Mechanism of HIV spread from lymphocytes to epithelia

Anatomical and physiological aspects of the HIV infection pathogenesis in animal models

Understanding the entire pathogenesis of HIV infection, from penetration at the gates of infection to the induction of severe immunodeficiency, is an essential tool for the development of new treatment methods. Less than 40 years of research into the mechanisms of HIV infection that lead to the development of acquired immunodeficiency syndrome have accumulated a huge amount of information, but HIV's own unique variability identifies new whitespaces. Despite the constant improvement of the protocols of antiretroviral therapy and the success of its use, it has not yet been possible to stop the spread of HIV infection. The development of new protocols and the testing of new groups of antiretroviral drugs is possible, first of all, due to the improvement of animal models of the HIV infection pathogenesis. Their relevance, undoubtedly increases, but still depends on specific research tasks, since none of the in vivo models can comprehensively simulate the mechanism of the infection pathology in humans which leads to multi-organ damage. The aim of the review was to provide up-to-date information on known animal models of HIV infection, focusing on the method of their infection and anatomical, physiological and pathological features.

Embedding of HIV Egress within Cortical F-Actin

F-Actin remodeling is important for the spread of HIV via cell-cell contacts; however, the mechanisms by which HIV corrupts the actin cytoskeleton are poorly understood. Through live cell imaging and focused ion beam scanning electron microscopy (FIB-SEM), we observed F-Actin structures that exhibit strong positive curvature to be enriched for HIV buds. Virion proteomics, gene silencing, and viral mutagenesis supported a Cdc42-IQGAP1-Arp2/3 pathway as the primary intersection of HIV budding, membrane curvature and F-Actin regulation. Whilst HIV egress activated the Cdc42-Arp2/3 filopodial pathway, this came at the expense of cell-free viral release. Importantly, release could be rescued by cell-cell contact, provided Cdc42 and IQGAP1 were present. From these observations, we conclude that a proportion out-going HIV has corrupted a central F-Actin node that enables initial coupling of HIV buds to cortical F-Actin to place HIV at the leading cell edge. Whilst this initially prevents particle release, the maturation of cell-cell contacts signals back to this F-Actin node to enable viral release & subsequent infection of the contacting cell.

The role of polymorphonuclear neutrophils during HIV-1 infection

It is well-recognized that human immunodeficiency virus type-1 (HIV-1) mainly targets CD4⁺ T cells and macrophages. Nonetheless, during the past three decades, a huge number of studies have reported that HIV-1 can directly or indirectly target other cellular components of the immune system including CD8⁺ T cells, B cells, dendritic cells, natural killer cells, and polymorphonuclear neutrophils (PMNs), among others. PMNs are the most abundant leukocytes in the human circulation, and are known to play principal roles in the elimination of invading pathogens, regulating different immune responses, healing of injured tissues, and maintaining mucosal homeostasis. Until recently, little was known about the impact of HIV-1 infection on PMNs as well as the impact of PMNs on HIV-1 disease progression. This is because early studies focused on neutropenia and recurrent microbial infections, particularly, during advanced disease. However, recent studies have extended the investigation area to cover new aspects of the interactions between HIV-1 and PMNs. This review aims to summarize these advances and address the impact of HIV-1 infection on PMNs as well as the impact of PMNs on HIV-1 disease progression to better understand the pathophysiology of HIV-1 infection.

The oral mucosa immune environment and oral transmission of HIV/SIV

The global spread of human immunodeficiency virus (HIV) is dependent on the ability of this virus to efficiently cross from one host to the next by traversing a mucosal membrane. Unraveling how mucosal exposure of HIV results in systemic infection is critical for the development of effective therapeutic strategies. This review focuses on understanding the immune events associated with the oral route of transmission (via breastfeeding or sexual oral intercourse), which occurs across the oral and/or gastrointestinal mucosa. Studies in both humans and simian immunodeficiency virus (SIV) monkey models have identified viral changes and immune events associated with oral HIV/SIV exposure. This review covers our current knowledge of HIV oral transmission in both infants and adults, the use of SIV models in understanding early immune events, oral immune factors that modulate HIV/SIV susceptibility (including mucosal inflammation), and interventions that may impact oral HIV transmission rates. Understanding the factors that influence oral HIV transmission will provide the foundation for developing immune therapeutic and vaccine strategies that can protect both infants and adults from oral HIV transmission.

HIV-1 assembly, budding, and maturation

A defining property of retroviruses is their ability to assemble into particles that can leave producer cells and spread infection to susceptible cells and hosts. Virion morphogenesis can be divided into three stages: assembly, wherein the virion is created and essential components are packaged; budding, wherein the virion crosses the plasma membrane and obtains its lipid envelope; and maturation, wherein the virion changes structure and becomes infectious. All of these stages are coordinated by the Gag polyprotein and its proteolytic maturation products, which function as the major structural proteins of the virus. Here, we review our current understanding of the mechanisms of HIV-1 assembly, budding, and maturation, starting with a general overview and then providing detailed descriptions of each of the different stages of virion morphogenesis.

Primary human epithelial cell culture system for studying interactions between female upper genital tract and sexually transmitted viruses, HSV-2 and HIV-1

Evidence from clinical and epidemiological studies indicates that women are disproportionately susceptible to sexually transmitted viral infections. To understand the underlying biological basis for this increased susceptibility, more studies are needed to examine the acute events in the female reproductive tract following exposure to viruses during sexual transmission. The epithelial lining of the female reproductive tract is the primary barrier that sexually transmitted viruses, such as HIV-1 and HSV-2 need to infect or traverse, in order to initiate and establish productive infection. We have established an ex-vivo primary culture system to grow genital epithelial cells from upper reproductive tract tissues of women. Using these cultures, we have extensively examined the interactions between epithelial cells of the female genital tract and HSV-2 and HIV-1. In this review, we describe in detail the experimental protocol to grow these cultures, monitor their differentiation and inoculate with HSV-2 and HIV-1. Prospective use of these cultures to re-create the microenvironment in the reproductive tract is discussed.

The regulated secretory pathway in CD4(+) T cells contributes to human immunodeficiency virus type-1 cell-to-cell spread at the virological synapse

Direct cell-cell spread of Human Immunodeficiency Virus type-1 (HIV-1) at the virological synapse (VS) is an efficient mode of dissemination between CD4(+) T cells but the mechanisms by which HIV-1 proteins are directed towards intercellular contacts is unclear. We have used confocal microscopy and electron tomography coupled with functional virology and cell biology of primary CD4(+) T cells from normal individuals and patients with Chediak-Higashi Syndrome and report that the HIV-1 VS displays a regulated secretion phenotype that shares features with polarized secretion at the T cell immunological synapse (IS). Cell-cell contact at the VS re-orientates the microtubule organizing center (MTOC) and organelles within the HIV-1-infected T cell towards the engaged target T cell, concomitant with polarization of viral proteins. Directed secretion of proteins at the T cell IS requires specialized organelles termed secretory lysosomes (SL) and we show that the HIV-1 envelope glycoprotein (Env) localizes with CTLA-4 and FasL in SL-related compartments and at the VS. Finally, CD4(+) T cells that are disabled for regulated secretion are less able to support productive cell-to-cell HIV-1 spread. We propose that HIV-1 hijacks the regulated secretory pathway of CD4(+) T cells to enhance its dissemination.

Virus-host interactions in HIV pathogenesis: directions for therapy

The challenge of controlling HIV infection involves an understanding of the heterogeneity of the virus, its wide cellular host range, its primary routes of transmission, and the immunologic and intrinsic cellular factors that can prevent its transmission and replication. Identification of HIV-infected individuals who have survived more than 10 years without signs of the infection and without therapy encourages studies examining the natural mechanisms for resistance to infection and disease. Within the immune system, emphasis should be given to the innate or natural response that appears within minutes of the infection and offers the optimal time for controlling HIV. All these parameters in HIV pathogenesis underline the information needed to develop optimal anti-HIV therapies and an effective AIDS vaccine.

Virological synapses allow HIV-1 uptake and gene expression in renal tubular epithelial cells

In animal models of HIV-associated nephropathy, the expression of HIV regulatory genes in epithelial cells is sufficient to cause disease, but how the CD4-negative epithelial cells come to express HIV genes is unknown. Here, we co-cultured T cells infected with fluorescently tagged HIV with renal tubular epithelial cells and observed efficient virus transfer between these cells. The quantity of HIV transferred was much greater than that achieved by exposure to large amounts of cell-free virus and occurred without a requirement for CD4 or Env. The transfer required stable cell-cell adhesion, which could be blocked by sulfated polysaccharides or poly-anionic compounds. We found that the internalization of virus could lead to de novo synthesis of viral protein from incoming viral RNAs even in the presence of a reverse transcriptase inhibitor. These results illustrate an interaction between infected T cells and nonimmune cells, supporting the presence of virological synapses between HIV-harboring T cells and renal tubular epithelial cells, allowing viral uptake and gene expression in epithelial cells.

DC-specific ICAM-3-grabbing nonintegrin mediates internalization of HIV-1 into human podocytes

Human immunodeficiency virus (HIV)-1 has been demonstrated to contribute to the pathogenesis of HIV-associated nephropathy. In renal biopsy studies, podocytes have been reported to be infected by HIV-1. However, the mechanism involved in HIV-1 internalization into podocytes is not clear. In the present study, we evaluated the occurrence of HIV-1 internalization into conditionally immortalized human podocytes and the mechanism involved. Human podocytes rapidly internalized R5 and X4 HIV-1 primary strains via an endocytosis-dependent pathway, without establishing a productive infection. The HIV-1 internalization was dendritic cell-specific ICAM-3-grabbing nonintegrin (DC-SIGN) receptor mediated. The role of DC-SIGN was confirmed by using specific blocking antibodies and transfection with small interfering (si) RNA/DC-SIGN. Since podocyte HIV-1 trafficking was not altered by pH-modulating agents, it appeared that HIV-1 routing occurred through nonacid vesicular compartments. Interestingly, transfection of podocytes with neither siRNA/caveolin-1 nor siRNA/clathrin heavy chain inhibited podocyte viral accumulation. Thus it appears that clathrin-coated vesicles and caveosomes may not be contributing to HIV-1-associated membrane traffic.

T cell polarization at the virological synapse

Cell-to-cell spread of HIV-1 between CD4(+) T cells takes place at multimolecular structures called virological synapses. A defining feature of the virological synapse is polarization of viral assembly and budding at sites of T cell-T cell contact. Recent work is beginning to address how viral proteins are targeted to the virological synapse and the molecular mechanisms that regulate HIV-1 egress by cell-to-cell spread. This review discusses our current understanding of these processes and considers how T cell polarization during other forms of intercellular communication may provide insight into HIV-1 assembly and dissemination.

Quantitative comparison of HTLV-1 and HIV-1 cell-to-cell infection with new replication dependent vectors

We have developed an efficient method to quantify cell-to-cell infection with single-cycle, replication dependent reporter vectors. This system was used to examine the mechanisms of infection with HTLV-1 and HIV-1 vectors in lymphocyte cell lines. Effector cells transfected with reporter vector, packaging vector, and Env expression plasmid produced virus-like particles that transduced reporter gene activity into cocultured target cells with zero background. Reporter gene expression was detected exclusively in target cells and required an Env-expression plasmid and a viral packaging vector, which provided essential structural and enzymatic proteins for virus replication. Cell-cell fusion did not contribute to infection, as reporter protein was rarely detected in syncytia. Coculture of transfected Jurkat T cells and target Raji/CD4 B cells enhanced HIV-1 infection two fold and HTLV-1 infection ten thousand fold in comparison with cell-free infection of Raji/CD4 cells. Agents that interfere with actin and tubulin polymerization strongly inhibited HTLV-1 and modestly decreased HIV-1 cell-to-cell infection, an indication that cytoskeletal remodeling was more important for HTLV-1 transmission. Time course studies showed that HTLV-1 transmission occurred very rapidly after cell mixing, whereas slower kinetics of HIV-1 coculture infection implies a different mechanism of infectious transmission. HTLV-1 Tax was demonstrated to play an important role in altering cell-cell interactions that enhance virus infection and replication. Interestingly, superantigen-induced synapses between Jurkat cells and Raji/CD4 cells did not enhance infection for either HTLV-1 or HIV-1. In general, the dependence on cell-to-cell infection was determined by the virus, the effector and target cell types, and by the nature of the cell-cell interaction.

Cell-free virus particles released from infected cells can be transmitted to target cells by diffusion or may be conveyed directly to target cells via specific intercellular contacts; the latter is referred to as cell-to-cell infection. Microscopic imaging has shown how viral proteins and virus particles move within and between cells, accumulating at sites of cell-cell contact. While we suspect that these images represent virus infection, it has been difficult to accurately quantify virus replication and provirus formation in most cell-to-cell infection experiments. Retroviral vectors that encode reporter proteins have been invaluable tools for analyzing retrovirus replication and restriction, but they have had limited utility in cell-to-cell infection studies due to high background noise resulting from reporter expression in the producer cells. We report the construction and characterization of retroviral vectors that express reporter protein exclusively in target cells and only after completing a full replication cycle. We have validated this approach and have begun to analyze cell and virus determinants for cell-to-cell infection with vectors for two human retroviruses that infect T cells. We show that the mechanism of transmission and ensuing virus replication depend on the particular virus, the effector and target cell types, and on the specific type of cell-cell interaction.

The role of the local microenvironment in regulating susceptibility and immune responses to sexually transmitted viruses in the female genital tract

Sexually transmitted viruses cause chronic infections that have serious long-term health consequences. Based on the evidence from clinical and epidemiological studies, women carry a disproportionately higher burden of sexually transmitted diseases. The reasons for this are not well understood and possibly relate to a variety of social, behavioral and economic factors. In addition to these factors there are biological reasons that contribute to the higher prevalence in women. In this context it is critical to focus on and understand the local microenvironment of the female genital tract, since the majority of viral infections in women occur by heterosexual transmission. The genital tract is also the target site for initiation and maintenance of protective immune responses that could prevent or eliminate viral infections. The epithelial cells of the genital tract provide the first line of defense against viral entry. The interactions between each sexually transmitted virus and the genital epithelium are distinct and determine the outcome of exposure. They are also influenced by a number of factors in the local genital milieu. Among these factors are the female sex hormones that regulate both the susceptibility as well as immune responses to viral infections in the genital tract. Better understanding of the interactions of viruses with the local environment in the female genital tract will lead to development of novel methods to prevent sexually transmitted infections as well as to enhance innate and adaptive immunity.

Replication of HIV-1 in Vivo and in Vitro

A complex relationship exists between HIV and its cellular targets. The lethal effect of HIV on circulating CD4(+) helper T lymphocytes parallels the degree of the infected individual's immunodeficiency and ultimately the transition to AIDS and death. However, as with other members of the Lentivirus family of retroviruses, the ubiquitous, mobile macrophage is also a prime target for HIV infection, and apparently, in most instances, is the initial infected cell, since most people are infected with a CCR5 chemokine-tropic virus. Unlike the lymphocyte, the macrophage is apparently a more stable viral host, capable of a long infected life as an HIV reservoir and a chronic source of infectious virus. Published in vitro studies have indicated that whereas lymphocytes replicate HIV solely on their plasma membrane, macrophages have been envisaged to predominantly replicate HIV within cytoplasmic vacuoles, and thus have been likened to a "Trojan horse," when it comes to the immune system. Recent studies have revealed an ingenious way by which the cultured monocyte-derived macrophage (MDM) replicates HIV and releases it into the medium. The key macrophage organelle appears to be what is alternatively referred to as the "late endosome" (LE) or the "multivesicular body" (MVB), which have a short and a long history, respectively. Proof of the association is that chemically, LE/MVB and their vesicles possess several pathopneumonic membrane markers (e.g., CD63) that are found on released HIV particles. The hypothesis is that HIV usurps this vesicle-forming mechanism and employs it for its own replication. Release of the intravacuolar virus from the cell is hypothesized to occur by a process referred to as exocytosis, resulting from the fusion of virus-laden LE/MVB with the plasma membrane of the macrophage. Interestingly, LE/MVB are also involved in the infection stage of MDM by HIV. Close review of the literature reveals that along with the Golgi, which contributes to the formation of LE/MVB, the MVB was first identified as a site of HIV replication by macrophages many years ago, but the full implication of this observation was not appreciated at the time. As in many other areas of HIV research, what has been totally lacking is an in vivo confirmation of the in vitro phenomenon. Herein, the ultrastructure of HIV interaction with cells in vitro and in vivo is explored. It is shown that while HIV is regularly found in LE/MVB in vitro, it is infrequently the case in vivo. Therefore, the results challenge the "Trojan horse" concept.

Bluetongue virus outer capsid protein VP5 interacts with membrane lipid rafts via a SNARE domain

Bluetongue virus (BTV) is a nonenveloped double-stranded RNA virus belonging to the family Reoviridae. The two outer capsid proteins, VP2 and VP5, are responsible for virus entry. However, little is known about the roles of these two proteins, particularly VP5, in virus trafficking and assembly. In this study, we used density gradient fractionation and methyl beta cyclodextrin, a cholesterol-sequestering drug, to demonstrate not only that VP5 copurifies with lipid raft domains in both transfected and infected cells, but also that raft domain integrity is required for BTV assembly. Previously, we showed that BTV nonstructural protein 3 (NS3) interacts with VP2 and also with cellular exocytosis and ESCRT pathway proteins, indicating its involvement in virus egress (A. R. Beaton, J. Rodriguez, Y. K. Reddy, and P. Roy, Proc. Natl. Acad. Sci. USA 99:13154-13159, 2002; C. Wirblich, B. Bhattacharya, and P. Roy J. Virol. 80:460-473, 2006). Here, we show by pull-down and confocal analysis that NS3 also interacts with VP5. Further, a conserved membrane-docking domain similar to the motif in synaptotagmin, a protein belonging to the SNARE (soluble N-ethylmaleimide-sensitive fusion attachment protein receptor) family was identified in the VP5 sequence. By site-directed mutagenesis, followed by flotation and confocal analyses, we demonstrated that raft association of VP5 depends on this domain. Together, these results indicate that VP5 possesses an autonomous signal for its membrane targeting and that the interaction of VP5 with membrane-associated NS3 might play an important role in virus assembly.

Virological consequences of early events following cell-cell contact between human immunodeficiency virus type 1-infected and uninfected CD4+ cells

Human immunodeficiency virus type 1 (HIV-1)-infected cells transmit viral products to uninfected CD4(+) cells very rapidly. However, the natures of the transmitted viral products and the mechanism of transmission, as well as the relative virological consequences, have not yet been fully clarified. We studied the virological events occurring a few hours after contact between HIV-1-infected and uninfected CD4(+) cells using a coculture cell system in which the virus expression in target cells could be monitored through the induction of a green fluorescent protein reporter gene driven by HIV-1 long terminal repeats. Within 16 h of coculture, we observed two phenomena not related to the cell-free virus infection, i.e., the formation of donor-target cell fusions and a fusion-independent internalization of viral particles likely occurring at least in part through intercellular connections. Both events depended on the expression of Env and CD4 in donor and target cells, respectively, whereas the HIV-1 internalization required clathrin activity in target cells. Importantly, both phenomena were also observed in cocultures of primary CD4(+) lymphocytes, while primary macrophages supported only HIV-1 endocytosis. By investigating the virological consequences of these events, we noticed that while fused cells released infectious HIV-1 particles, albeit with reduced efficiency compared with donor cells, no virus expression was detectable upon HIV-1 endocytosis in target cells. In sum, the HIV-1 transmission following contact between an HIV-1-infected and an uninfected CD4(+) cell can occur through different mechanisms, leading to distinguishable virological outcomes.

Risk of breast, ovary, and uterine corpus cancers among 85,268 women with AIDS

By linking HIV/AIDS and cancer surveillance data in 12 US regions, breast and reproductive cancer risks with AIDS were compared to those in the general population. Trends in standardized incidence ratios (SIRs) were assessed by CD4 count, AIDS-relative time, and calendar time. Standardized incidence ratios were indirectly adjusted for cancer risk factors using data from AIDS cohort participants and the general population. With AIDS, 313 women developed breast cancer (SIR 0.69, 95% confidence interval (CI) 0.62–0.77), 42 developed ovary cancer (SIR 1.05, 95% CI, 0.75–1.42), and 31 developed uterine corpus cancer (SIR 0.57, 95% CI, 0.39–0.81). Uterine cancer risk was reduced significantly after age 50 (SIR 0.33). Breast cancer risk was reduced significantly both before (SIR 0.71) and after (SIR 0.66) age 50, and was lower for local or regional (SIR 0.54) than distant (SIR 0.89) disease. Breast cancer risk varied little by CD4 count (P_trend=0.47) or AIDS-relative time (P_trend=0.14) or after adjustment for established cancer risk factors. However, it increased significantly between 1980 and 2002 (P_trend=0.003), approaching the risk of the general population. We conclude that the cancer deficit reflected direct or indirect effects of HIV/AIDS and that anti-HIV therapy reduced these effects.

Virological Synapse for Cell-Cell Spread of Viruses

Cell-to-cell spread of retroviruses via virological synapse (VS) contributes to overall progression of disease. VS are specialized pathogen-induced cellular structures that facilitate cell-to-cell transfer of HIV-1 and HTLV-1. VS provide a mechanistic explanation for cell-associated retroviral replication. While VS share some common features with neurological or immunological synapses, they also exhibit important differences. The role of VS might not be limited to human retroviruses and the emerging role of a plant synapse suggests that VS might well be conserved structures for cell-cell spreading of both animal and plant viruses. Dissection of the VS is just at its beginning, but already offers ample information and fascinating insights into mechanisms of viral replication and cell-to-cell communication.

Detection of human immunodeficiency virus type 1 RNA by in situ hybridization in oral mucosa epithelial cells from anti-HIV-1 positive patients

Several in vitro studies have shown that HIV-1 can infect CD4 negative epithelial cells of different origin including normal human oral keratinocytes, but whether this infection of mucosal epithelial cells occurs in vivo is still unclear. In this report, the presence and cell types infected by HIV-1 in paraffin embedded oral mucosa biopsies from 17 anti-HIV-1 positive patients have been examined by in situ hybridization and immunohistochemistry. As controls, oral mucosa biopsies from eight patients without HIV-1 infection markers were also analyzed. The results showed that 8 out of the 17 anti-HIV-1 positive patients had HIV-1 RNA detectable in plasma. Positive hybridization signals were observed in the mucosa biopsies from 14 of the 17 anti-HIV-1 patients (82.3%). The mean percentage of cells showing HIV-1 RNA was 2.64% +/- 1.77% (range: 1% to 5.5%). No differences in the mean percentage of HIV-1 infected cells were found between patients with and without HIV-1 RNA in plasma (3.01% +/- 1.57% vs. 3.4% +/- 1.27% respectively), or between untreated patients and patients under antiretroviral therapy (2.83% +/- 1.63% vs. 3.42% +/- 1.29% respectively). Immunohistochemical detection of S-100 antigen, cytokeratin and CD4 showed that hybridization signals appeared in cytokeratin positive cells and CD4 positive cells but not in S-100 positive cells. In conclusion, this study has demonstrated that HIV-1 infects and replicates in oral mucosa epithelial cells in vivo and that these cells could represent a reservoir of the virus that may escape to the currently used antiretroviral therapy.

Statins Could Be Used to Control Replication of Some Viruses, Including HIV-1

Statins are mainly known for their plasma cholesterol-lowering properties and are widely used for the prevention of cardiovascular diseases. They however also exert pleiotropic effects through a variety of mechanisms, among which several immunosuppressive effects that are unrelated to their cholesterol-lowering activity. Interestingly, there has been recent evidence of antiviral effects, including preliminary studies on the efficacy of statins against HIV-1. This paper more particularly focuses on the specific inhibition of the binding of leukocyte function-associated antigen-1 (LFA-1) to intercellular adhesion molecule (ICAM-1) by statins, independently of the inhibition of HMGCoA reductase. Targeting the statin-binding site within LFA-1 or regulating LFA-1 affinity by inhibiting prenylation of the small GTPases could prove useful to treat inflammatory, autoimmune diseases and possibly viral infections, including HIV-1.

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Induction of systemic and mucosal cross-clade neutralizing antibodies in BALB/c mice immunized with human immunodeficiency virus type 1 clade A virus-like particles administered by different routes of inoculation

We have recently developed a candidate human immunodeficiency virus type 1 (HIV-1) vaccine model, based on virus-like particles (VLPs) expressing gp120 from a Ugandan HIV-1 isolate of clade A (HIV-VLP(A)s), which shows the induction of neutralizing antibodies as well as cytotoxic T lymphocytes (CTL) in BALB/c mice by intraperitoneal (i.p.) administration. In the present study, immunization experiments based on a multiple-dose regimen have been performed with BALB/c mice to compare different routes of administration. i.p. and intranasal (i.n.), but not oral, administration induce systemic as well as mucosal (vaginal and intestinal) immunoglobulin G (IgG) and IgA responses. These immune sera exhibit >50% ex vivo neutralizing activity against both autologous and heterologous primary isolates. Furthermore, the administration of HIV-VLP(A)s by the i.n. immunization route induces a specific CTL activity, although at lower efficiency than the i.p. route. The HIV-VLP(A)s represent an efficient strategy to stimulate both arms of immunity; furthermore, the induction of specific humoral immunity at mucosal sites, which nowadays represent the main port of entry for HIV-1 infection, is of great interest. All these properties, and the possible cross-clade in vivo protection, could make these HIV-VLP(A)s a good candidate for a mono- and multicomponent worldwide preventive vaccine approach not restricted to high-priority regions, such as sub-Saharan countries.

Human immunodeficiency virus (HIV)-1 proteins and cytoskeleton: partners in viral life and host cell death

Cytoskeletal components play a major role in the human immunodeficiency virus-1 (HIV-1) infection. A wide variety of molecules belonging to the microfilament system, including actin filaments and actin binding proteins, as well as microtubules have a key role in regulating both cell life and death. Cell shape maintenance, cell polarity and cell movements as well as cytoplasmic trafficking of molecules determining cell fate, including apoptosis, are in fact instructed by the cytoskeleton components. HIV infection and viral particle production seem to be controlled by cytoskeleton as well. Furthermore, HIV-associated apoptosis failure can also be regulated by the actin network function. In fact, HIV protein gp120 is able to induce cytoskeleton-driven polarization, thus sensitizing T cells to CD95/Fas-mediated apoptosis. The microfilament system seems thus to be a sort of cytoplasmic supervisor of the viral particle, the host cell and the bystander cell's very fate.

HIV-1 cell to cell transfer across an Env-induced, actin-dependent synapse

Direct cell–cell transfer is an efficient mechanism of viral dissemination within an infected host, and human immunodeficiency virus 1 (HIV-1) can exploit this mode of spread. Receptor recognition by HIV-1 occurs via interactions between the viral surface envelope glycoprotein (Env), gp120, and CD4 and a chemokine receptor, CCR5 or CXCR4. Here, we demonstrate that the binding of CXCR4-using HIV-1–infected effector T cells to primary CD4⁺/CXCR4⁺ target T cells results in rapid recruitment to the interface of CD4, CXCR4, talin, and lymphocyte function–associated antigen 1 on the target cell, and of Env and Gag on the effector cell. Recruitment of these membrane molecules into polarized clusters was dependent on Env engagement of CD4 and CXCR4 and required remodelling of the actin cytoskeleton. Transfer of Gag from effector to target cell was observed by 1 h after conjugate formation, was independent of cell–cell fusion, and was probably mediated by directed virion fusion with the target cell. We propose that receptor engagement by Env directs the rapid, actin-dependent recruitment of HIV receptors and adhesion molecules to the interface, resulting in a stable adhesive junction across which HIV infects the target cell.

Dangerous liaisons at the virological synapse

Cell-to-cell viral transmission facilitates the propagation of HIV-1 and human T cell leukemia virus type 1. Mechanisms of cell-to-cell transmission by retroviruses were not well understood until the recent description of virological synapses (VSs). VSs function as specialized sites of immune cell-to-cell contact that direct virus infection. Deciphering the molecular mechanisms of VS formation provides a fascinating insight into how pathogens subvert immune cell communication programs and achieve viral spread.

The tyrosine-based YXXØ targeting motif of murine leukemia virus envelope glycoprotein affects pathogenesis

Retroviruses, such as human and simian immunodeficiency viruses (HIV and SIV), and murine leukemia viruses (MuLV), harbor a tyrosine-based motif in the intracytoplasmic domain of their envelope glycoprotein. This motif can act as an endocytosis signal or as a targeting signal, restricting viral budding at specific cell surface membrane domains. In the present study, proviral DNA of the ecotropic Cas-Br-E strain of MuLV was modified by substitution or deletion of the critical tyrosine residue. Mutant viruses lost basolateral targeting in polarized MDCK epithelial cells while expression level of the glycoprotein at the cell surface was not affected. This suggests that the tyrosine-based motif in MuLV does not act as an endocytosis signal. Only a small delay in the appearance of disease was observed in inoculated mice. In contrast, a striking change in the pathology was observed with enlarged thymus and lymph nodes in animals inoculated with mutant viruses.

Spread of hepatitis B viruses in vitro requires extracellular progeny and may be codetermined by polarized egress

Viruses can spread by different mechanisms: via intracellular particles through cell junctions to neighboring cells or via secreted virions to adjacent or remote cells. The observation of clusters of hepadnavirus-infected cells both in vivo and in primary hepatocytes neither proves the first mechanism nor excludes the second. In order to test which mechanism, if not both, is used by hepatitis B viruses in order to spread, we used primary duck hepatocytes and duck hepatitis B virus (DHBV) as an infection model. If extracellular progeny virus alone determines spreading, neutralizing antisera or drugs blocking virus binding to hepatocytes should abolish secondary infection. In order to test this, we used DHBV envelope-specific neutralizing antisera, as well as suramin, a known inhibitor of infection. Both reagents strongly reduced hepatocellular attachment of viral particles and almost completely abolished primary infection, whereas an ongoing intracellular infection was not affected as long as no progeny virus was released. In contrast, incubation of infected primary hepatocytes with these reagents during release of progeny virus completely prevented secondary infection. Moreover, the combination of electron and immunofluorescence microscopy analyses revealed the residence of viral particles in cytoplasmic vesicles preferentially located near the basolateral membrane of infected hepatocytes. Taken together, these data strongly suggest that hepatitis B viruses mainly spread by secreted, extracellular progeny and point to polarized egress of viral particles into intercellular compartments, which restricts their diffusion and favors transmission of virus to adjacent cells.

Mucosal AIDS vaccines

Debates are still being waged over what is the best strategy for developing a potent AIDS vaccine. All the obvious approaches to making AIDS vaccines have been tried in the past two decades without much success. It is clear that new thinking and a revision of prevailing dogmas needs to be in place if we really want a vaccine. Conventional envelope-based antibody-inducing vaccines do not appear to hold promise, and broadly-neutralizing antibodies are now being searched as an alternative to the failed approach with subunit vaccines. The current consensus is that cellular immune responses, especially those mediated by CD8 cytotoxic/suppressor (CTL) and CD4 helper T lymphocytes, are needed to control HIV. Vaccines capable of inducing cell-mediated responses are, therefore, considered critical for controlling the spread of HIV. DNA-based vaccines triggering CTL reaction are currently thought to be an answer, but will they fulfill the promise? In the following paragraphs, a critical assessment of the state of the art will be provided in an attempt to analyze what we know and still don't know. The focus of this review is primarily on mucosal vaccines-a relatively new area in AIDS research. The update on V-1 Immunitor, the first mucosal AIDS vaccine available commercially, is provided within this context. Some of the reviewed concepts may be disputable, but without departure from the uninspiring consensus no substantial progress in the AIDS vaccine field can be envisioned.

Human immunodeficiency virus type 1 infection and replication in normal human oral keratinocytes

Recent epidemiologic studies show increasing human immunodeficiency virus type 1 (HIV-1) transmission through oral-genital contact. This paper examines the possibility that normal human oral keratinocytes (NHOKs) might be directly infected by HIV or might convey infectious HIV virions to adjacent leukocytes. PCR analysis of proviral DNA constructs showed that NHOKs can be infected by CXCR4-tropic (NL4-3 and ELI) and dualtropic (89.6) strains of HIV-1 to generate a weak but productive infection. CCR5-tropic strain Ba-L sustained minimal viral replication. Antibody inhibition studies showed that infection by CXCR4-tropic viral strains is mediated by the galactosylceramide receptor and the CXCR4 chemokine coreceptor. Coculture studies showed that infectious HIV-1 virions can also be conveyed from NHOKs to activated peripheral blood lymphocytes, suggesting a potential role of oral epithelial cells in the transmission of HIV infection.

Target cells in vaginal HIV transmission

Understanding the mechanisms of HIV transmission to women will be crucial to the development of effective strategies to curb this epidemic. Current data suggest that HIV has at least two routes to penetrate the vaginal epithelium and reach lymphoid tissues, trans-epithelial migration of infected Langerhans cells or virus penetration into the lamina propria through loss of epithelial integrity resulting in direct infection of lymphocytes, dendritic cells and macrophages.

Enhanced transcytosis of R5-tropic human immunodeficiency virus across tight monolayer of polarized human endometrial cells under pro-inflammatory conditions

Most cases of human immunodeficiency virus (HIV) infection worldwide occur following sexual contact, implying that the virus may breach the protective epithelial barrier lining the genital tract. HIV infection is known to preferentially occur when the genital epithelial integrity is altered, particularly when epithelial micro-ulcerations occur during heterosexual intercourse or ulcerations appear, due to sexually transmitted infections or else in the context of ectopy of the endocervical mucosa, which may leave the genital tissue. We report that R5-tropic infectious HIV-1 isolates are capable of in vitro transcytosis through a tight and polarized monolayer of human endometrial HEC-1 cells. Transcytosis of HIV particles was increased 2-fold within a pro-inflammatory micro-environment. Our findings suggest that transcytosis may be a relevant mechanism for the passage of virus through the genital mucosa in vivo, particularly when inflammatory cells and mediators are present in the vicinity of the mucosal surface.

Transepithelial transport of HIV-1 by M cells is receptor-mediated

Human colon carcinoma Caco-2 cell monolayers undergo conversion into cells that share morphological and functional features of M cells when allowed to interact with B lymphocytes. A lymphotropic (X4) HIV-1 strain crosses M cell monolayers and infects underlying CD4(+) target cells. Transport requires both lactosyl cerebroside and CXCR4 receptors, which are expressed on the apical surface of Caco-2 and M cells. Antibodies specific for each receptor block transport. In contrast, a monotropic (R5) HIV-1 strain is unable to cross M cell monolayers and infect underlying monocytes, despite efficient transport of latex beads. Caco-2 and M cells do not express CCR5, but transfection of these cells with CCR5 cDNA restores transport of R5 virus, which demonstrates that HIV-1 transport across M cells is receptor-mediated. The follicle-associated epithelium covering human gut lymphoid follicles expresses CCR5, but not CXCR4, and lactosyl cerebroside, suggesting that HIV-1 infection may occur through M cells and enterocytes at these sites.

Early pathogenesis of transmucosal feline immunodeficiency virus infection

To identify the early target cells and tissues in transmucosal feline immunodeficiency virus (FIV) infection, cats were exposed to a clade C FIV isolate via the oral-nasal or vaginal mucosa and multiple tissues were examined by virus isolation coculture (VI), DNA PCR, catalyzed tyramide signal-amplified in situ hybridization (TSA-ISH), and immunohistochemistry between days 1 and 12 postinoculation (p.i.). FIV RNA was detected in tonsil and oral or vaginal mucosa as early as 1 day p.i. by TSA-ISH and in retropharyngeal, tracheobronchial, or external iliac lymph nodes and sometimes in spleen or blood mononuclear cells by day 2, indicating that regional and distant spread of virus-infected cells occurred rapidly after mucosal exposure. By day 8, viral RNA, DNA, and culturable virus were uniformly detected in regional and distant tissues, connoting systemic infection. TSA-ISH proved more sensitive than DNA PCR in detecting early FIV-infected cells. In mucosal tissues, the earliest demonstrable FIV-bearing cells were either within or subjacent to the mucosal epithelium or were in germinal centers of regional lymph nodes. The FIV(+) cells were of either of two morphological types, large stellate or small round. Those FIV RNA(+) cells which could be colabeled for a phenotype marker, were labeled for either dendritic-cell-associated protein p55 or T-lymphocyte receptor antigen CD3. These studies indicate that FIV crosses mucous membranes within hours after exposure and rapidly traffics via dendritic and T cells to systemic lymphoid tissues, a pathway similar to that thought to occur in the initial phase of infection by the human and simian immunodeficiency viruses.

Cell-associated HIV-1 infection of salivary gland epithelial cell lines

In this study we investigated the potential of cell-associated primary isolates of HIV-1 to infect epithelial cells of the oral cavity, using two cell lines originating in the salivary gland, HSY and HSG. Neither of these cell lines expresses surface CD4; however, both express the alternative epithelial receptor galactosylceramide and the coreceptor CXCR4. The ability of these cell lines to be infected via cell-associated infection was studied using a lab-adapted strain and primary isolates of HIV-1. Both HIV-1 phenotypes were able to infect these cell lines. The ligand for CXCR4 was able to block infection of both cell lines, whereas the ligands for CCR5 were able to block infection in HSY only, indicating that these receptors were utilized for entry into the epithelial cells. These findings demonstrate that epithelial cells of the oral cavity can be productively infected with HIV-1 by cell-borne virus in the absence of CD4.

Practical prevention of vaginal and rectal transmission of HIV by adapting the oral defense: use of commercial lubricants

HIV is transmitted to 6.4 million human beings per year and the majority of these transmissions are sexual. Condoms are highly effective and are recommended as the primary preventive. However, the fact that there are millions of sexual transmissions each year indicates that many people do not use condoms and that additional preventives are needed. The mechanisms of natural prevention of oral transmission by saliva may be adaptable to the susceptible vagina and rectum. The objective of our study was to reduce the sexual transmission of HIV by mimicking saliva's targeting of the transmitting infected leukocytes and any cell-free HIV in seminal fluid. The previously recommended anti-HIV topical microbicide, nonoxynol-9, has not prevented HIV transmission in humans, probably because it causes mucosal irritation and attracts CD4(+) cells. To identify effective preparations that are nonirritating, we studied the anti-HIV activity of commercially available, over-the-counter (OTC) lubricants and vaginal preparations that are judged safest by the U.S. Food and Drug Administration (FDA), and are nonirritating. The effect of OTC preparations on both the production of HIV by infected leukocytes and cell-free HIV suspended in seminal fluid was measured under simulated in vivo conditions. We surveyed 22 OTC vaginal preparations and excluded those with low inhibitory activity and those that were inhibitory but likely to be irritating. Three included preparations are highly active against both HIV-infected leukocytes suspended in seminal fluid and active against cell-free HIV, under in vitro conditions that simulate in vivo conditions. Since the preparations identified here as anti-HIV substances have the advantages of being widely available, inexpensive, acceptable, in the safest U.S. FDA category, and may be used by recipient women or men, they should be tested in clinical trials to help prevent sexual transmission of HIV.

Active and selective transcytosis of cell-free human immunodeficiency virus through a tight polarized monolayer of human endometrial cells

We report that both primary and laboratory-adapted infectious human immunodeficiency virus type 1 (HIV-1) isolates in a cell-free form are capable of transcytosis through a tight and polarized monolayer of human endometrial cells. Trancytosis of cell-free HIV occurs in a strain-selective fashion and appears to be dependent on interactions between HIV envelope glycoproteins and lectins on the apical membrane of the epithelial cells. These findings provide new insights into the initial events occurring during heterosexual transmission of the virus.

Amplification of extracellular matrix and oncogenes in tat-transfected human salivary gland cell lines with expression of laminin, fibronectin, collagens I, III, IV, c-myc and p53

Considerable progress has been made in the transfer of foreign genes into salivary glands in vivo using adenovirus vectors in rats. In an attempt to avoid the transient expression inherent, when using these vectors, retroviral vectors and human cell lines where used here in attempt to develop an in vitro model of HIV-associated salivary gland disease. The HIV-1-tat protein is increasingly implicated in the pathogenesis of the AIDS through altering the expression of strategic cellular genes. The purpose of this study was to transfect human salivary gland (HSG) cell lines in vitro, with the pHIV-1/LTR-tat plasmid, and examine the effect of tat on expression of matrix and basement membrane genes known to be important in the pathogenesis of salivary gland disease. HSG cells were transfected with HIV-1-tat plasmid by the lipofection method. Transfection was confirmed by polymerase chain reaction (PCR) and Southern blot, which verified that tat-specific DNA was present. Tat-mRNA was analysed by Northern blotting and quantified by reverse transcriptase polymerase chain reaction (RT-PCR) to demonstrate its expression. Numerous clones were found to contain integrated tat DNA sequences and analysis of mRNA showed stable expression of tat-specific RNA. Further analysis of mRNA expression for various marker proteins important in HIV pathogenesis showed that the HSG cell line transfected with HIV-1-tat, was associated with significant induction of mRNA expression for extracellular matrix protein. Tat-amplified transcription of the major basement membrane protein laminin, as well as of fibronectin, collagen I and III, and c-myc oncogene was demonstrated. Conversely, expression of p53 suppressor gene mRNA was reduced. Post-transfection expression of collagen IV was erratic and inconclusive. It was concluded that the presence of HIV-tat in this in vitro model of salivary ductal epithelial cell model alters the mRNA expression of several matrix, basement membrane and oncoproteins known to be involved in HIV pathogenesis. These cell lines provide a useful system for studying the role of tat in the immunopathogenesis of HIV-associated salivary gland disease.

Innate antiviral defenses in body fluids and tissues

Innate, non-specific, resistance mechanisms are important barriers to pathogens, particularly delaying virus multiplication at the onset of infections. These innate defense mechanisms include a series of mechanical barriers, pre-existing inhibitory molecules, and cellular responses with antimicrobial activity. The antiviral activities of these innate inhibitors reside in a variety of partly characterized substances. This review presents the innate antiviral inhibitors in cell cultures, urine, serum, the gastrointestinal tract, the nervous system, tissues of crustaceans, and saliva. Medical adaptation of the innate antiviral defense mechanisms may be useful for prevention and treatment of viral infections.

Parameters of human immunodeficiency virus infection of human cervical tissue and inhibition by vaginal virucides

Heterosexual transmission of human immunodeficiency virus (HIV) is the most frequent mode of infection worldwide. However, the immediate events between exposure to infectious virus and establishment of infection are still poorly understood. This study investigates parameters of HIV infection of human female genital tissue in vitro using an explant culture model. In particular, we investigated the role of the epithelium and virucidal agents in protection against HIV infection. We have demonstrated that the major target cells of infection reside below the genital epithelium, and thus HIV must cross this barrier to establish infection. Immune activation enhanced HIV infection of such subepithelial cells. Furthermore, our data suggest that genital epithelial cells were not susceptible to HIV infection, appear to play no part in the transfer of infectious virus across the epithelium, and thus may provide a barrier to infection. In addition, experiments using a panel of virucidal agents demonstrated differential efficiency to block HIV infection of subepithelial cells from partial to complete inhibition. This is the first demonstration that virucidal agents designed for topical vaginal use block HIV infection of genital tissue. Such agents have major implications for world health, as they will provide women with a mechanism of personal and covert protection from HIV infection.

Secretory leukocyte protease inhibitor inhibits infection of monocytes and lymphocytes with human immunodeficiency virus type 1 but does not interfere with transcytosis of cell-associated virus across tight epithelial barriers

In the present study, we demonstrate that recombinant human secretory leukocyte protease inhibitor (rhSLPI) inhibits infection of lymphocyte- and monocyte-derived tumor cell lines and peripheral blood lymphocytes with laboratory-adapted isolates and with the primary isolate, NDK, of free human immunodeficiency virus type 1 (HIV-1). In contrast, rhSLPI did not exhibit inhibitory activity toward transcytosis of cell-associated HIV-1 through a tight monolayer of endometrial epithelial cells. These observations indicate that the inhibitory effect of SLPI is restricted to free HIV-1 in corporal fluids.