Wellington and York Partners: Using Travel Promotions for Traveling to Zurich, Switzerland

Travel promotions can be a blessing in disguise. It can help travelers save money when you are going on a trip somewhere. It may include accommodation and restaurant discounts, as well as a way to give exclusive access to tourist spots.

Switzerland may not be the top country chosen by most travelers. Most people love to go to France, New York, or Spain. It is less likely that travelers go to Zurich unless they have a sweet tooth. The country isn’t only known for their remarkable yet expensive watches. Tourists can also enjoy the food products like cheese and chocolates. It is a great place for adventurers and hikers because of the famous mountain range called the Swiss Alps.

Zurich is also one of the famous places in Switzerland. According to surveys, it has the highest quality of life compared to other places in the world. Travelers can enjoy a lot of incredible accommodations, as well as outstanding cuisine. There are a lot of restaurant services you can find in the city. After a gastronomic adventure, an ideal way to do is enjoy a great shopping experience in various shops. You can even see celebrities strolling around the place.

The sweet in Switzerland

Switzerland is a haven for lovers of confections because of their rich history of chocolate making which is traced back to the 17th century. Chocolate making can be considered an art and a lucrative industry in the country. Milk chocolate is one of the best-known products.

You can find a great-tasting Swiss chocolate in local shops. A lot of supermarkets around the world showcase a lot of sweet treats from Switzerland but to taste the fresh and delicious confection, better to head to a local sweet shop than the grocery stores.

The selection of Swiss chocolates will not disappoint you. If you go to local chocolatiers, you can even get a free taste of their products. While you’re in Zurich, don’t just focus on the wrapped chocolate treats. There are other selections of goodies such as Swiss hot chocolate which is far different from the ones sold in supermarkets.

Switzerland has astonished people who go in and out of the country. Travelers can see a combination of rolling mountains, crystal blue lakes, stunning architectures, as well as a busy yet astounding city streets. It may be small since travelers can cross the country by train in just 5 hours. However, it is worth the time to experience the Swiss sights and sounds. The public transit is reliable and a good way to go around the tourist spots.

The lack of travel reviews should not be a hesitation for would-be travelers planning to take a trip to the country since it is still extremely underrated.

Stellamc.info

Stellamc.info

HIV-1 dynamics drive CD4+ T cell turnover

The hallmark of HIV-1 infection is a progressive reduction in CD4+ T cells, which leads to a general decline in immune function and is the primary factor responsible for the clinical course of disease. Although the discovery of CD4 as a receptor for HIV-1 in the 1980s (MILESTONE 3) could help explain thesusceptibility of CD4+T cells to infection, the mechanisms responsible for their decline remained elusive. In 1995, two seminal studies by the groups of George Shaw and David Ho published in Nature provided important insights on the dynamics and pathophysiology of HIV-1 infection, including pivotal observations concerning CD4+ T cell decline.

The advent of new quantitative assays for measuring HIV-1 RNA concentrations (viral load) and experimental drugs that could potently inhibit HIV-1 replication enabled both groups to perform experiments in which the rates of CD4+ T cell and viral turnover could be extrapolated from measurements of changes in plasma viral load and CD4+ T cell counts following antiviral therapy. In both studies, abrupt inhibition of viral replication led to a substantial rise in CD4+ T cell numbers and revealed a scenario in which continuous and highly productive viral replication drove rapid turnover of CD4+ T cells.

The initial stage of HIV-1 infection is followed by an asymptomatic period that can last for years before disease progresses and results in the development of AIDS. Given that this asymptomatic period is accompanied by relatively stable levels of CD4+ T cells and viral load, loss of these cells was initially thought to involve a gradual process of destruction. However, these new findings challenged this view, supporting a model of accelerated CD4+ T cell destruction, which Ho and colleagues likened to a ‘tap-and-drain’ scenario.

In this analogy, the destruction of CD4+ T cells (the drain) is counterbalanced by homeostatic production of T cells (the tap) during the asymptomatic period; however, once production of T cells becomes exhausted, this balance is disrupted, resulting in eventual loss of CD4+ T cells (emptying of the sink) and AIDS. Although the mechanisms underlying this imbalance were an area of debate and later evidence pointed to the existence of other (potentially non-mutually exclusive) models of CD4+ T cell depletion (MILESTONE 10), the findings nonetheless had important clinical implications.

In both studies, evolving resistance to the antiviral drug led to a rise in viral load and a concurrent decrease in CD4+ T cell numbers to pretreatment levels. The previously unrecognized regenerative capacity of CD4+ T cells in HIV-1 infection along with the idea of drug resistance and immune escape fueled the search for effective antiviral strategies.

The findings also raised questions about the utility of CD4+ T cell count as a predictor of long-term survival (at the time, it was the main surrogate marker of disease progression). Just one year later, John Mellors and colleagues linked viral load and HIV prognosis in a paper published in Science. By measuring plasma HIV-1 RNA concentrations in a cohort of 180 HIV-seropositive men who were followed for >10 years, they reported that plasma viral load (irrespective of duration of infection) was a better predictor of patient outcome (that is, progression to AIDS or death) than number of CD4+ T cells.

Thomas Quinn and colleagues later revealed that viral load was also a risk factor for viral transmission. Of the factors they measured (including various behavioral and biological risk factors) in a study of 415 couples who were followed for up to 30 months, viral load was the best predictor. Indeed, measurements of viral load are now routinely used for the clinical assessment and monitoring of patients infected with HIV-1, alongside CD4+ T cell count.

These findings spurred the development of antiviral therapies and therapeutic strategies (such as combination therapies) to reduce viral load in individuals infected with HIV-1, with the aim of improving long-term patient outcomes and potentially preventing viral transmission (MILESTONE 14, 20, 21).

First HIV protease inhibitor approved: key to combination antiretroviral therapy

The first antiretroviral therapies (ARTs) for people with HIV were nucleoside reverse-transcriptase inhibitors (NRTIs), but these drugs were only partially effective. The addition of an orally administered protease inhibitor, the first of which was approved in 1995, reduced HIV plasma concentrations and increased CD4+ cell counts to levels that enabled patients to have fairly normal life expectancies. This combination—two nucleoside analogs and a protease inhibitor—is now the cornerstone of ART.

The HIV genome encodes a long polypeptide that must be cleaved into functional proteins by the HIV protease. Following virus uncoating and reverse transcription of the RNA genome, a polypeptide is produced that contains all viral gene products, including the structural proteins and enzymes. The HIV protease then cleaves this polypeptide into its constituent viral proteins. Inhibiting the activity of the protease is therefore an attractive means to prevent virion production.

The first protease inhibitors were peptidomimetic molecules designed to look like the peptide linkages in the precursor polypeptide and therefore compete with the substrate. However, like most peptidomimetic proteins, the early protease inhibitors had poor pharmacokinetic properties, namely, low oral absorption and rapid elimination. Key medicinal chemistry–led structural changes improved these properties. For example, substituting a pyridine with the less electron-rich thiazole to produce ritonavir improved both metabolic stability and aqueous solubility. This molecule was also more potent in animal studies than its parent, predominantly because it also had a lower inhibitory constant (Ki).

These drugs wowed the community in early clinical trials. The addition of a protease inhibitor to two NRTIs approximately halved the number of patients whose disease progressed to AIDS or death. In 90% of patients taking the three-drug combination, the number of HIV RNA particles in the blood went from >20,000 particles per milliliter to <500 in 24 weeks.

The first protease inhibitor to be approved by the US Food and Drug Administration (FDA) was saquinavir, in December 1995, a mere 97 days after the FDA received its marketing application. Within months, two other protease inhibitors, ritonavir and indinavir, were also approved. There are currently ten FDA-approved protease inhibitors on the market for HIV.

The remarkable results from the clinical trials of this first wave of protease inhibitors also highlighted important aspects of the biology of HIV infection. First, the clearance rate of virus was independent of initial viral loads and suggested that, on average, half of plasma virions turn over every two days. Second, the number of CD4+ cells destroyed and replenished each day was close to the total number of infected cells. The potential to generate viral diversity (and resulting drug-resistant clones) is therefore substantial, arguing for early initiation of ART.

ART has changed the course of HIV. In the US, mortality among patients with advanced HIV infection declined from 29.4 per 100 person-years in 1995 to 8.8 once ART including a protease inhibitor became the standard of care. In geographic locations with high rates of HIV infection, ART has also changed economics and demographics. Places with high rates of infection, such as Swaziland, saw a 10- to 15-year decrease in life expectancy during the peak of HIV deaths. In the neighboring rural KwaZulu-Natal region of South Africa, where an estimated 29% of adults are HIV positive, adult life expectancy (the mean age to which a 15-year-old could expect to live) increased from 49 to 61 years in the ~10 years after government programs for HIV treatment were initiated. Because many people with HIV were dying during their most economically productive years, the knock-on societal and economic effects are substantial.

Since 1995, new protease inhibitors and combinations with improved dosing have become available, but the protease inhibitors developed in the mid-1990s changed the course of the disease and formed the foundations of ART.

Identification of CCR5 and CXCR4 as HIV-1 co-receptors

Following the discovery of CD4 as the main receptor for HIV-1 in the mid-1980s (MILESTONE 3), it became clear that expression of a CD4transgene rendered human cells, but not mouse cells, permissive for infection with HIV-1. There was also a growing awareness that different HIV-1 isolates have different tropisms in vitro for the infection of different human CD4+ cell types. Macrophage-tropic virus strains (which infect primary macrophages and T cells, but not immortalized T cell lines) predominate during the asymptomatic phase of infection, whereas T cell–tropic strains (which infect primary T cells and T cell lines, but not primary macrophages) become more common during progression to AIDS. The viral envelope protein Env, a ligand for CD4, was known to be the main viral determinant of this cell tropism. Together, these observations led to the suggestion that additional human-specific receptors for Env are required for infection, the expression of which determines cell tropism.

In May 1996, Berger and colleagues identified, in an unbiased manner, the first of these co-receptors for HIV-1. They developed a method to study Env–receptor-mediated cell fusion by expressing a phage T7 polymerase in a CD4+ mouse cell line and a reporter gene linked to the T7 promoter in a second, Env-expressing mouse cell line. Expression of the reporter would occur only in the cytoplasm of fused cells. By screening a cDNA plasmid library from HeLa cells for cofactors that would enable fusion of these nonhuman cells, they cloned a G-protein-coupled receptor of unknown ligand and function, but with the greatest homology to the receptor for the chemokine CXCL8. This cofactor was named ‘fusin’ in the original paper and was renamed later that year as CXCR4 when its ligand was identified as CXCL12. Importantly, fusin was shown to enable entry mediated by Env from T cell–tropic HIV-1 but not macrophage-tropic HIV-1, which led to a race to identify the second cofactor for macrophage tropism.

That the T cell–tropic factor fusin had homology to an α-chemokine receptor fit well with an observation made the previous year by Cocchi et al. that the β-chemokines RANTES (CCL5), MIP-1α (CCL3) and MIP-1β (CCL4) produced by CD8+ T cells inhibit infection with macrophage-tropic HIV-1. Thus, it seemed likely that a β-chemokine receptor was the cofactor for infection of macrophages.

Five papers published within eight days of each other in June 1996 identified CCR5 as the second co-receptor for HIV-1. Another study by Berger’s group, using the same fusion assay that had identified fusin, described the role of CCR5 in macrophage infection. Deng et al. showed that CD4 and CCR5 function cooperatively in mouse cells to permit membrane fusion with macrophage-tropic HIV-1. Similarly, Choe et al. described that macrophage-tropic HIV-1 uses CCR5, as well as CCR3, to facilitate infection. In keeping with the switch in viral tropism that accompanies pathogenesis in vivo, Dragic et al. identified CCR5 as a second co-receptor for macrophage-tropic HIV-1 in primary CD4+ T cells, and Doranz et al. showed that a dual-tropic ‘intermediate’ HIV-1 isolate used both fusin and CCR5.

Discovery of CXCR4 and CCR5 as co-receptors provided an explanation for the long-standing puzzle of Env-related differences in HIV-1 tropism and opened up the possibility of developing new antiretroviral drug therapies to block infection. Soon thereafter it was recgonized that individual differences in the expression or activity of these co-receptors could underlie susceptibility to infection and disease progression, and this was confirmed by three papers published later in 1996. Liu et al., Samson et al. and Dean et al. described a 32-base-pair deletion in the coding region of CCR5 that was variously shown to protect homozygotes from infection, partially protect heterozygotes from infection and delay disease progression in heterozygotes. The lack of an obvious phenotype associated with the mutation, together with the later description of the Berlin patient (MILESTONE 18), gave hope that pharmacological or genetic targeting of CCR5 could be a safe and effective therapeutic approach.

Latent integrated HIV-1 forms a stable, inducible viral reservoir

The advent of combination therapy for HIV treatment in the mid-1990s had a huge impact on patient morbidity and mortality (MILESTONE 14). Together, reverse-transcriptase inhibitors and newly developed protease inhibitors caused plasma virus to fall to undetectable levels within 2–4 months. It was presumed that integration of HIV-1 DNA into the host genome could enable persistence of the virus, and indeed, a 1995 paper by Chun et al. had detected integrated proviruses in some infected patients. These proviruses were found in resting CD4+ T cells, which did not produce virus unless activated. But in 1995 it was not clear how much of an obstacle this potential latent reservoir of integrated proviruses would be to ultimate eradication of the virus by the new combination therapy.

A 1997 paper from the group of David Ho described two phases of viral decline in infected patients treated with combination therapy: a first phase in which virus levels dropped by around 99% within the first two weeks of treatment and a second phase of slower decline, which the authors concluded was driven by loss of long-lived infected cells. Extrapolating from these decay characteristics, they predicted that around 2–3 years of effective treatment would be required to eradicate HIV-1, perhaps longer if the virus persisted in sanctuary sites. Sadly, this estimate proved too optimistic.

Papers from the groups of Robert Siliciano and Douglas Richman, published in 1997, began to characterize and quantify the latent reservoir of HIV-1 virus in patients. Chun et al. provided the first snapshot of the latent reservoir of virus, in a group of 14 asymptomatic HIV-1-infected donors. Looking in the lymph nodes, the authors found that around 0.5% of resting CD4+ T cells harbored HIV-1 DNA, but that less than 0.05% of resting cells contained integrated provirus. The relatively small size of this latent reservoir was a surprise, but the authors cautioned against underestimating its importance, due to the long survival of memory CD4+ T cells. In work published later that year, Finzi et al. looked at 22 patients successfully treated with drugs for up to 30 months and reported the sobering finding that, despite apparently complete suppression of virus replication, highly purified resting CD4+ T cells from each of these individuals could be induced to make replicating virus. Furthermore, levels of inducible replication-competent virus did not decline with increasing time on therapy and the inducible viruses had not acquired mutations conferring drug resistance, suggesting that they were derived from long-lived cells that were infected before the initiation of therapy. Similar findings were also reported in the same issue of Science by Wong et al. and in PNAS by Chun et al.

Today, we know that the latent reservoir of HIV-1 is a formidable obstacle to eradication of the virus. We know that the reservoir is maintained at least in part by clonal expansion of CD4+ T cells containing integrated provirus and that it is hard to measure, as the vast majority of integrated proviruses are defective. The reservoir also extends beyond quiescent cells in the blood and lymph nodes, to sanctuary sites such as the brain that are poorly penetrable to antiretroviral drugs. Nevertheless, achieving at least a functional cure of HIV infection remains a hotly pursued goal and there is intense interest in learning more about the HIV reservoir and how to prevent the virus from rebounding so that medication might be stopped.

Identification of host-encoded HIV restriction factors

Numerous positively acting cellular factors and pathways support HIV replication, but in the 1990s evidence emerged that suggested that cells express dominantly acting factors that suppress HIV replication. For instance, HIV replication is affected by the animal origin of target cells and requirements for the viral accessory genes vif and vpu vary significantly between human cell lines. These observations hinted that primate cells express antiviral proteins (termed restriction factors) that block infection. The existence of restriction factors has important implications for understanding viral replication and pathogenesis, host range and HIV evolution, and for developing animal models.

In 2002, Sheehy et al. reported the isolation of a human gene that inhibits HIV-1 infection and is suppressed by the viral Vif protein—the first identification of an HIV restriction factor. In this initial report, a subtracted cDNA screen using cells that were permissive and non-permissive to infection by vif-deficient HIV-1 identified the human APOBEC3G gene as responsible for suppression of vif-deficient HIV-1 infection. Subsequent studies determined that APOBEC3G can restrict HIV-1 by incorporating itself into nascent virions through its RNA-binding activity and subsequently hypermutating newly synthesized viral DNA through its cytidine deaminase activity, leading to a catastrophically altered nucleotide sequence. It was also found that Vif–APOBEC3G binding leads to degradation of the restricting factor, enabling HIV-1 to circumvent this intrinsic immune response.

This initial study represents an important milestone in HIV/AIDS research, as it revealed an integral part of the host’s first line of defense against HIV and suggested that further HIV restriction factors might exist.

APOBEC3 proteins do not alone account for the observed infection restriction phenotype in non-permissive cells. Since the discovery of APOBEC3G, numerous restriction factors that target diverse components of HIV-1, HIV-2 and SIV during various stages of their life cycles have been identified. For instance, identification of the monkey TRIM5α and TRIMCyp proteins in 2004 that restrict HIV through interactions with the viral capsid revealed that this class of molecules is responsible for the majority of restriction phenomena in mammalian cells following virus entry. Later, the discovery of tetherin (also known as BST2), a transmembrane protein, revealed a crucial function of the lentiviral vpu gene. In the absence of vpu expression, tetherin physically tethers nascent virions to the surface of infected cells and the virions are subsequently internalized into endosomes, thus preventing onward transmission.

Similarly, an important function of the HIV-2 (and SIV) Vpx accessory protein was elucidated through the discovery of SAMHD1, a deoxynucleotide triphosphohydrolase that limits reverse transcription of incoming viral RNA genomes: the viral Vpx protein induces ubiquitin–proteasome-dependent degradation of SAMHD1.

More recently, further restriction factors with distinct or unknown mechanisms of antiviral activity have been implicated as having a role in the outcome of initial HIV infections (for example, Mx2, SERINC3 and SERINC5, and ZAP), highlighting the important and complex involvement of restriction factors in the life cycle of HIV and in the evolutionary battle between host and virus. Indeed, a major raison d’être for lentiviral accessory proteins is to remove or displace host antiviral proteins.

During this evolutionary battle, humans have not emerged as the victors, as HIV-related illnesses cause thousands of deaths each year. HIV has an extraordinary degree of genetic plasticity that has enabled the virus to adapt to new host proteins when crossing species barriers and during the evolutionary arms race. Despite HIV’s ability to evade host restriction factors, the discovery of these factors and understanding of how they interface with viral accessory proteins provide remarkable insight into the evolution of HIV. Their discovery also provides targets for new antivirals and valuable knowledge in the development of primate animal models for HIV/AIDS, which may lead to HIV losing the battle.