Requirement of human immunodeficiency virus type 1 nef for in vivo replication and pathogenicity

Apoptosis regulation by the tyrosine-protein kinase CSK

C-terminal Src kinase (CSK) is a cytosolic tyrosine-protein kinase with an important role in regulating critical cellular decisions, such as cellular apoptosis, survival, proliferation, cytoskeletal organization and many others. Current knowledge on the CSK mechanisms of action, regulation and functions is still at an early stage, most of CSK's known actions and functions being mediated by the negative regulation of the SRC family of tyrosine kinases (SFKs) through phosphorylation. As SFKs play a vital role in apoptosis, cell proliferation and survival regulation, SFK inhibition by CSK has a pro-apoptotic effect, which is mediated by the inhibition of cellular signaling cascades controlled by SFKs, such as the MAPK/ERK, STAT3 and PI3K/AKT signaling pathways. Abnormal functioning of CSK and SFK activation can lead to diseases such as cancer, cardiovascular and neurological manifestations. This review describes apoptosis regulation by CSK, CSK inhibition of the SFKs and further explores the clinical relevance of CSK in important pathologies, such as cancer, autoimmune, autoinflammatory, neurologic diseases, hypertension and HIV/AIDS.

HIV-1 and drug abuse comorbidity: Lessons learned from the animal models of NeuroHIV

Various research studies that have investigated the association between HIV infection and addiction underpin the role of various drugs of abuse in impairing immunological and non-immunological pathways of the host system, ultimately leading to augmentation of HIV infection and disease progression. These studies have included both in vitro and in vivo animal models wherein investigators have assessed the effects of various drugs on several disease parameters to decipher the impact of drugs on both HIV infection and progression of HIV-associated neurocognitive disorders (HAND). However, given the inherent limitations in the existing animal models of HAND, these investigations only recapitulated specific aspects of the disease but not the complex human syndrome. Despite the inability of HIV to infect rodents over the last 30 years, multiple strategies have been employed to develop several rodent models of HAND. While none of these models can accurately mimic the overall pathophysiology of HAND, they serve the purpose of modeling some unique aspects of HAND. This review provides an overview of various animal models used in the field and a careful evaluation of methodological strengths and limitations inherent in both the model systems and study designs to understand better how the various animal models complement one another.

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.

HIV-1 Nef-POTEE; A novel interaction modulates macrophage dissemination via mTORC2 signaling pathway

AIMS

Human immunodeficiency virus -1 [HIV-1] Nef, localizes in different cellular compartments and modulates several cellular pathways. Nef promotes virus pathogenicity through alteration in cell surface receptor expression, apoptosis, protein trafficking etc. Nef regulates viral pathogenesis through interaction with different host proteins. Thus, molecular mechanisms of pathogenesis could be deciphered by identifying novel Nef interacting proteins.

MAIN METHODS

HIV-1 Nef interacting proteins were identified by pull down assay and MALDI-TOF analysis. The interaction was further validated through mammalian two hybrid assay. Functional role of this interaction was identified by immunoprecipitation assay, cell invasion and cell migration studies. Fold Change in mRNA levels of CD163, CD206, CCL17 and CCL18 was analyzed using qPCR.

KEY FINDINGS

In current study, C. elegans protein ACT4C and its human homolog POTEE was identified to be interacting with Nef. This interaction activates mTORC2 complex, which in-turn activates AKT and PKC-α. The activation of mTORC2 complex was found to be initiated by the interaction of Nef, mTORC2, Rictor to POTEE. The cellular phenotype and functions affected by Nef-POTEE interaction resulted in significant increase in cell invasion and migration of macrophages (MΦ).

SIGNIFICANCE

MΦ is primary target of HIV-1 infection where HIV-1 replicates and polarizes immunosuppressive M2 phenotype. Combine effect of M2 phenotype and Viral-host protein interactions compromise the MΦ associated physiological functions. Infected MΦ dissemination into other system also leads to HIV-1 induced malignancies. Therefore, targeting POTEE-Nef interaction can lead to formulating better therapeutic strategy against HIV-1.

Killed whole-HIV vaccine; employing a well established strategy for antiviral vaccines

The development of an efficient prophylactic HIV vaccine has been one of the major challenges in infectious disease research during the last three decades. Here, we present a mini review on strategies employed for the development of HIV vaccines with an emphasis on a well-established vaccine technology, the killed whole-virus vaccine approach. Recently, we reported an evaluation of the safety and the immunogenicity of a genetically modified and killed whole-HIV-1 vaccine designated as SAV001 [1]. HIV-1 Clade B NL4-3 was genetically modified by deleting the nef and vpu genes and substituting the coding sequence of the Env signal peptide with that of honeybee melittin to produce an avirulent and replication efficient HIV-1. This genetically modified virus (gmHIV-1 NL4-3 ) was propagated in a human T cell line followed by virus purification and inactivation by aldrithiol-2 and γ-irradiation. We found that SAV001 was well tolerated with no serious adverse events. HIV-1 NL4-3 -specific polymerase chain reaction showed no evidence of vaccine virus replication in participants receiving SAV001 and in human T cells infected in vitro. Furthermore, SAV001 with an adjuvant significantly increased the antibody response to HIV-1 structural proteins. Moreover, antibodies in the plasma from these vaccinations neutralized tier I and tier II of HIV-1 B, A, and D subtypes. These results indicated that the killed whole-HIV vaccine is safe and may trigger appropriate immune responses to prevent HIV infection. Utilization of this killed whole-HIV vaccine strategy may pave the way to develop an effective HIV vaccine.

In vivo analysis of Nef's role in HIV-1 replication, systemic T cell activation and CD4(+) T cell loss

Background

Nef is a multifunctional HIV-1 protein critical for progression to AIDS. Humans infected with nef(−) HIV-1 have greatly delayed or no disease consequences. We have contrasted nef(−) and nef(+) infection of BLT humanized mice to better characterize Nef’s pathogenic effects.

Results

Mice were inoculated with CCR5-tropic HIV-1_JRCSF (JRCSF) or JRCSF with an irreversibly inactivated nef (JRCSFNefdd). In peripheral blood (PB), JRCSF exhibited high levels of viral RNA (peak viral loads of 4.71 × 10⁶ ± 1.23 × 10⁶ copies/ml) and a progressive, 75% loss of CD4⁺ T cells over 17 weeks. Similar losses were observed in CD4⁺ T cells from bone marrow, spleen, lymph node, lung and liver but thymocytes were not significantly decreased. JRCSFNefdd also had high peak viral loads (2.31 × 10⁶ ± 1.67 × 10⁶) but induced no loss of PB CD4⁺ T cells. In organs, JRCSFNefdd produced small, but significant, reductions in CD4⁺ T cell levels and did not affect the level of thymocytes. Uninfected mice have low levels of HLA-DR⁺CD38⁺CD8⁺ T cells in blood (1–2%). Six weeks post inoculation, JRCSF infection resulted in significantly elevated levels of activated CD8⁺ T cells (6.37 ± 1.07%). T cell activation coincided with PB CD4⁺ T cell loss which suggests a common Nef-dependent mechanism. At 12 weeks, in JRCSF infected animals PB T cell activation sharply increased to 19.7 ± 2.9% then subsided to 5.4 ± 1.4% at 14 weeks. HLA-DR⁺CD38⁺CD8⁺ T cell levels in JRCSFNefdd infected mice did not rise above 1–2% despite sustained high levels of viremia. Interestingly, we also noted that in mice engrafted with human tissue expressing a putative protective HLA-B allele (B42:01), JRCSFNefdd exhibited a substantial (200-fold) reduced viral load compared to JRCSF.

Conclusions

Nef expression was necessary for both systemic T cell activation and substantial CD4⁺ T cell loss from blood and tissues. JRCSFNefdd infection did not activate CD8⁺ T cells or reduce the level of CD4⁺ T cells in blood but did result in a small Nef-independent decrease in CD4⁺ T cells in organs. These observations strongly support the conclusion that viral pathogenicity is mostly driven by Nef. We also observed for the first time substantial host-specific suppression of HIV-1 replication in a small animal infection model.

Electronic supplementary material

The online version of this article (doi:10.1186/s12977-015-0187-z) contains supplementary material, which is available to authorized users.

Ectopic expression of anti-HIV-1 shRNAs protects CD8(+) T cells modified with CD4ζ CAR from HIV-1 infection and alleviates impairment of cell proliferation

Chimeric antigen receptors (CARs) are artificially engineered receptors that confer a desired specificity to immune effector T cells. As an HIV-1-specific CAR, CD4ζ CAR has been extensively tested in vitro as well as in clinical trials. T cells modified with this CAR mediated highly potent anti-HIV-1 activities in vitro and were well-tolerated in vivo, but exerted limited effects on viral load and reservoir size due to poor survival and/or functionality of the transduced cells in patients. We hypothesize that ectopic expression of CD4ζ on CD8(+) T cells renders them susceptible to HIV-1 infection, resulting in poor survival of those cells. To test this possibility, highly purified CD8(+) T cells were genetically modified with a CD4ζ-encoding lentiviral vector and infected with HIV-1. CD8(+) T cells were vulnerable to HIV-1 infection upon expression of CD4ζ as evidenced by elevated levels of p24(Gag) in cells and culture supernatants. Concurrently, the number of CD4ζ-modified CD8(+) T cells was reduced relative to control cells upon HIV-1 infection. To protect these cells from HIV-1 infection, we co-expressed two anti-HIV-1 shRNAs previously developed by our group together with CD4ζ. This combination vector was able to suppress HIV-1 infection without impairing HIV-1-dependent effector activities of CD4ζ. In addition, the number of CD4ζ-modified CD8(+) T cells maintained similar levels to that of the control even under HIV-1 infection. These results suggest that protecting CD4ζ-modified CD8(+) T cells from HIV-1 infection is required for prolonged HIV-1-specific immune surveillance.

Studies of retroviral infection in humanized mice

Many important aspects of human retroviral infections cannot be fully evaluated using only in vitro systems or unmodified animal models. An alternative approach involves the use of humanized mice, which consist of immunodeficient mice that have been transplanted with human cells and/or tissues. Certain humanized mouse models can support robust infection with human retroviruses including different strains of human immunodeficiency virus (HIV) and human T cell leukemia virus (HTLV). These models have provided wide-ranging insights into retroviral biology, including detailed information on primary infection, in vivo replication and pathogenesis, latent/persistent reservoir formation, and novel therapeutic interventions. Here we describe the humanized mouse models that are most commonly utilized to study retroviral infections, and outline some of the important discoveries that these models have produced during several decades of intensive research.

Clearly different mechanisms of enhancement of short-lived Nef-mediated viral infectivity between SIV and HIV-1

Background

One of the major functions of Nef is in the enhancement of the infectivity of the human and simian immunodeficiency viruses (HIV and SIV, respectively). However, the detailed mechanism of the enhancement of viral infectivity by Nef remains unclear. Additionally, studies of mechanisms by which Nef enhances the infectivity of SIV are not as intensive as those of HIV-1.

Methods

We generated short-lived Nef constructed by fusing Nef to a proteasome-mediated protein degradation sequence to characterize the Nef role in viral infectivity.

Results

The apparent expression level of the short-lived Nef was found to be extremely lower than that of the wild-type Nef. Moreover, the expression level of the short-lived Nef increased with the treatment with a proteasome inhibitor. The infectivity of HIV-1 with the short-lived Nef was significantly lower than that with the wild-type Nef. On the other hand, the short-lived Nef enhanced the infectivity of SIV_mac239, an ability observed to be interestingly equivalent to that of the wild-type Nef. The short-lived Nef was not detected in SIV_mac239, but the wild-type Nef was, suggesting that the incorporation of Nef into SIV_mac239 is not important for the enhancement of SIV_mac239 infectivity.

Conclusions

Altogether, the findings suggest that the mechanisms of infectivity enhancement by Nef are different between HIV-1 and SIV_mac239. Lastly, we propose the following hypothesis: even when the expression level of a protein is extremely low, the protein may still be sufficiently functional.

A consensus surface activation marker signature is partially dependent on human immunodeficiency virus type 1 Nef expression within productively infected macrophages

Background

The high prevalence of HIV-associated comorbidities including neurocognitive disorder, high levels of residual inflammatory mediators in the plasma and cerebrospinal fluid and the resurgence of HIV replication upon interruption of antiviral treatment in HIV-1 infected individuals, strongly suggests that despite therapy HIV persists in its cellular targets which include T-lymphocytes and cells of the myeloid lineage. These reservoirs present a major barrier against eradication efforts. Knowledge of the molecular mechanisms used by HIV to modulate innate macrophage immune responses and impair viral clearance is quite limited. To explore the role of HIV in potentially modulating macrophage function through changes in protein expression, we used single-cell analyses with flow cytometry to determine whether, in unpolarized cultures, macrophage surface marker phenotype was altered by HIV infection in a manner that was independent of host genetic background.

Results

These analyses revealed that at several time points post-infection, GFP + HIV-infected macrophages were significantly enriched in the CD14+ fraction (3 to 5-fold, p = .0001) compared to bystander, or uninfected cells in the same culture. However, the enrichment and higher levels of CD14 on HIV expressing macrophages did not depend on the production of HIV Nef. Sixty to eighty percent of macrophages productively infected with HIV after day 28 post-infection were also enriched in the population of cells expressing the activation markers CD69 (2 to 4-fold, p < .0001) and CD86 (2 to 4-fold, p < .0001 ) but suppressed amounts of CD68 (3 to 10-fold, p < .0001) compared to bystander cells. Interestingly, there was no enrichment of CD69 on the surface of HIV producing cells that lacked Nef or expressed a variant of Nef mutated in its SH3-binding domain.

Conclusions

These findings suggest that HIV actively regulates the expression of a subset of surface molecules involved in innate and inflammatory immune signaling in primary human macrophages through Nef-dependent and Nef-independent mechanisms acting within productively infected cells.

HIV-1 Nef sequence and functional compartmentalization in the gut is not due to differential cytotoxic T lymphocyte selective pressure

The gut is the largest lymphoid organ in the body and a site of active HIV-1 replication and immune surveillance. The gut is a reservoir of persistent infection in some individuals with fully suppressed plasma viremia on combination antiretroviral therapy (cART) although the cause of this persistence is unknown. The HIV-1 accessory protein Nef contributes to persistence through multiple functions including immune evasion and increasing infectivity. Previous studies showed that Nef’s function is shaped by cytotoxic T lymphocyte (CTL) responses and that there are distinct populations of Nef within tissue compartments. We asked whether Nef’s sequence and/or function are compartmentalized in the gut and how compartmentalization relates to local CTL immune responses. Primary nef quasispecies from paired plasma and sigmoid colon biopsies from chronically infected subjects not on therapy were sequenced and cloned into Env⁻ Vpu⁻ pseudotyped reporter viruses. CTL responses were mapped by IFN-γ ELISpot using expanded CD8+ cells from blood and gut with pools of overlapping peptides covering the entire HIV proteome. CD4 and MHC Class I Nef-mediated downregulation was measured by flow cytometry. Multiple tests indicated compartmentalization of nef sequences in 5 of 8 subjects. There was also compartmentalization of function with MHC Class I downregulation relatively well preserved, but significant loss of CD4 downregulation specifically by gut quasispecies in 5 of 7 subjects. There was no compartmentalization of CTL responses in 6 of 8 subjects, and the selective pressure on quasispecies correlated with the magnitude CTL response regardless of location. These results demonstrate that Nef adapts via diverse pathways to local selective pressures within gut mucosa, which may be predominated by factors other than CTL responses such as target cell availability. The finding of a functionally distinct population within gut mucosa offers some insight into how HIV-1 may persist in the gut despite fully suppressed plasma viremia on cART.

Nef functions in BLT mice to enhance HIV-1 replication and deplete CD4+CD8+ thymocytes

Background

The outcome of untreated HIV-1 infection is progression to AIDS and death in nearly all cases. Some important exceptions are the small number of patients infected with HIV-1 deleted for the accessory gene, nef. With these infections, disease progression is entirely suppressed or greatly delayed. Whether Nef is critical for high levels of replication or is directly cytotoxic remains controversial. The major problem in determining the role of Nef in HIV/AIDS has been the lack of tractable in vivo models where Nef’s complex pathogenic phenotype can be recapitulated.

Results

Intravenous inoculation (3000 to 600,000 TCIU) of BLT humanized mice with HIV-1_LAI reproducibly establishes a systemic infection. HIV-1_LAI (LAI) replicates to high levels (peak viral load in blood 8,200,000 ± 1,800,000 copies of viral RNA/ml, range 3,600,000 to 20,400,000; n = 9) and exhaustively depletes CD4⁺ T cells in blood and tissues. CD4⁺CD8⁺ thymocytes were also efficiently depleted but CD4⁺CD8^- thymocytes were partially resistant to cell killing by LAI. Infection with a nef-deleted LAI (LAINefdd) gave lower peak viral loads (1,220,000 ± 330,000, range 27,000 to 4,240,000; n = 17). For fourteen of seventeen LAINefdd-infected mice, there was little to no loss of either CD4⁺ T cells or thymocytes. Both LAI- and LAINefdd-infected mice had about 8% of total peripheral blood CD8⁺ T cells that were CD38⁺HLA-DR⁺ compared <1% for uninfected mice. Three exceptional LAINefdd-infected mice that lost CD4⁺ T cells received 600,000 TCIU. All three exhibited peak viral loads over 3,000,000 copies of LAINefdd RNA/ml. Over an extended time course, substantial systemic CD4⁺ T cell loss was observed for the three mice, but there was no loss of CD4⁺CD8⁺ or CD4⁺CD8^- thymocytes.

Conclusion

We conclude Nef is necessary for elevated viral replication and as a result indirectly contributes to CD4⁺ T cell killing. Further, Nef was not necessary for the activation of peripheral blood CD8⁺ T cells following infection. However, CD4⁺CD8⁺ thymocyte killing was dependent on Nef even in cases of elevated LAINefdd replication and T cell loss. This depletion of thymic T cell precursors may be a significant factor in the elevated pathogenicity of CXCR4 trophic HIV-1.

HIV Nef is secreted in exosomes and triggers apoptosis in bystander CD4+ T cells

The HIV accessory protein negative factor (Nef) is one of the earliest and most abundantly expressed viral proteins. It is also found in the serum of infected individuals (Caby MP, Lankar D, Vincendeau-Scherrer C, Raposo G, Bonnerot C. Exosomal-like vesicles are present in human blood plasma. Int Immunol 2005;17:879-887). Extracellular Nef protein has deleterious effects on CD4(+) T cells (James CO, Huang MB, Khan M, Garcia-Barrio M, Powell MD, Bond VC. Extracellular Nef protein targets CD4(+) T cells for apoptosis by interacting with CXCR4 surface receptors. J Virol 2004;78:3099-3109), the primary targets of HIV, and can suppress immunoglobulin class switching in bystander B cells (Qiao X, He B, Chiu A, Knowles DM, Chadburn A, Cerutti A. Human immunodeficiency virus 1 Nef suppresses CD40-dependent immunoglobulin class switching in bystander B cells. Nat Immunol 2006;7:302-310). Nevertheless, the mode of exit of Nef from infected cells remains a conundrum. We found that Nef stimulates its own export via the release of exosomes from all cells examined. Depending on its intracellular location, these Nef exosomes form at the plasma membrane, late endosomes or both compartments in Jurkat, SupT1 and primary T cells, respectively. Nef release through exosomes is conserved also during HIV-1 infection of peripheral blood lymphocytes (PBLs). Released Nef exosomes cause activation-induced cell death of resting PBLs in vitro. Thus, HIV-infected cells export Nef in bioactive vesicles, which facilitate the depletion of CD4(+) T cells that is a hallmark of acquired immunodeficiency syndrome (AIDS).

Inhibition of HIV-1 infection and replication by enhancing viral incorporation of innate anti-HIV-1 protein A3G: a non-pathogenic Nef mutant-based anti-HIV strategy

APOBEC3G (A3G) is a cellular protein that has been identified as an innate anti-human immunodeficiency virus type 1 (HIV-1) factor. One of the major functions of HIV-1 virion infectivity protein (Vif) protein is to target A3G for ubiquitination/proteasome-mediated degradation and, as a result, evade the host innate defense mechanism. Thus, we wished to devise a strategy to restore the anti-HIV activity of A3G by actively targeting it into HIV-1 virions and countering HIV-1 Vif-targeted degradation. In the current study we performed a series of proof-of-concept experiments for this strategy using as a delivery vehicle of A3G, a derivate of non-pathogenic Nef mutant Nef7 that is capable of being efficiently incorporated into HIV-1 virions. We demonstrate that the Nef7.A3G fusion protein retains several important properties of Nef7; that is, the higher virion incorporation efficiency, no PAK-2 (p21-activated kinase 2) activation, and no CD4 and major histocompatibility complex I down-regulation. Meanwhile, we show that virion incorporated Nef7.A3G possesses the anti-HIV infectivity function of A3G. Moreover, we show that virus-like particle-mediated inverse fusion delivery of Nef7.A3G into HIV-infected CD4+ T lymphocytes leads to potent inhibition of HIV-1 replication in these cells. Taken together, these results indicate that Nef7.A3G can effectively restrict HIV infection and replication by restoring the virion incorporation of A3G, even in the presence of Vif.

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.

Nef does not contribute to replication differences between R5 pre-AIDS and AIDS HIV-1 clones from patient ACH142

AIDS-associated, CCR5-tropic (R5) HIV-1 clones, isolated from a patient that never developed CXCR4-tropic HIV-1, replicate to a greater extent and cause greater cytopathic effects than R5 HIV-1 clones isolated before the onset of AIDS. Previously, we showed that HIV-1 Env substantially contributed to the enhanced replication of an AIDS clone. In order to determine if Nef makes a similar contribution, we cloned and phenotypically analyzed nef genes from a series of patient ACH142 derived R5 HIV-1 clones. The AIDS-associated Nef contains a series of residues found in Nef proteins from progressors [1]. In contrast to other reports [1-3], this AIDS-associated Nef downmodulated MHC-I to a greater extent and CD4 less than pre-AIDS Nef proteins. Additionally, all Nef proteins enhanced infectivity similarly in a single round of replication. Combined with our previous study, these data show that evolution of the HIV-1 env gene, but not the nef gene, within patient ACH142 significantly contributed to the enhanced replication and cytopathic effects of the AIDS-associated R5 HIV-1 clone.

Alterations in intracellular potassium concentration by HIV-1 and SIV Nef

Background

HIV-1 mediated perturbation of the plasma membrane can produce an alteration in the transmembrane gradients of cations and other small molecules leading to cell death. Several HIV-1 proteins have been shown to perturb membrane permeability and ion transport. Xenopus laevis oocytes have few functional endogenous ion channels, and have proven useful as a system to examine direct effects of exogenously added proteins on ion transport.

Results

HIV-1 Nef induces alterations in the intracellular potassium concentration in CD4+ T-lymphoblastoid cells, but not intracellular pH. Two electrode voltage-clamp recording was used to determine that Nef did not form ion channel-like pores in Xenopus oocytes.

Conclusion

These results suggest that HIV-1 Nef regulates intracellular ion concentrations indirectly, and may interact with membrane proteins such as ion channels to modify their electrical properties.

Human immunodeficiency virus bearing a disrupted central DNA flap is pathogenic in vivo

The central DNA flap is an important component of lentiviral vectors, but its significance in the context of wild-type human immunodeficiency virus (HIV) is currently unclear. To address this issue, we have compared the in vitro infection kinetics of NL4-3 with those of a flap-deficient mutant and evaluated the in vivo growth characteristics of these viruses by using the SCID-hu mouse model of HIV infection. Flap-deficient virus was only modestly attenuated in vitro, as assessed by single-round and spreading infection assays, and exhibited levels of replication and pathogenesis close to those of the wild-type in vivo. Hence, an intact central flap is not essential for HIV replication.

A novel small reporter gene and HIV-1 fitness assay

Most currently available HIV-1 reporter gene constructs are large and disrupt the nef reading frame. This report describes a novel reporter gene based on the small murine heat stable antigen (HSA) protein, which is expressed on the surface of infected cells. This HSA reporter can be inserted in the vpr reading frame, leaving nef intact. Nine amino acids from the extracellular domain of HSA are replaced with an influenza hemagglutinin (HA) antibody epitope (HSA-HA). Like the parental reporter protein, this novel reporter is expressed on the surface of infected cells. Antibodies for HSA and HA specifically detect reporter viruses with each construct, indicating disruption of the original HSA antibody epitope. Finally, a strategy is developed to detect each reporter virus by real-time PCR quantitation. The growth of viruses tagged with each reporter allows precise assessment of the relative growth of viruses differing in mutations of interest. Moreover, the availability of these reporters in either of two half-genome plasmids allows convenient production of reporter and non-reporter HIV-1 by co-transfection of appropriately paired plasmids. These paired reporter viruses offer a potentially useful standardized method for measurement of HIV-1 fitness in competition assays.

Human immunodeficiency virus type 1 Vpr-mediated G2 arrest requires Rad17 and Hus1 and induces nuclear BRCA1 and gamma-H2AX focus formation

Eukaryotic cells have evolved a complex mechanism for sensing DNA damage during genome replication. Activation of this pathway prevents entry into mitosis to allow for either DNA repair or, in the event of irreparable damage, commitment to apoptosis. Under conditions of replication stress, the damage signal is initiated by the ataxia-telangiectasia-mutated and Rad3-related kinase ATR. We recently demonstrated that the human immunodeficiency virus type 1 (HIV-1) gene product viral protein R (Vpr) arrests infected cells in the G(2) phase via the activation of ATR. In the present study, we show that the activation of ATR by Vpr is analogous to activation by certain genotoxic agents, both mechanistically and in its downstream consequences. Specifically, we show a requirement for Rad17 and Hus1 to induce G(2) arrest as well as Vpr-induced phosphorylation of histone 2A variant X (H2AX) and formation of nuclear foci containing H2AX and breast cancer susceptibility protein 1. These results demonstrate that G(2) arrest mediated by the HIV-1 gene product Vpr utilizes the cellular signaling pathway whose physiological function is to recognize replication stress. These findings should contribute to a greater understanding of how HIV-1 manipulates the CD4(+)-lymphocyte cell cycle and apoptosis induction in the progressive CD4(+)-lymphocyte depletion characteristic of HIV-1 pathogenesis.

Characterization of a thymus-tropic HIV-1 isolate from a rapid progressor: role of the envelope

Loss of T cell homeostasis usually precedes the onset of AIDS. We hypothesized that rapid progressors may be transmitted with HIV-1 that is particularly able to perturb T cell homeostasis. To this end, we have tested two transmitted, syncytium-inducing (SI) viral isolates from a rapid progressor in two thymus models. One of the isolates (R3A) exhibited markedly rapid kinetics of replication and thymocyte depletion. These phenotypes mapped to the envelope, as a recombinant NL4-3 virus encoding the R3A envelope had similar phenotypes, even in the absence of nef. Notably, the viruses with high pathogenic activity in the thymus (R3A and NL4-R3A) did not show enhanced replication or cytopathicity in PHA-stimulated PBMCs. Furthermore, NL4-R3A did not enhance replication of the coinfected NL4-3 virus in the thymus, suggesting an intrinsic advantage of the R3A envelope. The R3A envelope showed higher entry activity in infecting human T cells and in depleting CD4+ thymocytes when expressed in trans. These data suggest that SI viruses with unique envelope functions which can overcome barriers to transmission may hasten disease progression by perturbing T cell homeostasis.

Nef stimulates human immunodeficiency virus type 1 replication in primary T cells by enhancing virion-associated gp120 levels: coreceptor-dependent requirement for Nef in viral replication

The Nef protein enhances human immunodeficiency virus type 1 (HIV-1) replication through an unknown mechanism. We and others have previously reported that efficient HIV-1 replication in activated primary CD4(+) T cells depends on the ability of Nef to downregulate CD4 from the cell surface. Here we demonstrate that Nef greatly enhances the infectivity of HIV-1 particles produced in primary T cells. Nef-defective HIV-1 particles contained significantly reduced quantities of gp120 on their surface; however, Nef did not affect the levels of virion-associated gp41, indicating that Nef indirectly stabilizes the association of gp120 with gp41. Surprisingly, Nef was not required for efficient replication of viruses that use CCR5 for entry, nor did Nef influence the infectivity or gp120 content of these virions. Nef also inhibited the incorporation of CD4 into HIV-1 particles released from primary T cells. We propose that Nef, by downregulating cell surface CD4, enhances HIV-1 replication by inhibiting CD4-induced dissociation of gp120 from gp41. The preferential requirement for Nef in the replication of X4-tropic HIV-1 suggests that the ability of Nef to downregulate CD4 may be most important at later stages of disease when X4-tropic viruses emerge.

Early steps of retrovirus replicative cycle

During the last two decades, the profusion of HIV research due to the urge to identify new therapeutic targets has led to a wealth of information on the retroviral replication cycle. However, while the late stages of the retrovirus life cycle, consisting of virus replication and egress, have been partly unraveled, the early steps remain largely enigmatic. These early steps consist of a long and perilous journey from the cell surface to the nucleus where the proviral DNA integrates into the host genome. Retroviral particles must bind specifically to their target cells, cross the plasma membrane, reverse-transcribe their RNA genome, while uncoating the cores, find their way to the nuclear membrane and penetrate into the nucleus to finally dock and integrate into the cellular genome. Along this journey, retroviruses hijack the cellular machinery, while at the same time counteracting cellular defenses. Elucidating these mechanisms and identifying which cellular factors are exploited by the retroviruses and which hinder their life cycle, will certainly lead to the discovery of new ways to inhibit viral replication and to improve retroviral vectors for gene transfer. Finally, as proven by many examples in the past, progresses in retrovirology will undoubtedly also provide some priceless insights into cell biology.

Interactions of processed Nef (58-206) with virion proteins of HIV type 1

The Nef protein plays a major role in vivo in promoting HIV and SIV replication and pathogenesis. In vitro, Nef has been shown to down-regulate cell surface molecules, such as CD4 and MHC-I, alter T cell signaling, and enhance virion infectivity. These effects are attributed to interactions of Nef with cellular proteins. In addition, HIV Nef is incorporated into viral particles, mainly localizing in the virion cores. However, no report has been published to date regarding Nef interactions with virion proteins. By immunoprecipitation, Nef was found to bind to viral enzymes. Using yeast two-hybrid and GST pulldown procedures to find out direct potential partners of Nef, Nef was consistently found to interact with viral integrase (IN). The interaction between Nef and IN was stronger when Nef was present as the viral protease-cleaved isoform. We hypothesize that the interaction of Nef with viral integrase or other virion proteins may explain the presence of Nef in viral cores. In addition, this interaction suggests that Nef may accompany the reverse transcription and the preintegration complexes during the early steps of the infection cycle and potentially affect infectivity during these steps.

Chimeric human immunodeficiency virus type 1 (HIV-1) virions containing HIV-2 or simian immunodeficiency virus Nef are resistant to cyclosporine treatment

The viral protein Nef and the cellular factor cyclophilin A are both required for full infectivity of human immunodeficiency virus type 1 (HIV-1) virions. In contrast, HIV-2 and simian immunodeficiency virus (SIV) do not incorporate cyclophilin A into virions or need it for full infectivity. Since Nef and cyclophilin A appear to act in similar ways on postentry events, we determined whether chimeric HIV-1 virions that contained either HIV-2 or SIV Nef would have a direct effect on cyclophilin A dependence. Our results show that chimeric HIV-1 virions containing either HIV-2 or SIV Nef are resistant to treatment by cyclosporine and enhance the infectivity of virions with mutations in the cyclophilin A binding loop of Gag. Amino acids at the C terminus of HIV-2 and SIV are necessary for inducing cyclosporine resistance. However, transferring these amino acids to the C terminus of HIV-1 Nef is insufficient to induce cyclosporine resistance in HIV-1. These results suggest that HIV-2 and SIV Nef are able to compensate for the need for cyclophilin A for full infectivity and that amino acids present at the C termini of these proteins are important for this function.

Compensatory link between fusion and endocytosis of human immunodeficiency virus type 1 in human CD4 T lymphocytes

Virions of the type 1 human immunodeficiency virus (HIV-1) can enter target cells by fusion or endocytosis, with sharply different functional consequences. Fusion promotes productive infection of the target cell, while endocytosis generally leads to virion inactivation in acidified endosomes or degradation in lysosomes. Virion fusion and endocytosis occur equally in T cells, but these pathways have been regarded as independent because endocytosis of HIV virions requires neither CD4 nor CCR5/CXCR4 engagement in HeLa-CD4 cells. Using flow cytometric techniques to assess the binding and entry of green fluorescent protein (GFP)-Vpr-labeled HIV virions into primary peripheral blood mononuclear cells, we have found that HIV fusion and endocytosis are restricted to the CD4-expressing subset of cells and that both pathways commonly require the initial binding of HIV virions to surface CD4 receptors. Blockade of CXCR4-tropic HIV virion fusion with AMD3100, a CXCR4-specific entry inhibitor, increased virion entry via the endocytic pathway. Similarly, inhibition of endosome acidification with bafilomycin A1, concanamycin A, or NH(4)Cl enhanced entry via the fusion pathway. Although fusion remained dependent on CD4 and chemokine receptor binding, the endosome inhibitors did not alter surface expression of CD4 and CXCR4. These results suggest that fusion in the presence of the endosome inhibitors likely occurs within nonacidified endosomes. However, the ability of these inhibitors to impair vesicle trafficking from early to late endosomes in some cells could also increase the recycling of these virion-containing endosomes to the cell surface, where fusion occurs. In summary, our results reveal an unexpected, CD4-mediated reciprocal relationship between the pathways governing HIV virion fusion and endocytosis.

Antibody-Dependent Cellular Cytotoxicity via Humoral Immune Epitope of Nef Protein Expressed on Cell Surface

Antibodies against various proteins of HIV type 1 (HIV-1) can be detected in HIV-1-infected individuals. We previously reported that the level of Ab response against one Nef epitope is correlated with HIV-1 disease progression. To elucidate the mechanism for this correlation, we examined Ab-dependent cellular cytotoxicity (ADCC) against target cells expressing Nef. We observed efficient cytotoxicity against Nef-expressing target cells in the presence of patient plasma and PBMCs. This ADCC activity was correlated with the dilution of plasma from HIV-1-infected patients. Addition of a specific synthetic peptide (peptide 31:FLKEKGGLE) corresponding to the Nef epitope reduced cell lysis to approximately 50%. These results suggest that PBMCs of HIV-1-infected patients may exert ADCC via anti-Nef Abs in the patients' own plasma and serve as a mechanism used by the immune system to regulate HIV-1 replication.

HIV-1 Nef protein in the nucleus influences adipogenesis as well as viral transcription through the peroxisome proliferator-activated receptors

BACKGROUND

Although the HIV-1 Nef protein (27 kDa) localizes primarily in cytoplasm, there is considerable evidence suggesting its occasional localization in the nucleus. Nef is known to play an important role in transcriptional events and viral replication, but the actual target of Nef in the nucleus remains to be identified.

OBJECTIVE

To examine the functional roles of Nef in the nucleus and its possible interactions with other unknown factors in the nucleus.

METHODS

High-density microarray analysis was used to screen directly the unique functions of Nef on host gene transcription. The nuclear localization of Nef and its effects on the expression of peroxisome proliferator-activated receptors (PPAR) was examined using PPAR promoter/reporter assay and immunoblotting. A long terminal repeat/reporter assay was used to investigated the effects of Nef and PPAR on viral transcription.

RESULTS

Nef in the nucleus suppressed PPAR gamma expression and reduced fatty acid levels in human T and macrophage cell lines. Expression of Nef or PPAR suppressed viral replication; the effect of PPAR gamma or retinoid X receptor-alpha on viral replication were reduced by coexpression of Nef in MT(-)4 T cells.

CONCLUSION

Nef may be involved in both viral replication and the wasting syndrome associated with AIDS.

Signaling but not trafficking function of HIV-1 protein Nef is essential for Nef-induced defects in human intrathymic T-cell development

The HIV-1 gene nef is important for progression toward AIDS and cellular depletion of the infected thymus. Expression of the Nef protein alone impairs human thymopoiesis. Here, we performed a structure-function analysis of the Nef protein by comparing the effect on T-cell development of different nef alleles, either wild type or defective for selected functions, expressed by human thymocytes. We show that Nef-mediated impaired thymopoiesis is not due to altered surface marker trafficking, nor dependent on oligomerization of Nef. By contrast, mutations in the myristoylation site and in signaling sites of Nef, ie, sites important for interaction with phosphofurin acidic cluster sorting protein-1 (PACS-1), Src homology domain 3 (SH3) domains, and p21-activated kinase 2 (PAK2), were found to be critical for its effect on T-cell development. These results point to sites in Nef to target therapeutically for restoration of thymopoiesis in HIV infection.

HIV-1 replication and pathogenesis in the human thymus

How HIV replicates and causes destruction of the thymus, and how to restore thymic function, are among the most important questions of HIV-1 pathogenesis and therapy in adult as well as pediatric patients. The thymus appears to function, albeit at reduced levels, throughout the life of adults, to respond to T cell depletion induced by HIV and to be suppressed by HIV. In this review, we summarize recent findings concerning HIV replication and pathogenesis in the human thymus, focusing on mechanistic insights gleaned from studies in the SCID-hu Thy/Liv mouse and human fetal-thymus organ culture (HF-TOC) models. First, we discuss HIV viral determinants and host factors involved in the replication of HIV in the thymus. Second, we consider evidence that both viral factors and host factors contribute to HIV-induced thymocyte depletion. We thus propose that multiple mechanisms, including depletion and suppression of progenitor cells, paracrine and direct lytic depletion of thymocytes, and altered thymocyte selection are involved in HIV-induced pathology in the thymus. With the SCID-hu Thy/Liv mouse and HF-TOC models, it will be important in the coming years to further clarify the virological, cell biological, and immunological mechanisms of HIV replication and pathogenesis in human thymus, and to correlate their significance in HIV disease progression.

Activation of the signal transducer and activator of transcription 1 signaling pathway in thymocytes from HIV-1-infected human thymus

OBJECTIVE

To identify HIV-induced host factors in the severe combined immunodeficient (SCID)-hu Thy/Liv mouse that may contribute to HIV pathogenesis in the thymus.

DESIGN

To identify genes specifically altered by HIV-1 infection using the cDNA microarray assay, SCID-hu Thy/Liv organs derived from the same donors were used. Therefore, no genetic variations existed between HIV and mock-infected samples. In addition, the 12-14 day post-infection timepoint was chosen because no significant thymocyte depletion was detected in HIV-infected Thy/Liv organs, so mRNA from the same cell types could be compared.

METHODS

Using SCID-hu Thy/Liv mice constructed from the same donor tissues, we analysed the expression of 9183 host genes in response to HIV infection with cDNA microarrays. Expression of selected genes with more than threefold induction was confirmed by measuring RNA (reverse transcriptase-polymerase chain reaction; RT-PCR) and proteins.

RESULTS

HIV-1 (JD or NL4-3) infection of the SCID-hu Thy/Liv mouse led to more than threefold induction of 19 genes, 12 of which were IFN-inducible and six were unknown EST clones. We confirmed induction by RT-PCR and protein blots. Both signal transducer and activator of transcription (STAT)1 and STAT2 proteins were induced, and STAT1 was also activated by phosphorylation at the Tyr701 and Ser727 sites in human thymus infected with HIV-JD or NL4-3. Treatment of human fetal thymus organ culture or human thymocytes with recombinant HIV-1 gp120 proteins also led to induction or activation of STAT1.

CONCLUSION

HIV-1 infection of the thymus led to activation of the STAT1 signaling pathway in thymocytes, which may contribute to HIV-1 pathogenesis in the thymus.

Expression of simian immunodeficiency virus (SIV) nef in astrocytes during acute and terminal infection and requirement of nef for optimal replication of neurovirulent SIV in vitro

As the most numerous cells in the brain, astrocytes play a critical role in maintaining central nervous system homeostasis, and therefore, infection of astrocytes by human immunodeficiency virus (HIV) or simian immunodeficiency virus (SIV) in vivo could have important consequences for the development of HIV encephalitis. In this study, we establish that astrocytes are infected in macaques during acute SIV infection (10 days postinoculation) and during terminal infection when there is evidence of SIV-induced encephalitis. Additionally, with primary adult rhesus macaque astrocytes in vitro, we demonstrate that the macrophage-tropic, neurovirulent viruses SIV/17E-Br and SIV/17E-Fr replicate efficiently in astrocytes, while the lymphocyte-tropic, nonneurovirulent virus SIV(mac)239 open-nef does not establish productive infection. Furthermore, aminoxypentane-RANTES abolishes virus replication, suggesting that these SIV strains utilize the chemokine receptor CCR5 for entry into astrocytes. Importantly, we show that SIV Nef is required for optimal replication in primary rhesus macaque astrocytes and that normalizing input virus by particle number rather than by infectivity reveals a disparity between the ability of a Nef-deficient virus and a virus encoding a nonmyristoylated form of Nef to replicate in these central nervous system cells. Since the myristoylated form of Nef has been implicated in functions such as CD4 and major histocompatibility complex I downregulation, kinase association, and enhancement of virion infectivity, these data suggest that an as yet unidentified function of Nef may exist to facilitate SIV replication in astrocytes that may have important implications for in vivo pathogenesis.

Double-stranded nef RNA interferes with human immunodeficiency virus type 1 replication

RNA interference (RNAi) has been reported to be post-transcriptional gene silencing (PTGS) by approximately 500 nucleotide-(nt)-long double-stranded (ds) RNA that specifically targets homologous sequences of messenger RNA. In this report, we describe inhibition of HIV-1 transcription by synthetic dsRNAs constructed with mutated nef genes (nef dsRNAs) derived from long-term non-progressors (LTNPs) using cotransfection of the target gene-expressing plasmid and dsRNA. The effects of nef dsRNAs were examined with luciferase (Luc) reporter which is combined with the HIV-1 (SF2) LTR in persistently HIV-1-infected T cell and macrophage cell lines. At 48 hr, a defective nef dsRNA (556 nt) suppressed Luc activity more potently than did SF2 full-length nef dsRNA (744 nt), suggesting that approximately 500 nt-long nef dsRNA could interfere with the HIV-1 transcription.

Human immunodeficiency virus type 1 Nef-mediated downregulation of CD4 correlates with Nef enhancement of viral pathogenesis

The nef gene products encoded by human immunodeficiency virus type 1 (HIV-1) and simian immunodeficiency virus type 1 (SIV-1) increase viral loads in infected hosts and accelerate clinical progression to AIDS. Nef exhibits a spectrum of biological activities, including the ability to downregulate surface expression of CD4 and major histocompatibility complex (MHC) class I antigens, to alter the state of T-cell activation, and to enhance the infectivity of viral particles. To determine which of these in vitro functions most closely correlates with the pathogenic effects of Nef in vivo, we constructed recombinant HIV-1 NL4-3 viruses carrying mutations within the nef gene that selectively impair these functions. These mutant viruses were evaluated for pathogenic potential in severe combined immunodeficiency (SCID) mice implanted with human fetal thymus and liver (SCID-hu Thy/Liv mice), in which virus-mediated depletion of thymocytes is known to be Nef dependent. Disruption of the polyproline type II helix (Pxx)4 within Nef (required for binding of Hck and p21-activated kinase-like kinases, downregulation of MHC class I, and enhancement of HIV-1 infectivity in vitro but dispensable for CD4 downregulation) did not impair thymocyte depletion in virus-infected Thy/Liv human thymus implants. Conversely, three separate point mutations in Nef that compromised its ability to downregulate CD4 attenuated thymocyte depletion while not diminishing viral replication. These findings indicate that the functional ability of Nef to downregulate CD4 and not MHC class I downregulation, Hck or PAK binding, or (Pxx)4-associated enhancement of infectivity most closely correlates with Nef-mediated enhancement of HIV-1 pathogenicity in vivo. Nef-mediated CD4 downregulation merits consideration as a new target for the development of small-molecule inhibitors.

TCR gamma delta+ and CD161+ thymocytes express HIV-1 in the SCID-hu mouse, potentially contributing to immune dysfunction in HIV infection

The vast diversity of the T cell repertoire renders the adaptive immune response capable of recognizing a broad spectrum of potential antigenic peptides. However, certain T cell rearrangements are conserved for recognition of specific pathogens, as is the case for TCRgammadelta cells. In addition, an immunoregulatory class of T cells expressing the NK receptor protein 1A (CD161) responds to nonpeptide Ags presented on the MHC-like CD1d molecule. The effect of HIV-1 infection on these specialized T cells in the thymus was studied using the SCID-hu mouse model. We were able to identify CD161-expressing CD3(+) cells but not the CD1d-restricted invariant Valpha24/Vbeta11/CD161(+) NK T cells in the thymus. A subset of TCRgammadelta cells and CD161-expressing thymocytes express CD4, CXCR4, and CCR5 during development in the thymus and are susceptible to HIV-1 infection. TCRgammadelta thymocytes were productively infectable by both X4 and R5 virus, and thymic HIV-1 infection induced depletion of CD4(+) TCRgammadelta cells. Similarly, CD4(+)CD161(+) thymocytes were depleted by thymic HIV-1 infection, leading to enrichment of CD4(-)CD161(+) thymocytes. Furthermore, compared with the general CD4-negative thymocyte population, CD4(-)CD161(+) NK T thymocytes exhibited as much as a 27-fold lower frequency of virus-expressing cells. We conclude that HIV-1 infection and/or disruption of cells important in both innate and acquired immunity may contribute to the overall immune dysfunction seen in HIV-1 disease.

HIV-1 replication cycle

The HIV-1 is a formidable pathogen with establishment of a persistent infection based on the ability to integrate the proviral genome into chronically infected cells, and by the rapid evolution made possible by a high mutation rate and frequent recombination during the viral replication. HIV-1 has a variety of novel genes that facilitate viral persistence and regulation of HIV replication, but this virus also usurps cellular machinery for HIV replication, particularly during gene expression and virion assembly and budding. Recent success with antiretroviral therapy may be limited by the emergence HIV drug resistance and by toxicities and other requirements for successful long-term therapy. Further investigation of HIV-1 replication may allow identification of novel targets of antiretroviral therapy that may allow continued virus suppression in patients of failing current regiments, particularly drugs that target HIV-1 entry and HIV-1 integration.

The expression of HIV-1 tat and nef genes induces cell-specific changes in growth properties and morphology of different types of rat cells

Among the viral regulatory genes the tat and nef genes of HIV-1 encode the proteins playing a central role in viral replication and exerting pleiotropic effects on the survival and growth of the cells. These effects differ in various cell types, possibly due to the use of genes from different HIV-1 isolates. In this work, we studied the effects of the tat and nef genes on three types of cultured rat cells: primary embryo fibroblasts, pseudonormal Rat-2, and pheochromocytoma PC12. Both genes affected growth properties and morphology of cells, the effects being cell-specific. The proliferative activity of both Rat-2 and PC12 cells was considerably increased after transfection with the tat gene. In primary rat embryo fibroblasts the tat gene induced multilayered foci. More importantly, it was shown that the efficiency of transformation was higher in cells coexpressing tat and nef. The nef gene caused considerable suppression of Rat-2 cell proliferation, but no changes in their morphology. The nef gene transfection of PC12 cells also led to suppression of their proliferative activity. In addition, cellular agglomerates which were morphologically similar to multinuclear syncytial cells were detected in these cells for the first time.

Impact of cytokines on replication in the thymus of primary human immunodeficiency virus type 1 isolates from infants

Early infection of the thymus with the human immunodeficiency virus (HIV) may explain the more rapid disease progression among children infected in utero than in children infected intrapartum. Therefore, we analyzed infection of thymocytes in vitro by HIV type 1 primary isolates, obtained at or near birth, from 10 children with different disease outcomes. HIV isolates able to replicate in the thymus and impact thymopoiesis were present in all infants, regardless of the timing of viral transmission and the rate of disease progression. Isolates from newborns utilized CCR5, CXCR4, or both chemokine receptors to enter thymocytes. Viral expression was observed in discrete thymocyte subsets postinfection with HIV isolates using CXCR4 (X4) and isolates using CCR5 (R5), despite the wider distribution of CXCR4 in the thymus. In contrast to previous findings, the X4 primary isolates were not more cytopathic for thymocytes than were the R5 isolates. The cytokines interleukin-2 (IL-2), IL-4, and IL-7 increased HIV replication in the thymus by inducing differentiation and expansion of mature CD27(+) thymocytes expressing CXCR4 or CCR5. IL-2 and IL-4 together increased expression of CXCR4 and CCR5 in this population, whereas IL-4 and IL-7 increased CXCR4 but not CCR5 expression. IL-2 plus IL-4 increased the viral production of all pediatric isolates, but IL-4 and IL-7 had a significantly higher impact on the replication of X4 isolates compared to R5 isolates. Our studies suggest that coreceptor use by HIV primary isolates is important but is not the sole determinant of HIV pathogenesis in the thymus.

Nef enhances human immunodeficiency virus type 1 infectivity in the absence of matrix

Nef enhances the serine phosphorylation of the human immunodeficiency virus type 1 matrix (MA) protein, which suggests that MA may be a functional target of Nef. Using mutants that remain infectious despite the absence of most or all of MA, we show in the present study that the ability of Nef to enhance virus infectivity is not compromised even if MA is entirely replaced by a heterologous lipid anchor.

Cytopathic killing of peripheral blood CD4(+) T lymphocytes by human immunodeficiency virus type 1 appears necrotic rather than apoptotic and does not require env

An important unresolved issue of AIDS pathogenesis is the mechanism of human immunodeficiency virus (HIV)-induced CD4(+) T-lymphocyte destruction. We show here that HIV type 1 (HIV-1) exerts a profound cytopathic effect upon peripheral blood CD4(+) T lymphocytes that resembles necrosis rather than apoptosis. Necrotic cytopathology was found with both laboratory-adapted strains and primary isolates of HIV-1. We carefully investigated the role of env, which has been previously implicated in HIV cytopathicity. HIV-1 stocks with equivalent infectivity were prepared from constructs with either an intact or mutated env coding region and pseudotyped with the glycoprotein of vesicular stomatitis virus (VSV-G) so that the HIV envelope was not rate-limiting for infection. Infected Jurkat T cells died whether or not env was intact; however, the expression of env accelerated death significantly. The accelerated death was blocked by protease inhibitors, indicating that it was due to reinfection by newly produced virus in env(+) cultures. Accordingly, we found no disparity in kinetics in CD4(lo) Jurkat cells. In highly infected peripheral blood T cells, profound necrosis occurred equivalently with both env(+) and env(-) stocks of HIV-1. We also found that HIV-1 cytopathicity was undiminished by the absence of nef. However, viral stocks made by complementation or packaging of HIV-1 genomes with the natural protein-coding sequences replaced by the green fluorescent protein were highly infectious but not cytopathic. Thus, env can accelerate cell death chiefly as an entry function, but one or more viral functions other than env or nef is essential for necrosis of CD4(+) T cells induced by HIV-1.

Interaction between Nef and phosphatidylinositol-3-kinase leads to activation of p21-activated kinase and increased production of HIV

The negative factor (Nef) is one of six accessory proteins from primate lentiviruses (HIV-1, HIV-2, and SIV). It leads to high levels of viremia and the progression to AIDS in monkeys and humans. In this study, we demonstrated that Nef from HIV-1 binds to the regulatory subunit (p85) of phosphatidylinositol-3-kinase (PI3K). This interaction depended on the C-terminus of p85 and Nef. Moreover, PI3K was required to activate the Nef-associated p21-activated kinase (PAK). Finally, inhibition of PI3K blocked the activation of PAK and decreased the production of viral particles to levels observed with the Nef-deleted provirus. We conclude that Nef assembles a multiprotein signaling complex which is required for the optimal replication of HIV-1.

Nef: agent of cell subversion

Primate lentiviruses encode a small protein designated Nef that has been shown to be a major determinant of virus pathogenicity. Nef regulates multiple host factors in order to optimize the cellular environment for virus replication. The mechanisms by which this small protein modulates distinct host cell properties provide intriguing insight into the intricate interaction between virus and host.

Restoration of wild-type infectivity to human immunodeficiency virus type 1 strains lacking nef by intravirion reverse transcription

Human immunodeficiency virus type 1 (HIV-1) Nef protein exerts several effects, both on infected cells and as a virion protein, which work together to enhance viral replication. One of these activities is the ability to enhance infectivity and the formation of proviral DNA. The mechanism of this enhancement remains incompletely understood. We show that virions with nef deleted can be restored to wild-type infectivity by stimulating intravirion reverse transcription. Particle composition and measures of reverse transcriptase activity remain the same for Nef(+) and Nef(-) virions both before and after natural endogenous reverse transcription (NERT) treatment. The effect of NERT treatment on virions pseudotyped with murine leukemia virus envelope protein was similar to that on particles pseudotyped with HIV-1 envelope protein. However, virions pseudotyped with vesicular stomatitis virus G envelope protein showed no influence of Nef on NERT enhancement of infectivity. These observations suggest that Nef may function at a level prior to reverse transcription. Since NERT treatment results in partial disassembly of the viral core, we speculate that Nef may function at the level of core particle disassembly.

The pathogenicity of human immunodeficiency virus (HIV) type 1 Nef in CD4C/HIV transgenic mice is abolished by mutation of its SH3-binding domain, and disease development is delayed in the absence of Hck

The human immunodeficiency virus type 1 (HIV-1) Nef protein is an important determinant of AIDS pathogenesis. We have previously reported that HIV-1 Nef is responsible for the induction of a severe AIDS-like disease in CD4C/HIV transgenic (Tg) mice. To understand the molecular mechanisms of this Nef-induced disease, we generated Tg mice expressing a mutated Nef protein in which the SH3 ligand-binding domain (P(72)XXP(75)XXP(78)) was mutated to A(72)XXA(75)XXQ(78). This mutation completely abolished the pathogenic potential of Nef, although a partial downregulation of the CD4 cell surface expression was still observed in these Tg mice. We also studied whether Hck, one of the effectors previously found to bind to this PXXP motif of Nef, was involved in disease development. Breeding of Tg mice expressing wild-type Nef on an hck(-/-) (knockout) background did not abolish any of the pathological phenotypes. However, the latency of disease development was prolonged. These data indicate that an intact PXXP domain is essential for inducing an AIDS-like disease in CD4C/HIV Tg mice and suggest that interaction of a cellular effector(s) with this domain is required for the induction of this multiorgan disease. Our findings indicate that Hck is an important, but not an essential, effector of Nef and suggest that another factor(s), yet to be identified, may be more critical for disease development.

Human immunodeficiency virus type 1 IIIB selected for replication in vivo exhibits increased envelope glycoproteins in virions without alteration in coreceptor usage: separation of in vivo replication from macrophage tropism

Analysis of viral replication and pathogenicity after in vivo selection of human immunodeficiency virus type 1 (HIV-1) attenuated in vitro will help to define the functions involved in replication and pathogenesis in vivo. Using the SCID-hu Thy/Liv mouse and human fetal thymus organ culture as in vivo models, we previously defined HIV-1 env determinants (HXB2/LW) which were reverted for replication in vivo (L. Su et al., Virology 227:46-52, 1997). In this study, we examined the replication of four highly related HIV-1 clones directly derived from Lai/IIIB or after selection in vivo to investigate the envelope gp120 determinants associated with replication in macrophages and in the thymus models in vivo. The LW/C clone derived from the IIIB-infected laboratory worker and HXB2/LW both efficiently infected monocyte-derived macrophages (MDM) and the human thymus. Although the laboratory worker (LW) isolates showed altered tropism from IIIB, they still predominantly used CXCR4 as coreceptors for infecting peripheral blood mononuclear cells, macrophages, and the thymus. Interestingly, a single amino acid mutation in the V3 loop associated with resistance to neutralizing antibodies was also essential for the replication activity of the LW virus in the thymus models but not for its activity in infecting MDM. The LW virions were equally sensitive to a CXCR4 antagonist. We further demonstrated that the LW HIV-1 isolate selected in vivo produced more infectious viral particles that contained higher levels of the Env protein gp120. Thus, selection of the laboratory-attenuated Lai/IIIB isolate in vivo leads to altered tropism but not coreceptor usage of the virus. The acquired replication activity in vivo is correlated with an early A-to-T mutation in the V3 loop and increased virion association of HIV-1 Env gp120, but it is genetically separable from the acquired replication activity in macrophages.

Differential effects of primary human immunodeficiency virus type 1 nef sequences on downregulation of CD4 and MHC class I

We analyzed two primary nef sequences, KS2 (subtype B) and K306 (subtype D), each directly isolated from patients. Cell lines constitutively expressing respective Nef proteins were constructed using a retroviral vector. There were significant differences in the ability to downregulate surface CD4 and MHC class I proteins between different nef sequences. When the nef sequence from NL432 was used as a reference, KS2 Nef demonstrated the highest ability to downregulate MHC class I, whereas it appeared to lack the ability to downregulate CD4. On the contrary, Nef from K306 decreased the level of surface CD4 to a greater extent, but was less effective on downregulation of MHC class I. These results showed that the levels of downregulation of CD4 and MHC class I could significantly vary among HIV strains and that two well-known functions of Nef, downregulation of CD4 and MHC class I, would be separated.

nef-deleted HIV-1 inhibits phagocytosis by monocyte-derived macrophages in vitro but not by peripheral blood monocytes in vivo

OBJECTIVE

HIV-1 infection impairs a number of macrophage effector functions, but the mechanism is unknown. We studied the role of HIV-1 Nef in modulating phagocytosis by human monocytes and monocyte-derived macrophages (MDM).

DESIGN AND METHODS

Using a flow cytometric assay, phagocytosis of Mycobacterium avium complex (MAC) by monocytes in whole blood of Sydney Blood Bank Cohort (SBBC) members infected with a nef-deleted (Delta nef) strain of HIV-1 was compared with that of monocytes from uninfected or wild-type (WT) HIV-infected subjects. The specific impact of Nef on phagocytosis by MDM was determined by either infecting cells in vitro with Delta nef strains of HIV-1 or electroporating Nef into uninfected MDM.

RESULTS

MAC phagocytic capacity of monocytes from SBBC members was equivalent to that of cells from uninfected individuals (P = 0.81); it was greater than that of cells from individuals infected with WT HIV-1 (P < 0.0001), irrespective of CD4 counts and HIV viral load. In contrast, in vitro infection of MDM with either Delta nef or WT strains of HIV-1 resulted in similar levels of HIV replication and equivalent impairment of phagocytosis via Fc gamma and complement receptors. Electroporation of Nef into MDM did not alter phagocytic capacity.

CONCLUSIONS

This study provides evidence demonstrating the complex indirect effect of Nef on phagocytosis by peripheral blood monocytes (infrequently infected with HIV-1) in vivo. Conversely, the fact that MDM infected with either Delta nef or WT HIV-1 in vitro (high multiplicity of infection) show comparably impaired phagocytosis, indicates that HIV-1 infection of macrophages can directly impair function, independent of Nef.

Low or no antibody responses to human immunodeficiency virus type 1 Nef in infected carriers with subtype E, in contrast to subtype B that showed antibodies preferentially recognizing subtype-specific Nef epitopes

The viral accessory gene product Nef has been shown to play an important role in human immunodeficiency virus type 1 (HIV-1)-induced pathogenesis. Only little information is available regarding the differences in the host immune responses against Nef protein and its function in vivo among different subtypes of HIV-1. In the present study, we showed marked differences in the immune responses to Nef protein between subtypes B and E. The amino acid sequence in subtype E Nef showed 72% homology with that in subtype B. Most murine monoclonal antibodies obtained by immunization with subtype B or E Nef protein showed cross-reactivity with both Nef proteins (80 and 67%, respectively). Next, we focused on the immune responses among infected Japanese and Thai individuals. Subtyping of the individuals into B and E was carried out by enzyme-linked immunosorbent assay (ELISA) using synthetic peptides corresponding to the V3 loop representing the principal neutralizing domain. Most of the sera from these individuals reacted strongly with Gag p24 proteins derived from subtypes B and E at similar levels. However, the immune responses among these individuals to Nef protein were markedly different. Some subtype B-infected Japanese and Thai individuals (40 and 35%, respectively) showed higher levels of anti-Nef antibodies, although these antibodies preferentially recognized epitopes specific to subtype B. On the other hand, most of the subtype E-infected Japanese and Thai individuals showed low or no antibody responses to Nef proteins. Thus, immune responses to Nef were markedly different between subtypes B- and E-infected carriers, suggesting different function(s) for Nef in AIDS pathogenesis. Further, vaccine design must take into account the different subtypes of HIV-1.

Separation of human immunodeficiency virus type 1 replication from nef-mediated pathogenesis in the human thymus

Human immunodeficiency virus type 1 (HIV-1) is frequently attenuated after long-term culture in vitro. The attenuation process probably involves mutations of functions required for replication and pathogenicity in vivo. Analysis of attenuated HIV-1 for replication and pathogenicity in vivo will help to define these functions. In this study, we examined the pathogenicity of an attenuated HIV-1 isolate in a laboratory worker accidentally exposed to a laboratory-adapted HIV-1 isolate. Using heterochimeric SCID-hu Thy/Liv mice as an in vivo model, we previously defined HIV-1 env determinants (HXB/LW) that reverted to replicate in vivo (L. Su, H. Kaneshima, M. L. Bonyhadi, R. Lee, J. Auten, A. Wolf, B. Du, L. Rabin, B. H. Hahn, E. Terwilliger, and J. M. McCune, Virology 227:46-52, 1997). Here we further demonstrate that HIV-1 replication in vivo can be separated from its pathogenic activity, in that the HXB/LW virus replicated to high levels in SCID-hu Thy/Liv mice, with no significant thymocyte depletion. Restoration of the nef gene in the recombinant HXB/LW genome restored its pathogenic activity, with no significant effect on HIV-1 replication in the thymus. Our results suggest that in vitro-attenuated HIV-1 lacks determinants for pathogenicity as well as for replication in vivo. Our data indicate that (i) the replication defect can be recovered in vivo by mutations in the env gene, without an associated pathogenic phenotype, and (ii) nef can function in the HXB/LW clone as a pathogenic factor that does not enhance HIV-1 replication in the thymus. Furthermore, the HXB/LW virus may be used to study mechanisms of HIV-1 nef-mediated pathogenesis in vivo.