HIV-1 Infection Results in Sphingosine-1-Phosphate Receptor 1 Dysregulation in the Human Thymus.

Regeneration of functional naïve T lymphocytes following the onset of human immunodeficiency virus (HIV) infection remains a crucial issue for people living with HIV (PLWH), even when adhering to antiretroviral therapy (ART). Thus far, reports on the impact of HIV-1 infection on the entry of thymic precursors and the egress of functional naïve T lymphocytes to and from the thymus are limited. We examined the impact of HIV-1 on Sphingosine-1-phosphate (S1P) signaling, which governs the egress of functional naïve thymocytes from the thymus to the periphery. Using in vitro experiments with primary human thymocytes and in vivo and ex vivo studies with humanized mice, we show that HIV-1 infection results in upregulation of the expression of S1P receptor 1 (S1PR1) in the human thymus. Intriguingly, this upregulation occurs during intrathymic infection (direct infection of the human thymic implant) as well as systemic infection in humanized mice. Moreover, considering the dysregulation of pro- and anti-inflammatory cytokines in infected thymi, the increased expression of S1PR1 in response to in vitro exposure to Interferon-Beta (IFN-ß) and Tumor Necrosis Factor-Alpha (TNF-α) indicates that cytokine dysregulation following HIV infection may contribute to upregulation of S1PR1. Finally, an increased presence of CD3hiCD69- (fully mature) as well as CD3hiCD69+ (less mature) T cells in the spleen during HIV infection in humanized mice, combined with earlier expression of S1PR1 during thymocyte development, suggests that upregulation of S1PR1 may translate to increased or accelerated egress from the thymus. The egress of thymocytes that are not functionally mature from the thymus to peripheral blood and lymphoid organs may have implications for the immune function of PLWH.

Characterization of T Follicular Helper Cells and T Follicular Regulatory Cells in HIV-Infected and Sero-Negative Individuals.

Humoral immune response is important in fighting pathogens by the production of specific antibodies by B cells. In germinal centers, T follicular helper (TFH) cells provide important help to B-cell antibody production but also contribute to HIV persistence. T follicular regulatory (TFR) cells, which inhibit the function of TFH cells, express similar surface markers. Since FOXP3 is the only marker that distinguishes TFR from TFH cells it is unknown whether the increase in TFH cells observed in HIV infection and HIV persistence may be partly due to an increase in TFR cells. Using multicolor flow cytometry to detect TFH and TFR cells in cryopreserved peripheral blood mononuclear cells from HIV-infected and non-infected participants in the UCLA Multicenter AIDS Cohort Study (MACS), we identified CD3+CXCR5+CD4+CD8-BCL6+ peripheral blood TFH (pTFH) cells and CD3+CXCR5+CD4+CD8-FOXP3+ peripheral blood TFR (pTFR) cells. Unlike TFR cells in germinal centers, pTFR cells do not express B cell lymphoma 6 (BCL6), a TFH cell master transcriptional regulator. Our major findings are that the frequency of pTFH cells, but not pTFR cells was higher in HIV-infected participants of the MACS and that pTFH cells expressed less CCR5 in HIV-infected MACS participants. Constitutive expression of CCR5 in TFR cells supports their potential to contribute to HIV persistence.

A Novel HIV-1 Nef Mutation in a Primary Pediatric Isolate Impairs MHC-Class I Downregulation and Cytopathicity.

HIV-1-induced cytopathicity of thymocytes is a major cause of reduced peripheral T cells and rapid disease progression observed in HIV-1-infected infants. Understanding the virulence factors responsible for thymocyte depletion has paramount importance in addressing the pathogenesis of disease progression in children. In this study, thymocyte depletion was analyzed following infection with two primary CXCR4-tropic HIV-1 pediatric isolates (PI), PI-2 and PI-2.1, which were serially derived from an in utero-infected infant. Although highly similar to each other, PI-2 showed markedly decreased thymocyte depletion in vitro compared with PI-2.1. Further analysis showed a novel deletion in the Nef protein (NefΔK7S) of PI-2, which was absent in PI-2.1. This deletion inhibited Nef-mediated major histocompatibility complex class I (MHC-I) downregulation in infected thymocytes in vitro and in vivo; in contrast, the mutated Nef continued to downregulate CD4 surface expression in vitro. These results suggest that HIV-1 Nef contributes to thymic damage in infants through selective functions.

Histoarchitectural Deterioration of Lymphoid Tissues in HIV-1 Infection and in Aging.

Impaired immunity is a common symptom of aging and advanced Human Immunodeficiency Virus type 1 (HIV-1) disease. In both diseases, a decline in lymphocytic function and cellularity leads to ineffective adaptive immune responses to opportunistic infections and vaccinations. Furthermore, despite sustained myeloid cellularity there is a background of chronic immune activation and a decrease in innate immune function in aging. In HIV-1 disease, myeloid cellularity is often more skewed than in normal aging, but similar chronic activation and innate immune dysfunction typically arise. Similarities between aging and HIV-1 infection have led to several investigations into HIV-1-mediated aging of the immune system. In this article, we review various studies that report alterations of leukocyte number and function during aging, and compare those alterations with those observed during progressive HIV-1 disease. We pay particular attention to changes within lymphoid tissue microenvironments and how histoarchitectural changes seen in these two diseases affect immunity. As we review various immune compartments including peripheral blood as well as primary and secondary lymphoid organs, common themes arise that help explain the decline of immunity in the elderly and in HIV-1-infected individuals with advanced disease. In both conditions, lymphoid tissues often show signs of histoarchitectural deterioration through fat accumulation and/or fibrosis. These structural changes can be attributed to a loss of communication between leukocytes and the surrounding stromal cells that produce the extracellular matrix components and growth factors necessary for cell migration, cell proliferation, and lymphoid tissue function. Despite the common general impairment of immunity in aging and HIV-1 progression, deterioration of immunity is caused by distinct mechanisms at the cellular and tissue levels in these two diseases.

TGF-ß Sustains Tumor Progression through Biochemical and Mechanical Signal Transduction.

Transforming growth factor β (TGF-β) signaling transduces immunosuppressive biochemical and mechanical signals in the tumor microenvironment. In addition to canonical SMAD transcription factor signaling, TGF-β can promote tumor growth and survival by inhibiting proinflammatory signaling and extracellular matrix (ECM) remodeling. In this article, we review how TGF-β activated kinase 1 (TAK1) activation lies at the intersection of proinflammatory signaling by immune receptors and anti-inflammatory signaling by TGF-β receptors. Additionally, we discuss the role of TGF-β in the mechanobiology of cancer. Understanding how TGF-β dampens proinflammatory responses and induces pro-survival mechanical signals throughout cancer development is critical for designing therapeutics that inhibit tumor progression while bolstering the immune response.

Urbanization and anticoagulant poisons promote immune dysfunction in bobcats.

Understanding how human activities influence immune response to environmental stressors can support biodiversity conservation across increasingly urbanizing landscapes. We studied a bobcat (Lynx rufus) population in urban southern California that experienced a rapid population decline from 2002-2005 due to notoedric mange. Because anticoagulant rodenticide (AR) exposure was an underlying complication in mange deaths, we aimed to understand sublethal contributions of urbanization and ARs on 65 biochemical markers of immune and organ function. Variance in immunological variables was primarily associated with AR exposure and secondarily with urbanization. Use of urban habitat and AR exposure has pervasive, complex and predictable effects on biochemical markers of immune and organ function in free-ranging bobcats that include impacts on neutrophil, lymphocyte and cytokine populations, total bilirubin and phosphorus. We find evidence of both inflammatory response and immune suppression associated with urban land use and rat poison exposure that could influence susceptibility to opportunistic infections. Consequently, AR exposure may influence mortality and has population-level effects, as previous work in the focal population has revealed substantial mortality caused by mange infection. The secondary effects of anticoagulant exposure may be a worldwide, largely unrecognized problem affecting a variety of vertebrate species in human-dominated environments.

CD31, a Valuable Marker to Identify Early and Late Stages of T Cell Differentiation in the Human Thymus.

Although CD31 expression on human thymocytes has been reported, a detailed analysis of CD31 expression at various stages of T cell development in the human thymus is missing. In this study, we provide a global picture of the evolution of CD31 expression from the CD34+ hematopoietic precursor to the CD45RA+ mature CD4+ and CD8+ single-positive (SP) T cells. Using nine-color flow cytometry, we show that CD31 is highly expressed on CD34+ progenitors and stays high until the early double-positive stage (CD3-CD4+CD8α+ß-). After ß-selection, CD31 expression levels become low to undetectable. CD31 expression then increases and peaks on CD3highCD4+CD8+ double-positive thymocytes. However, following positive selection, CD31 expression differs dramatically between CD4+ and CD8+ lineages: homogeneously high on CD8 SP but lower or negative on CD4 SP cells, including a subset of CD45RA+CD31- mature CD4+ thymocytes. CD31 expression on TCRγδ thymocytes is very similar to that of CD4 SP cells. Remarkably, there is a substantial subset of semimature (CD45RA-) CD4 SP thymocytes that lack CD31 expression. Moreover, FOXP3+ and ICOS+ cells are overrepresented in this CD31- subpopulation. Despite this CD31-CD45RA- subpopulation, most egress-capable mature CD45RA+ CD4 SP thymocytes express CD31. The variations in CD31 expression appear to coincide with three major selection processes occurring during thymopoiesis: ß-selection, positive selection, and negative selection. Considering the ability of CD31 to modulate the TCR's activation threshold via the recruitment of tyrosine phosphatases, our results suggest a significant role for CD31 during T cell development.

Sphingosine-1-phosphate/sphingosine-1-phosphate receptor 1 signaling is required for migration of naive human T cells from the thymus to the periphery.

BACKGROUND: The mechanisms that govern the egress of mature thymocytes from the human thymus to the periphery remain understudied yet are of utmost importance to the field of basic immunology, as well as T-cell reconstitution in various immunodeficiencies. We examined the expression and function of sphingosine-1-phosphate (S1P) receptors in human thymocyte egress. OBJECTIVES: We aimed to determine whether S1P receptors (S1P-Rs) play a role in mature human thymocyte egress and to identify the thymocyte population or populations that express S1P-Rs and respond to S1P by migrating across a concentration gradient. METHODS: Human thymocytes were exposed to S1P in Transwell plate migration assays coupled to flow cytometry to evaluate the response to S1P of thymocytes at different stages of maturation. Constitutive S1P-R expression was quantified by means of real-time PCR in sorted thymocyte subsets and flow cytometry. S1P-R1 and Kruppel-like factor 2 expression were monitored after S1P exposure by using flow cytometry and quantitative PCR. RESULTS: S1P-R1 was the prevalent S1P receptor on mature human thymocytes (CD3(hi)CD27(+)CD69(-)), the population that also demonstrated the greatest response to S1P in migration assays. Pretreatment with FTY720, an S1P-R1 nonselective modulator significantly reduced migration and suggested a role for S1P-R2 in retaining thymocytes in the tissue. Lastly, surface S1P-R1 expression, as well S1PR1 and Kruppel-like factor 2 (KLF2) transcripts, were significantly decreased in mature thymocytes on exposure to S1P. CONCLUSION: Mature human thymocytes rely on S1P-R1 to migrate toward S1P. Taken in the context of murine work demonstrating that S1P is required for thymocyte egress to the periphery, our data highlight a new key chemokine for human thymocyte egress.

BST2/Tetherin is constitutively expressed on human thymocytes with the phenotype and function of Treg cells.

In contrast to peripheral plasmacytoid DCs (pDCs), thymic pDCs constitutively express low levels of IFN-α. This leads to induction of interferon secondary genes (ISGs) in medullary thymocytes, raising the question whether IFN-α may play a role in T-cell development. When characterizing further differences between peripheral and thymic pDCs, we found that thymic pDCs have a phenotype consistent with an "activated signature" including expression of TNF-α and bone marrow stromal cell antigen 2 (BST2), but no expression of ILT7. Given that BST2 is induced by IFN-α, and IFN-α secretion is controlled by interaction between ILT7 and BST2, this regulatory pathway is apparently lost in thymic pDCs. Further, we also show that BST2 is constitutively expressed on a subset of medullary thymocytes at the mRNA and protein level reflecting a history of IFN-α transduced signals. The majority of BST2(+) thymocytes express CCR5 rendering them prevalent targets for R5-tropic HIV infection. Moreover, BST2(+) thymocytes express Foxp3 and CD25, consistent with the phenotype of natural Treg cells, and exert suppressive activity as they impair the proliferation of autologous CD3(+) thymocytes. Collectively, our results suggest that low levels of IFN-α secreted by thymic pDCs play an important role in the development of natural Treg cells.

Human T-Cell Development and Thymic Egress: An Infectious Disease Perspective.

Emigration of mature naïve CD4 SP T cells from the human thymus to the periphery is not fully understood, although elucidation of the mechanisms that govern egress of T cells is crucial to understanding both basic immunology and the immune response in diseases such as HIV infection. Recent work has brought to light the requirement for sphingosine-1-phosphate (S1P) and its receptors in a variety of fields including mature naïve T-cell egress from the thymus of mice. We are examining the expression and function of this novel requisite T-cell egress receptor within the human thymus, characterizing changes observed in the expression and function of this receptor in infectious diseases. To perform this work, we use a variety of humanized murine models reviewed in this article. Future work in the field of T-cell egress, especially as it pertains to S1P receptors, should advance the fields of basic T-cell immunology and immunopathology and open new avenues for exploration into novel therapeutics.

The plasmacytoid dendritic cell as the Swiss army knife of the immune system: molecular regulation of its multifaceted functions.

Plasmacytoid dendritic cells (pDC) have been regarded as the "professional type I IFN-producing cells" of the immune system following viral recognition that relies on the expression of TLR7 and TLR9. Furthermore, pDC link the innate and adaptive immune systems via cytokine production and Ag presentation. More recently, their ability to induce tolerance and cytotoxicity has been added to their "immune skills." Such a broad range of actions, resembling the diverse functional features of a Swiss army knife, requires strong and prompt molecular regulation to prevent detrimental effects, including autoimmune pathogenesis or tumor escape. Over the last decades, we and other investigators have started to unravel some aspects of the signaling pathways that regulate the various functions of human pDC. In this article, we review aspects of the molecular regulatory mechanisms to control pDC function in light of their multifaceted roles during immunity, autoimmunity, and cancer.

MicroRNA-146a regulates survival and maturation of human plasmacytoid dendritic cells.

During microbial infections, plasmacytoid dendritic cells (pDCs) are a main source of type I interferons α/ß (IFN-α/-ß). Nucleic acids from microbes are sensed by Toll-like receptors 7/9 (TLR7/9), which are selectively expressed in pDCs. Activated pDCs also produce proinflammatory cytokines and upregulate costimulatory molecules. Together, this equips pDCs with the ability to prime T, B, and NK cells and conventional DCs, thereby initiating adaptive immune responses. To avoid deleterious effects to the host, tight regulation of pDC activation is required. Despite data linking aberrant activation of pDCs with autoimmune diseases, little is known about mechanisms controlling pDC activation. Here, we investigated the role of microRNA-146a (miR-146a) in TLR pathway regulation in human pDCs. MiR-146a expression was induced upon TLR7/9 signaling. Furthermore, ectopic miR-146a expression effectively impaired TLR-mediated signaling in pDCs as TLR-induced nuclear factor-κB activation was reduced. This consequently diminished the production of proinflammatory cytokines and reduced pDC survival. Moreover, miR-146a-expressing pDCs had decreased ability to induce CD4(+) T-cell proliferation likely due to reduced expression levels of major histocompatibility complex class II and costimulatory molecules. Our data unravel the crucial immunomodulatory role of miR-146a in pDCs and may add to our understanding of aberrant responses in autoimmune diseases.

HIV-1 infection of hematopoietic progenitor cells in vivo in humanized mice.

HIV infection has been associated with defective hematopoiesis since the earliest days of the HIV/AIDS epidemic. Generation of all hematopoietic lineages suffers in the face of infection. The mechanisms by which HIV impairs normal blood cell development remain unclear, and direct infection of intermediate hematopoietic progenitors has not been established as a source of HIV-associated hematopoietic pathology. Here, we demonstrate infection of multiple subsets of highly purified intermediate hematopoietic progenitors by wild-type HIV both in vitro and in vivo. Although direct infection is clearly cytotoxic, we find that some infected progenitors can survive and harbor proviral DNA. We report intermediate hematopoietic progenitors to be a novel target of infection and their permissivity to infection increases with development. Further, the nonobese diabetic severe combined immunodeficiency common γ chain knockout-bone marrow-liver-thymus humanized mouse provides a unique model for studying the impact of HIV infection on bone marrow-based human hematopoiesis.

IL-21-stimulated human plasmacytoid dendritic cells secrete granzyme B, which impairs their capacity to induce T-cell proliferation.

Plasmacytoid dendritic cells (pDCs) play a crucial role during innate immunity by secreting bulk amounts of type I interferons (IFNs) in response to Toll-like receptor (TLR)-mediated pathogen recognition. In addition, pDCs can also contribute to adaptive immunity by activation of antigen-specific T cells. Furthermore, it is well established that pDCs contribute to the pathogenesis of autoimmune diseases, including lupus. Interleukin-21 (IL-21) is a cytokine produced by activated CD4(+) T and natural killer T (NKT) cells and has a pleiotropic role in immunity by controlling myeloid DC-, NKT-, T-, and B-cell functions. It has remained elusive whether IL-21 affects pDCs. Here we investigate the role of IL-21 in human pDC activation and function and observe that IL-21 activates signal transducer and activator of transcription 3 in line with the finding that pDCs express the IL-21 receptor. Although IL-21 did not affect TLR-induced type I IFNs, IL-6, and TNF-α nor expression of major-histocompatibility-complex class II or costimulatory molecules, IL-21 markedly increased expression of the serine protease granzyme B (GrB). We demonstrate that GrB induction was, in part, responsible for IL-21-mediated downmodulation of CD4(+) T-cell proliferation induced by TLR preactivated pDCs. Collectively, our data provide evidence that pDCs are important cells to consider when investigating the role of IL-21 in immunity or pathogenesis.

GLI2 regulates TGF-ß1 in human CD4+ T cells: implications in cancer and HIV pathogenesis.

Elevated levels of the immunoregulatory cytokine TGF-ß1 in cancer and HIV infection have been linked to the suppression of protective immune responses. The transcriptional regulation of TGF-ß1 is complex and still not completely understood. We report here for the first time that the transcription factor GLI2 regulates the expression of TGF-ß1 in human CD4(+) T cells. In silico screening revealed five novel putative GLI binding sites in the human TGF-ß1 promoter. At least two of these sites within the human TGF-ß1 promoter are regulated by the GLI2 activator as knockdown of GLI2 in regulatory CD4(+)CD25(hi) T cells, high producers of TGF-ß1, significantly decreased TGF-ß1 transcription. Additionally, naïve CD4(+) T cells, low producers of TGF-ß1, increased their basal level of TGF-ß1 mRNA following lentiviral infection with GLI2. The transcriptional regulation of TGF-ß1 by GLI2 is a new extension to Sonic Hedgehog (SHH) and TGF-ß1 cross-regulation and may provide insight into the detrimental elevation of TGF-ß1 leading to pathogenesis in cancer and HIV infection.

A doxycycline-dependent human immunodeficiency virus type 1 replicates in vivo without inducing CD4+ T-cell depletion.

A novel genetic approach for the control of virus replication was used for the design of a conditionally replicating human immunodeficiency virus (HIV) variant, HIV-rtTA. HIV-rtTA gene expression and virus replication are strictly dependent on the presence of a non-toxic effector molecule, doxycycline (dox), and thus can be turned on and off at will in a graded and reversible manner. The in vivo replication capacity, pathogenicity and genetic stability of this HIV-rtTA variant were evaluated in a humanized mouse model of haematopoiesis that harbours lymphoid and myeloid components of the human immune system (HIS). Infection of dox-fed BALB Rag/γc HIS (BRG-HIS) mice with HIV-rtTA led to the establishment of a productive infection without CD4(+) T-cell depletion. The virus did not show any sign of escape from dox control for up to 10 weeks after the onset of infection. No reversion towards a functional Tat-transactivating responsive (TAR) RNA element axis was observed, confirming the genetic stability of the HIV-rtTA variant in vivo. These results demonstrate the proof of concept that HIV-rtTA replicates efficiently in vivo. HIV-rtTA is a promising tool for fundamental research to study virus-host interactions in vivo in a controlled fashion.

IFN-α is constitutively expressed in the human thymus, but not in peripheral lymphoid organs.

Type I interferons have been typically studied for their effects in the context of bacterial or viral infections. However in this report, we provide evidence that Interferon-alpha (IFN-α) expressing cells are present in the thymus in the absence of infection. We show that pDC express the highest level of IFN-α and that MxA, which is exclusively expressed after engagement of the type I IFN receptor by IFN-α/ß, is expressed in normal fetal and post-natal thymus, but not in the periphery. The highest level of MxA is expressed in mature thymocytes and pDC located in the medulla and at the cortico-medullary junction. The anti-microbial peptide LL-37, which is expressed in the thymus, when complexed with eukaryotic nucleic acids, induces the secretion of IFN-α by thymic pDC. This results in the upregulation of MxA expression in responsive thymocytes. We propose that the secretion of IFN-α in the thymus may function to regulate the rate of T cell development and modulate the requirements for the selection of developing T cells.

Immune dysregulation after cardiothoracic surgery and incidental thymectomy: maintenance of regulatory T cells despite impaired thymopoiesis.

Thymectomy is performed in infants during cardiothoracic surgery leaving many patients with reduced thympopoiesis. An association between immune disorders and regulatory T cells (Treg) after incidental thymectomy has not been investigated. Questionnaires soliciting symptoms of atopic or autoimmune disease and biomarkers were measured in children and adults with congenital heart disease and either reduced or preserved thymopoiesis. Tregs were examined. Atopic or autoimmune-like symptoms and elevated anti-dsDNA antibodies were common after surgery in individuals with low thymopoiesis. Total Treg number and function were maintained but with fewer naïve Treg. TCR spectratypes were similar to other memory T cells. These data suggest that thymectomy does not reduce total Treg number but homeostasis is affected with reduced naïve Treg. Prevalence of autoimmune or atopic symptoms after surgery is not associated with total number or proportion of Tregs but appears to be due to otherwise unknown factors that may include altered Treg homeostasis.

Correlation of immune activation during late pregnancy and early postpartum with increases in plasma HIV RNA, CD4/CD8 T cells, and serum activation markers.

A previously observed rise in the plasma viral load postpartum in both treated and untreated HIV-positive women remains unexplained. Virological and immunological markers were evaluated in HIV-negative controls and HIV-positive pregnant women with and without antiretroviral treatment. Plasma HIV RNA, CD4/CD8 T cells, and serum activation markers were sequentially measured during the third trimester, at delivery, and 2 to 8 weeks postpartum in a cohort of HIV-positive pregnant women (n = 96) enrolled in a maternal-fetal HIV transmission study and a control group of HIV-negative pregnant women (n = 28). Mean plasma HIV RNA (P = 0.003) increased from delivery to postpartum, and mean CD4 T cells (P = 0.002) and serum ß2-microglobulin (P < 0.0001) increased from the third trimester through postpartum among the HIV-positive women. Mean CD8 T cells increased from the third trimester through postpartum in women receiving zidovudine (ZDV) and in those not treated (P < 0.05) but remained stable in those on highly active antiretroviral therapy (HAART) and the HIV-negative controls. Increases in serum ß2-microglobulin were correlated with increases in HIV RNA (P = 0.01). HIV-positive pregnant women showed postpartum increases in plasma HIV RNA, CD4 T cells, and serum ß2-microglobulin regardless of the treatment regimen. The rise in CD4 T cells and ß2-microglobulin was also observed in HIV-negative pregnant women, suggesting hormonal changes and/or labor-induced cytokines may contribute to immune activation. Immune activation correlated with increased plasma HIV RNA in postpartum women despite treatment, although HAART appeared to blunt the effect. The observed rise in plasma HIV RNA postpartum, which correlated with markers of immune activation, may have implications for enhanced transmission to infants through early breast-feeding and to sexual partners.

Signaling through the P38 and ERK pathways: a common link between HIV replication and the immune response.

One of the defining characteristics of HIV is its ability to manipulate the human immune response to promote its own replication. Since the beginning of the epidemic, there has been controversy whether a robust immune response to the virus is beneficial or detrimental for the host. Therefore, the effects of HIV on signaling pathways and cytokine production need to be characterized in order to distinguish between protective immune responses and inappropriate immune activation. Cytokine and biomarker expression during HIV infection results from the combined effects of intracellular signaling pathways orchestrated by kinases like P38 and ERK. The P38 and ERK Mitogen-Activated Protein Kinase (MAPK) pathways govern the regulation of cytokines (IL-2, IL-10, and TNF-α) as well biomarkers (PD-1, Fas/FasL, among others) that are skewed in chronic HIV infection. HIV utilizes the P38 and ERK pathways to produce new virions and to deplete CD4+ T cells from the host's immune system. Understanding the interplay between HIV and the cytokines induced by activation of the P38 and ERK pathways may provide insights into HIV immunopathogenesis and the development of a protective vaccine.