HIV-1 Nef increases T cell activation in a stimulus-dependent manner

HIV-1 Nef-mediated inhibition of T cell migration and its molecular determinants

Lymphocyte trafficking is a multistep, intricate process and involves a number of host factors such as integrins and chemokine receptors on lymphocytes, adhesion molecules on endothelial cells, and chemokines present in the local microenvironment. Previous studies have shown that HIV-1 Nef inhibits T cell chemotaxis in response to the physiological ligand SDF-1alpha [( 1) ]. In this study, we aimed to gain a better understanding of the inhibitory mechanisms and to define the molecular determinants of HIV-1 Nef for this phenotype. We showed that HIV-1 Nef inhibited transwell and transendothelial migration of T cells. Specifically, HIV-1 Nef protein impaired T cell chemotaxis toward SDF-1alpha without altering CXCR4 expression. Moreover, we showed that HIV-1 Nef protein down-modulated LFA-1 expression on T lymphocytes and diminished adhesion and polarization of T lymphocytes and as a result, led to decreased migration across the endothelium. Furthermore, we showed that the myristoylation site and DeltaSD domain played important roles in Nef-mediated inhibition of transwell and transendothelial migration and polarization of T lymphocytes; however, different sites or domains were needed for Nef-mediated LFA-1 down-modulation and impaired adhesion of T lymphocyte. Taken together, these results demonstrated that HIV-1 Nef inhibited T lymphocyte migration at multiple steps and suggest that membrane localization and intracellular signaling events likely contribute to the inhibitory effects of Nef on T cell migration and subsequently, the pathobiology of the HIV-1 Nef protein.

Measurement of human immunodeficiency virus type 1 preintegration transcription by using Rev-dependent Rev-CEM cells reveals a sizable transcribing DNA population comparable to that from proviral templates

Preintegration transcription is an early process in human immunodeficiency virus type 1 infection and has been suggested to occur at a low level. The templates have also been suggested to represent a small population of nonintegrated viral DNA, particularly the two-long-terminal-repeat (2-LTR) circles. However, these determinations were made by either using PCR amplification of viral transcripts in bulk cell populations or utilizing the LTR-driving reporter cells that measure the synthesis of Tat. The intrinsic leakiness of LTR often makes the measurement of low-level viral transcription inaccurate. Since preintegration transcription also generates Rev, to eliminate the nonspecificity associated with the use of LTR alone we have developed a novel Rev-dependent indicator cell, Rev-CEM, to measure preintegration transcription based on the amount of Rev generated. In this report, using Rev-CEM cells, we demonstrate that preintegration transcription occurs on a much larger scale than expected. The transcribing population derived from nonintegrated viral DNA was comparable (at approximately 70%) to that derived from provirus in a productive viral replication cycle. Nevertheless, each nonintegrated viral DNA template exhibited a significant reduction in the level of transcriptional activity in the absence of integration. We also performed flow cytometry sorting of infected cells to identify viral templates. Surprisingly, our results suggest that the majority of 2-LTR circles are not active in directing transcription. It is likely that the nonintegrated templates are from the predominant DNA species, such as the full-length, linear DNA. Our results also suggest that a nonintegrating lentiviral vector can be as effective as an integrating vector in directing gene expression in nondividing cells, with the proper choice of an internal promoter.

At the crossroads of homoeostasis and disease: roles of the PACS proteins in membrane traffic and apoptosis

The endomembrane system in mammalian cells has evolved over the past two billion years from a simple endocytic pathway in a single-celled primordial ancestor to complex networks supporting multicellular structures that form metazoan tissue and organ systems. The increased organellar complexity of metazoan cells requires additional trafficking machinery absent in yeast or other unicellular organisms to maintain organ homoeostasis and to process the signals that control proliferation, differentiation or the execution of cell death programmes. The PACS (phosphofurin acidic cluster sorting) proteins are one such family of multifunctional membrane traffic regulators that mediate organ homoeostasis and have important roles in diverse pathologies and disease states. This review summarizes our current knowledge of the PACS proteins, including their structure and regulation in cargo binding, their genetics, their roles in secretory and endocytic pathway traffic, interorganellar communication and how cell-death signals reprogramme the PACS proteins to regulate apoptosis. We also summarize our current understanding of how PACS genes are dysregulated in cancer and how viral pathogens ranging from HIV-1 to herpesviruses have evolved to usurp the PACS sorting machinery to promote virus assembly, viral spread and immunoevasion.

The co-receptor signaling model of HIV-1 pathogenesis in peripheral CD4 T cells

HIV-mediated CD4 depletion is the hallmark of AIDS and is the most reliable predictor of disease progression. While HIV replication is associated with CD4 depletion in general, plasma viremia by itself predicts the rate of CD4 loss only minimally in untreated patients. To resolve this paradox, I hypothesize the existence of a subpopulation of R5_X4-signaling viruses. I also suggest that the gradual evolution and emergence of this subpopulation are largely responsible for the slow depletion of peripheral CD4 T cells.

Is the high virulence of HIV-1 an unfortunate coincidence of primate lentiviral evolution?

In the subset of primate lentiviruses that contain a vpu gene - HIV-1 and its simian precursors - the Nef protein has lost the ability to down-modulate CD3, block T cell activation and suppress programmed death. Vpu counteracts a host restriction factor induced by the inflammatory cytokine interferon-alpha. I propose that the acquisition of vpu may have allowed the viral lineage that gave rise to HIV-1 to evolve towards greater pathogenicity by removing the selective pressure for a protective Nef function that prevents damagingly high levels of immune activation.

Implications of Nef: host cell interactions in viral persistence and progression to AIDS

The HIV and SIV Nef accessory proteins are potent enhancers of viral persistence and accelerate progression to AIDS in HIV-1-infected patients and non-human primate models. Although relatively small (27-35 kD), Nef can interact with a multitude of cellular factors and induce complex changes in trafficking, signal transduction, and gene expression that together converge to promote viral replication and immune evasion. In particular, Nef recruits several immunologically relevant cellular receptors to the endocytic machinery to reduce the recognition and elimination of virally infected cells by the host immune system, while simultaneously interacting with various kinases to promote T cell activation and viral replication. This review provides an overview on selected Nef interactions with host cell proteins, and discusses their possible relevance for viral spread and pathogenicity.

Biphasic decay kinetics suggest progressive slowing in turnover of latently HIV-1 infected cells during antiretroviral therapy

Background

Mathematical models based on kinetics of HIV-1 plasma viremia after initiation of combination antiretroviral therapy (cART) inferred HIV-infected cells to decay exponentially with constant rates correlated to their strength of virus production. To further define in vivo decay kinetics of HIV-1 infected cells experimentally, we assessed infected cell-classes of distinct viral transcriptional activity in peripheral blood mononuclear cells (PBMC) of five patients during 1 year after initiation of cART

Results

In a novel analytical approach patient-matched PCR for unspliced and multiply spliced viral RNAs was combined with limiting dilution analysis at the single cell level. This revealed that HIV-RNA⁺ PBMC can be stratified into four distinct viral transcriptional classes. Two overlapping cell-classes of high viral transcriptional activity, suggestive of a virion producing phenotype, rapidly declined to undetectable levels. Two cell classes expressing HIV-RNA at low and intermediate levels, presumably insufficient for virus production and occurring at frequencies exceeding those of productively infected cells matched definitions of HIV-latency. These cells persisted during cART. Nevertheless, during the first four weeks of therapy their kinetics resembled that of productively infected cells.

Conclusion

We have observed biphasic decays of latently HIV-infected cells of low and intermediate viral transcriptional activity with marked decreases in cell numbers shortly after initiation of therapy and complete persistence in later phases. A similar decay pattern was shared by cells with greatly enhanced viral transcriptional activity which showed a certain grade of levelling off before their disappearance. Thus it is conceivable that turnover/decay rates of HIV-infected PBMC may be intrinsically variable. In particular they might be accelerated by HIV-induced activation and reactivation of the viral life cycle and slowed down by the disappearance of such feedback-loops after initiation of cART.

HIV envelope-CXCR4 signaling activates cofilin to overcome cortical actin restriction in resting CD4 T cells

Binding of the HIV envelope to the chemokine coreceptors triggers membrane fusion and signal transduction. The fusion process has been well characterized, yet the role of coreceptor signaling remains elusive. Here, we describe a critical function of the chemokine coreceptor signaling in facilitating HIV infection of resting CD4 T cells. We find that static cortical actin in resting T cells represents a restriction and that HIV utilizes the Galphai-dependent signaling from the chemokine coreceptor CXCR4 to activate a cellular actin-depolymerizing factor, cofilin, to overcome this restriction. HIV envelope-mediated cofilin activation and actin dynamics are important for a postentry process that leads to viral nuclear localization. Inhibition of HIV-mediated actin rearrangement markedly diminishes viral latent infection of resting T cells. Conversely, induction of active cofilin greatly facilitates it. These findings shed light on viral exploitation of cellular machinery in resting T cells, where chemokine receptor signaling becomes obligatory.

HIV-1 Nef: at the crossroads

The development of anti-virals has blunted the AIDS epidemic in the Western world but globally the epidemic has not been curtailed. Standard vaccines have not worked, and attenuated vaccines are not being developed because of safety concerns. Interest in attenuated vaccines has centered on isolated cases of patients infected with HIV-1 containing a deleted nef gene. Nef is a multifunctional accessory protein that is necessary for full HIV-1 virulence. Unfortunately, some patients infected with the nef-deleted virus eventually lose their CD4⁺ T cells to levels indicating progression to AIDS.

This renders the possibility of an attenuated HIV-1 based solely on a deleted nef remote. In this review we discuss the knowledge gained both from the study of these patients and from in vitro investigations of Nef function to assess the possibility of developing new anti-HIV-1 drugs based on Nef. Specifically, we consider CD4 downregulation, major histocompatibility complex I downregulation, Pak2 activation, and enhancement of virion infectivity. We also consider the recent proposal that simian immunodeficiency viruses are non-pathogenic in their hosts because they have Nefs that downregulate CD3, but HIV-1 is pathogenic because its Nef fails to downregulate CD3. The possibility of incorporating the CD3 downregulation function into HIV-1 Nef as a therapeutic option is also considered. Finally, we conclude that inhibiting the CD4 downregulation function is the most promising Nef-targeted approach for developing a new anti-viral as a contribution to combating AIDS.

HIV-1 Nef protein expression in human CD34+ progenitors impairs the differentiation of an early T/NK cell precursor

HIV-1 impairs the production of T cells, through mechanisms that are still unknown. Here, we investigated the effect of the expression of HIV-1 Nef on the T-cell potential of human hematopoietic CD34(+) precursors. Those progenitors were transduced by using lentiviral vectors expressing Nef and cultured on OP9-DL1 cells allowing the differentiation of T cell from human hematopoietic precursors. We demonstrate that Nef impairs the generation of a CD3epsilon(+)CD5(+) CD1a(+) precursor stage that has initiated a D-J rearrangement of the TCRbeta locus. Onward stages of T-cell development were also affected with a quantitative reduction of CD4(+) intraCD3epsilon(+) Immature single positive cells (ISP), Double Positive (DP) CD4(+)CD8(+) TCRalphabeta T cells and CD56(+) NK cells. But B cell production was not affected. Limiting dilution analyses demonstrated a significant reduction in the frequency of T/NK progenitors among Nef-expressing CD34(+) cells. Altogether, these data demonstrate that Nef interferes with the differentiation of a primitive lymphoid human precursor with a T/NK potential.

Inefficient Nef-mediated downmodulation of CD3 and MHC-I correlates with loss of CD4+T cells in natural SIV infection

Recent data suggest that Nef-mediated downmodulation of TCR-CD3 may protect SIVsmm-infected sooty mangabeys (SMs) against the loss of CD4⁺ T cells. However, the mechanisms underlying this protective effect remain unclear. To further assess the role of Nef in nonpathogenic SIV infection, we cloned nef alleles from 11 SIVsmm-infected SMs with high (>500) and 15 animals with low (<500) CD4⁺ T-cells/µl in bulk into proviral HIV-1 IRES/eGFP constructs and analyzed their effects on the phenotype, activation, and apoptosis of primary T cells. We found that not only efficient Nef-mediated downmodulation of TCR-CD3 but also of MHC-I correlated with preserved CD4⁺ T cell counts, as well as with high numbers of Ki67⁺CD4⁺ and CD8⁺CD28⁺ T cells and reduced CD95 expression by CD4⁺ T cells. Moreover, effective MHC-I downregulation correlated with low proportions of effector and high percentages of naïve and memory CD8⁺ T cells. We found that T cells infected with viruses expressing Nef alleles from the CD4low SM group expressed significantly higher levels of the CD69, interleukin (IL)-2 and programmed death (PD)-1 receptors than those expressing Nefs from the CD4high group. SIVsmm Nef alleles that were less active in downmodulating TCR-CD3 were also less potent in suppressing the activation of virally infected T cells and subsequent cell death. However, only nef alleles from a single animal with very low CD4⁺ T cell counts rendered T cells hyper-responsive to activation, similar to those of HIV-1. Our data suggest that Nef may protect the natural hosts of SIV against the loss of CD4⁺ T cells by at least two mechanisms: (i) downmodulation of TCR-CD3 to prevent activation-induced cell death and to suppress the induction of PD-1 that may impair T cell function and survival, and (ii) downmodulation of MHC-I to reduce CTL lysis of virally infected CD4⁺ T cells and/or bystander CD8⁺ T cell activation.

The accessory Nef protein is commonly considered a “pathogenicity” factor of primate lentiviruses. However, SIVs do not cause disease in their natural hosts, although they all encode nef genes and sustain high levels of viremia. To better understand the role of Nef in natural nonpathogenic SIV infection, we compared the function of Nef alleles from two groups of SIVsmm-infected sooty mangabeys: (i) those that maintained normal CD4⁺ T cell counts and (ii) a small subset (10%–15%) of animals that exhibited a considerable loss of CD4⁺ helper T cells. We found that the efficiency of two specific Nef functions, i.e., downmodulation of TCR-CD3 and MHC-I, correlated with preserved CD4⁺ T cell homeostasis, as well as with other immunological features, such as high numbers of proliferating CD4⁺ Ki67⁺ T cells. Moreover, lack of CD3 surface expression was associated with low levels of apoptosis and PD-1 expression by virally infected T cells. Thus, the ability of Nef to remove TCR-CD3 and MHC-I from the cell surface may help the natural hosts of SIV to maintain normal CD4⁺ T cell counts despite high levels of viral replication by preventing activation-induced cell death and CTL lysis of infected T cells and/or CD8⁺ T cell activation.

The second chance story of HIV-1 DNA: Unintegrated? Not a problem!

Accumulation of high levels of unintegrated viral DNA is a common feature of retroviral infection. It was recently discovered that coinfection of cells with integrated and unintegrated HIV-1 can result in complementation, allowing viral replication in the absence of integration. This new mode of HIV-1 replication has numerous implications for the function of unintegrated viral DNA and its application as a therapeutic vector.

Viral pathogenesis, modulation of immune receptor signaling and treatment

During the co-evolution of viruses and their hosts, the latter have equipped themselves with an elaborate immune system to defend themselves from the invading viruses. In order to establish a successful infection, replicate and persist in the host, viruses have evolved numerous strategies to counter and evade host antiviral immune responses as well as exploit them for productive viral replication. These strategies include those that target immune receptor transmembrane signaling. Uncovering the exact molecular mechanisms underlying these critical points in viral pathogenesis will not only help us understand strategies used by viruses to escape from the host immune surveillance but also reveal new therapeutic targets for antiviral as well as immunomodulatory therapy. In this chapter, based on our current understanding of transmembrane signal transduction mediated by multichain immune recognition receptors (MIRRs) and the results of sequence analysis, we discuss the MIRR-targetingviral strategies of immune evasion and suggest their possible mechanisms that, in turn, reveal new points of antiviral intervention. We also show how two unrelated enveloped viruses, human immunodeficiency virus and human cytomegalovirus, use a similar mechanism to modulate the host immune response mediated by two functionally different MIRRs-T-cell antigen receptor and natural killer cell receptor, NKp30. This suggests that it is very likely that similar general mechanisms can be or are used by other viral and possibly nonviral pathogens.

SCHOOL model and new targeting strategies

Protein-protein interactions play a central role in biological processes and thus are an appealing target for innovative drug design a nd development. They can be targeted bysmall molecule inhibitors, peptides and peptidomimetics, which represent an alternative to protein therapeutics that carry many disadvantages. In this chapter, I describe specific protein-protein interactions suggested by a novel model of immune signaling, the Signaling Chain HOmoOLigomerization (SCHOOL) model, to be critical for cell activation mediated by multichain immune recognition receptors (MIRRs) expressed on different cells of the hematopoietic system. Unraveling a long-standing mystery of MIRR triggering and transmembrane signaling, the SCHOOL model reveals the intrareceptor transmembrane interactions and interreceptor cytoplasmic homointeractions as universal therapeutic targets for a diverse variety of disorders mediated by immune cells. Further, assuming that the general principles underlying MIRR-mediated transmembrane signaling mechanisms are similar, the SCHOOL model can be applied to any particular receptor of the MIRR family. Thus, an important application of the SCHOOL model is that global therapeutic strategies targeting key protein-protein interactions involved in MIRR triggering and transmembrane signal transduction may be used to treat a diverse set of immune-mediated diseases. This assumes that clinical knowledge and therapeutic strategies can be transferred between seemingly disparate disorders, such as T-cell-mediated skin diseases and platelet disorders, or combined to develop novel pharmacological approaches. Intriguingly, the SCHOOL model unravels the molecular mechanisms underlying ability of different human viruses such as human immunodeficiency virus, cytomegalovirus and severe acute respiratory syndrome coronavirus to modulate and/or escape the host immune response. It also demonstrates how the lessons learned from viral pathogenesis can be used practically for rational drug design. Application of this model to platelet collagen receptor signaling has already led to the development of a novel concept of platelet inhibition and the invention of new platelet inhibitors, thus proving the suggested hypothesis and highlighting the importance and broad perspectives of the SCHOOL model in the development of new targeting strategies.

Human immunodeficiency virus type 1 Nef recruits the guanine exchange factor Vav1 via an unexpected interface into plasma membrane microdomains for association with p21-activated kinase 2 activity

Alterations of T-cell receptor signaling by human immunodeficiency virus type 1 (HIV-1) Nef involve its association with a highly active subpopulation of p21-activated kinase 2 (PAK2) within a dynamic signalosome assembled in detergent-insoluble membrane microdomains. Nef-PAK2 complexes contain the GTPases Rac and Cdc42 as well as a factor providing guanine nucleotide exchange factor (GEF) activity for Rac/Cdc42. However, the identity of this GEF has remained controversial. Previous studies suggested the association of Nef with at least three independent GEFs, Vav, DOCK2/ELMO1, and betaPix. Here we used a broad panel of approaches to address which of these GEFs is involved in the functional interaction of Nef with PAK2 activity. Biochemical fractionation and confocal microscopy revealed that Nef recruits Vav1, but not DOCK2/ELMO1 or betaPix, to membrane microdomains. Transient RNAi knockdown, analysis of cell lines defective for expression of Vav1 or DOCK2 as well as use of a betaPix binding-deficient PAK2 variant confirmed a role for Vav1 but not DOCK2 or betaPix in Nef's association with PAK2 activity. Nef-mediated microdomain recruitment of Vav1 occurred independently of the Src homology 3 domain binding PxxP motif, which is known to connect Nef to many cellular signaling processes. Instead, a recently described protein interaction surface surrounding Nef residue F195 was identified as critical for Nef-mediated raft recruitment of Vav1. These results identify Vav1 as a relevant component of the Nef-PAK2 signalosome and provide a molecular basis for the role of F195 in formation of a catalytically active Nef-PAK2 complex.

Cooperative binding of the class I major histocompatibility complex cytoplasmic domain and human immunodeficiency virus type 1 Nef to the endosomal AP-1 complex via its mu subunit

Human immunodeficiency virus type 1 Nef provides immune evasion by decreasing the expression of major histocompatibility complex class I (MHC-I) at the surfaces of infected cells. The endosomal clathrin adaptor protein complex AP-1 is a key cellular cofactor for this activity, and it is recruited to the MHC-I cytoplasmic domain (CD) in the presence of Nef by an uncharacterized mechanism. To determine the molecular basis of this recruitment, we used an MHC-I CD-Nef fusion protein to represent the MHC-I CD/Nef complex during protein interaction assays. The MHC-I CD had no intrinsic ability to bind AP-1, but it conferred binding activity when fused to Nef. This activity was independent of the canonical leucine-based AP-binding motif in Nef; it required residue Y320 in the MHC-I CD and residues E62-65 and P78 in Nef, and it involved the mu but not the gamma/sigma subunits of AP-1. The impaired binding of mutants encoding substitutions of E62-65 or P78 in Nef was rescued by replacing the Y320SQA sequence in the MHC-I CD with YSQL, suggesting that Nef allows the YSQA sequence to act as if it were a canonical mu-binding motif. These data identify the mu subunit of AP-1 (mu1) as the key target of the MHC-I CD/Nef complex, and they indicate that both Y320 in the MHC-I CD and E62-65 in Nef interact directly with mu1. The data support a cooperative binding model in which Nef functions as a clathrin-associated sorting protein that allows recognition of an incomplete, tyrosine-based mu-binding signal in the MHC-I CD by AP-1.

HIV-1 Nef employs two distinct mechanisms to modulate Lck subcellular localization and TCR induced actin remodeling

The Nef protein acts as critical factor during HIV pathogenesis by increasing HIV replication in vivo via the modulation of host cell vesicle transport and signal transduction processes. Recent studies suggested that Nef alters formation and function of immunological synapses (IS), thereby modulating exogenous T-cell receptor (TCR) stimulation to balance between partial T cell activation required for HIV-1 spread and prevention of activation induced cell death. Alterations of IS function by Nef include interference with cell spreading and actin polymerization upon TCR engagement, a pronounced intracellular accumulation of the Src kinase Lck and its reduced IS recruitment. Here we use a combination of Nef mutagenesis and pharmacological inhibition to analyze the relative contribution of these effects to Nef mediated alterations of IS organization and function on TCR stimulatory surfaces. Inhibition of actin polymerization and IS recruitment of Lck were governed by identical Nef determinants and correlated well with Nef's association with Pak2 kinase activity. In contrast, Nef mediated Lck endosomal accumulation was separable from these effects, occurred independently of Pak2, required integrity of the microtubule rather than the actin filament system and thus represents a distinct Nef activity. Finally, reduction of TCR signal transmission by Nef was linked to altered actin remodeling and Lck IS recruitment but did not require endosomal Lck rerouting. Thus, Nef affects IS function via multiple independent mechanisms to optimize virus replication in the infected host.

Nef-mediated enhancement of virion infectivity and stimulation of viral replication are fundamental properties of primate lentiviruses

Nef is a multifunctional accessory protein of primate lentiviruses. Recently, it has been shown that the ability of Nef to downmodulate CD4, CD28, and class I major histocompatibility complex is highly conserved between most or all primate lentiviruses, whereas Nef-mediated downregulation of T-cell receptor-CD3 was lost in the lineage that gave rise to human immunodeficiency virus type 1 (HIV-1). Whether or not other Nef activities are preserved between different groups of primate lentiviruses remained to be determined. Here, we show that nef genes from a large variety of HIVs and simian immunodeficiency viruses (SIVs) enhance virion infectivity and stimulate viral replication in human cells and/or in ex vivo infected human lymphoid tissue (HLT). Notably, nef alleles from unpassaged SIVcpz and SIVsmm enhanced viral infectivity, replication, and cytopathicity in cell culture and in ex vivo infected HLT as efficiently as those from HIV-1 and HIV-2, their human counterparts. Furthermore, nef genes from several highly divergent SIVs that have not been found in humans were also highly active in human cells and/or tissues. Thus, most primate lentiviral Nefs enhance virion infectivity and stimulate viral replication. Moreover, our data show that SIVcpz and SIVsmm Nefs do not require adaptive changes to perform these functions in human cells or tissues and support the idea that nef alleles from other primate lentiviruses would also be capable of promoting efficient virus spread in humans.

Human immunodeficiency virus type 1 Nef protein modulates the lipid composition of virions and host cell membrane microdomains

Background

The Nef protein of Human Immunodeficiency Viruses optimizes viral spread in the infected host by manipulating cellular transport and signal transduction machineries. Nef also boosts the infectivity of HIV particles by an unknown mechanism. Recent studies suggested a correlation between the association of Nef with lipid raft microdomains and its positive effects on virion infectivity. Furthermore, the lipidome analysis of HIV-1 particles revealed a marked enrichment of classical raft lipids and thus identified HIV-1 virions as an example for naturally occurring membrane microdomains. Since Nef modulates the protein composition and function of membrane microdomains we tested here if Nef also has the propensity to alter microdomain lipid composition.

Results

Quantitative mass spectrometric lipidome analysis of highly purified HIV-1 particles revealed that the presence of Nef during virus production from T lymphocytes enforced their raft character via a significant reduction of polyunsaturated phosphatidylcholine species and a specific enrichment of sphingomyelin. In contrast, Nef did not significantly affect virion levels of phosphoglycerolipids or cholesterol. The observed alterations in virion lipid composition were insufficient to mediate Nef's effect on particle infectivity and Nef augmented virion infectivity independently of whether virus entry was targeted to or excluded from membrane microdomains. However, altered lipid compositions similar to those observed in virions were also detected in detergent-resistant membrane preparations of virus producing cells.

Conclusion

Nef alters not only the proteome but also the lipid composition of host cell microdomains. This novel activity represents a previously unrecognized mechanism by which Nef could manipulate HIV-1 target cells to facilitate virus propagation in vivo.

Association of Nef with p21-activated kinase 2 is dispensable for efficient human immunodeficiency virus type 1 replication and cytopathicity in ex vivo-infected human lymphoid tissue

Interaction of the human immunodeficiency virus type 1 (HIV-1) Nef protein with p21-activated kinase 2 (PAK2) has been proposed to play a role in T-cell activation, viral replication, apoptosis, and progression to AIDS. However, these hypotheses were based on results obtained using Nef mutants impaired in multiple functions. Recently, it was reported that Nef residue F191 is specifically involved in PAK2 binding. However, only a limited number of Nef activities were investigated in these studies. To further evaluate the role of F191 in Nef function and to elucidate the biological relevance of Nef-PAK2 interaction, we performed a comprehensive analysis of HIV-1 Nef mutants carrying F191H and F191R mutations. We found that the F191H mutation reduces and the F191R mutation disrupts the association of Nef with PAK2. Both mutants upregulated the major histocompatibility complex II (MHC-II)-associated invariant chain and downregulated CD4, MHC-I, and CD28, although with reduced efficiency for the latter. Furthermore, the F191H/R changes neither affected the levels of interleukin-2 receptor expression and apoptosis of HIV-1-infected primary T cells nor reduced Nef-mediated induction of NFAT. Unexpectedly, the F191H change markedly reduced and the F191R mutation disrupted the ability of Nef to enhance virion infectivity in P4-CCR5 indicator cells but not in TZM-bl cells or peripheral blood mononuclear cells. Most importantly, all HIV-1 Nef mutants replicated efficiently and caused CD4+ T-cell depletion in ex vivo-infected human lymphoid tissue. Altogether, our data show that the interaction of Nef with PAK2 does not play a major role in T-cell activation, viral replication, and apoptosis.

T cell activation and proliferation characteristic for HIV-Nef transgenic mice is lymphopenia induced

The HIV-Nef protein has been implicated in generating high viral loads and T cell activation. Transgenic (tg) mice with constitutive T cell-specific Nef expression show a dramatic reduction in T cell number and highly increased T cell turnover. Previous studies in Nef tg mice attributed this T cell activation to a direct effect of Nef at the cellular level. Given the strongly reduced peripheral T cell numbers, we examined whether this enhanced T cell division might instead be lymphopenia induced. Adoptively transferred naive wild-type T cells into lymphopenic Nef tg mice showed high T cell turnover and obtained the same effector/memory phenotype as the autologous Nef tg T cells, supporting the idea that the microenvironment determines the phenotype of the T cells present. Moreover, in bone marrow chimeras from mixtures of wild-type and Nef tg bone marrow, with a full T cell compartment containing a small proportion of Nef tg T cells, Nef tg T cells kept a naive phenotype. These results demonstrate that T cell activation in the Nef tg mice is lymphopenia induced rather than due to a direct T cell-activating effect of Nef.

In search of a function for the most frequent naturally-occurring length polymorphism (MFNLP) of the HIV-1 LTR: retaining functional coupling, of Nef and RBF-2, at RBEIII?

Although the prototypical HIV-1 LTR sequences were determined 22 years ago from the initial isolate, elucidating which transcription factors are critical to replication in vivo, has been difficult. One approach has been to examine HIV-1 LTRs that have gone through the gamut of in vivo mutation and selection, in search of absolutely conserved sequences. In this vein, RBEIII sequences are virtually 100% conserved in naturally occurring HIV-1 LTRs. This is because when they are mutated, the MFNLP recreates an RBEIII site. Here, I enumerate some retroviral mutation mechanisms, which could generate the MFNLP. I then review the literature corresponding to the MFNLP, highlighting the discovery in 1999, that RBEIII and MFNLP sequences, bind USF and TFII-I cooperatively, within the context of earlier and later work that suggests a role in HIV-1 activation, through T-cell receptor engagement and the MAPK cascade. One exception to the nearly absolute conservation of RBEIII, has been a group of long term non progressors (LTNP). These patients harbor deletions to the Nef gene. However, the Nef gene overlaps with the LTR, and the LTNP deletions abrogate RBEIII, in the absence of an MFNLP. I suggest that the MFNLP retains functional coupling between the MAPK-mediated effects of Nef and the HIV-1 LTR, through RBEIII. I propose that difficult-to-revert-mutations, to either Nef or RBEIII, result in the convergent LTNP Nef/LTR deletions recently observed. The potential exploitation of this highly conserved protein-binding site, for chimeric transcription factor repression (CTFR) of HIV-1, functionally striving to emulate the LTNP deletions, is further discussed.

Modulation of the immunological synapse: a key to HIV-1 pathogenesis?

AIDS is the result of a constant struggle between the lentivirus HIV and the immune system. Infection with HIV interferes directly with the function of CD4(+) T cells and manipulates the host immune response to the virus. Recent studies indicate that the viral protein Nef, a central player in HIV pathogenesis, impairs the ability of infected lymphocytes to form immunological synapses with antigen-presenting cells and affects T-cell-receptor-mediated stimulation. An integrative picture of the abnormal behaviour of HIV-infected lymphocytes is therefore emerging. We propose that modulating lymphocyte signalling, apoptosis and intracellular trafficking ensures efficient spread of the virus in the hostile environment of the immune system.

Human immunodeficiency virus 1 Nef protein downmodulates the ligands of the activating receptor NKG2D and inhibits natural killer cell-mediated cytotoxicity

Natural killer (NK) cells are a major component of the host innate immune defence against various pathogens. Several viruses, including Human immunodeficiency virus 1 (HIV-1), have developed strategies to evade the NK-cell response. This study was designed to evaluate whether HIV-1 could interfere with the expression of NK cell-activating ligands, specifically the human leukocyte antigen (HLA)-I-like MICA and ULBP molecules that bind NKG2D, an activating receptor expressed by all NK cells. Results show that the HIV-1 Nef protein downmodulates cell-surface expression of MICA, ULBP1 and ULBP2, with a stronger effect on the latter molecule. The activity on MICA and ULBP2 is well conserved in Nef protein variants derived from HIV-1-infected patients. In HIV-1-infected cells, cell-surface expression of NKG2D ligands increased to a higher extent with a Nef-deficient virus compared with wild-type virus. Mutational analysis of Nef showed that NKG2D ligand downmodulation has structural requirements that differ from those of other reported Nef activities, including HLA-I downmodulation. Finally, data demonstrate that Nef expression has functional consequences on NK-cell recognition, causing a decreased susceptibility to NK cell-mediated lysis. These findings provide a novel insight into the mechanisms evolved by HIV-1 to escape from the NK-cell response.

Interaction of the Wiskott-Aldrich syndrome protein with sorting nexin 9 is required for CD28 endocytosis and cosignaling in T cells

The Wiskott-Aldrich syndrome protein (WASp) plays a major role in coupling T cell antigen receptor (TCR) stimulation to induction of actin cytoskeletal changes required for T cell activation. Here, we report that WASp inducibly binds the sorting nexin 9 (SNX9) in T cells and that WASp, SNX9, p85, and CD28 colocalize within clathrin-containing endocytic vesicles after TCR/CD28 costimulation. SNX9, implicated in clathrin-mediated endocytosis, binds WASp via its SH3 domain and uses its PX domain to interact with the phosphoinositol 3-kinase regulatory subunit p85 and product, phosphoinositol (3,4,5)P3. The data reveal ligation-induced CD28 endocytosis to be clathrin- and phosphoinositol 3-kinase-dependent and TCR/CD28-evoked CD28 internalization and NFAT activation to be markedly enhanced by SNX9 overexpression, but severely impaired by expression of an SNX9 mutant (SNX9DeltaPX) lacking p85-binding capacity. CD28 endocytosis and CD28-evoked actin polymerization also are impaired in WASp-deficient T cells. These findings suggest that SNX9 couples WASp to p85 and CD28 so as to link CD28 engagement to its internalization and to WASp-mediated actin remodeling required for CD28 cosignaling. Thus, the WASp/SNX9/p85/CD28 complex enables a unique interface of endocytic, actin polymerizing, and signal transduction pathways required for CD28-mediated T cell costimulation.

CD4 and MHC-I downregulation are conserved in primary HIV-1 Nef alleles from brain and lymphoid tissues, but Pak2 activation is highly variable

HIV-1 compartmentalization in the CNS has been demonstrated for gag, pol, and env genes. However, little is known about tissue compartmentalization of nef genes and their functional characteristics in brain. We have cloned 97 nef genes and characterized 10 Nef proteins from autopsy brain and lymphoid tissues from 2 patients with AIDS and HIV-1-associated dementia. Distinct compartmentalization of brain versus lymphoid nef genes was demonstrated within each patient. CD4 and MHC-I downregulation were conserved in all tissue-derived Nefs. However, MHC-I downregulation by brain-derived Nefs was weaker than downregulation by lymphoid-derived Nefs. The motifs KEEE- or EKEE- at the PACS-1 binding site represented brain-specific signature patterns in these 2 patients and contributed to the reduced MHC-I downregulation activity of brain-derived Nefs from these patients. Pak2 association was highly variable in Nefs from both patients. Three of 10 tissue-derived Nefs coimmunoprecipitated activated Pak2, with strong association demonstrated for only 2 Nefs. The ability of Nef to associate with activated Pak2 did not correlate with brain or lymphoid tissue origin. Nef genes from viruses isolated from brain by coculture with PBMC were not closely related to sequences amplified directly from brain tissue, suggesting that viral selection or adaptation occurred during coculture. This study of tissue-derived HIV-1 Nefs demonstrates that CD4 and MHC-I downregulation are highly conserved Nef functions, while Pak2 association is variable in late stage AIDS patients.

Expression of Nef downregulates CXCR4, the major coreceptor of human immunodeficiency virus, from the surfaces of target cells and thereby enhances resistance to superinfection

Lentiviral Nef proteins are key factors for pathogenesis and are known to downregulate functionally important molecules, including CD4 and major histocompatibility complex class I (MHC-I), from the surfaces of infected cells. Recently, we demonstrated that Nef reduces cell surface levels of the human immunodeficiency virus type 1 (HIV-1) entry coreceptor CCR5 (N. Michel, I. Allespach, S. Venzke, O. T. Fackler, and O. T. Keppler, Curr. Biol. 15:714-723, 2005). Here, we report that Nef downregulates the second major HIV-1 coreceptor, CXCR4, from the surfaces of HIV-infected primary CD4 T lymphocytes with efficiencies comparable to those of the natural CXCR4 ligand, stromal cell-derived factor-1 alpha. Analysis of a panel of mutants of HIV-1(SF2) Nef revealed that the viral protein utilized the same signature motifs for downmodulation of CXCR4 and MHC-I, including the proline-rich motif P(73)P(76)P(79)P(82) and the acidic cluster motif E(66)E(67)E(68)E(69.) Expression of wild-type Nef, but not of specific Nef mutants, resulted in a perinuclear accumulation of the coreceptor. Remarkably, the carboxy terminus of CXCR4, which harbors the classical motifs critical for basal and ligand-induced receptor endocytosis, was dispensable for the Nef-mediated reduction of surface exposure. Functionally, the ability of Nef to simultaneously downmodulate CXCR4 and CD4 correlated with maximum-level protection of Nef-expressing target cells from fusion with cells exposing X4 HIV-1 envelopes. Furthermore, the Nef-mediated downregulation of CXCR4 alone on target T lymphocytes was sufficient to diminish cells' susceptibility to X4 HIV-1 virions at the entry step. The downregulation of chemokine coreceptors is a conserved activity of Nef to modulate infected cells, an important functional consequence of which is an enhanced resistance to HIV superinfection.

Immune cell signaling: a novel mechanistic model reveals new therapeutic targets

Multichain immune recognition receptors (MIRRs) represent a family of surface receptors that is expressed on different cells and that transduces extracellular signals, leading to many biological responses. The most intriguing common structural feature of MIRR family members is that the extracellular recognition domains and the intracellular signaling domains are located on separate subunits. It is not clear how extracellular ligand binding triggers MIRRs and initiates intracellular signal-transduction processes. In this article, I suggest that the structural similarity of the MIRRs provides the basis for the similarity in the mechanisms of MIRR-mediated transmembrane signaling. This hypothesis assumes that the therapeutic strategies learned from a novel mechanistic model of MIRR-mediated signal transduction, the signaling chain homo-oligomerization model, are generalized for this entire family and have important implications for the treatment of many disorders that are mediated by immune cells, including HIV.

The HIV-1 pathogenicity factor Nef interferes with maturation of stimulatory T-lymphocyte contacts by modulation of N-Wasp activity

The Nef protein is a key determinant of human immunodeficiency virus (HIV) pathogenicity that, among other activities, sensitizes T-lymphocytes for optimal virus production. The initial events by which Nef modulates the T-cell receptor (TCR) cascade are poorly understood. TCR engagement triggers actin rearrangements that control receptor clustering for signal initiation and dynamic organization of signaling protein complexes to form an immunological synapse. Here we report that Nef potently interferes with cell spreading and formation of actin-rich circumferential rings in T-lymphocytes upon surface-supported TCR stimulation. These effects were conserved among Nef proteins from different lentiviruses and occurred in HIV-1-infected primary human T-lymphocytes. This novel Nef activity critically depended on its Src homology 3 domain binding motif and required efficient association with Pak2 activity. Notably, whereas overall signaling microcluster formation immediately following TCR engagement occurred normally in Nef-expressing cells, the viral protein inhibited the concomitant activation of the actin organizer N-Wasp. During the subsequent maturation phase of the stimulatory contact, Nef interfered with the translocation of N-Wasp to the cell periphery, the overall induction of tyrosine phosphorylation, and the selective recruitment of phosphorylated LAT to stimulatory contacts. Consistent with such a critical role of N-Wasp in this process, Nef also blocked morphological changes induced by the known N-Wasp regulators Rac1 and Cdc42. Together, our results demonstrate that Nef alters both the amount and composition of signaling microclusters. We propose modulation of actin dynamics as an important mechanism for Nef-induced alterations of TCR signaling.

Functional characterization of HIV-1 Nef mutants in the context of viral infection

Nef is an important pathogenesis factor of HIV-1 with a multitude of effector functions. We have designed a broad panel of isogenic viruses encoding defined mutants of HIV-1(SF2) Nef and analyzed their biological activity in the context of productive HIV-1 infection. Analysis of subcellular localization, virion incorporation, downregulation of cell surface CD4 and MHC-I, enhancement of virion infectivity and facilitation of HIV replication in primary human T lymphocytes mostly confirmed the mapping of Nef determinants previously reported upon isolated expression of Nef. However, reduced activity in downregulation of CD4, infectivity enhancement and virion incorporation of a Nef variant (Delta12-39) lacking an amphipatic helix required for binding of a cellular kinase complex and the association of Nef with MHC-I/AP-1 suggested a novel role of this N-terminal motif. The SH3 binding motif of Nef was partially required for infectivity enhancement and replication but not for receptor downmodulation. In contrast to previous results obtained using other Nef alleles, non-myristoylated SF2-Nef was only partly defective when expressed during HIV infection and was present in HIV-1 particles. Importantly, incorporation of Nef into HIV-1 virions was not required for any of the tested Nef activities. Altogether, this study provides a broad characterization and mapping of multiple Nef activities in HIV-infected cells. The results emphasize that multiple activities govern Nef's effects on HIV replication and argue against a role of virion incorporation for Nef's activity as pathogenicity factor.

A hydrophobic binding surface on the human immunodeficiency virus type 1 Nef core is critical for association with p21-activated kinase 2

The interaction of human immunodeficiency virus type 1 (HIV-1) Nef with p21-activated kinase 2 (Pak2) has been proposed to play an important role in T-cell activation and disease progression during viral infection. However, the mechanism by which Nef activates Pak2 is poorly understood. Mutations in most Nef motifs previously reported to be required for Pak2 activation (G2, PxxP72, and RR105) also affect other Nef functions, such as CD4 or major histocompatibility complex class I (MHC-I) downregulation. To better understand Nef interactions with Pak2, we performed mutational analysis of three primary HIV-1 Nef clones that exhibited similar capacities for downregulation of CD4 and MHC-I but variable abilities to associate with activated Pak2. Our results demonstrate that Nef amino acids at positions 85, 89, 187, 188, and 191 (L, H, S, R, and F in the clade B consensus, respectively) are critical for Pak2 association. Mutation of these Nef residues dramatically altered association with Pak2 without affecting Nef expression levels or CD4 and MHC-I downregulation. Furthermore, compensation occurred at positions 89 and 191 when both amino acids were substituted. Since residues 85, 89, 187, 188, and 191 cluster on the surface of the Nef core domain in a region distinct from the dimerization and SH3-binding domains, we propose that these Nef residues form part of a unique binding surface specifically involved in association with Pak2. This binding surface includes exposed and recessed hydrophobic residues and may participate in an as-yet-unidentified protein-protein interaction to facilitate Pak2 activation.

Activation of p21-activated kinase 2 by human immunodeficiency virus type 1 Nef induces merlin phosphorylation

The accessory human immunodeficiency virus type 1 (HIV-1) protein Nef activates the autophosphorylation activity of p21-activated kinase 2 (PAK2). Merlin, a cellular substrate of PAK2, is homologous to the ezrin-radixin-moesin family and plays a critical role in Rac signaling. To assess the possible impact on host cell metabolism of Nef-induced PAK2 activation, we investigated the phosphorylation of merlin in Nef expressing cells. Here we report that Nef induces merlin phosphorylation in multiple cell lines independently of protein kinase A. This intracellular phosphorylation of merlin directly correlates with in vitro assay of the autophosphorylation activity of Nef-activated PAK2. Importantly, merlin phosphorylation induced by Nef was also observed in human primary T cells. The finding that Nef induces phosphorylation of the key signaling molecule merlin suggests several possible roles for PAK2 activation in HIV pathogenesis.

Modulation of specific surface receptors and activation sensitization in primary resting CD4+ T lymphocytes by the Nef protein of HIV-1

The human immunodeficiency virus type 1 (HIV-1) pathogenicity factor Nef increases viral replication in vivo. In immortalized cell lines, Nef affects the cell surface levels of multiple receptors and signal transduction pathways. Resting CD4+ T lymphocytes are important targets for HIV-1 infection in vivo-they actively transcribe and express HIV-1 genes and contribute to the local viral burden and long-lived viral reservoirs in patients undergoing antiretroviral therapy. In vitro, this primary cell type has, however, thus far been highly refractory to experimental manipulation, and the biological activities exerted by HIV-1 Nef in these cells are largely unknown. Using nucleofection for gene delivery, we find that Nef induces a drastic and moderate down-regulation of CD4 and major histocompatibility complex type 1 (MHC-I), respectively, but does not alter surface levels of other receptors, the down-modulation of which has been reported in cell line studies. In contrast, Nef markedly up-regulated cell surface levels of the MHC-II invariant chain CD74. The effect of Nef on these three surface receptors was also detected upon HIV-1 infection of activated primary CD4+ T lymphocytes. Nef expression alone was insufficient to activate resting CD4+ T lymphocytes, but Nef modestly enhanced the responsiveness of cells to exogenous T cell activation. Consistent with such a signal transduction activity, a subpopulation of Nef localized to lipid raft clusters at the plasma membrane. This study establishes the analysis of Nef functions in these primary HIV target cells. Our data support the involvement of modulation of a defined set of cell surface receptors and sensitization to activation rather than an autonomous activation function in the role of Nef in HIV-1 pathogenesis.

Nef is physically recruited into the immunological synapse and potentiates T cell activation early after TCR engagement

The HIV-1 protein Nef enhances viral pathogenicity and accelerates disease progression in vivo. Nef potentiates T cell activation by an unknown mechanism, probably by optimizing the intracellular environment for HIV replication. Using a new T cell reporter system, we have found that Nef more than doubles the number of cells expressing the transcription factors NF-kappaB and NFAT after TCR stimulation. This Nef-induced priming of TCR signaling pathways occurred independently of calcium signaling and involved a very proximal step before protein kinase C activation. Engagement of the TCR by MHC-bound Ag triggers the formation of the immunological synapse by recruiting detergent-resistant membrane microdomains, termed lipid rafts. Approximately 5-10% of the total cellular pool of Nef is localized within lipid rafts. Using confocal and real-time microscopy, we found that Nef in lipid rafts was recruited into the immunological synapse within minutes after Ab engagement of the TCR/CD3 and CD28 receptors. This recruitment was dependent on the N-terminal domain of Nef encompassing its myristoylation. Nef did not increase the number of cell surface lipid rafts or immunological synapses. Recently, studies have shown a specific interaction of Nef with an active subpopulation of p21-activated kinase-2 found only in the lipid rafts. Thus, the corecruitment of Nef and key cellular partners (e.g., activated p21-activated kinase-2) into the immunological synapse may underlie the increased frequency of cells expressing transcriptionally active forms of NF-kappaB and NFAT and the resultant changes in T cell activation.

Nef enhances c-Cbl phosphorylation in HIV-infected CD4+ T lymphocytes

The multifunctional HIV-1 protein Nef possesses several motifs that interact with signaling molecules in infected T cells. In order to determine whether Nef influences T cell activation, cells were infected with Nef-positive and Nef-negative clones of HIV. CD28 expression and changes in tyrosine phosphorylation were monitored. We observed no Nef-dependent changes in CD28 expression or function. However, infection with Nef-positive virus led to changes in tyrosine phosphorylation. This Nef-induced phosphorylation was observed in unstimulated cells, and c-Cbl was identified as one of the proteins whose phosphorylation was upregulated by Nef. Furthermore, Lck is required for Nef-mediated c-Cbl tyrosine phosphorylation. These results suggest that Nef modifies T cell signaling in the absence of T cell receptor engagement and co-stimulation.

The HIV-1 Nef protein as a target for antiretroviral therapy

HIV-1 Nef is a peripheral membrane protein that affects both signal transduction and membrane trafficking in infected cells. Alterations in these cellular processes enhance the efficiency of viral replication and the pathogenesis of AIDS in vivo. The precise mechanisms by which Nef functions are not fully elucidated. Nef is not an enzyme but appears to act as a linker molecule, mediating a variety of protein-protein interactions. Structural, biochemical and mutational data have allowed tentative identification of the key interactive surfaces on Nef, their cellular partners and their roles in Nef activity. Nef contains an SH3-binding surface through which it can interact with cellular Src-family tyrosine kinases and/or activator molecules for small GTPases involved in signal transduction. This SH3-binding surface is important for the ability of Nef to facilitate the activation of host T-lymphocytes, a process which renders the cells more permissive for viral replication. Nef also contains two relatively unstructured, solvent-exposed loops, through which it interacts with the cellular proteins that coat vesicles involved in membrane trafficking. These surfaces are important for Nef-mediated alterations in the subcellular distribution of transmembrane proteins, a process which causes diverse effects, including the assembly of maximally infectious viral particles and viral evasion of the host immune system. These data provide precise molecular targets within the Nef protein. Molecules that bind these interactive surfaces are predicted to inhibit Nef activity and provide the basis for novel chemotherapeutic agents for the treatment of HIV-infection.

Displacement of SATB1-bound histone deacetylase 1 corepressor by the human immunodeficiency virus type 1 transactivator induces expression of interleukin-2 and its receptor in T cells

One hallmark of human immunodeficiency virus type 1 (HIV-1) infection is the dysregulation of cytokine gene expression in T cells. Transfection of T cells with human T-cell leukemia type 1 or 2 transactivator results in the induction of the T-cell-restricted cytokine interleukin-2 (IL-2) and its receptor (IL-2Ralpha). However, no T-cell-specific factor(s) has been directly linked with the regulation of IL-2 and IL-2Ralpha transcription by influencing the promoter activity. Thymocytes from SATB1 (special AT-rich sequence binding protein 1) knockout mice have been shown to ectopically express IL-2Ralpha, suggesting involvement of SATB1 in its negative regulation. Here we show that SATB1, a T-cell-specific global gene regulator, binds to the promoters of human IL-2 and IL-2Ralpha and recruits histone deacetylase 1 (HDAC1) in vivo. SATB1 also interacts with Tat in HIV-1-infected T cells. The functional interaction between HIV-1 Tat and SATB1 requires its PDZ-like domain, and the binding of the HDAC1 corepressor occurs through the same. Furthermore, Tat competitively displaces HDAC1 that is bound to SATB1, leading to increased acetylation of the promoters in vivo. Transduction with SATB1 interaction-deficient soluble Tat (Tat 40-72) and reporter assays using a transactivation-negative mutant (C22G) of Tat unequivocally demonstrated that the displacement of HDAC1 itself is sufficient for derepression of these promoters in vivo. These results suggest a novel mechanism by which HIV-1 Tat might overcome SATB1-mediated repression in T cells.

Rodent cells support key functions of the human immunodeficiency virus type 1 pathogenicity factor Nef

After infection with human immunodeficiency virus (HIV), progression toward immunodeficiency is governed by a complex interplay of viral and host determinants. The viral accessory protein Nef is a key factor for the development of AIDS. Strains of HIV and simian immunodeficiency virus that lack functional nef genes either do not induce AIDS or do so only after a significant delay. The validity of a transgenic-small-animal model for de novo infection by HIV will depend on its ability to recapitulate the actions of critical factors of viral pathogenicity, such as Nef. We assessed the ability of rat, mouse, and hamster cells to support key effector functions of Nef. In cell lines from rodents, the subcellular distribution of wild-type HIV type 1 strain SF2 Nef and mutants was comparable to that in human cells. Nef downregulated human CD4 from the cell surface, was associated with p21-activated kinase activity, and enhanced the infectivity of HIV-1 virions. Importantly, these Nef-induced effects, as well as the downregulation of rat CD4 and major histocompatibility complex class I molecules, could also be demonstrated in primary T lymphocytes and macrophages from human CD4-transgenic rats. Thus, HIV-1 Nef exerts key functions in rodent cells. In line with our ongoing efforts to establish a transgenic-rat model of HIV disease, these results indicate that important aspects of viral pathogenesis could be addressed in a transgenic-rodent model permissive for de novo infection and that such a model would be valuable for evaluating the function of Nef in vivo.

Human N-myristoyltransferases form stable complexes with lentiviral nef and other viral and cellular substrate proteins

Nef is a multifunctional virulence factor of primate lentiviruses that facilitates viral replication in the infected host. All known functions of Nef require that it be myristoylated at its N terminus. This reaction is catalyzed by N-myristoyltransferases (NMTs), which transfer myristate from myristoyl coenzyme A (myristoyl-CoA) to the N-terminal glycine of substrate proteins. Two NMT isoforms (NMT-1 and NMT-2) are expressed in mammalian cells. To provide a better mechanistic understanding of Nef function, we used biochemical and microsequencing techniques to isolate and identify Nef-associated proteins. Through these studies, NMT-1 was identified as an abundant Nef-associated protein. The Nef-NMT-1 complex is most likely a transient intermediate of the myristoylation reaction of Nef and is modulated by agents which affect the size of the myristoyl-CoA pool in the cell. We also examined two other proteins that bear an N-terminal myristoylation signal, human immunodeficiency virus type 1 Gag and Hck protein tyrosine kinase, and found that Gag bound preferentially the NMT-2 isoform, while Hck bound mostly to NMT-1. Recognition of different NMT isoforms by these viral and cellular substrate proteins suggests nonoverlapping roles for these enzymes in vivo and reveals a potential for the development of inhibitors that target the myristoylation of specific viral substrates more selectively.

Impact of Nef-mediated downregulation of major histocompatibility complex class I on immune response to simian immunodeficiency virus

Functional activities that have been ascribed to the nef gene product of simian immunodeficiency virus (SIV) and human immunodeficiency virus (HIV) include CD4 downregulation, major histocompatibility complex (MHC) class I downregulation, downregulation of other plasma membrane proteins, and lymphocyte activation. Monkeys were infected experimentally with SIV containing difficult-to-revert mutations in nef that selectively eliminated MHC downregulation but not these other activities. Monkeys infected with these mutant forms of SIV exhibited higher levels of CD8(+) T-cell responses 4 to 16 weeks postinfection than seen in monkeys infected with the parental wild-type virus. Furthermore, unusual compensatory mutations appeared by 16 to 32 weeks postinfection which restored some or all of the MHC-downregulating activity. These results indicate that nef does serve to limit the virus-specific CD8 cellular response of the host and that the ability to downregulate MHC class I contributes importantly to the totality of nef function.

The increased replicative capacity of a late-stage simian immunodeficiency virus mne variant is evident in macrophage- or dendritic cell-T-cell cocultures

Human and simian immunodeficiency virus (HIV and SIV) may co-opt antigen capture and presentation functions of antigen presenting cells (APCs) to facilitate infection of CD4+ T-cells. To address whether the replicative capacity of SIV in the host may be associated with the extent of viral replication in response to APC-T-cell interactions, we compared the replicative phenotypes of cloned early and late-stage SIVmne variants of known pathogenicity. Here, we show that the highly pathogenic late variant SIVmne027 replicates more efficiently in both macrophage- and dendritic cell (DC)-T-cell cocultures than the minimally pathogenic early virus SIVmneCl8. Contact between either macrophages or DC and T-cells increases replication of SIVmne027. Our analysis also demonstrates that mutations in pol and nef contribute to the greater replicative capacity of SIVmne027 in DC- or macrophage-T-cell cocultures. Together, these data suggest that variant viruses that evolve to replicate vigorously in response to APC-T-cell interactions may have increased replicative capacity in vivo.

In vivo mutational analysis of the N-terminal region of HIV-1 Nef reveals critical motifs for the development of an AIDS-like disease in CD4C/HIV transgenic mice

HIV-1 Nef is a critical determinant of pathogenicity in humans and transgenic (Tg) mice. To gain a better understanding of the molecular mechanisms by which Nef induces an AIDS-like disease in Tg mice, a mutational analysis of the N-terminal domain, involved in anchoring Nef to the plasma membrane, was carried out. The pathogenic effects of these Nef mutant alleles were evaluated in Tg mice by FACS analysis and by histopathological assessment. Mutation of the myristoylation site (G2A) completely abrogated the development of the AIDS-like organ disease in Tg mice, although partial downregulation of the CD4 cell surface protein and depletion of peripheral CD4+ T-cells, but not of CD4(+)CD8+ thymocytes, still occurred. Despite that, the peripheral CD4+ T cells expressing Nef(G2A) show normal spontaneous proliferation in vivo or after stimulation in vitro, including in an allogenic mixed leukocyte reaction (MLR). Three other internal deletion mutants of Nef, spanning amino acids 8-17 (Nef(Delta8-17)), 25-35 (Nef(Delta25-35)), and 57-66 (Nef(Delta57-66)), were also studied. Nef(Delta8-17) retained full pathogenic potential, although Nef(Delta25-35) and Nef(Delta57-66) Tg mice were free of organ disease. However, Nef(Delta25-35) Tg mice exhibited disorganization of thymic architecture and a partial depletion of peripheral CD4+ T cells. These data indicate that myristoylation and other regions at the N-terminus of Nef (aa 25-35 and 57-66) are involved in mediating severe T-cell phenotypes and organ disease, although residues 8-17 are dispensable for these Nef functions. In addition, these results indicate that at least some of the CD4+ T-cell phenotypes can develop independently of the other AIDS-like organ phenotypes. This apparent segregation of different Nef-mediated phenotypes suggests distinct mechanisms of Nef action in different populations of target cells, and may be relevant to human AIDS.

Substitution of HIV Type 1 Nef with HTLV-1 p12

Human retroviruses, such as HTLV-1 and HIV-1, encode accessory proteins, which regulate viral pathogenesis. The p12 protein of HTLV-1 is encoded from the pX-I open reading frame, and is critical for efficient virus replication in rabbits. Although dispensable for infection, replication, and immortalization of activated lymphocytes in culture, p12 expression is important for infection of quiescent lymphocytes. Similar to HTLV-1 p12, Nef is important for virus infectivity in SIV animal models. We questioned whether p12 could replace Nef in HIV-1, and reconstitute virus replication in culture. We found that p12 could complement for effects of Nef on HIV-1 infection of Magi-CCR5 cells or macrophages.

Hyper-responsiveness to stimulation of human immunodeficiency virus-infected CD4+ T cells requires Nef and Tat virus gene products and results from higher NFAT, NF-kappaB, and AP-1 induction

A chronic state of immune hyperactivation is a feature of human immunodeficiency virus type-1 (HIV-1) infection. Studies on the molecular mechanisms by which HIV-1 can modulate the activation state of T cells indicate that both Nef and Tat can alter T cell activation. However, the vast majority of data has been obtained from experiments performed with vectors encoding a single virus protein. We demonstrate that infection of human CD4(+) T lymphocytes with fully infectious HIV-1 leads to a hyper-responsiveness of the interleukin-2 promoter. Hypersensitivity in HIV-1-infected T cells was observed upon stimulation with various agents that are engaging different signal transduction pathways. Experiments performed with recombinant heat stable antigen-encoding HIV-1 indicated that the virus-infected cells are the cells with an enhanced response. Both Nef and Tat are involved in this virus-mediated enhancing effect on interleukin-2 promoter activity. Interestingly, whereas Nef seems to be acting mainly through hyperactivation of nuclear factor of activated T cells (NFAT), Tat acts in an NFAT-independent manner. Mobility shift experiments demonstrated that the HIV-1-associated priming of human T cells for stimulation results in a greater induction of transcription factors recognized as essential players in T cell activation, i.e. NFAT, NF-kappaB, and AP-1. A hyper-responsive state was also established upon HIV-1 infection of a more natural cellular reservoir, i.e. primary CD4(+) T lymphocytes. Considering that the HIV-1 life cycle is tightly regulated by the T cell signaling machinery, the priming for activation of a major viral reservoir represents a means by which this retrovirus can create an ideal cellular microenvironment for its propagation and maintenance.

CD4+ T cells from CD4C/HIVNef transgenic mice show enhanced activation in vivo with impaired proliferation in vitro but are dispensable for the development of a severe AIDS-like organ disease

The cellular and molecular mechanisms of dysfunction and depletion of CD4+ T lymphocytes over the course of human immunodeficiency virus type 1 (HIV-1) infection are still incompletely understood, but chronic immune activation is thought to play an important role in disease progression. We studied CD4+ T-cell biology in CD4C/HIV transgenic (Tg) mice, in which Nef expression is sufficient to induce a severe AIDS-like disease including a preferential decrease of CD4+ T cells. We show here that Nef-expressing Tg CD4+ T cells exhibit an activated/memory-like phenotype which appears to be independent of antigenic stimulation, as documented in experiments involving breeding with AD10 TcR Tg mice. In addition, in vivo bromodeoxyuridine incorporation showed that a larger proportion of Tg than non-Tg CD4+ T cells entered the S phase. However, in vitro, Tg CD4+ T cells were found to have a very limited capacity to divide in response to stimulation with anti-CD3 and anti-CD28 or in allogeneic mixed leukocyte reactions. Interestingly, despite these observations, the deletion of Tg CD4+ T cells had little impact on the development of other AIDS-like organ phenotypes. Thus, the Nef-induced chronic activation of CD4+ T cells may exhaust the T-cell pool and may contribute to the thymic atrophy and the low number of CD4+ T cells observed in these Tg mice.