lymphocytes to HGV proteins.
The HCV envelope protein 2 (E2) has been shown to interact with CD81, a member of the tetraspanin family, which is expressed on the cell surface of virtually all nucleated cells. Of note, ligation of CD81 with HCV E2 alters cellular functions of T cells and natural killer cells. As the HGV polyprotein also contains an envelope E2 protein with homology to HCV E2, we focused our studies on a possible interaction of HGV E2 with CD81 and its effects on CCR5 expression.
Discussion by authors
CCR5 serves as a co-receptor for M-tropic HIV strains. Thus, the level of CCR5 expression is an important factor for HIV transmission and disease progression.
In this study, we report that CCR5 expression is markedly reduced on CD4 and CD8 T lymphocytes from HGV-infected individuals, irrespective of HIV co-infection. Moreover, we demonstrate that serum proteins from HGV RNA-positive individuals down-regulate CCR5 expression on T lymphocytes. In particular, we show that the HGV E2 protein binds to CD81 and that triggering of CD81 by either a stimulatory antibody or immobilized HGV E2 induces reduced CCR5 expression. This process may contribute importantly to the delayed HIV progression in patients co-infected with HGV due to impaired cell-to-cell transmission of HIV within the infected host.
Our findings bear some similarities to observations in chronic hepatitis C. The HCV E2 protein also interacts with CD81, and this interaction alters cellular functions of natural killer cells and T lymphocytes. Although it is unclear at present whether binding of HCV E2 to CD81 actually modifies expression of CCR5, Lichterfeld et al. observed a reduced CCR5 expression on lymphocytes in chronic hepatitis C due to altered subcellular compartmentalization of CCR5. Likewise we demonstrated here that cross-linking of CD81 with HGV E2 leads to intra-cellular accumulation of CCR5. Moreover, we demonstrate that exposure of lymphocytes to HGV E2 induces secretion of RANTES, a natural ligand of CCR5, which induces internalization of CCR5. Thus, our findings suggest that down-regulated CCR5 expression after interaction of HGV E2 with CD81 may be mediated by RANTES secretion and subsequent binding of this chemokine to CCR5. Several groups have shown that internalization of chemokine receptors following ligand binding is an effective mechanism to block cellular entry of HIV. However, it remains currently unclear why HGV E2 specifically induces RANTES but not MIP-1[alpha] or MIP-1[beta].
The difference between our results and the lack of any effect on CCR5 expression observed in infection studies in vitro is explained by the fact that unlike natural HGV infection only minimal amounts of HGV E2 protein are presumably generated in such an in vitro model. However, the fact that Xiang et al. observed increased resistance to HIV infection in their in vitro infection experiments indicates that other mechanisms than down-regulation of CCR5 may contribute to delayed progression of HIV disease in HGV infection. Interestingly, Vallet et al. recently proposed HGV-infection to be associated with an increased incidence of the CCR5-[Delta]32 allele However, as all HGV-infected individuals in our study were homozygous for the CCR5 wild-type gene, effects due to the CCR5-[Delta]32 allele have not contributed to our observations.
Taken together, our study demonstrates an association between HGV-infection and decreased surface expression of CCR5 mediated by an interaction of HGV E2 with CD81 and subsequent release of RANTES. This finding may at least in part explain the reported epidemiological association between HGV infection and prolonged survival of HIV-infected subjects.
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