[Chemokine receptors and its importance in the replication cycle of human immunodeficiency virus: clinical and therapeutic implications]

Human immunodeficiency virus (HIV) infection of host cells begins with binding of viral envelope (Env) surface (SU) glycoprotein to specific receptors present at plasma cell membrane. One of these receptors is the CD4 molecule which can be found namely in T-helper lymphocytes (T-CD4+), macrophages and dendritic cells. Binding of SU glycoprotein to CD4 molecule, enables HIV to adsorb to target cell membrane and also initiates conformational changes in SU glycoprotein that enable it to bind to a second receptor (coreceptor). This coreceptor belongs to a family of plasma cell membrane molecules that acts, in vivo, as chemokine receptors. The SU glycoprotein binding to coreceptor molecule triggers further conformational changes and consequently the exposure of a hydrophobic region of viral envelope transmembrane glycoprotein (TM), named fusion peptide that ultimately leads to viral envelope fusion with target cell membrane. Nowadays, 19 of these chemokine receptors have been thus considered, in vitro, as coreceptors for HIV. Interestingly, despite this extensive range of potential coreceptors, only CCR5 and CXCR4 seem to be relevant in HIV transmission and in the pathogenesis of HIV infection. Identification of cell surface coreceptors, specific for HIV envelope SU glycoprotein, has provided an elucidative explanation for molecular mechanisms involved in viral cell tropism and pathogenesis. Furthermore, the recognition of a coreceptor-mediated HIV's entry has also provided novel viral and cellular targets for antiretroviral intervention. During the last few years, the inhibition of HIV entry has become an incontestable target for anti-HIV drug discovery. Enfuvirtide is one example of these new antiretroviral molecules. It is the only member of fusion inhibitors targeting fusion peptide region, which prevents HIV entry by blocking the TM-mediated fusion between viral envelope and plasma cell membrane. More recently, CCR5-specific antagonists have been described, including monoclonal antibodies, modified chemokines and more importantly small-molecules inhibitors, such as maraviroc and vicriviroc. These drugs prevent SU glycoprotein binding to CCR5 coreceptor, and thus inhibiting HIV entry into target cell. This review will focus on the influence of coreceptor engagement, by HIV Env glycoproteins, in viral replication cycle and the importance of targeting its coreceptor function, by specific inhibitors, as a new and promising class of antiretrovirals.

Prediction of R5, X4, and R5X4 HIV-1 coreceptor usage with evolved neural networks

The HIV-1 genome is highly heterogeneous. This variation affords the virus a wide range of molecular properties, including the ability to infect cell types, such as macrophages and lymphocytes, expressing different chemokine receptors on the cell surface. In particular, R5 HIV-1 viruses use CCR5 as co-receptor for viral entry, X4 viruses use CXCR4, whereas some viral strains, known as R5X4 or D-tropic, have the ability to utilize both co-receptors. X4 and R5X4 viruses are associated with rapid disease progression to AIDS. R5X4 viruses differ in that they have yet to be characterized by the examination of the genetic sequence of HIV-1 alone. In this study, a series of experiments was performed to evaluate different strategies of feature selection and neural network optimization. We demonstrate the use of artificial neural networks trained via evolutionary computation to predict viral co-receptor usage. The results indicate identification of R5X4 viruses with predictive accuracy of 75.5%.

Tropism switch in patients infected with HIV-1 and its clinical implications for the treatment with CCR5-receptor inhibitors

CCR5 receptor inhibitors are currently being introduced into clinical practice. In some instances treatment failure is related to the selection of pre-existing CXCR4-tropic minority virus strains. Up to date it is unclear whether the outgrowth of a CXCR4 using reservoir is associated with accelerated HIV-disease. In any case, treatment with CCR5 inhibitors should only be initiated in the absence of a relevant CXCR4-tropic minority. Otherwise treatment failure and the accumulation of mutations may ensue. Tropism tests, clinical data and other laboratory parameters help to determine the risk for an individual patient to harbour CXCR4 tropic virus strains, although the negative predictive value of each of these parameters and tests is quite low. If treatment fails re-assessment of viral tropism can help to differentiate between failure due to the development of CCR5 inhibitor resistance or the selection of CXCR4-tropic virus strains. This article presents and discusses available data on viral tropism and tropism testing in the context of CCR5 inhibitor treatment.

Genotypic coreceptor analysis

HIV infects target cells by binding of its envelope gp120 protein to CD4 and a coreceptor on the cell surface. In vivo, the different HIV-strains use either CCR5 or CXCR4 as coreceptor. CCR5-using strains are named R5 viruses, while CXCR4-using strains are named X4. X4 viruses usually occur in the later stages. Coreceptor usage is a marker for disease progression. Additionally interest on coreceptors continually raises as a consequence of the development of a new class of antiretroviral drugs, namely the coreceptor antagonists or blockers. These specific drugs block the CCR5 or the CXCR4 coreceptors. So far, the CXCR4 blockers are not allowed to be used in the clinical practice due to their severe side effects. On the other hand, CCR5 blockers are currently in clinical practice, although they can only be administered after a baseline determination of the coreceptor usage of the predominant viral strain. Most of the coreceptor analyses in clinical cohorts have been performed with commercially available phenotypic assays. As for resistance testing of NRTIs, NNRTIs and PIs, efforts have also been made to predict the coreceptor usage from the genotype of the viruses. Different rules have been published based on the amino acid sequence of the Env-V3 region of HIV-gp120, which is known to be the major determinant of coreceptor usage. Among these, the most widely used is the 11/25 rule. Recently, bioinformatics driven prediction systems have been developed. Three of the interpretation systems are freely available via internet: WetCat, WebPSSM, geno2pheno[coreceptor]. All three systems focus on the Env-V3 region and take the amino acid sequence only into account. They learn from phenotypic and corresponding genotypic data. So far, two cohorts have been analyzed with such a genotypic approach and provided frequencies of R5 virus strains that are within the range of those reported with phenotypic assays. For one of the systems, geno2pheno[coreceptor], additional clinical data (e.g. CD4+T-cell counts) or structural information can be used to improve the prediction. Such genotypic systems provide the possibility for rapid screening of patients who may be administered with CCR5 blockers like the recently licensed Maraviroc.

The role of receptors in the HIV-1 entry process

The advent of highly active antiretroviral therapy (HAART) has revolutionised the management of HIV. A wide selection of antiretroviral agents with various mechanisms of action is now available, allowing patients and physicians a greater choice of effective therapy. This article details the development of the HIV entry inhibitors, describing the physiology and pharmacology involved in their design, and their use in individuals at all stages of HIV infection. We focus on the CCR5 antagonists, a novel class of HIV entry inhibitor and detail the findings of the recent MERIT and MOTIVATE trials, designed to investigate CCR5 antagonist use in the antiretroviral naive and highly treatment experienced populations, respectively. Drug resistance and toxicities have emerged as major treatment challenges in the HAART era and the development of novel antiretroviral agents remains paramount. This article discusses how the entry inhibitors may meet many of these challenges and preserve the reduced morbidity and mortality we have come to expect from HAART use.

The epidemiology of HIV coreceptor tropism

According to their cellular coreceptor tropism, HIV variants are termed R5 if they use CCR5 as a coreceptor, whereas viruses with a preference for CXCR4 are termed X4. The prevalence of R5, X4 and dual/mixed (D/M) strains shows considerable variation in different patient populations. In treatment naive patients, R5 strains are found in 80-90%, compared to only 50-55% in patients with antiretroviral exposure. The most important predictor of R5 tropism seems to be a higher CD4 T-cell count in both naive and antiretrovirally pretreated patients. A low HIV plasma viremia seems to be associated with R5 tropism only in untreated patients. As the benefit of the new antiretroviral drug class of the CCR5 coreceptor antagonists will be probably limited to the HIV-infected patients harbouring R5 strains, determination of viral coreceptor tropism has become an important diagnostic prerequisite for the treatment of HIV infection. This review will focus on current knowledge of the epidemiology of HIV coreceptor tropism.

Phenotypic assays for the determination of coreceptor tropism in HIV-1 infected individuals

Coreceptor tropism antagonists represent a new class of antiretrovirals for the treatment of HIV infection. The knowledge of patients' viral population tropism before the initiation of and during therapy with such compounds may be critical in order to optimize treatment strategies. In this review we focus on the characteristics of phenotypic assays for the determination of HIV coreceptor tropism. Beside traditional phenotypic assays, there are at least four phenotypic recombinant virus assays (RVA) available to predict coreceptor usage: Trofile (Monogram Biosciences), Phenoscript (VIRalliance), XtrackC/ PhenX-R (inPheno) and a platform developed by Virco. Trofile and Phenoscript represent single-cycle assays and are able to determine coreceptor tropism without cocultivation of HIV particles in cell culture. Trofile offers the most clinically validated data with currently about 25,000 analysed samples. The detection of minority variants is a limitation of all population-based assays and varies between 1 and 10%, depending on the assay used. XtrackC/PhenX-R and Virco's platform combine genotypic and phenotypic assays to analyze a patient's sample for tropism. Although all assays are validated for the assessment of coreceptor tropism in different HIV-1 subtypes, there is still a need for further evaluations. Furthermore, the establishment of cut-offs for X4 minority species will be difficult, and is affected by many factors like patient sample quality, the input volume, viral load, the detection limits and PCR variations. Overall, RVAs confirm efficiency and accuracy thus making them suitable for the clinical management of HIV infected individuals treated with coreceptor antagonists.

Determination of HIV-1 coreceptor tropism in clinical practise

Several studies showed that the upcoming drug class of CCR5 coreceptor antagonists have potent virological and immunological activity in treatment experienced patients. In patients failing a CCR5 antagonists-based regimen, the emergence of CXCR4-tropic viral variants has been demonstrated. Clonal analysis of viral isolates from a limited number of patients revealed that these CXCR4-tropic strains did not develop by mutation of a CCR5-tropic virus during therapy, but emerged from a minor population of CXCR4-tropic variants already present in the patients at baseline. Obviously, screening for CXCR4-tropic strains with a functional assay and subsequent exclusion of positive individuals from clinical studies could not completely avoid the selection of CXCR4-tropic strains during failure. But emergence of CXCR4-tropic viruses on therapy may require a critical threshold of CXCR4 viral load at baseline, which may not be the case in patients with a very low proportion of CXCR4-using variants. Therefore, this review addresses to what extent currently available methods are suitable to detect CXCR4-tropic strains in clinical settings. Available functional assays are based on recombinant viruses. These assays are generally restricted to a few laboratories and cannot be easily included in daily clinical settings. Whereas minority detection limits of sequence analyses are generally high with 15 to 30%, functional assays achieve lower detection limits for minorities of 5%. Sequence analyses require an additional interpretation step, and the accuracy of interpretation from clinical samples by current predictions systems has to be improved. In consequence, new methods are arising: genotyping may be improved by hybridisation assays, which quantify CXCR4-tropic viruses by their homology down to 1% minorities, and functional non-infectious cell fusion assays may overcome security restrictions and make phenotypic methods suitable for routine clinical laboratory practise. The highly sensitive detection of CXCR4-tropic viruses may provide the opportunity to clarify the conditions of clinical relevance for CXCR4-tropic minorities.

Antibody-Mediated Rejection of Cardiac Allografts in CCR5-Deficient Recipients

Rejected MHC-mismatched cardiac allografts in CCR5(-/-) recipients have low T cell infiltration, but intense deposition of C3d in the large vessels and capillaries of the graft, characteristics of Ab-mediated rejection. The roles of donor-specific Ab and CD4 and CD8 T cell responses in the rejection of complete MHC-mismatched heart grafts by CCR5(-/-) recipients were directly investigated. Wild-type C57BL/6 and B6.CCR5(-/-) (H-2(b)) recipients of A/J (H-2(a)) cardiac allografts had equivalent numbers of donor-reactive CD4 T cells producing IFN-gamma, whereas CD4 T cells producing IL-4 were increased in CCR5(-/-) recipients. Numbers of donor-reactive CD8 T cells producing IFN-gamma were reduced 60% in CCR5(-/-) recipients. Day 8 posttransplant serum titers of donor-specific Ab were 15- to 25-fold higher in CCR5(-/-) allograft recipients, and transfer of this serum provoked cardiac allograft rejection in RAG-1(-/-) recipients within 14 days, whereas transfer of either serum from wild-type recipients or immune serum from CCR5-deficient recipients diluted to titers observed in wild-type recipients did not mediate this rejection. Wild-type C57BL/6 and B6.CCR5(-/-) recipients rejected A/J cardiac grafts by day 11, whereas rejection was delayed (day 12-60, mean 21 days) in muMT(-/-)/CCR5(-/-) recipients. These results indicate that the donor-specific Ab produced in CCR5(-/-) heart allograft recipients is sufficient to directly mediate graft rejection, and the absence of recipient CCR5 expression has differential effects on the priming of alloreactive CD4 and CD8 T cells.

Controls for lung dendritic cell maturation and migration during respiratory viral infection

Dendritic cells are ideally suited to orchestrate the innate and adaptive immune responses to infection, but we know little about how these cells respond to infection with common respiratory viruses. Paramyxoviral infections are the most frequent cause of serious respiratory illness in childhood and are associated with an increased risk of asthma. We therefore used a high-fidelity mouse model of paramyxoviral respiratory infection triggered by Sendai virus to examine the response of conventional and plasmacytoid dendritic cells (cDCs and pDCs, respectively) in the lung. We found that pDCs are scarce at baseline but become the predominant population of lung dendritic cells during infection. This recruitment allows for a source of IFN-alpha locally at the site of infection. In contrast, cDCs rapidly differentiate into myeloid cDCs and begin to migrate from the lung to draining lymph nodes within 2 h after viral inoculation. These events cause the number of lung cDCs to decrease rapidly and remain decreased at the site of viral infection. Maturation and migration of lung cDCs depends on Ccl5 and Ccr5 signals because these events are significantly impaired in Ccl5(-/-) and Ccr5(-/-) mice. cDCs failure to migrate to draining lymph nodes in Ccl5(-/-) or Ccr5(-/-) mice is associated with impaired up-regulation of CCR7 that would normally direct this process. Our results indicate that pDCs and cDCs respond distinctly to respiratory paramyxoviral infection with patterns of movement that should serve to coordinate the innate and adaptive immune responses, respectively.

CC-chemokine receptors in chronic obstructive pulmonary disease

Chronic obstructive pulmonary disease (COPD) is one of the leading causes of mortality and disability in the world, with a prevalence that is expected to increase in the next decades. The disease is characterized by a chronic inflammatory response of the airways and lungs to noxious particles and gases, mostly cigarette smoke. The molecular and cellular mechanisms that lead to this exaggerated influx of cells belonging to both the innate and adaptive immune system are not yet completely unravelled. However, there is now growing evidence that the recruitment of these inflammatory cells in response to cigarette smoke is largely regulated by chemokines acting as ligands for chemokine receptors. Several of these receptors, which fall mainly in the CC- or CXC-category, have been implicated in the pathogenesis of COPD. In this review we will focus mainly on the CC-family, as the involvement of CXC-receptors in COPD has already been extensively reviewed. In patients with COPD, several CC-chemokines like MIP-1alpha, MIP-3alpha, RANTES and MCP-1 are upregulated, suggesting the contribution of their respective receptor in the pathogenesis of the disease. Using knock out mice, this contribution has been further confirmed for CCR5 and CCR6, evidenced by an attenuated accumulation of inflammatory cells like macrophages, dendritic cells, neutrophils and CD8(+) T-lymphocytes upon cigarette smoke-exposure. Moreover, mice deficient for CCR5 or CCR6 are partially protected from the development of pulmonary emphysema, another hallmark of COPD. These data suggest that chemokine receptors are potential therapeutic targets to reduce the chronic inflammation and parenchymal destruction in COPD.

Identification of a postendocytic sorting sequence in CCR5

The chemokine receptor 5 (CCR5), a member of the G protein-coupled receptor family (GPCR), is used by human immunodeficiency virus type 1 (HIV-1) with a R5 tropism as an entry receptor in addition to CD4. It is a key target for an antiviral action aiming at inhibiting the HIV-1 entry process. Only few data are available today regarding the mechanism involved in the intracellular trafficking process of CCR5. Understanding how CCR5 cell surface expression is regulated is particularly important with regard to HIV-1 entry inhibition. We set out to investigate whether CCR5 molecular determinants were involved in the postendocytic recycling and degradative pathways. We constructed progressive deletion mutants of the C-terminal domain of CCR5 that we stably expressed in HEK293 cells. All of the deletion mutants were expressed at the cell surface and were functional HIV-1 receptors. The deletion mutants were internalized after stimulation, but they lost their ability to recycle to the plasma membrane. They were rerouted toward a lysosomal degradative pathway. We identified here a sequence of four amino acids, present at the extreme C terminus of CCR5, that is necessary for the recycling of the internalized receptor, independently of its phosphorylation. A detailed analysis of this sequence indicated that the four amino acids acted as a postsynaptic density 95/discs-large/zona occludens (PDZ) interacting sequence. These results show that the CCR5 cytoplasmic domain bears a sequence similar to the "recycling signals" previously identified in other GPCRs. Drugs able to disrupt the recycling pathway of CCR5 may constitute promising tools for therapeutic treatment.

CCR1 and CCR5 chemokine receptors are involved in fever induced by LPS (E. coli) and RANTES in rats

This study, besides examining the involvement of CCR1 and CCR5 receptors in the LPS-induced fever (lipopolysaccharide, Escherichia coli) in male Wistar rats, evaluated if RANTES (regulated on activation, normal T cells expressed and secreted) injected into the preoptic area of the anterior hypothalamus (AH/POA) would promote an integrated febrile response via these receptors. Moreover, the effects of selective and non-selective cyclooxygenase blockers on both fever and the level of prostaglandin (PG)E(2) in the cerebrospinal fluid (CSF) after injection of RANTES into the AH/POA were also investigated. Met-RANTES, CCR1 and CCR5 receptor antagonist, reduced LPS-evoked fever dose dependently. RANTES microinjected into the AH/POA increased the rectal temperature of rats dose dependently and caused a significant decrease in the tail skin temperature and an increase (at 2.5 and 5 h) of the levels of PGE(2) in the CSF. Met-RANTES prevented the fever induced by RANTES. Ibuprofen abolished the fever caused by RANTES between 60 min and 2.5 h, and it reduced the temperature until the end of observation period. Celecoxib blocked the RANTES-induced fever, while indomethacin reduced it in the last 60 min of the experimental period. At 2.5 and 5 h all antipyretics brought the CSF PGE(2) level near to the control. These results indicate that CCR1 and CCR5 receptors are involved in the fever induced by systemic LPS and intrahypothalamic RANTES. RANTES promotes an integrated febrile response accompanied by an increase of CSF PGE(2). The inhibitory effects of celecoxib and ibuprofen suggest that PGE(2) was generated via COX-2. As indomethacin dissociates fever and the decrease of PGE(2) level during the RANTES-induced fever, an alternative COX-2-independent pathway or other mechanisms of action of celecoxib and ibuprofen might be considered.

Expression of the chemokine receptor CCR5 in psoriasis and results of a randomized placebo controlled trial with a CCR5 inhibitor

Several reports have indicated that the chemokine receptor CCR5 and its ligands, especially CCL5 (formerly known as RANTES), may play a role in the pathogenesis of psoriasis. The purpose of this investigation was to examine the expression of CCR5 and its ligands in chronic plaque psoriasis and to evaluate the clinical and immunohistochemical effect of a CCR5 receptor inhibitor. Immunohistochemical analysis showed low but significant increased total numbers of CCR5 positive cells in epidermis and dermis of lesional skin in comparison to non-lesional skin. However, relative expression of CCR5 proportional to the cells observed revealed that the difference between lesional and non-lesional skin was only statistically significant in the epidermis for CD3 positive cells and in the dermis for CD68 positive cells. Quantification of mRNA by reverse transcriptase-polymerase chain reaction only showed an increased expression of CCL5 (RANTES) in lesional skin. A randomized placebo-controlled clinical trial in 32 psoriasis patients revealed no significant clinical effect and no changes at the immunohistochemical level comparing patients treated with placebo or a CCR5 inhibitor SCH351125. We conclude that although CCR5 expression is increased in psoriatic lesions, this receptor does not play a crucial role in the pathogenesis of psoriasis.

CD4-independent use of the CCR5 receptor by sequential primary SIVsm isolates

Background

CD4-independence has been taken as a sign of a more open envelope structure that is more accessible to neutralizing antibodies and may confer altered cell tropism. In the present study, we analyzed SIVsm isolates for CD4-independent use of CCR5, mode of CCR5-use and macrophage tropism. The isolates have been collected sequentially from 13 experimentally infected cynomolgus macaques and have previously been shown to use CCR5 together with CD4. Furthermore, viruses obtained early after infection were neutralization sensitive, while neutralization resistance appeared already three months after infection in monkeys with progressive immunodeficiency.

Results

Depending whether isolated early or late in infection, two phenotypes of CD4-independent use of CCR5 could be observed. The inoculum virus (SIVsm isolate SMM-3) and reisolates obtained early in infection often showed a pronounced CD4-independence since virus production and/or syncytia induction could be detected directly in NP-2 cells expressing CCR5 but not CD4 (CD4-independent-HIGH). Conversely, late isolates were often more CD4-dependent in that productive infection in NP-2/CCR5 cells was in most cases weak and was revealed only after cocultivation of infected NP-2/CCR5 cells with peripheral blood mononuclear cells (CD4-independent-LOW). Considering neutralization sensitivity of these isolates, newly infected macaques often harbored virus populations with a CD4-independent-HIGH and neutralization sensitive phenotype that changed to a CD4-independent-LOW and neutralization resistant virus population in the course of infection. Phenotype changes occurred faster in progressor than long-term non-progressor macaques. The phenotypes were not reflected by macrophage tropism, since all isolates replicated efficiently in macrophages. Infection of cells expressing CCR5/CXCR4 chimeric receptors revealed that SIVsm used the CCR5 receptor in a different mode than HIV-1.

Conclusion

Our results show that SIVsm isolates use CCR5 independently of CD4. While the degree of CD4 independence and neutralization sensitivity vary over time, the ability to productively infect monocyte-derived macrophages remains at a steady high level throughout infection. The mode of CCR5 use differs between SIVsm and HIV-1, SIVsm appears to be more flexible than HIV-1 in its receptor requirement. We suggest that the mode of CCR5 coreceptor use and CD4-independence are interrelated properties.

CCR5 signaling through phospholipase D involves p44/42 MAP-kinases and promotes HIV-1 LTR-directed gene expression

The chemokine receptor CCR5 plays an important role as an entry gate for the human immunodeficiency virus-1 (HIV-1) and for viral postentry events. Among signal transducers used by chemoattractant receptors, the phosphatidylcholine-specific phospholipase D (PLD) produces large amounts of second messengers in most cell types. However, the relevance of PLD isoforms to CCR5 signaling and HIV-1 infection process remains unexplored. We show here that CCR5 activation by MIP-1beta in HeLa-MAGI cells triggered a rapid and substantial PLD activity, as assessed by mass choline production. This activity required the activation of ERK1/2-MAP kinases and involved both PLD1 and PLD2. MIP-1beta also promoted the activation of an HIV-1 long terminal repeat (LTR) by the transactivator Tat in HeLa P4.2 cells through a process involving ERK1/2. Expression of wild-type and catalytically inactive PLDs dramatically boosted and inhibited the LTR activation, respectively, without altering Tat expression. Wild-type and inactive PLDs also respectively potentiated and inhibited HIV-1(BAL) replication in MAGI cells. Finally, in monocytic THP-1 cells, antisense oligonucleotides to both PLDs dramatically inhibited the HIV-1 replication. Thus, PLD is activated downstream of ERK1/2 upon CCR5 activation and plays a major role in promoting HIV-1 LTR transactivation and virus replication, which may open novel perspectives to anti-HIV-1 strategies.

Chemokines in hepatitis C virus infection: Pathogenesis, prognosis and therapeutics

Hepatitis C virus infection and its associated liver inflammatory disease is a major global health problem affecting over 170 million people worldwide. Following viral infection, multiple pro-inflammatory mediators contribute to recruitment of immune cells to the liver and to the generation of an anti-viral immune response. However, when this vigorous immune response fails to eliminate the virus, chronic infection is established. This in turn, results in an ongoing process of inflammation, regeneration and fibrosis that in many cases leads to the development of cirrhosis and of hepatocellular carcinoma. Multiple recent publications mark chemokines and their receptors as key players in leukocyte recirculation through the inflamed liver. Furthermore, chemokines may also be involved in liver regeneration, fibrosis, and in malignant transformation, which is induced by the persistence of inflammation. Accumulating data indicates that distinct chemokines and chemokine receptors may be associated with different stages of the chronic hepatitis C virus infection-associated liver disease. Multiple small molecules and peptide antagonizing chemokines and their receptors are in advanced phase 3 and phase 2 clinical trials. In the near future, such drugs are expected to enter clinical use raising the question whether they may be applicable for the treatment of chronic viral infection-associated liver disease. In this review, recent advances in understanding the role of chemokines and their receptors in the pathogenesis of chronic viral infection-associated liver disease are presented. Furthermore, the clinical implications of these novel findings, which mark chemokines as prognostic markers and therapeutic targets for immune-modulation during chronic liver viral infection, are documented.

Coreceptor switch in R5-tropic simian/human immunodeficiency virus-infected macaques

The basis for the switch from CCR5 to CXCR4 coreceptor usage seen in approximately 50% of human immunodeficiency virus type 1 (HIV-1) subtype B-infected individuals as disease advances is not well understood. Among the reasons proposed are target cell limitation and better immune recognition of the CXCR4 (X4)-tropic compared to the CCR5 (R5)-tropic virus. We document here X4 virus emergence in a rhesus macaque (RM) infected with R5-tropic simian/human immunodeficiency virus, demonstrating that coreceptor switch can happen in a nonhuman primate model of HIV/AIDS. The switch to CXCR4 usage in RM requires envelope sequence changes in the V3 loop that are similar to those found in humans, suggesting that the R5-to-X4 evolution pathways in the two hosts overlap. Interestingly, compared to the inoculating R5 virus, the emerging CXCR4-using virus is highly neutralization sensitive. This finding, coupled with the observation of X4 evolution and appearance in an animal with undetectable circulating virus-specific antibody and low cellular immune responses, lends further support to an inhibitory role of antiviral immunity in HIV-1 coreceptor switch.

CCR5 receptor: biologic and genetic implications in age-related diseases

The CC chemokine receptor 5 (CCR5) is a member of CC-chemokine receptor family. CCR5 has the characteristic structure of a seven transmembrane G protein-coupled receptor (GPCR), which regulates trafficking and effector functions of memory/effector Th1 cells, macrophages, NK cells, and immature dendritic cells. CCR5 and its ligands are important molecules in viral pathogenesis. CCR5 represents the co-receptor for macrophage (M) and dual (T cell and M)-tropic immunodeficiency viruses. Recent evidence has also demonstrated the role of CCR5 in a variety of human diseases, ranging from infectious and inflammatory diseases to cancer. In this article, we describe the involvement of CCR5 in two age-related diseases, atherosclerosis and Alzheimer's disease, suggesting a possible role of chemokine system on these diseases' pathophysiology. Finally, we review the data on the probable association between CCR5Delta32 deletion and cardiovascular diseases and Alzheimer's disease.

HIV-1 coreceptors and their inhibitors

Entry of human immunodeficiency virus (HIV) into target cells is mediated by the viral Envelope glycoprotein (Env) and its coordinated interaction with a receptor (CD4) and a coreceptor (usually the chemokine receptors CCR5 or CXCR4). This review describes the identification of chemokine receptors as coreceptors for HIV-1 Env-mediated fusion, the determinants of chemokine receptor usage, and the impact of nonfunctional chemokine receptor alleles on HIV-1 resistance and disease progression. Due to the important role of chemokine receptors in HIV-1 entry, inhibitors of these coreceptors are good candidates for blocking entry and development of antiretroviral therapies. We discuss the different CCR5- and CXCR4-based antiretroviral drugs that have been developed thus far, highlighting the most promising drug candidates. Resistance to these coreceptor inhibitors as well as the impact of these drugs on clinical monitoring and treatment are also discussed.

Chemokine receptor CCR2 but not CCR5 or CCR6 mediates the increase in pulmonary dendritic cells during allergic airway inflammation

Increased numbers of pulmonary dendritic cells (DCs) are recruited to the lungs during allergic airway inflammation and contribute to the maintenance of the inflammatory immune response. The chemokine receptors that directly control DC accumulation into the lungs are largely unknown. To explore this issue, we generated mixed bone marrow chimeric mice containing both wild-type and knockout cells for a given chemokine receptor. After induction of allergic airway inflammation, we specifically tracked and compared chemokine receptor knockout vs wild-type DC populations through various lung compartments. Using this approach, we show that CCR2, but not CCR5 or CCR6, directly controls the accumulation of DCs into allergic lungs. Furthermore, the size of inflammatory monocyte populations in peripheral blood was strikingly CCR2 dependent, suggesting that CCR2 primarily mediates the release of monocytic DC precursors into the bloodstream.