Intracellular pathogen detection by RIG-I-like receptors

The RIG-I-like receptors (RLRs) RIG-I, MDA5, and LGP2 trigger innate immune responses against viral infections that serve to limit virus replication and to stimulate adaptive immunity. RLRs are cytosolic sensors for virus-derived RNA and thus responsible for intracellular immune surveillance against infection. RLR signaling requires the adapter protein MAVS to induce type I interferon, interferon-stimulated genes, and proinflammatory cytokines. This review focuses on the molecular and cell biological requirements for RLR signal transduction.

Modifications in the human T cell proteome induced by intracellular HIV-1 Tat protein expression

The effects of the human immunodeficiency virus type 1 (HIV-1) Tat protein on cellular gene expression were analysed using a Jurkat cell line that was stably transfected with tat gene in a doxycycline-repressible expression system. Expressed Tat protein (aa 1-101) was proved to present basically a nuclear localisation, and to be fully functional to induce HIV LTR transactivation. Tat expression also resulted in protection from Tunicamycin-induced apoptosis as determined by DNA staining and TUNEL assays. We applied proteomics methods to investigate changes in differential protein expression in the transfected Jurkat-Tat cells. Protein identification was performed using 2-D DIGE followed by MS analysis. We identified the down-regulation of several cytoskeletal proteins such as actin, beta-tubulin, annexin II, as well as gelsolin, cofilin and the Rac/Rho-GDI complex. Down-expression of these proteins could be involved in the survival of long-term reservoirs of HIV-infected CD4+ T cells responsible for continuous viral production. In conclusion, in addition to its role in viral mRNA elongation, the proteomic approach has provided insight into the way that Tat modifies host cell gene expression.

Critical negative regulation of type 1 T cell immunity and immunopathology by signaling adaptor DAP12 during intracellular infection

Transmembrane signaling adaptor DAP12 has increasingly been recognized for its important role in innate responses. However, its role in the regulation of antimicrobial T cell responses has remained unknown. In our current study, we have examined host defense, T cell responses, and tissue immunopathology in models of intracellular infection established in wild-type and DAP12-deficient mice. During mycobacterial infection, lack of DAP12 leads to pronounced proinflammatory and Th1 cytokine responses, overactivation of Ag-specific CD4 and CD8 T cells of type 1 phenotype, and heightened immunopathology both in the lung and lymphoid organs. DAP12-deficient airway APC display enhanced NF-kappaB activation and cytokine responses upon TLR stimulation or mycobacterial infection in vitro. Of importance, adoptive transfer of Ag-loaded DAP12-deficient APC alone could lead to overactivation of transferred transgenic or endogenous wild-type T cells in vivo. We have further found that the immune regulatory role by DAP12 is not restricted only to intracellular bacterial infection, since lack of this molecule also leads to uncontrolled type 1 T cell activation and severe immunopathology and tissue injury during intracellular viral infection. Our study thus identifies DAP12 as an important novel immune regulatory molecule that acts, via APC, to control the level of antimicrobial type 1 T cell activation and immunopathology.

Adenovirus type 5 exerts multiple effects on the expression and activity of cytosolic phospholipase A2, cyclooxygenase-2, and prostaglandin synthesis

In this study, we examine how infection of murine and human fibroblasts by adenovirus (Ad) serotype 5 (Ad5) affects the expression and activity of cytosolic phospholipase A2 (cPLA2), cyclooxygenase-2 (COX-2), and production of PGs. Our experiments showed that infection with Ad5 is accompanied by the rapid activation of cPLA2 and the cPLA2-dependent release of [3H]arachidonic acid ([3H]AA). Increased expression of COX-2 was also observed after Ad infection, as was production of PGE2 and PGI2. Later, however, as the infection progressed, release of [3H]AA and production of PGs stopped. Late-stage Ad5-infected cells also did not release [3H]AA or PGs following treatment with a panel of biologically diverse agents. Experiments with UV-inactivated virus confirmed that Ad infection is accompanied by the activation of a host-dependent response that is later inhibited by the virus. Investigations of the mechanism of suppression of the PG pathway by Ad5 did not reveal major effects on the expression or activity of cPLA2 or COX-2. We did note a change in the intracellular position of cPLA2 and found that cPLA2 did not translocate normally in infected cells, raising the possibility that Ad5 interferes with the PG pathway by interfering with the intracellular movement of cPLA2. Taken together, these data reveal dynamic interactions between Ad5 and the lipid mediator pathways of the host and highlight a novel mechanism by which Ad5 evades the host immune response. In addition, our results offer insight into the inflammatory response induced by many Ad vectors lacking early region gene products.

The CD16+Monocyte Subset Is More Permissive to Infection and Preferentially Harbors HIV-1 In Vivo

HIV-1 persists in peripheral blood monocytes in individuals receiving highly active antiretroviral therapy (HAART) with viral suppression, despite these cells being poorly susceptible to infection in vitro. Because very few monocytes harbor HIV-1 in vivo, we considered whether a subset of monocytes might be more permissive to infection. We show that a minor CD16+ monocyte subset preferentially harbors HIV-1 in infected individuals on HAART when compared with the majority of monocytes (CD14highCD16-). We confirmed this by in vitro experiments showing that CD16+ monocytes were more susceptible to CCR5-using strains of HIV-1, a finding that is associated with higher CCR5 expression on these cells. CD16+ monocytes were also more permissive to infection with a vesicular stomatitis virus G protein-pseudotyped reporter strain of HIV-1 than the majority of monocytes, suggesting that they are better able to support HIV-1 replication after entry. Consistent with this observation, high molecular mass complexes of apolipoprotein B mRNA-editing enzyme, catalytic polypeptide-like 3G (APOBEC3G) were observed in CD16+ monocytes that were similar to those observed in highly permissive T cells. In contrast, CD14highCD16- monocytes contained low molecular mass active APOBEC3G, suggesting this is a mechanism of resistance to HIV-1 infection in these cells. Collectively, these data show that CD16+ monocytes are preferentially susceptible to HIV-1 entry, more permissive for replication, and constitute a continuing source of viral persistence during HAART.

Principles of intracellular viral recognition

In mammalian cells, the products of microbial infection are recognized by pathogen recognition receptor (PRR) proteins. Virus recognition is mediated in part by PRRs that comprise a subset of Toll-like receptors or a family of RNA helicases, the latter of which contain caspase activation and recruitment domains, both of which induce interferons alpha and beta and antiviral immune defenses. Recent studies show that PRR engagement of specific pathogen-associated molecular patterns (PAMPs) within viral products, including viral proteins and nucleic acid, is facilitated by the discrete subcellular distribution of PRRs to sites that intersect with processes of virus entry and replication. PAMP structure and the subcellular context of PRR distribution form a basis of self versus nonself discrimination during the antiviral response. Understanding the virus/host interface of PRR function and PAMP recognition will advance therapeutic strategies for immune response regulation.

Screening and identification of severe acute respiratory syndrome-associated coronavirus-specific CTL epitopes

Severe acute respiratory syndrome (SARS) is a highly contagious and life-threatening disease that emerged in China in November 2002. A novel SARS-associated coronavirus was identified as its principal etiologic agent; however, the immunopathogenesis of SARS and the role of special CTLs in virus clearance are still largely uncharacterized. In this study, potential HLA-A*0201-restricted spike (S) and nucleocapsid protein-derived peptides were selected from an online database and screened for potential CTL epitopes by in vitro refolding and T2 cell-stabilization assays. The antigenicity of nine peptides which could refold with HLA-A*0201 molecules was assessed with an IFN-gamma ELISPOT assay to determine the capacity to stimulate CTLs from PBMCs of HLA-A2(+) SARS-recovered donors. A novel HLA-A*0201-restricted decameric epitope P15 (S411-420, KLPDDFMGCV) derived from the S protein was identified and found to localize within the angiotensin-converting enzyme 2 receptor-binding region of the S1 domain. P15 could significantly enhance the expression of HLA-A*0201 molecules on the T2 cell surface, stimulate IFN-gamma-producing CTLs from the PBMCs of former SARS patients, and induce specific CTLs from P15-immunized HLA-A2.1 transgenic mice in vivo. Furthermore, significant P15-specific CTLs were induced from HLA-A2.1-transgenic mice immunized by a DNA vaccine encoding the S protein; suggesting that P15 was a naturally processed epitope. Thus, P15 may be a novel SARS-associated coronavirus-specific CTL epitope and a potential target for characterization of virus control mechanisms and evaluation of candidate SARS vaccines.

Maintenance of HIV-specific CD4+ T cell help distinguishes HIV-2 from HIV-1 infection

Unlike HIV-1-infected people, most HIV-2-infected subjects maintain a healthy CD4+ T cell count and a strong HIV-specific CD4+ T cell response. To define the cellular immunological correlates of good prognosis in HIV-2 infection, we conducted a cross-sectional study of HIV Gag-specific T cell function in HIV-1- and HIV-2-infected Gambians. Using cytokine flow cytometry and lymphoproliferation assays, we show that HIV-specific CD4+ T cells from HIV-2-infected individuals maintained proliferative capacity, were not terminally differentiated (CD57-), and more frequently produced IFN-gamma or IL-2 than CD4+ T cells from HIV-1-infected donors. Polyfunctional (IFN-gamma+/IL-2+) HIV-specific CD4+ T cells were found exclusively in HIV-2+ donors. The disparity in CD4+ T cell responses between asymptomatic HIV-1- and HIV-2-infected subjects was not associated with differences in the proliferative capacity of HIV-specific CD8+ T cells. This study demonstrates that HIV-2-infected donors have a well-preserved and functionally heterogeneous HIV-specific memory CD4+ T cell response that is associated with delayed disease progression in the majority of infected people.

NS1 interaction with CKII alpha: novel protein complex mediating parvovirus-induced cytotoxicity

During a productive infection, the prototype strain of the parvovirus minute virus of mice (MVMp) induces dramatic morphological alterations in permissive A9 fibroblasts, culminating in cell lysis at the end of infection. These cytopathic effects (CPE) result from rearrangements and destruction of the cytoskeletal micro- and intermediate filaments, while other structures such as the nuclear lamina and particularly the microtubule network remain protected throughout the infection (J. P. F. Nüesch et al., Virology 331:159-174, 2005). In order to unravel the mechanism(s) by which parvoviruses trigger CPE, we searched for NS1 interaction partners by differential affinity chromatography, using distinct NS1 mutants debilitated specifically for this function. Thereby, we isolated an NS1 partner polypeptide, whose interaction with NS1 correlated with the competence of the viral product for CPE induction, and further identified it by tandem mass spectrometry and Western blotting analyses to consist of the catalytic subunit of casein kinase II, CKIIalpha. This interaction of NS1 with CKIIalpha suggested interference by the viral protein with intracellular signaling. Using permanent cell lines expressing dominant-negative CKIIalpha mutants, we were able to show that this kinase activity was indeed specifically involved in parvoviral CPE and progeny particle release. Furthermore, the NS1/CKIIalpha complex proved to be able to specifically phosphorylate viral capsids, indicating a mediator function of NS1 for CKII activity and specificity, at least in vitro. Altogether our data suggest that parvovirus-induced CPE is mediated by NS1 interference with intracellular CKII signaling.

Different domains of CD81 mediate distinct stages of hepatitis C virus pseudoparticle entry

The CD81 tetraspanin was first identified as a hepatitis C virus (HCV) receptor by its ability to bind the soluble ectodomain of envelope glycoprotein E2 (sE2). More recently, it has been suggested that CD81 is necessary but not sufficient for HCV entry into target cells. Here we present further evidence that putative human hepatocyte-specific factors act in concert with CD81 to mediate sE2 binding and HCV pseudoparticle (HCVpp) entry. Moreover, we show that CD81-mediated HCVpp entry entails E2 binding to residues in the large extracellular loop as well as molecular events mediated by the transmembrane and intracellular domains of CD81. The concept that CD81 receptor function progresses in stages is further supported by our finding that anti-CD81 monoclonal antibodies inhibit HCVpp entry by different mechanisms. The half-life of CD81-HCVpp binding was determined to be approximately 17 min, and we propose that binding is followed by CD81 oligomerization, partitioning into cholesterol-rich membrane domains, or other, lateral protein-protein interactions. This results in the formation of a receptor-virus complex that undergoes endocytosis and pH-dependent membrane fusion.

IL-12-independent Th1 polarization in human mononuclear cells infected with varicella-zoster virus

T helper type 1 (Th1) cells perform a critical role in fighting intracellular organisms, and interleukin-12 (IL-12) is known to promote a Thl response. This study was conducted to identify whether an IL-12-independent Th1 reaction is induced by the varicella-zoster virus (VZV) in human beings. It was found that different intracellular microorganisms could induce IFNgamma but not IL-12 production. Induction of IFNgamma production by VZV was associated with IFNalpha production and phosphorylation of both the signal transducer and activator of transcription-1 (STAT-1) and STAT-4 in lymphocytes. In contrast, Bacillus Calmette-Guerin (BCG) induced IL-12 production in association with STAT-4 but not STAT-1 activation. Anti-IFNalpha but not anti-IL-12 antibodies blocked the VZV-induced Th1 polarization. A patient with an IL-12 receptor beta1 chain deficiency showed a normal VZV- but not a normal BCG-induced Th1 reaction, further supporting the concept of an IFNalpha-mediated, IL-12-independent Th1 reaction in response to certain intracellular infections. Identification of the early Th1 polarization induced by IFNalpha versus IL-12 in response to specific viruses may enable the development of better therapeutic strategies tailored to different infections.

Innate Sensors of Microbial Infection

The innate immune system utilizes multiple families of pattern-recognition receptors (PRRs) to protect the host from infection. Each of these families contributes certain elements to the complement of innate effector functions that is elicited during an infection. Here we review the families of PRRs and explore examples of their cooperativity.

Protein kinase R mediates intestinal epithelial gene remodeling in response to double-stranded RNA and live rotavirus

As sentinels of host defense, intestinal epithelial cells respond to the viral pathogen rotavirus by activating a gene expression that promotes immune cell recruitment and activation. We hypothesized that epithelial sensing of rotavirus might target dsRNA, which can be detected by TLR3 or protein kinase R (PKR). Accordingly, we observed that synthetic dsRNA, polyinosinic acid:cytidylic acid (poly(I:C)), potently induced gene remodeling in model intestinal epithelia with the specific pattern of expressed genes, including both classic proinflammatory genes (e.g., IL-8), as well as genes that are classically activated in virus-infected cells (e.g., IFN-responsive genes). Poly(I:C)-induced IL-8 was concentration dependent (2-100 mug/ml) and displayed slower kinetics compared with IL-8 induced by bacterial flagellin (ET(50) approximately 24 vs 8 h poly(I:C) vs flagellin, respectively). Although model epithelia expressed detectable TLR3 mRNA, neither TLR3-neutralizing Abs nor chloroquine, which blocks activation of intracellular TLR3, attenuated epithelial responses to poly(I:C). Conversely, poly(I:C)-induced phosphorylation of PKR and inhibitors of PKR, 2-aminopurine and adenine, ablated poly(I:C)-induced gene expression but had no effect on gene expression induced by flagellin, thus suggesting that intestinal epithelial cell detection of dsRNA relies on PKR. Consistent with poly(I:C) detection by an intracellular molecule such as PKR, we observed that both uptake of and responses to poly(I:C) were polarized to the basolateral side. Lastly, we observed that the pattern of pharmacologic inhibition of responses to poly(I:C) was identical to that seen in response to infection by live rotavirus, indicating a potentially important role for PKR in activating intestinal epithelial gene expression in rotavirus infection.

Intraepithelial cell neutralization of HIV-1 replication by IgA

HIV is transmitted sexually through mucosal surfaces where IgA Abs are the first line of immune defense. In this study, we used paired IgA and IgG mAbs against HIV gp160 to study intraepithelial cell neutralization and inhibition of HIV replication. African green monkey kidney cells, Vero C1008, polarizable epithelial cells transfected to express the polymeric Ig receptor (pIgR), were transfected with HIV proviral DNA, and intracellular neutralization mediated by the mAbs was assessed. D47A and D19A IgA, which neutralized HIV in a conventional assay, potently inhibited intracellular HIV replication as assessed by infecting HeLa-CD4-long terminal repeat/beta-galactosidase cells (human cervical carcinoma cell line) and CEMx174 cells (human T cell line) with apical supernatant, basolateral medium, and cell lysate from transfected cells. D47A also inhibited the production of virus as assessed by direct assay of p24. In contrast, D47 and D19 IgG, sharing the same V regions, but which were not transcytosed by the pIgR, did not inhibit intracellular HIV replication, nor did D47A and D19A IgA in pIgR- cells, incapable of transcytosing IgA. Confocal immunofluorescence microscopy showed prominent colocalization of HIV protein and D47A, in agreement with the intracellular neutralization data. D10A, which did not neutralize HIV in the conventional assay, and irrelevant IgA did not show intracellular neutralization or colocalization. Control studies with two kinds of conditioned medium confirmed that HIV neutralization had indeed occurred inside the cells. Thus, during its transcytosis through epithelial cells, HIV-specific IgA can neutralize HIV replication.

Down-modulation of the CXCR4 co-receptor by intracellular expression of a single chain variable fragment (SFv) inhibits HIV-1 entry into primary human brain microvascular endothelial cells and post-mitotic neurons

Our laboratories previously demonstrated that expression of a single chain variable antibody fragment (SFv), anti-CXCR4 SFv, in human lymphoid cells suppresses surface display of the chemokine co-receptor CXCR4 and inhibits infectious entry of human immunodeficiency virus type I (HIV-1). We now sought to extend these results to two types of central nervous system (CNS) cells, primary isolated human brain microvascular endothelial cells (MVECs), and post-mitotic differentiated human neurons, both of which normally express significant levels of CXCR4. The anti-CXCR4 SFv expression construct was delivered using an HIV-1-based vector, and control cells received LacZ-expressing viral particles. Upon intracellular expression of the anti-CXCR4 SFv, immunostaining revealed a marked reduction in surface display of CXCR4 on both cell types. Consequently, post-mitotic neurons expressing the anti-CXCR4 SFv were significantly protected from HIV-1 infection, as measured by HIV-1 p24 antigen production, and partial protection was observed in human brain MVECs. The ability to selectively down-modulate the surface expression of CXCR4 in CNS cells may allow for the development of clinical molecular therapy strategies against HIV-1-related neurodegenerative disorders and neuroinvasion.

Blocking HIV-1 Vif Restores a Natural Mechanism of Intracellular Antiviral Defense

AIDS has become the greatest pandemic in the human history counting approximately 40 millions people worldwide. To purge HIV-1 infection, new therapeutic approaches need to be searched in alternative and/or in addition to the current pharmacological ones. Recently, several independent laboratories have unveiled a non-immune intracellular anti-HIV-1 defense strategy based on the cytidine deaminase APOBEC3G, which restricts HIV-1 production by directly mutating the proviral DNA in infected cells. To counteract this defense pathway, HIV-1 has developed an evasion strategy by acquiring the accessory protein Vif, which blocks the action of APOBEC3G by inducing its proteasome-mediated degradation.

Intracellular retention of the MHC class I-related chain B ligand of NKG2D by the human cytomegalovirus UL16 glycoprotein

Infection by human CMV induces expression of the cellular MHC class I-related chain A (MICA) and chain B (MICB) surface proteins, which function as ligands for the activating NKG2D receptor. Engagement of NKG2D triggers NK cells and costimulates Ag-specific effector CD8 alphabeta T cells. The potency of MHC class I-related chain-NKG2D in stimulating these anti-viral immune responses may be countered by a CMV-encoded transmembrane glycoprotein, UL16, which specifically binds MICB as well as two of the UL16-binding proteins that are ligands of NKG2D. However, the function and significance of these interactions are undefined. Using a stably transfected B cell line, we show that expression of UL16 results in loss of surface MICB. This effect is caused by the failure of newly synthesized MICB to mature and transit the secretory pathway due to physical association with UL16. The intracellular retention of these protein complexes is mediated by a tyrosine-based motif in the cytoplasmic tail sequence of UL16, which determines localization to or retrieval from the trans-Golgi network. Deletion of this motif restores surface expression of MICB, whereas UL16 may be redirected to endosomal compartments. Predictably, the retention of MICB abrogates the stimulatory function of NKG2D. These results suggest a potential mechanism of viral immune evasion. However, this activity remains to be confirmed with CMV-infected fibroblasts or endothelial cells, in particular because MICB is normally coexpressed with MICA, which is not retained by UL16.

Mechanism of the protective effect of heteropolyoxotungstate against herpes simplex virus type 2

The effects of heteropolyoxotungstate (K(7)[PTi(2)W(10)O(40)]. 6H(2)O; PM-19) on the replication of herpes simplex virus type 2 (HSV-2) were examined using a semiquantitative polymerase chain reaction of intracellular viral DNA established by us and also other methods. Vero cells were infected with HSV-2 strains: either the standard strain 169, or the acyclovir-resistant strain YS-4C-1. PM-19 was added at various stages during the replication of HSV-2. PM-19 strongly inhibited the synthesis of viral genomic DNA when it was added at the time of infection. The addition of PM-19 60-90 min after viral inoculation time-dependently decreased the antiviral activity and increased the relative yield of viral DNA, and the addition of PM-19 was completely ineffective at times later than 90 min. These results suggested that PM-19 inhibited viral penetration but did not affect the synthesis of viral DNA. Furthermore, PM-19 strongly inhibited a second round of infection.

HIV-1 protein Tat reduces the glutamate-induced intracellular Ca2+ increase in cultured cortical astrocytes

The trans-activator protein Tat of the human immunodeficiency virus type 1 (HIV-1) is regarded as an injurious molecule in the pathogenesis of HIV-1 associated encephalopathy (HIVE). We investigated the effects of Tat on neuroligand-induced intracellular Ca2+ increase in cultured astroglial cells. Rat cortical astrocytes, human glioblastoma cells and glial restricted precursor cells, from a human embryonic teratocarcinoma cell line, were incubated with recombinant Tat (100 ng/mL for 60 min) which induced a significant reduction of glutamate or ATP-induced intracellular Ca2+ increase ("glutamate response", "ATP response"). The reduction of the glutamate response was also observed following cell incubation with cell extracts of HeLa-T4+ cells transiently transfected with an expression plasmid coding for Tat. However, inactivation of the transcriptional trans-activity of Tat, by using a mutant form of Tat, as well as inhibition of de novo protein synthesis by cycloheximide abolished the effect on the glutamate response. This suggests that Tat acts upon induction of a so far unknown cellular gene whose gene product causes the reduction of glutamate responses. As the effect of Tat resembles the effect of TNFalpha on glutamate responses [Köller et al. (2001) Brain Res., 893, 237-243] which is locally released within the brains of HIVE patients, we also tested for synergistic effects of Tat and TNFalpha on the glutamate response. Low concentrations of Tat in combination with subthreshold concentrations of TNFalpha also elicited a marked reduction of astroglial glutamate responses. Our data suggest that Tat and TNFalpha, both by itself and synergistically, induce astroglial dysfunction.

Visualization of intracellular movement of vaccinia virus virions containing a green fluorescent protein-B5R membrane protein chimera

We produced an infectious vaccinia virus that expressed the B5R envelope glycoprotein fused to the enhanced green fluorescent protein (GFP), allowing us to visualize intracellular virus movement in real time. Previous transfection studies indicated that fusion of GFP to the C-terminal cytoplasmic domain of B5R did not interfere with Golgi localization of the viral protein. To determine whether B5R-GFP was fully functional, we started with a B5R deletion mutant that made small plaques and inserted the B5R-GFP gene into the original B5R locus. The recombinant virus made normal-sized plaques and acquired the ability to form actin tails, indicating reversal of the mutant phenotype. Moreover, immunogold electron microscopy revealed that both intracellular enveloped virions (IEV) and extracellular enveloped virions contained B5R-GFP. By confocal microscopy of live infected cells, we visualized individual fluorescent particles, corresponding to IEV in size and shape, moving from a juxtanuclear location to the periphery of the cell, where they usually collected prior to association with actin tails. The fluorescent particles could be seen emanating from cells at the tips of microvilli. Using a digital camera attached to an inverted fluorescence microscope, we acquired images at 1 frame/s. At this resolution, IEV movement appeared saltatory; in some frames there was no net movement, whereas in others movement exceeded 2 microm/s. Further studies indicated that IEV movement was reversibly arrested by the microtubule-depolymerizing drug nocodazole. This result, together with the direction, speed, and saltatory motion of IEV, was consistent with a role for microtubules in intracellular transport of IEV.

Entry of poliovirus into cells is blocked by valinomycin and concanamycin A

Poliovirus contains a virus particle devoid of a lipid envelope that does not require an intact pH to enter into susceptible cells. Thus, the blockade of pH gradient generated in endosomes is not sufficient to impede the translocation of poliovirus particles to the cytoplasm, suggesting that translocation takes place at the plasma membrane. Measuring both viral protein synthesis and eIF4G-1 cleavage mediated by poliovirus protease 2A has been used to monitor productive entry of poliovirus into cells. Translation of the input poliovirus RNA produces enough 2A(pro) to cleave eIF4G-1, providing a sensitive assay to estimate poliovirus RNA delivery to the cytoplasm followed by its translation. Combination of concanamycin A, a vacuolar proton-ATPase inhibitor, and valinomycin, an ionophore that promotes K(+) efflux from cells, powerfully prevented poliovirus infection. Moreover, modifying the ionic conditions of the culture medium (increasing the concentration of K(+) and decreasing the concentration of Na(+)), together with concanamycin A, also significantly interfered with poliovirus entry. These findings suggest that poliovirus RNA requires an intact concentration of K(+) ions inside the cells to be uncoated and to gain access to the cytoplasm. In addition, the actual contribution of concanamycin A (as well as other inhibitors of endocytosis) to the total inhibition of productive poliovirus entry points to the idea that at least some percentage of polioviral subparticles translocates from the endosomes.

Independent regulation of lymphocytic choriomeningitis virus-specific T cell memory pools: relative stability of CD4 memory under conditions of CD8 memory T cell loss

Infection of mice with a series of heterologous viruses causes a reduction of memory CD8(+) T cells specific to viruses from earlier infections, but the fate of the virus-specific memory CD4(+) T cell pool following multiple virus infections has been unknown. We have previously reported that the virus-specific CD4(+) Th precursor (Thp) frequency remains stable into long-term immunity following lymphocytic choriomeningitis virus (LCMV) infection. In this study, we questioned whether heterologous virus infections or injection with soluble protein CD4 Ags would impact this stable LCMV-specific CD4(+) Thp memory pool. Limiting dilution analyses for IL-2-producing cells and intracellular cytokine staining for IFN-gamma revealed that the LCMV-specific CD4(+) Thp frequency remains relatively stable following multiple heterologous virus infections or protein Ag immunizations, even under conditions that dramatically reduce the LCMV-specific CD8(+) CTL precursor frequency. These data indicate that the CD4(+) and CD8(+) memory T cell pools are regulated independently and that the loss in CD8(+) T cell memory following heterologous virus infections is not a consequence of a parallel loss in the memory CD4(+) T cell population.

Bleomycin-induced alterations in DNA replication: relationship to DNA damage

Bleomycin (BLM), a well-known DNA scission agent, is assumed to inhibit intracellular DNA replication by damaging the DNA template (cis-acting mechanism), although other DNA damaging compounds can alter DNA replication through modulation of crucial replication factor(s) (trans-acting mechanism). The present study examines the relationship between DNA damage and inhibition of replication caused by BLM in the well-defined simian virus 40 (SV40) intracellular and cell-free in vitro systems. Treatment of SV40-infected BSC-1 cells for 2 h with BLM at 50 microg/mL, induced 0.3 break/viral genome. Under the same treatment conditions, analysis of replication intermediates on two-dimensional gels showed a decrease in both mass of SV40 replication intermediates and replication activity. The mass of SV40 intermediates was decreased to about 30%, whereas replication activity was reduced to less than 5%. These results suggest that BLM inhibits both initiation and elongation phases of SV40 replication. In a cell-free DNA replication system, extracts from BLM-treated cells (50 micro/mL) were able to support SV40 DNA replication by only 50%. In this study, non-drug-treated DNA template was used, implying that BLM can induce a trans-acting effect. Finally, the drug-induced effects on SV40 DNA replication in cell-free and intracellular viral systems were compared to the effects on genomic DNA replication in BSC-1 cells. Overall, the results support the concept that BLM-induced inhibition of DNA replication occurs by both trans- (inhibition of replication of nondamaged template) and cis-acting mechanisms (template damage).

Expression of intracellular IFN-gamma in HSV-1-specific CD8+ T cells identifies distinct responding subpopulations during the primary response to infection

Cutaneous infection in the footpads of C57BL/6 mice with HSV-1 results in an accumulation of activated (CD44high CD25+) CD8+ T cells within the draining popliteal lymph node (PLN). These studies were undertaken to evaluate the frequency and phenotype of the CD8+ T cell population within the PLN, recognizing the single immunodominant HSV-1 epitope derived from the viral envelope glycoprotein, glycoprotein B (gB), using an intracellular IFN-gamma-staining assay. It revealed that approximately 6% of the CD8+ T cells were specific for the gB epitope. Phenotypic analysis of the IFN-gamma-producing gB-specific CD8+ T cells generated in the PLN during the course of the acute infection expressed the CD44high CD25+ phenotype on days 3-5 postinfection. Surprisingly, IFN-gamma-producing CD8+ T cells expressed the CD44high CD25- phenotype on days 5-8 postinfection, in contrast to expectations for a CD8+ effector T cell. IFN-gamma-producing CD25- CD8+ T cells were detected in the PLN on day 21 postinfection, long after infectious virus had been cleared. Throughout the response, the spleen was found to be the major reservoir of gB-specific CD8+ T cells, even during the peak of the response. In contrast to the gB-specific CD8+ T cell population within the PLN, the entire gB-specific CD8+ T cell population within the spleen was CD25-. Collectively, these results suggest the generation of subpopulations of virus-specific CD8+ T cells, distinguished by the expression of CD25, during the acute phase of the primary response to a localized viral infection.

Impaired CTL recognition of cells latently infected with Kaposi's sarcoma-associated herpes virus

Kaposi's sarcoma-associated herpes virus (KSHV) is a recently identified human gamma2-herpesvirus associated with Kaposi's sarcoma, primary effusion lymphoma, and Castleman's disease. We reasoned that CTL responses may provide host defense against this virus, and consequently, KSHV may have evolved strategies to evade the CTL-mediated immune surveillance. In this study six B cell lines latently infected with KSHV were found to express reduced levels of HLA class I surface molecules compared with B cell lines transformed by the related gamma-herpesvirus EBV. KSHV-infected cells also required higher concentrations of soluble peptides to induce efficient CTL-mediated lysis than control cell lines and were unable to process and/or present intracellularly expressed Ag. Incubation of the KSHV-infected cell lines with high concentrations of soluble HLA class I binding peptides did not restore the deficient HLA class I surface expression. To assess the underlying mechanisms of these phenomena, TAP-1 and TAP-2 gene expression was analyzed. While no attenuation in TAP-2 expression was observed, TAP-1 expression was significantly reduced in all KSHV cell lines compared with that in controls. These results indicate that KSHV can modulate HLA class I-restricted Ag presentation to CTL, which may allow latently infected cells to escape CTL recognition and persist in the infected host.

Direct measurement of CD8+ T cell responses in macaques infected with simian immunodeficiency virus

The simian immunodeficiency virus (SIV) macaque model system has been used extensively to study AIDS pathogenesis and to test candidate vaccines for their ability to protect against homologous or heterologous challenge with pathogenic SIV or SHIV. Recent studies suggest that stimulation of HIV-1-specific CTL responses is important for effective vaccination against HIV-1. While quantitative measurements of SIV-specific cytotoxic T lymphocyte (CTL) responses have been facilitated by the use of tetrameric peptide complexes, this technique is currently limited to the study of Mamu-A*01-positive rhesus macaques. Furthermore, very few SIV-specific CTL epitopes have been identified, and there is limited identification of other MHC alleles in macaques. In this study, cytokine flow cytometry (CFC) was used to quantify SIV-specific CD8+ antigen-reactive T cells in macaques infected with SIV. We found a strong correlation (r = 0.96, P < 0.001) between CD8+ antigen-reactive T cells stained with the Mamu-A*01 p11C, C-M tetramer and production of intracellular TNF-alpha in the CFC assay. Furthermore, the CFC assay was used to identify a novel SIV-specific CTL epitope in Envelope (SIV Env, a.a. 486-494, sequence AEVAELYRL). The use of the CFC assay facilitates the study of antigen-reactive T cell responses in SIV infection and vaccination.

Aminoglycoside-arginine conjugates that bind TAR RNA: synthesis, characterization, and antiviral activity

Regulation of HIV gene expression is crucially dependent on binding of the trans-activator protein, Tat, to the trans-activation response RNA element, TAR, found at the 5' end of all HIV-1 transcripts. Tat-TAR interaction is mediated by a short arginine-rich domain of the protein. Disruption of this interaction could, in theory, create a state of complete viral latency. A new class of small-molecule peptidomimetic TAR RNA binders, conjugates of aminoglycosides and arginine, was recently designed [Litovchick, A., Evdokimov, A. G., and Lapidot, A. (1999) FEBS Lett. 445, 73-79]. Two of these compounds, the tri-arginine derivative of gentamicin C (R3G) and the tetra-arginine derivative of kanamycin A (R4K), bind efficiently and specifically to TAR RNA. These compounds display negligible toxicity while being transported and accumulated in cell nuclei. Here we present a detailed synthesis and chemical characterization of the aminoglycoside-arginine conjugates R3G and R4K as well as GB4K, the tetra-gamma-guanidinobutyric derivative of kanamycin A. Their binding sites on TAR RNA were assigned by RNase A, uranyl nitrate, and lead acetate footprinting. The conjugates interact with TAR RNA in the widened major groove, formed by the UCU bulge and the neighboring base pairs of the upper stem portion of TAR, the binding site of Tat protein, and Tat-derived peptides (e.g., R52). Our results suggest an additional binding site of R4K and R3G compounds, in the lower stem-bulge region of TAR. The antiviral activity of the conjugates in cultured equine dermal fibroblasts infected with equine infectious anemia virus, used as a model system of HIV-infected cells, is also presented.

Role of matrix protein in the type D retrovirus replication cycle: importance of the arginine residue at position 55

We previously reported that a mutant of Mason-Pfizer monkey virus (M-PMV), which has an amino acid substitution in the matrix (MA) protein at position 55, MA-R55W, showed altered viral morphogenesis, reduced glycoprotein incorporation, and loss of infectivity. In this report, we show that two additional amino acid substitutions at this site in MA, R55F and R55Y, also result in similar altered morphogenesis, Env incorporation, and infectivity, demonstrating that these changes are not specific for the substitution of tryptophan in place of arginine 55. Attempts to isolate second site infectious revertants from cells transfected with the R55W mutant genome resulted only in the recovery of infectious viruses in which the codon at position 55 had reverted to one encoding arginine. In contrast, no revertants were obtained from the phenylalanine and tyrosine mutants in which three nucleotide changes had been engineered into the arginine codon. These results confirm that the arginine residue at position 55 is critical for intracellular targeting of M-PMV Gag molecules and support the concept that as part of a cytoplasmic transport retention signal R55 interacts with cellular trafficking components rather than other regions of Gag.

Human cytomegalovirus gene products US3 and US6 down-regulate trophoblast class I MHC molecules

The epidemiological correlation between human CMV (HCMV) infection and spontaneous fetal loss has been suggested, but the underlying mechanism is not well understood. Fetal cytotrophoblasts, which are in direct contact with the maternal immune system in the uterus during pregnancy, do not express HLA-A and HLA-B, but express the nonclassical class I HLA-G and HLA-C. It has been shown that both HLA-G and HLA-C are capable of inhibiting NK-mediated cell lysis. In our present study, using human trophoblast cell lines as well as other cell lines stably transfected with the human class I genes, we have demonstrated that HCMV US3 and US6 down-regulate the cell-surface expression of both HLA-G and HLA-C by two different mechanisms. HCMV US3 physically associates with both trophoblast class I MHC species, retaining them in the endoplasmic reticulum. In contrast, HCMV US6 inhibits peptide transport by TAP and thus specifically the intracellular trafficking of class I molecules. Therefore, these findings suggest for the first time a possible molecular mechanism underlying HCMV-related spontaneous pregnancy loss.

Acute infection of Sindbis virus induces phosphorylation and intracellular translocation of small heat shock protein HSP27 and activation of p38 MAP kinase signaling pathway

In general, viral infection is supposed to induce stress responses in the host cell. However, very few detailed observations about virus-induced stress responses have been reported. Here we investigated specific stress responses in Vero cells infected with Sindbis virus (SV), a single-stranded RNA virus, acute infection with which is known to cause apoptotic cell death in the host cells. Prior to the onset of apoptosis, p38 mitogen-activated protein kinase (MAPK) and c-Jun NH2-terminal kinases (JNKs) were activated. Subsequently, a 27-kDa heat shock protein (HSP27) became phosphorylated, and intracellular distribution of HSP27 was changed from the cytoplasm to the perinuclear region. These results indicate that the cellular signaling cascades activated by pro-inflammatory cytokines and environmental stresses are also activated as a result of lytic infection with SV. These responses may contribute to the delayed onset of apoptosis in the host cells and the facilitation of viral replication.

Intracellular transport of varicella-zoster glycoproteins

Previous observations have established that varicella-zoster virus (VZV) is enveloped in the trans-Golgi network (TGN) in cultures infected with VZV and that the glycoprotein gE is targeted to the TGN by a signal sequence (AYRV) and an acidic TGN signal patch in its cytosolic domain. Neither sequence is present in other VZV glycoproteins. Like gE, gI was targeted to the TGN when it was expressed in transfected cells, suggesting that gI also contains TGN targeting information (colocalized with gE and the AP-1 adaptin complex). In contrast, gB, gC, gH, and gL immunoreactivities were not detected in the TGN when they were expressed individually in transfected cells. In VZV-infected cells, gE, gI, gH, and gL were all concentrated in the TGN. Since VZV glycoproteins that lack targeting sequences (gB, gC, gH, and gL) concentrated in the TGN of infected cells, it is proposed that gE and gI, which have such sequences, serve as navigator glycoproteins, forming complexes that direct the signal-deficient glycoproteins to the TGN.

Molecular mechanisms of neurotropic herpesvirus invasion and spread in the CNS

Pseudorabies virus (PRV) is a herpesvirus in the subfamily alphaherpesvirinae (the alpha herpesviruses). After primary infection at mucosal surfaces, PRV infects the peripheral nervous system in its natural host (swine) with occasional invasion of the central nervous system. When other hosts (including cows and rodents) are infected, the infection almost always gives rise to fatal disease in the CNS as a result of infection of peripheral neurons and subsequent spread to the brain. Part of the ability to cause fatal CNS disease can be attributed to a viral glycoprotein called gE. Viruses lacking gE are thought to be less virulent because they do not spread efficiently from cell to cell. Based on a set of gE mutations we have constructed, we suggest that these two phenotypes of cell-cell spread and virulence reflect separate functions of the gE protein. In this report, we show that viruses carrying these new gE mutations have marked reduction in virulence, yet spread efficiently in defined neural circuits in the rat brain. As such, they offer new insight and opportunities for understanding of viral disease and host response to injury, as well as in the construction of viral tracers of neuronal connections.

Phenotypic knock-out of the latent membrane protein 1 of Epstein-Barr virus by an intracellular single-chain antibody

Epstein-Barr virus (EBV) causes lymphoproliferative diseases in immunocompromised patients and is associated with endemic Burkitt lymphoma, nasopharyngeal carcinoma and some cases of Hodgkin disease. The latent membrane protein 1 (LMP1) of EBV is a transmembrane protein that is essential for the transformation of B lymphocytes. LMP1-mediated up-regulation of Bcl-2 is thought to be an important element in this process. As an approach to explore novel treatments for EBV-associated lymphomas, we constructed a single-chain antibody (sFv) directed against LMP1 to achieve functional inhibition of this oncoprotein in EBV-transformed B lymphocytes. We demonstrated that intracellular expression of an endoplasmic reticulum (ER)-targeted form of this sFv markedly reduced LMP1 protein levels. We also observed a decrease in intracellular level of this protein which correlated with a marked reduction of Bcl-2 expression in EBV-transformed B lymphocytes. We further demonstrated that anti-LMP1 sFv-mediated reduction of Bcl-2 correlated with increased sensitivity of these cells to drug-induced cell death. Therefore, these data suggest that an anti-LMP1 sFv used in combination with conventional chemotherapy may be useful for gene therapy of EBV-associated lymphomas in immunocompromised patients.

A rapid and simple procedure to detect the presence of MVM in conditioned cell fluids or culture media

During the manufacture of biopharmaceuticals, numerous adventitious agents have been detected in Master Cell Banks, end-of-production cells as well as bulk harvest fluid. Recently, a number of large-scale production bioreactors have become infected with Minute Virus of Mice (MVM) during cGMP (current good manufacturing practices) operations, and this has resulted in both the loss of product and the need for major cleaning validation procedures to be put in place. We have developed a simple DNA extraction/PCR assay to detect the presence of MVM in cell culture supernatant (conditioned cell fluids). This highly specific assay can detect 10 or fewer genome equivalents (copies) of MVM following PCR and gel electrophoresis visualization. For routine high-throughput detection, 300-100 copies could be consistently detected. The extraction procedure was shown to reliably detect MVM at a concentration of 1 TCID50/ml. The combination of the extraction/PCR procedure establishes a powerful, sensitive, specific assay that can detect the presence of MVM sequences with a 1-day turnaround time.

HIV genetic variation is directed and restricted by DNA precursor availability

The effects of deoxynucleoside triphosphate (dNTP) imbalances on the fidelity of human immunodeficiency virus type 1 (HIV-1) replication were investigated. Using detergent permeabilized virions and biased dNTP concentrations different types of hypermutants were readily produced. However, the mutant spectrum was different from naturally occurring hypermutants demonstrating that the host cell may restrict variation. Using a genetic screen based on the blue/white beta-galactosidase complementation assay, G --> A hypermutants were recovered from HIV-infected thymidine treated U937 cells. Furthermore, hypermutants were recovered from 1 to 2% of resting or activated peripheral blood mononuclear cells indicating that small proportions of primary cells had distorted intracellular [dTTP] and [dCTP]. Such imbalances may underlie a proportion of somatic and germline point mutations and shape to some extent the evolution of mammalian and viral genomes.

Intracellular thiol redox status affects rat cytomegalovirus infection of vascular cells

There is increasing evidence for cytomegalovirus (CMV) induced vascular pathology during acute infection in the immunocompromised host. Inflammation is involved in such processes, which is frequently associated with increased levels of oxidative mediators and reduced anti-oxidant protection. A relation between viral infection and oxidative stress has been recognized for human immunodeficiency virus and herpes simplex virus-1 infections, but little is known in this respect for CMV infections. We investigated if there is a relation between CMV infection of vascular cells and the intracellular redox status using an in vitro rat model. We measured intracellular glutathione levels and rat CMV (RCMV) permissiveness of rat heart endothelial cell lines (RHEC), rat smooth muscle cells (RSMC), and compared these with fully CMV-permissive rat fibroblasts (REF and Rat 2). In addition, the effects of the anti-oxidant N-acetylcysteine (NAC) and the glutathione synthesis inhibitor buthionine sulfoximide (BSO) on CMV permissiveness and replication were investigated in these cell lines. Finally, we investigated infection of vascular cells under inflammatory conditions in an in vivo rat model for acute CMV infection. The results show a very high endogenous glutathione level in RHEC compared to REF, Rat 2 cells and RSMC. This is associated with a low CMV permissiveness in RHEC as opposed to full permissiveness in REF, Rat 2 cells and RSMC in vitro. In addition, modulation of the intracellular thiol redox status affected CMV infection and replication only in RHEC, but not in RSMC and Rat 2 cells. During acute infection in vivo under immunosuppressed conditions rat endothelial cells first become activated and subsequently infected leading to vascular damage and pathology. This study suggests that a high endogenous thiol redox status may contribute to the apparent barrier function of endothelial cells with respect of CMV infection and that oxidative stress may facilitate CMV infection of the vascular wall.

Marked inhibition of the intracellular multiplication of Legionella pneumophila in monocytes isolated from carriers of human T lymphotrophic virus type I

Intracellular growth patterns of Legionella pneumophila were examined in monocytes obtained from carriers of human T-lymphotropic virus type I (HTLV-1) and controls who were HTLV-1 seronegative. All subjects were seronegative for antibodies against L. pneumophila. Bacterial growth was determined 0, 1, 2, and/or 3 days after infecting peripheral blood mononuclear cells (PBMCs) with the bacteria. The intracellular growth of L. pneumophila was markedly inhibited in HTLV-1 carriers compared with normal controls. When the lymphocytes were depleted from the HTLV-1 carrier PBMC cultures before infection, this inhibition was abolished. Inhibition reappeared, however, when the 72-h culture supernatants of PBMCs from HTLV-1 carriers were added to the lymphocyte-depleted cultures. Culture supernatants of infected and uninfected PBMCs from HTLV-1 carriers exhibited markedly increased levels of interferon-gamma (IFN-gamma) and tumor necrosis factor alpha (TNF-alpha) compared with the HTLV-1 seronegative controls. In the HTLV-1 carriers, IFN-gamma was produced by the CD4+ lymphocytes, whereas TNF-alpha was secreted by the monocytes. Addition of anti-IFN-gamma or anti-TNF-alpha antibodies to the HTLV-1 carrier PBMC cultures diminished the inhibition of intracellular growth of L. pneumophila. Results suggest that the monocytes are activated in HTLV-1 carriers. These findings may explain why an opportunistic infection by certain intracellular pathogens is rarely seen among HTLV-1 carriers.

Concerted use of immunologic and ultrastructural analyses in diagnostic medicine: immunoelectron microscopy and correlative microscopy

Electron microscopy (EM) is a valuable tool in diagnostic medicine, and in some cases, can be enhanced by immunological methods. A major medical application of EM, diagnostic virology, can frequently be augmented by employment of immunological reagents. Three immunoelectron microscopy (IEM) methods, aggregation, coating, and gold labeling, provide means for serotyping viruses; aggregation by antibody can also be used to concentrate viruses in dilute suspension or to serotype them. As a research tool, IEM can be useful in studying the relationship of various pathogen proteins to the infected cells or tissues. Delineating the subcellular location of viral components can yield information about how virions are constructed, and hence, suggest methods and compounds for inhibiting that process. Conversely, labeling virus-infected cells with antibodies against various cell receptors and proteins can yield information about the association of the proteins with budding virions. Another research example is the identification by immunological staining of virus-infected cells for subsequent ultrastructural identification of the specific cell type involved. Electron microscopy and immunolabeling methods are also useful in the diagnosis of immune complex disorders, including various forms of postinfectious immune complex glomerulonephritis. Precise analysis of immune complex deposits can be accomplished by using EM to pinpoint their location and immunohistology to probe their composition. Finally, a variety of optical microscopic techniques, including some involving immunofluorescent labeling, can be used to identify areas of interest in inhomogeneous tissues for further study by EM.