Recent advances in the understanding of HIV accessory protein function
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Introduction In addition to the structural proteins required for viral replication, HIV expresses several additional proteins. Although these proteins are not required for viral replication in tissue culture systems, they are important factors for the development of pathogenesis. These factors, known as accessory proteins, include the HIV proteins Nef, Vpr, Vpu, and Vif. The accessory proteins are a defining feature of lentiviruses, and are necessary for HIV to cause AIDS. Because the accessory proteins play an important role in causing the clinical manifestations of HIV disease, they have become potential targets for antiviral therapies, and many basic research studies have been designed to gain an understanding of their modes of action. The following discussion highlights a number of important insights that have emerged over the past year into how this important class of HIV protein functions. Nef Nef is a 27 kDa myristolated protein that is the first viral protein to accumulate to detectable levels in a cell following HIV infection [1]. It is clear that Nef is necessary for the maintenance of high viral loads and viral pathogenesis in HIV-infected individuals. This finding was originally demonstrated in experiments using the SIV model for HIV infection, whereby macaques infected with Nef mutant virus did not develop normal disease [2]. Instead, only low-level viral replication was detected in these animals, and the normal course of disease progression was severely delayed. However, if viral revertants emerged that restored normal Nef function, viral loads increased and the test animals progressed to AIDS in a normal fashion. These observations correlated with the findings of a study focused on an Australian cohort of patients infected with a Nef-defective derivative of HIV [3]. As observed in the SIV system, individuals exhibited only low-level HIV replication and significantly delayed progression to AIDS, thus confirming the role of Nef in HIV pathogenesis and the relevance of the SIV model to the study of HIV infection. Nef is clearly the most versatile of the accessory proteins, as it has been shown to have multiple activities that may influence HIV replication after infection. These activities can be divided into three separate classes that appear to be genetically distinct because they can be divided by their sensitivity to certain mutations in the Nef protein. For the purposes of this discussion, these three classes will be defined as Nef-mediated cell activation, altered intracellular trafficking of normal cell surface proteins, and the stimulation of the infectivity of virions produced after infection. Several lines of research suggest that Nef contributes to viral-mediated cellular activation by altering T-cell signal transduction and activation. Previous studies utilizing Nef transgenic mice revealed that Nef expression led to elevated T-cell signaling [4]. Additionally, recent reports have shown that Nef interacts with lipid rafts, which play an important role in T-cell signaling, as interaction with the rafts is required for Nef to prime T cells for activation [5]. Nef-mediated activation is thought to occur through the mimicking of signaling through the T-cell receptor. This concept was best demonstrated in a study by McMichael et al. utilizing a Jurkat cell line in which Nef expression could be induced by the addition of tetracycline [6]. Through the use of gene chip analysis, these studies revealed that gene expression stimulated by Nef expression was 97% identical to that observed after stimulation of the T-cell receptor. Use of mutant cell lines revealed that the tyrosine kinase Zap70 and the zeta chain of the T-cell receptor are required for Nef-mediated stimulation of the entire set of the normally induced genes. These studies support the model that Nef expression activates the infected T cell to provide an optimal environment for viral replication. One of the genes stimulated by Nef is FasL, the ligand for the death receptor Fas [7]. It is believed that the presence of FasL may lead to the death of reactive CD8 cells in vivo. One argument against this model is that the expression of FasL should lead to the death of cells expressing Nef through intercellular binding to Fas. However, recent results by Greene et al. have found that the virus can evade this potential dilemma through Nef interaction with the apoptosis stimulating kinase [8]. This interaction blocks apoptosis stimulating kinase function, leading to a block in the induction of apoptosis by signaling through Fas. Nef is believed to activate T cells by direct association with a factor designated the Nef-associated kinase (NAK) [9]. Two recent publications substantiate the model that NAK is the p21-associated kinase 2 (PAK2) [10,11]. These studies have found that, although only a fraction of the PAK2 present in cells is associated with Nef, most of the PAK2 kinase activity is associated with Nef. This finding suggests that binding of NAK to the SH3 domain of Nef leads to activation of its kinase activity, which could potentially activate a program of gene expression similar to that observed after T-cell activation [12]. Nef may also contribute to HIV pathogenicity by altering intracellular trafficking of normal cell surface proteins. One of the earliest observations related to the action of Nef is the downregulation of cell surface expression of CD4, the receptor for HIV [13]. Subsequent studies revealed that Nef acts post-translationally by increasing the rate of CD4 endocytosis and lysomal degradation [14]. The cytoplasmic tail of CD4, particularly a di-leucine repeat sequence contained in its membrane proximal region, is key for the effective CD4 response to Nef [14]. CD4 downregulation appears to be advantageous to viral production because an excess of CD4 on the cell surface has been found to inhibit env incorporation and virion budding [15,16]. Demonstration of direct binding of Nef to the cytoplasmic tail of CD4 has been difficult, suggesting that the interaction is relatively weak. Using nuclear magnetic resonance methods, studies of the interaction between CD4 and Nef suggest that the interaction is indeed weak, with a dissociation constant of slightly less than 1 μM. Subsequently, it was shown that Nef can also downregulate class I MHC for the purpose of immune evasion [17,18]. Studies in the past year add the co-stimulatory protein CD28 to the list of proteins whose cell surface expression are downregulated by the expression of Nef [5]. Interestingly, the downregulation of all three proteins can be genetically separated. This observation suggests that the ability of Nef to downregulate these proteins was independently selected. The mechanism of downregulation of MHC class I is distinct from that of CD4 and CD28 [5,19,20]. While CD4 and CD28 are downregulated through clatherin-mediated endocytosis, MHC class I downregulation is mediated by a distinct mechanism. Consistent with this model, Garcia et al. found that ikarugamycin, a naturally occurring anti-protozoan inhibitor of clatherin-mediated endocytosis, blocks Nef-mediated downregulation of CD4 cell surface expression [21]. After endocytosis, Nef redirects CD4 to late endosomes, and subsequently lysosomes, where it is degraded. Normally, CD4 is recycled to the cell surface. This alteration of the normal trafficking of CD4 is believed to be mediated by a di-acidic motif present in Nef. One group believes that this domain specifically interacts with the adaptor protein B-cop, which mediates delivery to late endosomes [22]. In contrast, another group finds that the di-acidic domain is not important for B-cop binding or CD4 downregulation [23]. Recent studies by Carl et al. have revealed that Nef-mediated downregulation of CD4 is important for normal viral replication and pathogenesis in the SIV model [24]. Macaques were infected with a virus expressing three point mutations in Nef, which had previously been shown to be involved in CD4 downregulation. Initially, only low-level viral replication was detected. Subsequent increases in viral replication coincided with reversion of the mutations or the appearance of second-site mutations. The mutated derivative of SIV Nef, which contained two point mutations, led to defects in both CD4 downregulation and the enhancement of infectivity. The accessory protein Nef can also work to stimulate the infectivity of HIV virions [25]. Although part of this effect is a consequence of CD4 downregulation, even in the absence of CD4, HIV-1 particles produced in the presence of Nef are significantly more infectious than virions produced in the absence of Nef. Nef is packaged into virions, where it can be cleaved by the viral protease during virion maturation [26]. Nef is also present in isolated virion cores [27]. Virions produced in the absence of Nef are less efficient for proviral DNA synthesis, although Nef does not appear to influence directly the process of reverse transcription [28]. This finding has led to a model suggesting that Nef plays a role in the uncoating of the viral core after fusion. An alternative model, suggesting that Nef is important for efficient viral entry, has been gaining support. This new model is supported by the observation that the importance of Nef for infectivity is dependent on the mechanism of virion entry. When entry is mediated by the vesticular stomatitis virus, which enters via receptor-mediated endocytosis, the presence of Nef does not significantly stimulate infectivity [29]. Consistent with the path of entry being important, it was recently shown that virions pseudotyped with the ebola virus envelope, which also enters via receptor-mediated endocytosis and acidification, are also not sensitive to the presence of Nef [30]. These studies suggest that Nef may only be important for the HIV entry pathway. This possibility is supported by a report that finds virion fusion is more efficient in the presence of Nef [31]. Finally, Zhou and Aiken have found new evidence that Nef is influencing virion entry. Their group has developed a system in which only virions that fuse together are infectious [32]. With this system, they have demonstrated that Nef can influence infectivity after introduction in trans, being introduced by intravirion fusion. Interestingly, Nef only complements when produced in the envelope protein containing component. This suggests that Nef has a role at the virion envelope. Although these new results argue that Nef plays a role in stimulating the efficiency of viral entry, more studies are required to determine the exact role that Nef plays in stimulating the infectivity of HIV. Vpr The Vpr protein, like the Nef protein, is present in viral particles. Although it has been suggested that Vpr can be incorporated into assembling virions in amounts equimolar with Gag (1500-2000 copies) [33], experimental studies suggest that much smaller amounts are present in particles, in the range of 100-200 copies [34,35]. A minor portion of the virion-associated Vpr is phosphorylated. Incorporation of Vpr into virions is mediated through specific interactions with the carboxyl terminal region of p55 Gag [36], which corresponds to p6 in the proteolytically processed protein. Interestingly, after maturation, Vpr is associated with the virion core, while the p6 protein is not [37]. Vpr also appears to be an HIV protein that provides multiple functions for HIV replication. These multiple activities include induction of cell-cycle arrest, transactivation of viral and cellular gene expression, participation in the nuclear import of the HIV genome, modulation of viral replication kinetics, and induction of apoptosis [38]. Although it was initially believed that many of these functions were the consequence of a single activity of Vpr, it has recently been shown that the activities leading to cell-cycle arrest, induction of apoptosis, and the activation of at least some genes can be separated through the use of certain mutations and cellular contexts [39]. For instance, the ability of Vpr to act as a glucocorticoid receptor co-activator and the ability to mediate cell-cycle arrest can be separated by mutagenesis [40]. Recently, a new function for Vpr has been proposed. Studies of Jurkat cells constitutively expressing Vpr revealed that these cells could not be efficiently infected. Subsequent characterization found that expression of Vpr lead to a decrease in the cell-surface expression of CD4 [41]. Understanding the mechanism of this new action of Vpr requires further study. Vpr has been demonstrated to play a role in the ability of HIV to infect non-dividing cells by facilitating the nuclear localization of the pre-integration complex [42]. Consistent with this model, Vpr is present in the reverse transcription complex and the pre-integration complex [43]. This potential activity has lead to extensive study of the nuclear localization activity contained in the Vpr protein. Various studies, including two published in the past year, have suggested that Vpr contains two sequences that can mediate nuclear localization [44,45]. A lack of sequence similarity and functional differences in the action of the two putative nuclear localization signals (NLS) suggests that they function through distinct pathways or provide distinct functions. For instance, one of the sequences may act like a nucleocytoplasmic transport factor by directly tethering the viral genome to the nuclear pore rather than a traditional NLS. Consistent with this model, Vpr is found to be associated with the nuclear pore when expressed in cells and can be biochemically demonstrated to bind to components of the nuclear pore complex [46]. The complexity of the NLS in Vpr has made the understanding of their function a contentious issue. One explanation for the complexity was recently revealed by the identification of a nuclear export signal (NES) in Vpr [45,47]. A NES functions to export proteins from the nucleus. When a protein contains both a NLS and a NES, as in the case of Vpr, it continually shuttles between the nuclear and cytoplasmic compartment. The reason for Vpr shuttling has yet to be defined. An additional activity of Vpr is demonstrated by its ability to block cell division [48]. Vpr expression causes cells to accumulate in the G2 phase of the cell cycle [49]. The expression of Vpr has been shown to prevent the activation of the cyclin B/p34cdc2 complex, which is a regulator of the cell cycle important for entry into mitosis [50,51]. Consistent with this model, it was recently shown that several yeast cell-cycle proteins are required for Vpr-mediated cell-cycle arrest in fission yeast [52]. Vpu The Vpu polypeptide is an integral membrane phosphoprotein that is primarily localized in the internal membranes of the cell [53]. The two functions of Vpu, the downmodulation of CD4 expression and the enhancement of virion release, can be genetically separated [54]. In HIV-infected cells, the simultaneous expression of both the viral receptor, CD4, and the viral envelope protein in the endoplasmic reticulum leads to the formation of complexes that trap both proteins within this compartment. The formation of intracellular Env-CD4 complexes decreases the amount of envelope protein, interfering with virion assembly. Vpu liberates the viral envelope by facilitating the ubiquitin-mediated degradation of CD4 molecules complexed with Env [55]. Proteosome-mediated degradation of CD4 complexed with Vpu is mediated by interaction between Vpu and βTrCP, a component of the SCF ubiquitin ligase complex [56]. Interestingly, Vpu itself is not degraded. Recent studies find that Vpu can act as a general inhibitor of βTrCP function [57]. This is best demonstrated by the observation that Vpu blocks the normal degradation of IκBa, which is typically degraded by interaction with βTrCP, after stimuli that activate the transcription factor nuclear factor-κB (NF-κB). Interestingly, NF-κB is a key player in the activation of the HIV LTR promoter. Therefore, Vpu may be a modulator of NF-κB-regulated viral and cellular gene expression. Vpu also stimulates HIV replication by facilitating the release of HIV from the surface of an infected cell. In the absence of Vpu, large numbers of virions can be seen attached to the surface of infected cells [58]. The cell-bound virus is infectious, as recently demonstrated by Bour et al.[57]. Using a cell culture system that selects for cell-cell transmission, these experiments led to the selection of a spontaneous Vpu mutation. Vpu is believed to form a channel-like structure in the plasma membrane that plays a role in virion release. Recent structural studies support the idea that Vpu forms ion channels in lipid bilayers [59]. It was found that pores were formed in a manner consistent with the self-assembly of oligomers and the structural findings. A study by Strebel et al. found that cell surface CD4 can inhibit virion release by disrupting Vpu function [60]. It is believed that Vpu-CD4 interactions block formation of the oligomers necessary to form the pores required for Vpu to facilitate virion release. Together, these results obtained in the past year support a model where Vpu facilitates virion release through the formation of ion-conducting channels in the plasma membrane. Vif Vif is a 23 kD a polypeptide that is essential for the replication of HIV in peripheral blood lymphocytes, macrophages, and certain cell lines [61]. In most cell lines, Vif is not required. These cell lines are called permissive for Vif mutants of HIV. Virions generated in permissive cells can infect non-permissive cells but the virus subsequently produced is not infectious. Complementation studies reveal that it is possible to restore the infectivity of HIV Vif mutants by expression of Vif in producer cells but not in target cells [62]. These results show that Vif must be present during virion assembly. Vif can be found at low levels in virions of HIV [63]. However, the incorporation of Vif may be non-specific, as Vif is also incorporated into heterologous retroviruses such as murine leukemia viruses [64]. Vif co-localizes with Gag [65] and has recently been shown to be present at higher levels in virions that contain unprocessed Gag proteins or mutations in the capsid region of the Gag precursor [65,66]. These results suggest that Vif packaging is regulated and that virion-associated Vif might not have a specific function. There are two obvious models for the permissive phenotype of Vif: either the non-permissive cell line lacks a factor that has a Vif-like function, or non-permissive cells contain an antiviral factor that is blocked by Vif. These models were tested using heterokaryons generated by the fusion of permissive and non-permissive cells. Vif-negative virus produced from the heterokaryons was non-infectious, revealing that non-permissive cells contain a naturally occurring antiviral factor that is overcome by Vif [67,68]. Further substantiation for a model that Vif is counteracting an antiviral cellular factor came from the finding that Vif proteins from different lentiviruses are species specific [69]. For instance, HIV Vif can stimulate the infectivity of HIV-2 and SIV produced in human non-permissive cells, while SIV Vif protein does not function in human cells. Together, these observations suggest that cellular factors, rather than viral components, are the target of Vif action. Vif-defective HIV strains can enter cells but cannot efficiently synthesize the proviral DNA [62]. It is not clear whether Vif facilitates reverse transcription per se, viral uncoating, or the overall stability of the viral nucleoprotein complex. Studies by Madani and Kabat reveal that Vif works at a post-penetration stage of the viral life-cycle [70]. These investigators found that murine leukemia virus-pseudotyped HIV capsids required Vif for replication, while murine leukemia virus produced from the same cells was infectious. Other results recently obtained by Pomerantz et al. support a role for Vif in reverse transcription [71]. By exposing Vif-defective particles to high concentrations of nucleotides, it is possible to stimulate reverse transcription within virions and to partially restore infectivity. Vif mutant virions have improperly packed nucleoprotein cores, as revealed by electron microscopic analyses [72]. A recent study has also shown that Vif-defective virions have cores that are less stable than wild-type virions [73]. Together, these observations suggest a model where Vif facilitates a post-entry step that is essential for the completion of reverse transcription. A new model for Vif function has been suggested in three recent studies that found Vif is an RNA binding protein that binds to intracellular genomic HIV RNA [74-76]. A recent report found that FIV Vif localized primarily in the nucleus is in support of an association with virion RNA [77]. To make these observations compatible with the results suggesting a post-entry role, it has been proposed that Vif promotes efficient reverse transcription after virion entry by modulating nucleic acid components within the virion core. Conclusion The accessory proteins represent an unexploited target for therapies directed to treat HIV infection. Although they can be shown to be non-essential for viral replication in certain cell culture contexts, they are clearly required for the generation of high viral loads and pathogenesis. Future studies should be focused on further elucidation of their mechanisms of action and their association with elements contributing to HIV disease.Keywords:
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Newcastle disease virus (NDV) substantially impacts the poultry industry worldwide and causes viral encephalitis and neurological disorders leading to brain damage, paralysis, and death. The mechanism of interaction between this neurotropic virus and the avian central nervous system (CNS) is largely unknown. Here, we report that host protein CARD11 presented brain-specific upregulated expression that inhibited NDV replication, which was not due to CARD11-Bcl10-MALT1 (CBM) complex-triggered activation of its downstream signaling pathways. The inhibitory mechanism of viral replication is through the CARD11 CC1 domain, and the viral large polymerase protein (L) competitively interacts with the X domain of the viral phosphoprotein (P), which hampers the P-L interaction, suppressing the viral polymerase activity and viral replication. An in vivo study indicated that CARD11 alleviated neuropathological lesions and reduced viral replication in chicken brains. These results provide insight into the interaction between NDV infection and the host defense in the CNS and a potential antiviral target for viral neural diseases.
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ABSTRACT Although antiviral agents which block human immunodeficiency virus (HIV) replication can result in long-term suppression of viral loads to undetectable levels in plasma, long-term therapy fails to eradicate virus, which generally rebounds after a single treatment interruption. Multiple structured treatment interruptions (STIs) have been suggested as a possible strategy that may boost HIV-specific immune responses and control viral replication. We analyze viral dynamics during four consecutive STI cycles in 12 chronically infected patients with a history (>2 years) of viral suppression under highly active antiretroviral therapy. We fitted a simple model of viral rebound to the viral load data from each patient by using a novel statistical approach that allows us to overcome problems of estimating viral dynamics parameters when there are many viral load measurements below the limit of detection. There is an approximate halving of the average viral growth rate between the first and fourth STI cycles, yet the average time between treatment interruption and detection of viral loads in the plasma is approximately the same in the first and fourth interruptions. We hypothesize that reseeding of viral reservoirs during treatment interruptions can account for this discrepancy, although factors such as stochastic effects and the strength of HIV-specific immune responses may also affect the time to viral rebound. We also demonstrate spontaneous drops in viral load in later STIs, which reflect fluctuations in the rates of viral production and/or clearance that may be caused by a complex interaction between virus and target cells and/or immune responses.
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Efforts to treat HCV patients are focused on developing antiviral combinations that lead to the eradication of infection. Thus, it is important to identify optimal combinations from the various viral inhibitor classes. Based on viral dynamic models, HCV entry inhibitors are predicted to reduce viral load in a monophasic manner reflecting the slow death rate of infected hepatocytes (t1/2 = 2-70 days) and the protection of naïve, un-infected cells from HCV infection. In contrast, replication inhibitors are predicted to reduce viral load in a biphasic manner. The initial rapid reduction phase is due to the inhibition of virus production and elimination of plasma virus (t1/2∼3 hours). The second, slower reduction phase results from the elimination of infected hepatocytes. Here we sought to compare the ability of HCV entry and replication inhibitors as well as combinations thereof to reduce HCV infection in persistently-infected Huh7 cells. Treatment with 5 × EC50 of entry inhibitors anti-CD81 Ab or EI-1 resulted in modest (≤ 1 log10 RNA copies/ml), monophasic declines in viral levels during 3 weeks of treatment. In contrast, treatment with 5 × EC50 of the replication inhibitors BILN-2016 or BMS-790052 reduced extracellular virus levels more potently (~2 log10 RNA copies/ml) over time in a biphasic manner. However, this was followed by a slow rise to steady-state virus levels due to the emergence of resistance mutations. Combining an entry inhibitor with a replication inhibitor did not substantially enhance the rate of virus reduction. However, entry/replication inhibitor and replication/replication inhibitor combinations reduced viral levels further than monotherapies (up to 3 log10 RNA copies/ml) and prolonged this reduction relative to monotherapies. Our results demonstrated that HCV entry inhibitors combined with replication inhibitors can prolong antiviral suppression, likely due to the delay of viral resistance emergence.
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HIV entry inhibitors such as maraviroc (MVC) prevent cell-free viruses from 24 entering the cells. In clinical trials, patients who were treated with MVC often 25 displayed viral loads that were above the limit of conventional viral load detection 26 in comparison with efavirenz-based regminens. We hypothesize that viruses 27 blocked by entry inhibitors may be redistri buted to plasma where they artificially 28 increase viral load measurements in contrast to use of antiretroviral drugs 29 (ARVs) that act intracellularly. 30 We infected PM-1 cells with CCR5-tropi c HIV-1 BaL or CXCR 4-tropic HIV-1 NL4-31 3 in the presence of inhibitory concentrati ons of efavirenz, ralt egravir, enfuvirtide, 32 maraviroc, and AMD3100, the latter three being entry inhibitors. Supernatant 33 viral load, reverse transcriptase enzyme activity, and intracellular nucleic acid 34 levels were measured at times up to 24 hr post-infection. Infectivity of 35 redistributed dual-tropic HIV-1 wa s assessed using TZM-bl cells. 36 Extracellular viral load analysis revealed that entry inhibitor-treated cells had 37 higher levels of virus in supernatant versus the other ARVs at 8 hours post-38 infection. By 24 hours, supernatant viral load was still higher for entry inhibitors
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Members of the genus Enterovirus have a significant effect on human health, especially in infants and children. Since the viral genome has limited coding capacity, Enteroviruses subvert a range of cellular processes for viral infection via the interaction of viral proteins and numerous cellular factors. Intriguingly, the capsid–receptor interaction plays a crucial role in viral entry and has significant implications in viral pathogenesis. Moreover, interactions between structural proteins and host factors occur directly or indirectly in multiple steps of viral replication. In this review, we focus on the current understanding of the multifunctionality of structural proteins in the viral life cycle, which may constitute valuable targets for antiviral and therapeutic interventions.
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Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is causing the current pandemic of coronavirus disease 2019 (COVID-19) that has killed nearly one million people so far. While this is a respiratory virus, surprisingly, it has been recognized that patients with cardiovascular disease are likely to be affected severely and die of COVID-19. This phenomenon cannot be explained by the generally accepted logic that the SARS-CoV-2 infection/replication is the sole determinant of the actions of the virus to define the fate of host cells. I herein propose the viral protein fragment theory of COVID-19 pathogenesis based on my observations in cultured human vascular cells that SARS-CoV-2 spike protein can activate cell signaling events without the rest of the viral components. It is generally thought that SARS-CoV-2 and other single-stranded RNA viruses attach to the host cells through the interactions between surface proteins of the viral capsid and the host cell receptors; the fusion and the entry of the viral components, resulting in the replication of the viruses; and the host cell responses are the consequence of these events. I hypothesize that, as humans are infected with SARS-CoV-2, the virus releases (a) fragment(s) of the spike protein that can target host cells for eliciting cell signaling without the rest of the viral components. Thus, COVID-19 patients are subjected to the intact virus infecting the host cells for the replication and amplification as well as the spike protein fragments that are capable of affecting the host cells. I propose that cell signaling elicited by the spike protein fragments that occur in cardiovascular cells would predispose infected individuals to develop complications that are seen in severe and fatal COVID-19 conditions. If this hypothesis is correct, then the strategies to treat COVID-19 should include, in addition to agents that inhibit the viral replication, therapeutics that inhibit the viral protein fragment-mediated cardiovascular cell signaling.
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In a typical HIV-1-infected patient, plasma viral load (pVL) increases steeply in the first week after acute infection, then decreases as the immune system becomes activated, resulting in antibody seroconversion 3–13 days after infection and a full western blot pattern approximately 3 months later [1–3]. The so-called viral set point or steady-state viral load is reached after approximately 40–276 days from the acute infection moment [1]. Especially in the first few weeks of infection, differences are obvious in patients, especially with regard to time to peak load and time to viral load drop from peak to nadir [1], but also in the absolute viral RNA count. The viral set point is thought to represent a trade-off between viral replication capacity and repression of the virus by the host immune system. HIV-1 RNA levels vary considerably between individuals and also throughout the infection course in a particular individual. The viral load at set point is an important parameter, as it is strongly predictive of clinical progression [4,5]. Both the innate replicating capacity (fitness) of the virus strain and the strength of the host immune system would intuitively be the most obvious contributors, but it has been suggested that age, sex, shared human leukocyte antigen (HLA) alleles and duration of infection also contribute to the phenomenon [6]. The involvement of virus characteristics could easily be measured by analyzing the HIV replication capacity in donor–recipient pairs, wherein the viral load should be similar if viral replication fitness is the main determinant of pVL. A cohort of transmission pairs, necessary to study comparative HIV-1 viral load dynamics, is not easy to establish. Viral relationships indicative of transmission should first be determined by phylogenetic analysis. Then, an acute phase plasma sample (to minimize the effect of immune pressure) of the recipient and a matching sample from the donor should be available. Hecht et al. [7] have analyzed early plasma samples from 24 such transmission pairs, all comprising men having sex with men (MSM), and reported a significant correlation between the HIV-1 RNA levels within the transmission pairs. However, they cautioned that these results should be reproduced in other cohorts to validate the finding. We here report a similar analysis in early samples from 56 sequence-verified HIV-1 transmission pairs, 60% MSM and 40% heterosexual, from The Netherlands. Recipients were sampled during primary infection, 20 recipients were in Fiebig et al. [8] stages III–IV (viral RNA+/− or indeterminate western blot) and 36 recipients were in Fiebig et al. [8] stages V (viral RNA+/western blot p31−) and VI (viral RNA+/western blot fully developed). HIV-1 blood pVL measurements were done using the Versant HIV-1 RNA 3.0 assay (Bayer Diagnostics Division, Tarrytown, New York, USA), NucliSens HIV-1 RNA (bioMérieux, Boxtel, The Netherlands) or m2000rt (Abbott Molecular Inc., Des Plaines, Illinois, USA). Viral loads of all couples were measured using the same assay. Samples from donors matched the time point when recipient samples were taken. Linear regression analysis was done with GraphPad Prism, version 5.01 (GraphPad Software, San Diego, California, USA) and correlation coefficients were calculated. In contrast to Hecht et al. [7], we do not find a strong correlation between plasma viral RNA levels within the pairs (Fig. 1). The Pearson coefficient of correlation (r) in our cohort was 0.25 for all 56 transmission pairs, 0.29 (range −0.17 to 0.65) for pairs when the recipients were in Fiebig et al. [8] stages III–IV and 0.06 (range −0.27 to 0.39) for pairs when the recipients were in Fiebig et al. [8] stages V–VI, suggesting that the correlation is completely lost when the infection progresses. The correlation coefficient (r) between viral RNA levels in donors and recipients was 0.55 in the 24 pairs studied by Hecht et al. [7], which were in similar early stages of HIV infection. A correlation coefficient (r) above 0.8 is usually denoted as strong and below 0.5 as weak, whereas r is equal to 1 represents a perfect correlation. So, in our transmission pairs, we detect only a weak correlation between viral RNA levels in acutely infected recipients and donors. Similar results were obtained for a transmission cohort [6] in Zambia where the viral RNA levels between 115 donor and seroconverting recipient pairs had a correlation coefficient (r) of 0.21 (P = 0.03). In this study, factors such as sex, age, HLA markers and duration of infection were also shown to contribute.Fig. 1: Relationship of HIV-1 RNA levels in 56 transmission pairs. Viral RNA levels in blood plasma from source individuals were correlated with viral RNA levels in recipients in the acute or early stages of infection. Correlations are shown for all 56 transmission pairs or for sources and recipients when the latter are separated according to the primary infection stage criteria of Fiebig et al. [8].The low correlation between pVL in donors and recipients suggests that viral traits do contribute to pVL early in infection, but that other factors are equally or more important.
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Viruses are obligatory parasites of their host cells. During the viral replication cycle, many host-cell components are co-opted or hijacked to favor viral replication and virus production. The interactions between the virus and the host induce multiple cellular responses that modify normal cell function and can lead either to clearing of the virus or to enhanced virus replication and pathogenesis. Most, if not all steps of the viral replication cycle are characterized by critical virus–host interactions. These begin with the earliest steps of the replication cycle (e.g. viral entry into the target cell) to later steps including mRNA synthesis, genome replication, and protein and viral genome trafficking within the nucleus and cytoplasm to sites of viral assembly. The understanding of the molecular details of these virus-cell interactions will ultimately help in the design of new and specific therapies against viral infection. The purpose of this review is to highlight various virus–cell interactions. Because of the extensive nature of this topic, we will be presenting general themes and specific examples of virus–cell interactions where appropriate rather than a comprehensive catalog of every virus–cell interaction that has been uncovered.
Viral Pathogenesis
Viral life cycle
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