DC-SIGN, a dendritic cell-specific HIV-1-binding protein that enhances trans-infection of T cells

Dendritic cells (DC) capture microorganisms that enter peripheral mucosal tissues and then migrate to secondary lymphoid organs, where they present these in antigenic form to resting T cells and thus initiate adaptive immune responses. Here, we describe the properties of a DC-specific C-type lectin, DC-SIGN, that is highly expressed on DC present in mucosal tissues and binds to the HIV-1 envelope glycoprotein gp120. DC-SIGN does not function as a receptor for viral entry into DC but instead promotes efficient infection in trans of cells that express CD4 and chemokine receptors. We propose that DC-SIGN efficiently captures HIV-1 in the periphery and facilitates its transport to secondary lymphoid organs rich in T cells, to enhance infection in trans of these target cells.

Gastrointestinal tract as a major site of CD4+ T cell depletion and viral replication in SIV infection

Human and simian immunodeficiency virus (HIV and SIV) replicate optimally in activated memory CD4(+) T cells, a cell type that is abundant in the intestine. SIV infection of rhesus monkeys resulted in profound and selective depletion of CD4+ T cells in the intestine within days of infection, before any such changes in peripheral lymphoid tissues. The loss of CD4+ T cells in the intestine occurred coincident with productive infection of large numbers of mononuclear cells at this site. The intestine appears to be a major target for SIV replication and the major site of CD4+ T cell loss in early SIV infection.

Multiple organ infection and the pathogenesis of SARS

After >8,000 infections and >700 deaths worldwide, the pathogenesis of the new infectious disease, severe acute respiratory syndrome (SARS), remains poorly understood. We investigated 18 autopsies of patients who had suspected SARS; 8 cases were confirmed as SARS. We evaluated white blood cells from 22 confirmed SARS patients at various stages of the disease. T lymphocyte counts in 65 confirmed and 35 misdiagnosed SARS cases also were analyzed retrospectively. SARS viral particles and genomic sequence were detected in a large number of circulating lymphocytes, monocytes, and lymphoid tissues, as well as in the epithelial cells of the respiratory tract, the mucosa of the intestine, the epithelium of the renal distal tubules, the neurons of the brain, and macrophages in different organs. SARS virus seemed to be capable of infecting multiple cell types in several organs; immune cells and pulmonary epithelium were identified as the main sites of injury. A comprehensive theory of pathogenesis is proposed for SARS with immune and lung damage as key features.

Control of early viral and bacterial distribution and disease by natural antibodies

Natural antibodies are often dismissed from immunological analysis as "background," but they may play an important role in conferring immunity against infections. In antibody-free mice infected with various viruses or with Listeria monocytogenes, viral or bacterial titers in peripheral organs, including the kidney and brain, were 10 to 100 times greater than in antibody-competent mice (and enhanced their susceptibility to some infections), and titers in secondary lymphoid organs were 10 to 100 times lower than in antibody-competent mice. Thus, natural antibodies play a crucial role by preventing pathogen dissemination to vital organs and by improving immunogenicity through enhanced antigen-trapping in secondary lymphoid organs.

Quantitative image analysis of HIV-1 infection in lymphoid tissue

Tracking human immunodeficiency virus-type 1 (HIV-1) infection at the cellular level in tissue reservoirs provides opportunities to better understand the pathogenesis of infection and to rationally design and monitor therapy. A quantitative technique was developed to determine viral burden in two important cellular compartments in lymphoid tissues. Image analysis and in situ hybridization were combined to show that in the presymptomatic stages of infection there is a large, relatively stable pool of virions on the surfaces of follicular dendritic cells and a smaller pool of productively infected cells. Despite evidence of constraints on HIV-1 replication in the infected cell population in lymphoid tissues, estimates of the numbers of these cells and the virus they could produce are consistent with the quantities of virus that have been detected in the bloodstream. The cellular sources of virus production and storage in lymphoid tissues can now be studied with this approach over the course of infection and treatment.

Increased production of IL-7 accompanies HIV-1-mediated T-cell depletion: implications for T-cell homeostasis

We hypothesized that HIV-1-mediated T-cell loss might induce the production of factors that are capable of stimulating lymphocyte development and expansion. Here we perform cross-sectional (n = 168) and longitudinal (n = 11) analyses showing that increased circulating levels of interleukin (IL)-7 are strongly associated with CD4+ T lymphopenia in HIV-1 disease. Using immunohistochemistry with quantitative image analysis, we demonstrate that IL-7 is produced by dendritic-like cells within peripheral lymphoid tissues and that IL-7 production by these cells is greatly increased in lymphocyte-depleted tissues. We propose that IL-7 production increases as part of a homeostatic response to T-cell depletion.

The dynamics of CD4+ T-cell depletion in HIV disease

The size and composition of the CD4+ T-cell population is regulated by balanced proliferation of progenitor cells and death of mature progeny. After infection with the human immunodeficiency virus, this homeostasis is often disturbed and CD4+ T cells are instead depleted. Such depletion cannot result simply from accelerated destruction of mature CD4+ T cells - sources of T-cell production must also fail. Ironically, this failure may be precipitated by physiological mechanisms designed to maintain homeostasis in the face of accelerated T-cell loss.

Population biology of HIV-1 infection: viral and CD4+ T cell demographics and dynamics in lymphatic tissues

Human immunodeficiency virus-1 (HIV-1) is usually transmitted through sexual contact and in the very early stages of infection establishes a persistent infection in lymphatic tissues (LT). Virus is produced and stored at this site in a dynamic process that slowly depletes the immune system of CD4+ T cells, setting the stage for AIDS. In this review, I describe the changes in viral and CD4+ T cell populations in LT over the course of infection and after treatment. I present recent evidence that productively infected CD4+ T cells play an important role in establishing persistent infection from the onset, and that the LT are the major reservoir where virus is produced and stored on follicular dendritic cells (FDCs). I discuss the methods used to define the size of viral and CD4+ T cell populations in LT and the nature of virus-host cell interactions in vivo. These experimental approaches have identified populations of latently and chronically infected cells in which virus can elude host defenses, perpetuate infection, and escape eradication by highly active antiretroviral treatment (HAART). I discuss the dramatic impact of HAART on suppressing virus production, reducing the pool of stored virus, and restoring CD4+ T cell populations. I discuss the contributions of thymopoiesis and other renewal mechanisms, lymphatic homeostasis and trafficking to these changes in CD4+ T cell populations in LT, and conclude with a model of immune depletion and repopulation based on the limited regenerative capacity of the adult and the uncompensated losses of productively infected cells that treatment stems. The prediction of this model is that immune regeneration will be slow, variable, and partial. It is nonetheless encouraging to know that even in late stages of infection, control of active replication of HIV-1 provides an opportunity for the immune system to recover from the injuries inflicted by infection.

Nef harbors a major determinant of pathogenicity for an AIDS-like disease induced by HIV-1 in transgenic mice

Transgenic (Tg) mice expressing the complete coding sequences of HIV-1 in CD4+ T cells and in cells of the macrophage/dendritic lineages develop severe AIDS-like pathologies: failure to thrive/weight loss, diarrhea, wasting, premature death, thymus atrophy, loss of CD4+ T cells, interstitial pneumonitis, and tubulo-interstitial nephritis. The generation of Tg mice expressing selected HIV-1 gene(s) revealed that nef harbors a major disease determinant. The latency and progression (fast/slow) of the disease were strongly correlated with the levels of Tg expression. Nef-expressing Tg thymocytes were activated and alpha-CD3 hyperresponsive with respect to tyrosine phosphorylation of several substrates, including LAT and MAPK. The similarity of this mouse model to human AIDS, particularly pediatric AIDS, suggests that Nef may play a critical role in human AIDS, independently of its role in virus replication.

Propagation and dissemination of infection after vaginal transmission of simian immunodeficiency virus

In the current global AIDS pandemic, more than half of new human immunodeficiency virus type 1 (HIV-1) infections are acquired by women through intravaginal HIV exposure. For this study, we explored pathogenesis issues relevant to the development of effective vaccines to prevent infection by this route, using an animal model in which female rhesus macaques were exposed intravaginally to a high dose of simian immunodeficiency virus (SIV). We examined in detail the events that transpire from hours to a few days after intravaginal SIV exposure through week 4 to provide a framework for understanding the propagation, dissemination, and establishment of infection in lymphatic tissues (LTs) during the acute stage of infection. We show that the mucosal barrier greatly limits the infection of cervicovaginal tissues, and thus the initial founder populations of infected cells are small. While there was evidence of rapid dissemination to distal sites, we also show that continuous seeding from an expanding source of production at the portal of entry is likely critical for the later establishment of a productive infection throughout the systemic LTs. The initially small founder populations and dependence on continuous seeding to establish a productive infection in systemic LTs define a small window of maximum vulnerability for the virus in which there is an opportunity for the host, vaccines, or other interventions to prevent or control infection.

Abortive HIV infection mediates CD4 T cell depletion and inflammation in human lymphoid tissue

The mechanism by which CD4 T cells are depleted in HIV-infected hosts remains poorly understood. In ex vivo cultures of human tonsil tissue, CD4 T cells undergo a pronounced cytopathic response following HIV infection. Strikingly, >95% of these dying cells are not productively infected but instead correspond to bystander cells. We now show that the death of these "bystander" cells involves abortive HIV infection. Inhibitors blocking HIV entry or early steps of reverse transcription prevent CD4 T cell death while inhibition of later events in the viral life cycle does not. We demonstrate that the nonpermissive state exhibited by the majority of resting CD4 tonsil T cells leads to accumulation of incomplete reverse transcripts. These cytoplasmic nucleic acids activate a host defense program that elicits a coordinated proapoptotic and proinflammatory response involving caspase-3 and caspase-1 activation. While this response likely evolved to protect the host, it centrally contributes to the immunopathogenic effects of HIV.

HIV-associated chronic immune activation

Systemic chronic immune activation is considered today as the driving force of CD4(+) T-cell depletion and acquired immunodeficiency syndrome (AIDS). A residual chronic immune activation persists even in HIV-infected patients in which viral replication is successfully inhibited by anti-retroviral therapy, with the extent of this residual immune activation being associated with CD4(+) T-cell loss. Unfortunately, the causal link between chronic immune activation and CD4(+) T-cell loss has not been formally established. This article provides first a brief historical overview on how the perception of the causative role of immune activation has changed over the years and lists the different kinds of immune activation characteristic of human immunodeficiency virus (HIV) infection. The mechanisms proposed to explain the chronic immune activation are multiple and are enumerated here, as well as the mechanisms proposed on how chronic immune activation could lead to AIDS. In addition, we summarize the lessons learned from natural hosts that know how to 'show AIDS the door', and discuss how these studies informed the design of novel immune modulatory interventions that are currently being tested. Finally, we review the current approaches aimed at targeting chronic immune activation and evoke future perspectives.

Defining total-body AIDS-virus burden with implications for curative strategies

In the quest for a functional cure or the eradication of HIV infection, it is necessary to know the sizes of the reservoirs from which infection rebounds after treatment interruption. Thus, we quantified SIV and HIV tissue burdens in tissues of infected nonhuman primates and lymphoid tissue (LT) biopsies from infected humans. Before antiretroviral therapy (ART), LTs contained >98% of the SIV RNA+ and DNA+ cells. With ART, the numbers of virus (v) RNA+ cells substantially decreased but remained detectable, and their persistence was associated with relatively lower drug concentrations in LT than in peripheral blood. Prolonged ART also decreased the levels of SIV- and HIV-DNA+ cells, but the estimated size of the residual tissue burden of 108 vDNA+ cells potentially containing replication-competent proviruses, along with evidence of continuing virus production in LT despite ART, indicated two important sources for rebound following treatment interruption. The large sizes of these tissue reservoirs underscore challenges in developing 'HIV cure' strategies targeting multiple sources of virus production.

HIV-1-driven regulatory T-cell accumulation in lymphoid tissues is associated with disease progression in HIV/AIDS

Regulatory T (Treg) cells accumulate in the lymphoid tissues of human immunodeficiency virus (HIV)-infected individuals, contributing to the inability of the immune system to control virus replication. We investigate here Treg-cell numbers and functional markers (FOXP3, CTLA-4, IDO, and TGF-beta1) in lymphoid tissues from untreated infected hosts with progressive or nonprogressive disease (HIV-infected humans and simian immunodeficiency virus [SIV]-infected macaques). We found that increased numbers of FOXP3(+) T cells as well as increased expression of Treg-cell-associated functional markers were detected only during progressive disease. Such increases were not correlated with immune activation. Of importance, a high-perforin/FOXP3 ratio was associated with nonprogressive disease, suggesting that the immune control of virus replication represents a balance between cell-mediated immune responses and Treg-cell-mediated counter regulation of such responses. Furthermore, using an in vitro model of Treg-cell-HIV interactions, we showed that exposure of Treg cells to HIV selectively promoted their survival via a CD4-gp120-dependent pathway, thus providing an underlying mechanism for the accumulation of Treg cells in infected hosts with active viral replication. Considered together, our findings imply that therapeutic manipulation of Treg-cell number and/or function could improve immune control of HIV infection.

Immunohistochemical detection of prion protein in lymphoid tissues of sheep with natural scrapie

The scrapie-associated form of the prion protein (PrPSc) accumulates in the brain and lymphoid tissues of sheep with scrapie. In order to assess whether detecting PrPSc in lymphoid tissue could be used as a diagnostic test for scrapie, we studied the localization and distribution of PrPSc in various lymphoid tissues collected at necropsy from 55 sheep with clinical scrapie. Samples collected from the spleen, palatine tonsil, ileum, and five different lymph nodes were immunohistochemically stained for PrPSc. PrPSc was found to be deposited in a reticular pattern in the center of both primary and secondary lymphoid follicles. In addition, granules of PrPSc were seen in the cytoplasm in macrophages associated with the lymphoid follicles. In 54 (98%) of the 55 scrapie-affected sheep, PrPSc was detected in the spleen, retropharyngeal lymph node, mesenteric lymph node, and the palatine tonsil. However, only in the palatine tonsils was PrPSc present in a consistently high percentage of the lymphoid follicles. PrP was not detected in any of the lymphoid tissues of 12 sheep that had no neurohistopathological signs of a scrapie infection. We conclude that the tonsils are the best-suited lymphoid tissue to be biopsied for the detection of PrPSc in the diagnosis of clinical scrapie in living sheep.

Nonredundant roles for B cell-derived IL-10 in immune counter-regulation

IL-10 plays a central role in restraining the vigor of inflammatory responses, but the critical cellular sources of this counter-regulatory cytokine remain speculative in many disease models. Using a novel IL-10 transcriptional reporter mouse, we found an unexpected predominance of B cells (including plasma cells) among IL-10-expressing cells in peripheral lymphoid tissues at baseline and during diverse models of in vivo immunological challenge. Use of a novel B cell-specific IL-10 knockout mouse revealed that B cell-derived IL-10 nonredundantly decreases virus-specific CD8(+) T cell responses and plasma cell expansion during murine cytomegalovirus infection and modestly restrains immune activation after challenge with foreign Abs to IgD. In contrast, no role for B cell-derived IL-10 was evident during endotoxemia; however, although B cells dominated lymphoid tissue IL-10 production in this model, myeloid cells were dominant in blood and liver. These data suggest that B cells are an underappreciated source of counter-regulatory IL-10 production in lymphoid tissues, provide a clear rationale for testing the biological role of B cell-derived IL-10 in infectious and inflammatory disease, and underscore the utility of cell type-specific knockouts for mechanistic limning of immune counter-regulation.

Virus quantification and identification of cellular targets in the lungs and lymphoid tissues of pigs at different time intervals after inoculation with porcine reproductive and respiratory syndrome virus (PRRSV)

Sixteen 6 week old conventional pigs were inoculated by aerosol with a European strain of porcine reproductive and respiratory syndrome virus (PRRSV). Virus replication was followed by virus titration and immunofluorescence in the lungs and in associated and distant lymphoid tissues at 3, 14, 21, 35, 42 and 82 days post-inoculation (DPI). PRRSV replication was detected in alveolar macrophages, lungs, tonsils, spleen, retropharyngeal lymph nodes, bronchial lymph nodes and thoracic aortic lymph nodes at 3 DPI. The same tissues, except retropharyngeal and thoracic aortic lymph nodes, were PRRSV positive at 14 DPI. Lungs and alveolar macrophages were PRRSV positive until 35 DPI. PRRSV was not detected in heart, peripheral blood mononuclear cells and bone marrow cells. Viremia was detected from 3 to 28 DPI. Not more than 2% of alveolar macrophages were PRRSV positive even during the acute stage of infection. 80 to 94% of the PRRSV infected cells in the lungs and in lung lavaged cells were identified as macrophages using a porcine macrophage specific monoclonal antibodies. In the lymph nodes and spleen, 100% of the infected cells were macrophages. Anti-PRRSV antibodies were detected by a blocking ELISA as early as 7 DPI. the antibody titre gradually increased to reach a geometric mean titre (GMT) of 160 at 35 DPI. It remained at that level until the end of the study. These findings clearly demonstrate that PRRSV has a tropism for macrophages. PRRSV mainly replicates in macrophages of the lymphoid tissues and lungs in the acute phase of infection and persists in the lung macrophages.

Large number of rebounding/founder HIV variants emerge from multifocal infection in lymphatic tissues after treatment interruption

Antiretroviral therapy (ART) suppresses HIV replication in most individuals but cannot eradicate latently infected cells established before ART was initiated. Thus, infection rebounds when treatment is interrupted by reactivation of virus production from this reservoir. Currently, one or a few latently infected resting memory CD4 T cells are thought be the principal source of recrudescent infection, but this estimate is based on peripheral blood rather than lymphoid tissues (LTs), the principal sites of virus production and persistence before initiating ART. We, therefore, examined lymph node (LN) and gut-associated lymphoid tissue (GALT) biopsies from fully suppressed subjects, interrupted therapy, monitored plasma viral load (pVL), and repeated biopsies on 12 individuals as soon as pVL became detectable. Isolated HIV RNA-positive (vRNA+) cells were detected by in situ hybridization in LTs obtained before interruption in several patients. After interruption, multiple foci of vRNA+ cells were detected in 6 of 12 individuals as soon as pVL was measureable and in some subjects, in more than one anatomic site. Minimal estimates of the number of rebounding/founder (R/F) variants were determined by single-gene amplification and sequencing of viral RNA or DNA from peripheral blood mononuclear cells and plasma obtained at or just before viral recrudescence. Sequence analysis revealed a large number of R/F viruses representing recrudescent viremia from multiple sources. Together, these findings are consistent with the origins of recrudescent infection by reactivation from many latently infected cells at multiple sites. The inferred large pool of cells and sites to rekindle recrudescent infection highlights the challenges in eradicating HIV.

Macrophage are the principal reservoir and sustain high virus loads in rhesus macaques after the depletion of CD4+ T cells by a highly pathogenic simian immunodeficiency virus/HIV type 1 chimera (SHIV): Implications for HIV-1 infections of humans

The highly pathogenic simian immunodeficiency virus/HIV type 1 (SHIV) chimeric virus SHIV(DH12R) induces a systemic depletion of CD4(+) T lymphocytes in rhesus monkeys during the initial 3-4 weeks of infection. Nonetheless, high levels of viral RNA production continue unabated for an additional 2-5 months. In situ hybridization and immunohistochemical analyses revealed that tissue macrophage in the lymph nodes, spleen, gastrointestinal tract, liver, and kidney sustain high plasma virus loads in the absence of CD4(+) T cells. Quantitative confocal immunofluorescence analysis indicated that greater than 95% of the virus-producing cells in these tissues are macrophage and less than 2% are T lymphocytes. Interestingly, the administration of a potent reverse transcriptase inhibitor blocked virus production during the early T cell phase but not during the later macrophage phase of the SHIV(DH12R) infection. When interpreted in the context of HIV-1 infections, these results implicate tissue macrophage as an important reservoir of virus in vivo. They become infected during the acute infection, gradually increase in number over time, and can be a major contributor to total body virus burden during the symptomatic phase of the human infection.

Pathogenesis of experimental Ebola virus infection in guinea pigs

The subtype Zaire of Ebola (EBO) virus (Mayinga strain) was adapted to produce lethal infections in guinea pigs. In many ways, the disease was similar to EBO infections in nonhuman primates and humans. The guinea pig model was used to investigate the pathologic events in EBO infection that lead to death. Analytical methods included immunohistochemistry, in situ hybridization, and electron microscopy. Cells of the mononuclear phagocyte system, primarily macrophages, were identified as the early and sustained targets of EBO virus. During later stages of infection, interstitial fibroblasts in various tissues were infected, and there was evidence of endothelial cell infection and fibrin deposition. The distribution of lesions, hematologic profiles, and increases in serum biochemical enzymes associated with EBO virus infection in guinea pigs was similar to reported findings in experimentally infected nonhuman primates and naturally infected humans.

Macrophage tropism of human immunodeficiency virus type 1 isolates from brain and lymphoid tissues predicts neurotropism independent of coreceptor specificity

The viral determinants that underlie human immunodeficiency virus type 1 (HIV-1) neurotropism are unknown, due in part to limited studies on viruses isolated from brain. Previous studies suggest that brain-derived viruses are macrophage tropic (M-tropic) and principally use CCR5 for virus entry. To better understand HIV-1 neurotropism, we isolated primary viruses from autopsy brain, cerebral spinal fluid, blood, spleen, and lymph node samples from AIDS patients with dementia and HIV-1 encephalitis. Isolates were characterized to determine coreceptor usage and replication capacity in peripheral blood mononuclear cells (PBMC), monocyte-derived macrophages (MDM), and microglia. Env V1/V2 and V3 heteroduplex tracking assay and sequence analyses were performed to characterize distinct variants in viral quasispecies. Viruses isolated from brain, which consisted of variants that were distinct from those in lymphoid tissues, used CCR5 (R5), CXCR4 (X4), or both coreceptors (R5X4). Minor usage of CCR2b, CCR3, CCR8, and Apj was also observed. Primary brain and lymphoid isolates that replicated to high levels in MDM showed a similar capacity to replicate in microglia. Six of 11 R5 isolates that replicated efficiently in PBMC could not replicate in MDM or microglia due to a block in virus entry. CD4 overexpression in microglia transduced with retroviral vectors had no effect on the restricted replication of these virus strains. Furthermore, infection of transfected cells expressing different amounts of CD4 or CCR5 with M-tropic and non-M-tropic R5 isolates revealed a similar dependence on CD4 and CCR5 levels for entry, suggesting that the entry block was not due to low levels of either receptor. Studies using TAK-779 and AMD3100 showed that two highly M-tropic isolates entered microglia primarily via CXCR4. These results suggest that HIV-1 tropism for macrophages and microglia is restricted at the entry level by a mechanism independent of coreceptor specificity. These findings provide evidence that M-tropism rather than CCR5 usage predicts HIV-1 neurotropism.

Predominant infection of CD150+ lymphocytes and dendritic cells during measles virus infection of macaques

Measles virus (MV) is hypothesized to enter the host by infecting epithelial cells of the respiratory tract, followed by viremia mediated by infected monocytes. However, neither of these cell types express signaling lymphocyte activation molecule (CD150), which has been identified as the receptor for wild-type MV. We have infected rhesus and cynomolgus macaques with a recombinant MV strain expressing enhanced green fluorescent protein (EGFP); thus bringing together the optimal animal model for measles and a virus that can be detected with unprecedented sensitivity. Blood samples and broncho-alveolar lavages were collected every 3 d, and necropsies were performed upon euthanasia 9 or 15 d after infection. EGFP production by MV-infected cells was visualized macroscopically, in both living and sacrificed animals, and microscopically by confocal microscopy and FACS analysis. At the peak of viremia, EGFP fluorescence was detected in skin, respiratory and digestive tract, but most intensely in all lymphoid tissues. B- and T-lymphocytes expressing CD150 were the major target cells for MV infection. Highest percentages (up to 30%) of infected lymphocytes were detected in lymphoid tissues, and the virus preferentially targeted cells with a memory phenotype. Unexpectedly, circulating monocytes did not sustain productive MV infection. In peripheral tissues, large numbers of MV-infected CD11c+ MHC class-II+ myeloid dendritic cells were detected in conjunction with infected T-lymphocytes, suggesting transmission of MV between these cell types. Fluorescent imaging of MV infection in non-human primates demonstrated a crucial role for lymphocytes and dendritic cells in the pathogenesis of measles and measles-associated immunosuppression.

Experimental inoculation of conventional pigs with porcine reproductive and respiratory syndrome virus and porcine circovirus 2

Postweaning multisystemic wasting syndrome (PMWS) is a disease of nursery and fattening pigs characterized by growth retardation, paleness of the skin, dyspnea, and increased mortality rates. Porcine circovirus 2 (PCV2) has been demonstrated to be the cause of PMWS. However, other factors are needed for full development of the syndrome, and porcine reproductive and respiratory syndrome virus (PRRSV) infection has been suggested to be one of them. Twenty-four conventional 5-week-old pigs were distributed in four groups: control (n = 5), PRRSV inoculated (n = 5), PCV2 inoculated (n = 7), and PRRSV and PCV2 inoculated (n = 7). The two groups inoculated with PRRSV showed growth retardation. Pigs inoculated with both PRRSV and PCV2 had increased rectal temperature. One of these pigs developed wasting, had severe respiratory distress, and died. The most important microscopic lesion in pigs inoculated with PCV2 was lymphocyte depletion with histiocytic infiltration of the lymphoid organs, more severe and in a wider range of tissues in doubly inoculated pigs. Interstitial pneumonia was observed in the three inoculated groups. PCV2 nucleic acid was found by in situ hybridization in larger amounts and in a wider range of lymphoid tissues in PRRSV- and PCV2-inoculated than in PCV2-inoculated pigs. TaqMan PCR was performed to quantify the PCV2 loads in serum during the experiment. PCV2 loads were higher in doubly inoculated pigs than in pigs inoculated with PCV2 alone. These findings indicate that severe disease can be reproduced in conventional 5-week-old pigs by inoculation of PRRSV and PCV2. Moreover, these results support the hypothesis that PRRSV infection enhances PCV2 replication.

Dynamics of viral load rebound and immunological changes after stopping effective antiretroviral therapy

BACKGROUND

This study addresses the dynamic of viral load rebound and immune system changes in a cohort of eight consecutive HIV-1-infected patients in very early stages [all the patients were taking highly active antiretroviral therapy (HAART} and were recruited in the coordinating center from a larger study] who decided to discontinue HAART after 1 year of treatment and effective virologic response. The safety of this procedure and the outcome with reintroduction of the same treatment was also investigated.

METHODS

Plasma, cerebrospinal fluid (CSF), and lymphatic tissue viral loads were measured at baseline; lymphocyte immunophenotyping and CD4 lymphocyte proliferative responses to mitogens and specific antigens were assessed. The same antiretroviral therapy was reintroduced as soon as plasma viral load became detectable (above 200 copies/ml).

RESULTS

At day 0, plasma viral load was below 20 copies/ml in all eight patients (and below 5 copies/ml in five of eight patients). A rebound in plasma viral load was detected in all patients from day 3 to day 31 with a mean doubling time of 2.01 (SE 0.29) days. Three out of eight patients achieved a peak plasma viral load at least 0.5 log10 above baseline, pretreatment values. Mutations associated with resistance to reverse transcriptase or protease inhibitors were not detected. After 31 days off therapy, CD4 lymphocytes decreased [mean 45% (SE 4) to 37% (SE 3); P = 0.04], CD8+CD28+ lymphocytes decreased [mean 59% (SE 5) to 43% (SE 4); P = 0.03], and CD8+CD38+ lymphocytes increased [mean 55% (SE 3) to 66% (SE 4); P = 0.009]. Mean stimulation indices of lymphocytes treated with phytohemagglutinin (PHA) and CD3 decreased from day 0 to day 31 from 34% (SE 8) to 17% (SE 9) (P = 0.06) and from 24% (SE 8) to 5% (SE 2) (P = 0.02), respectively. These changes were mainly contributed by the group of five patients with plasma viral load below 5 copies/ml at day 0. Viral load dropped below 20 copies/ml in all patients after 1 month of restarting the same antiretroviral regimen.

CONCLUSIONS

Discontinuation of HAART after 1 year of successful treatment is followed by a rapid rebound of viral load; this rapidly returns to undetectable levels following reintroduction of the same treatment. In patients with more effective control of virus replication (viremia below 5 copise/ml), discontinuation of treatment was associated with more severe impairment of immunologic parameters.

Progressive CD4+ central memory T cell decline results in CD4+ effector memory insufficiency and overt disease in chronic SIV infection

Primary simian immunodeficiency virus (SIV) infections of rhesus macaques result in the dramatic depletion of CD4⁺ CCR5⁺ effector–memory T (T_EM) cells from extra-lymphoid effector sites, but in most infections, an increased rate of CD4⁺ memory T cell proliferation appears to prevent collapse of effector site CD4⁺ T_EM cell populations and acute-phase AIDS. Eventually, persistent SIV replication results in chronic-phase AIDS, but the responsible mechanisms remain controversial. Here, we demonstrate that in the chronic phase of progressive SIV infection, effector site CD4⁺ T_EM cell populations manifest a slow, continuous decline, and that the degree of this depletion remains a highly significant correlate of late-onset AIDS. We further show that due to persistent immune activation, effector site CD4⁺ T_EM cells are predominantly short-lived, and that their homeostasis is strikingly dependent on the production of new CD4⁺ T_EM cells from central–memory T (T_CM) cell precursors. The instability of effector site CD4⁺ T_EM cell populations over time was not explained by increasing destruction of these cells, but rather was attributable to progressive reduction in their production, secondary to decreasing numbers of CCR5⁻ CD4⁺ T_CM cells. These data suggest that although CD4⁺ T_EM cell depletion is a proximate mechanism of immunodeficiency, the tempo of this depletion and the timing of disease onset are largely determined by destruction, failing production, and gradual decline of CD4⁺ T_CM cells.