Identification and characterization of murine DNAM-1 (CD226) and its poliovirus receptor family ligands

The leukocyte adhesion molecule DNAM-1 (CD226) is a member of the immunoglobulin superfamily and constitutively expressed on the majority of CD4+ and CD8+ T lymphocytes, natural killer (NK) cells, monocytes/macrophages, and a subset of B lymphocytes. The poliovirus receptor (PVR; CD155) and its family member nectin 2 (CD112) have recently been identified as the ligands for DNAM-1. Interaction of DNAM-1 with the ligands induces NK cell- and CD8+ T cell-mediated cytotoxicity and cytokine secretion. However, in vivo function of the receptor-ligand interaction has remained unclear. Here, we identified murine DNAM-1 and PVR homologues that physically and functionally bind each other. We demonstrated that ligand binding of murine DNAM-1 induced a costimulatory signal in antigen-specific CD8+ T cells. These results should provide a useful animal model to explore a role of DNAM-1 in immune responses in vivo.     

Degranulation of natural killer cells following interaction with HIV-1-infected cells is hindered by downmodulation of NTB-A by Vpu

Natural killer (NK) cell degranulation in response to virus-infected cells is triggered by interactions between invariant NK cell surface receptors and their ligands on target cells. Although HIV-1 Vpr induces expression of ligands for NK cell activation receptor, NKG2D, on infected cells, this is not sufficient to promote lytic granule release. We show that triggering the NK cell coactivation receptor NK-T- and -B cell antigen (NTB-A) alongside NKG2D promotes NK cell degranulation. Normally, NK cell surface NTB-A binds to NTB-A on CD4+ T cells. However, HIV-1 Vpu downmodulates NTB-A on infected T cells. Vpu associates with NTB-A through its transmembrane region without promoting NTB-A degradation. Cells infected with HIV-1 Vpu mutant elicited at least 50% more NK cells to degranulate than wild-type virus. Moreover, NK cells have a higher capacity to lyse HIV-infected cells with a mutant Vpu. Thus, Vpu downmodulation of NTB-A protects the infected cell from lysis by NK cells.     

Lysis of endogenously infected CD4+ T cell blasts by rIL-2 activated autologous natural killer cells from HIV-infected viremic individuals

Understanding the cellular mechanisms that ensure an appropriate innate immune response against viral pathogens is an important challenge of biomedical research. In vitro studies have shown that natural killer (NK) cells purified from healthy donors can kill heterologous cell lines or autologous CD4+ T cell blasts exogenously infected with several strains of HIV-1. However, it is not known whether the deleterious effects of high HIV-1 viremia interferes with the NK cell-mediated cytolysis of autologous, endogenously HIV-1-infected CD4+ T cells. Here, we stimulate primary CD4+ T cells, purified ex vivo from HIV-1-infected viremic patients, with PHA and rIL2 (with or without rIL-7). This experimental procedure allows for the significant expansion and isolation of endogenously infected CD4+ T cell blasts detected by intracellular staining of p24 HIV-1 core antigen. We show that, subsequent to the selective down-modulation of MHC class-I (MHC-I) molecules, HIV-1-infected p24(pos) blasts become partially susceptible to lysis by rIL-2-activated NK cells, while uninfected p24(neg) blasts are spared from killing. This NK cell-mediated killing occurs mainly through the NKG2D activation pathway. However, the degree of NK cell cytolytic activity against autologous, endogenously HIV-1-infected CD4+ T cell blasts that down-modulate HLA-A and -B alleles and against heterologous MHC-I(neg) cell lines is particularly low. This phenomenon is associated with the defective surface expression and engagement of natural cytotoxicity receptors (NCRs) and with the high frequency of the anergic CD56(neg)/CD16(pos) subsets of highly dysfunctional NK cells from HIV-1-infected viremic patients. Collectively, our data demonstrate that the chronic viral replication of HIV-1 in infected individuals results in several phenotypic and functional aberrancies that interfere with the NK cell-mediated killing of autologous p24(pos) blasts derived from primary T cells.     

HIV-1 Nef enhances dendritic cell-mediated viral transmission to CD4+ T cells and promotes T-cell activation

HIV-1 Nef enhances dendritic cell (DC)-mediated viral transmission to CD4(+) T cells, but the underlying mechanism is not fully understood. It is also unknown whether HIV-1 infected DCs play a role in activating CD4(+) T cells and enhancing DC-mediated viral transmission. Here we investigated the role of HIV-1 Nef in DC-mediated viral transmission and HIV-1 infection of primary CD4(+) T cells using wild-type HIV-1 and Nef-mutated viruses. We show that HIV-1 Nef facilitated DC-mediated viral transmission to activated CD4(+) T cells. HIV-1 expressing wild-type Nef enhanced the activation and proliferation of primary resting CD4(+) T cells. However, when co-cultured with HIV-1-infected autologous DCs, there was no significant trend for infection- or Nef-dependent proliferation of resting CD4(+) T cells. Our results suggest an important role of Nef in DC-mediated transmission of HIV-1 to activated CD4(+) T cells and in the activation and proliferation of resting CD4(+) T cells, which likely contribute to viral pathogenesis.     

Contribution of Vpu, Env, and Nef to CD4 down-modulation and resistance of human immunodeficiency virus type 1-infected T cells to superinfection

Human immunodeficiency virus type 1 (HIV-1) utilizes Vpu, Env, and Nef to down-modulate its primary CD4 receptor from the cell surface, and this function seems to be critical for the pathogenesis of AIDS. The physiological relevance of CD4 down-modulation, however, is currently not well understood. In the present study, we analyzed the kinetics of CD4 down-modulation and the susceptibility of HIV-1-infected T cells to superinfection using proviral HIV-1 constructs containing individual and combined defects in vpu, env, and nef and expressing red or green fluorescent proteins. T cells infected with HIV-1 mutants containing functional nef genes expressed low surface levels of CD4 from the first moment that viral gene expression became detectable. In comparison, Vpu and Env had only minor to moderate effects on CD4 during later stages of infection. Consistent with these quantitative differences, Nef inhibited superinfection more efficiently than Vpu and Env. Notably, nef alleles from AIDS patients were more effective in preventing superinfection than those derived from a nonprogressor of HIV-1 infection. Our data suggest that protection against X4-tropic HIV-1 superinfection involves both CD4-independent and CD4-dependent mechanisms of HIV-1 Nef. X4 was effectively down-regulated by simian immunodeficiency virus and HIV-2 but not by HIV-1 Nef proteins. Thus, maximal protection seems to involve an as-yet-unknown mechanism that is independent of CD4 or coreceptor down-modulation. Finally, we demonstrate that superinfected primary T cells show enhanced levels of apoptosis. Accordingly, one reason that HIV-1 inhibits CD4 surface expression and superinfection is to prevent premature cell death in order to expand the period of effective virus production.     

Minor contribution of HLA class I-associated selective pressure to the variability of HIV-1 accessory protein Vpu

Host HLA class I (HLA-I) allele-associated immune responses are major forces driving the evolution of HIV-1 proteins such as Gag and Nef. The viral protein U (Vpu) is an HIV-1 accessory protein responsible for CD4 degradation and enhancement of virion release by antagonizing tetherin/CD317. Although Vpu represents one of the most variable proteins in the HIV-1 proteome, it is still not clear to what extent HLA-I influence its evolution. To examine this issue, we enrolled 240 HLA-I-typed, treatment naïve, chronically HIV-infected subjects in Japan, and analyzed plasma HIV RNA nucleotide sequences of the vpu region. Using a phylogenetically-informed method incorporating corrections for HIV codon covariation and linkage disequilibrium among HLA alleles, we investigated HLA-associated amino acid mutations in the Vpu protein as well as in the translational products encoded by alternative reading frames. Despite substantial amino acid variability in Vpu, we identified only 4 HLA-associations in all possible translational products encoded in this region, suggesting that HLA-associated immune responses had minor effects on Vpu variability in this cohort. Rather, despite its size (81 amino acids), Vpu showed 103 codon–codon covariation associations, suggesting that Vpu conformation and function are preserved through many possible combinations of primary and secondary polymorphisms. Taken together, our study suggests that Vpu has been comparably less influenced by HLA-I-associated immune-driven evolution at the population level compared to other highly variable HIV-1 accessory proteins.     

Human immunodeficiency virus type 1 Vpr protein does not modulate surface expression of the CD4 receptor

The CD4 receptor is required for the entry of human immunodeficiency virus (HIV) into target cells. It has long been known that Nef, Env, and Vpu participate in the removal of the viral receptor from the cell surface. Recently, it has been proposed that the HIV type 1 (HIV-1) Vpr protein may also play a role in the downmodulation of CD4 from the surfaces of infected cells (L. Conti, B. Varano, M. C. Gauzzi, P. Matarrese, M. Federico, W. Malorani, F. Belardelli, and S. Gessani, J. Virol. 74:10207-10211, 2000). To investigate the possible role of Vpr in the downregulation of the viral receptor Vpr alleles from HIV-1 and simian immunodeficiency virus were transiently expressed in transformed T cells and in 293T fibroblasts, and their ability to modulate surface CD4 was evaluated. All Vpr alleles efficiently arrested cells in the G(2) stage of the cell cycle. However, none of the tested Vpr proteins altered the expression of CD4 on the cell surface. In comparison, HIV-1 Nef efficiently downmodulated surface CD4 in all the experimental settings. Transformed T cells and primary lymphocytes were challenged with wild-type, Nef-defective, and Vpr-defective viruses. A significant reduction in the HIV-induced downmodulation of surface CD4 was observed in viruses lacking Nef. However, Vpr-deletion-containing viruses showed no defect in their ability to remove CD4 from the surfaces of infected cells. Our results indicate that Vpr does not play a role in the HIV-induced downmodulation of the CD4 receptor.     

Nef-mediated downregulation of CD4 enhances human immunodeficiency virus type 1 replication in primary T lymphocytes

The accessory protein Nef plays a crucial role in primate lentivirus pathogenesis. Nef enhances human immunodeficiency virus type 1 (HIV-1) infectivity in culture and stimulates viral replication in primary T cells. In this study, we investigated the relationship between HIV-1 replication efficiency in CD4(+) T cells purified from human blood and two various known activities of Nef, CD4 downregulation and single-cycle infectivity enhancement. Using a battery of reporter viruses containing point mutations in nef, we observed a strong genetic correlation between CD4 downregulation by Nef during acute HIV-1 infection of activated T cells and HIV-1 replication efficiency in T cells. In contrast, HIV-1 replication ability was not significantly correlated with the ability of Nef to enhance single-cycle virion infectivity, as determined by using viruses produced in cells lacking CD4. These results demonstrate that CD4 downregulation by Nef plays a crucial role in HIV-1 replication in activated T cells and underscore the potential for the development of therapies targeting this conserved activity of Nef.     

Two sorting motifs, a ubiquitination motif and a tyrosine motif, are involved in HIV-1 and simian immunodeficiency virus Nef-mediated receptor endocytosis

HIV-1 and SIV Nef proteins downregulate cell surface CD4 and MHC class I (MHC-I) molecules of infected cells, which are necessary for efficient viral replication and pathogenicity. We previously reported that K144 in HIV-1 Nef is di-ubiquitinated, and K144R substitution impairs Nef-mediated CD4 downregulation. In this report, we extend the role of ubiquitination at this lysine residue from Nef-mediated CD4 downregulation to Nef-mediated MHC-I downregulation and from HIV Nef to SIV Nef. All HIV-1 Nef mutants that contain K144R substitution are inactive in MHC-I downregulation. Tested MHC-I alleles include HLA-ABC endogenously expressed and HLA-A2 exogenously expressed in Jurkat T cells. CD4 downregulation by SIV Nef involves K176 that aligns with K144 in HIV-1 Nef, as well as an N-terminal tyrosine motif Y28Y39 not present in HIV-1 Nef. Dual mutation at K176 and Y28Y39 completely impaired SIV Nef-mediated CD4 and MHC-I downregulation, whereas a single mutation at K176 or Y28Y39 did not. The involvement of tyrosine motif in SIV Nef-mediated CD4 and MHC-I downregulation prompted us to investigate a putative tyrosine motif (Y202Y/F203) in HIV-1 Nef that is conserved among HIV-1 species. Single mutation at the tyrosine motif Y202F203 in HIV-1 Nef (NA7) greatly impaired Nef-mediated CD4 downregulation, which is similar to what we observed previously with the single mutation at lysine K144. Thus, our study demonstrated that Nef-mediated receptor endocytosis involves the ubiquitination motif and tyrosine motif.     

Cell-surface expression of CD4 reduces HIV-1 infectivity by blocking Env incorporation in a Nef- and Vpu-inhibitable manner.

BACKGROUND: Human immunodeficiency virus-1 (HIV-1) infection decreases the cell-surface expression of its cellular receptor, CD4, through the combined actions of Nef, Env and Vpu. Such functional convergence strongly suggests that CD4 downregulation is critical for optimal viral replication, yet the significance of this phenomenon has so far remained a puzzle. RESULTS: We show that high levels of CD4 on the surface of HIV-infected cells induce a dramatic reduction in the infectivity of released virions by the sequestering of the viral envelope by CD4. CD4 is able to accumulate in viral particles while at the same time blocking incorporation of Env into the virion. Nef and Vpu, through their ability to downregulate CD4, counteract this effect. CONCLUSIONS: The CD4-mediated 'envelope interference' described here probably explains the plurality of mechanisms developed by HIV to downregulate the cell-surface expression of its receptor.     

Mechanisms of CD4 downregulation by the Nef and Vpu proteins of primate immunodeficiency viruses

Human immunodeficiency virus type 1 (HIV-1), human immunodeficiency virus type 2 (HIV-2), and simian immunodeficiency virus (SIV) are the etiological agents of acquired immunodeficiency syndrome (AIDS) in humans and a related disease in non-human primates. These viruses infect T cells and macrophages that express the surface glycoprotein, CD4, because this glycoprotein acts as a co-receptor for incoming virus particles. Once infection has occurred, however, the presence of CD4 poses problems for the virus life cycle, including the possibility of superinfection, premature binding of CD4 to nascent virus particles, and inhibition of virus release. Accordingly, primate immunodeficiency viruses have evolved at least two distinct mechanisms, mediated by the Nef and Vpu viral proteins, to "downregulate" CD4 in the host cells. Nef and Vpu are mainly expressed early and late, respectively, in the viral life cycle, ensuring continuous removal of CD4. Nef links mature CD4 to components of clathrin-dependent trafficking pathways at the plasma membrane, and perhaps in intracellular compartments, leading to internalization and delivery of CD4 to lysosomes for degradation. Vpu, on the other hand, interacts with newly-synthesized CD4 in the endoplasmic reticulum, linking CD4 to the SCF ubiquitin ligase and facilitating the entry of CD4 into the endoplasmic-reticulum-associated degradation pathway. These two mechanisms lead to a dramatic reduction of CD4 expression in infected cells and are essential for efficient virus replication and disease progression.     

HIV-1 accessory proteins--ensuring viral survival in a hostile environment

One of the features of primate immunodeficiency viruses (HIVs and SIVs) that distinguishes them from other retroviruses is the array of "accessory" proteins they encode. Here, we discuss recent advances in understanding the interactions of the HIV-1 Nef, Vif, Vpu, and Vpr proteins with factors and pathways expressed in cells of the immune system. In at least three instances, the principal activity of the accessory proteins appears to be evasion from various forms of cell-mediated (or intrinsic), antiviral resistance. Broadly speaking, the HIV-1 accessory proteins modify the local environment within infected cells to ensure viral persistence, replication, dissemination, and transmission.     

The switch from latent to productive infection in epstein-barr virus-infected B cells is associated with sensitization to NK cell killing

Following activation of Epstein-Barr virus (EBV)-infected B cells from latent to productive (lytic) infection, there is a concomitant reduction in the level of cell surface major histocompatibility complex (MHC) class I molecules and an impaired antigen-presenting function that may facilitate evasion from EBV-specific CD8+ cytotoxic T cells. In some other herpesviruses studied, most notably human cytomegalovirus (HCMV), evasion of virus-specific CD8+ effector responses via downregulation of surface MHC class I molecules is supplemented with specific mechanisms for evading NK cells. We now report that EBV differs from HCMV in this respect. While latently infected EBV-positive B cells were resistant to lysis by two NK lines and by primary polyclonal NK cells from peripheral blood, these effectors efficiently killed cells activated into the lytic cycle. Susceptibility to NK lysis coincided not only with downregulation of HLA-A, -B, and -C molecules that bind to the KIR family of inhibitory receptors on NK cells but also with downregulation of HLA-E molecules binding the CD94/NKG2A inhibitory receptors. Conversely, ULBP-1 and CD112, ligands for the NK cell-activating receptors NKG2D and DNAM-1, respectively, were elevated. Susceptibility of the virus-producing target cells to NK cell lysis was partially reversed by blocking ULBP-1 or CD112 with specific antibodies. These results highlight a fundamental difference between EBV and HCMV with regards to evasion of innate immunity.     

Simian and human immunodeficiency virus Nef proteins use different surfaces to downregulate class I major histocompatibility complex antigen expression

Simian immunodeficiency virus (SIV) and human immunodeficiency virus type 1 (HIV-1) Nef proteins are related regulatory proteins that share several functions, including the ability to downregulate class I major histocompatibility complex (MHC) and CD4 expression on the cell surface and to alter T-cell-receptor-initiated signal transduction in T cells. We compared the mechanisms used by SIV mac239 Nef and HIV-1 Nef to downregulate class I MHC and found that the ability of SIV Nef to downregulate class I MHC requires a unique C-terminal region of the SIV mac239 Nef molecule which is not found in HIV-1 Nef. Interestingly, mutation of the PxxP motif in SIV Nef, unlike in HIV-1 Nef, does not affect class I MHC downregulation. We also found that downregulation of class I MHC by SIV Nef requires a conserved tyrosine in the cytoplasmic domain of the class I MHC heavy chain and involves accelerated endocytosis of class I complexes, as previously found with HIV-1 Nef. Thus, while SIV and HIV-1 Nef proteins use a similar mechanism to downregulate class I MHC expression, they have evolved different surfaces for molecular interactions with cell factors that regulate class I MHC traffic. Mutations in the C-terminal domain of SIV mac239 Nef selectively disrupt class I MHC downregulation, having no detectable effect on other functions of Nef, such as the downregulation of CD4 and CD3 surface expression, the stimulation of SIV virion infectivity, and the induction of SIV replication from T cells infected in the absence of stimulation. The resulting mutants will be useful reagents for studying the importance of class I MHC downregulation for SIV replication and AIDS pathogenesis in infected rhesus macaques.     

Distinct mechanisms of CD4+ and CD8+ T-cell activation and bystander apoptosis induced by human immunodeficiency virus type 1 virions

Apoptosis of uninfected bystander T cells contributes to T-cell depletion during human immunodeficiency virus type 1 (HIV-1) infection. HIV-1 envelope/receptor interactions and immune activation have been implicated as contributors to bystander apoptosis. To better understand the relationship between T-cell activation and bystander apoptosis during HIV-1 pathogenesis, we investigated the effects of the highly cytopathic CXCR4-tropic HIV-1 variant ELI6 on primary CD4(+) and CD8(+) T cells. Infection of primary T-cell cultures with ELI6 induced CD4(+) T-cell depletion by direct cell lysis and bystander apoptosis. Exposure of primary CD4(+) and CD8(+) T cells to nonreplicating ELI6 virions induced bystander apoptosis through a Fas-independent mechanism. Bystander apoptosis of CD4(+) T cells required direct contact with virions and Env/CXCR4 binding. In contrast, the apoptosis of CD8(+) T cells was triggered by a soluble factor(s) secreted by CD4(+) T cells. HIV-1 virions activated CD4(+) and CD8(+) T cells to express CD25 and HLA-DR and preferentially induced apoptosis in CD25(+)HLA-DR(+) T cells in a CXCR4-dependent manner. Maximal levels of binding, activation, and apoptosis were induced by virions that incorporated MHC class II and B7-2 into the viral membrane. These results suggest that nonreplicating HIV-1 virions contribute to chronic immune activation and T-cell depletion during HIV-1 pathogenesis by activating CD4(+) and CD8(+) T cells, which then proceed to die via apoptosis. This mechanism may represent a viral immune evasion strategy to increase viral replication by activating target cells while killing immune effector cells that are not productively infected.     

BLV-CoCoMo-qPCR: Quantitation of bovine leukemia virus proviral load using the CoCoMo algorithm

Viral protein U (Vpu) is a type 1 membrane-associated accessory protein that is unique to human immunodeficiency virus type 1 (HIV-1) and a subset of related simian immunodeficiency virus (SIV). The Vpu protein encoded by HIV-1 is associated with two primary functions during the viral life cycle. First, it contributes to HIV-1-induced CD4 receptor downregulation by mediating the proteasomal degradation of newly synthesized CD4 molecules in the endoplasmic reticulum (ER). Second, it enhances the release of progeny virions from infected cells by antagonizing Tetherin, an interferon (IFN)-regulated host restriction factor that directly cross-links virions on host cell-surface. This review will mostly focus on recent advances on the role of Vpu in CD4 downregulation and Tetherin antagonism and will discuss how these two functions may have impacted primate immunodeficiency virus cross-species transmission and the emergence of pandemic strain of HIV-1.     

HIV-1 Gag Cytotoxic T Lymphocyte Epitopes Vary in Presentation Kinetics Relative to HLA Class I Downregulation

Although CD8⁺ cytotoxic T lymphocytes (CTLs) are protective in HIV-1 infection, the factors determining their antiviral efficiency are poorly defined. It is proposed that Gag targeting is superior because of very early Gag epitope presentation, allowing early killing of infected cells before Nef-mediated downregulation of human leukocyte antigen class I (HLA-I). To study Gag epitope presentation kinetics, three epitopes (SL9_77-85, KF11_162-172, and TW10_240-249) were genetically translocated from their endogenous location in the Rev-dependent (late) gag gene into the Rev-independent (early) nef gene with concomitant mutation of the corresponding endogenous epitopes to nonrecognized sequences. These viruses were compared to the index virus for CTL-mediated suppression of replication and the susceptibility of this antiviral activity to Nef-mediated HLA-I downregulation. SL9-specific CTLs gained activity after SL9 translocation to Nef, going from Nef sensitive to Nef insensitive, indicating that translocation accelerated infected cell recognition from after to before HLA-I downregulation. KF11-specific CTL antiviral activity was unchanged and insensitive to HLA-I downregulation before and after KF11 translocation, suggesting that already rapid recognition of infected cells was not accelerated. However, TW10-specific CTLs that were insensitive to Nef at the baseline became sensitive with reduced antiviral activity after translocation, indicating that translocation retarded epitope expression. Cytosolic peptide processing assays suggested that TW10 was inefficiently generated after translocation to Nef, compared to SL9 and KF11. As a whole, these data demonstrate that epitope presentation kinetics play an important role in CTL antiviral efficiency, that Gag epitopes are not uniformly presented early, and that the epitope context can play a major role in presentation kinetics.     

Human immunodeficiency virus type 1 Vpu protein interacts with CD74 and modulates major histocompatibility complex class II presentation

The human immunodeficiency virus type 1 (HIV-1) Vpu accessory protein is a transmembrane protein that down regulates CD4 expression and promotes the release of new virions. We screened a human leukocyte-specific yeast two-hybrid expression library to discover novel Vpu-interacting cellular proteins. The major histocompatibility complex class II (MHC II) invariant chain, also called Ii or CD74, was found to be one such protein. We show direct binding of Vpu and CD74 by using a yeast two-hybrid assay and coimmunoprecipitation from HIV-1-infected cells. The cytoplasmic region of Vpu was found to interact with the 30-amino-acid cytoplasmic tail of CD74. Human monocytic U937 cells infected with wild-type or Vpu-defective HIV-1 and transfected cells showed that Vpu down modulated the surface expression of mature MHC II molecules. The reduction in cell surface mature MHC II molecules correlated with decreased antigen presentation to T cells in culture. Thus, the Vpu protein also contributes to viral persistence by attenuating immune responses during HIV infection. This report further exemplifies the rich diversity and redundancy shown by HIV in immune evasion.     

HIV-1 Nef assembles a Src family kinase-ZAP-70/Syk-PI3K cascade to downregulate cell-surface MHC-I

HIV-1 Nef, which is required for the efficient onset of AIDS, enhances viral replication and infectivity by exerting multiple effects on infected cells. Nef downregulates cell-surface MHC-I molecules by an uncharacterized PI3K pathway requiring the actions of two Nef motifs-EEEE(65) and PXXP(75). We report that the Nef EEEE(65) targeting motif enables Nef PXXP(75) to bind and activate a trans-Golgi network-localized Src family tyrosine kinase (SFK). The Nef/SFK complex then recruits and phosphorylates the tyrosine kinase ZAP-70, which binds class I PI3K to trigger MHC-I downregulation in primary CD4+ T cells. In promonocytic cells, Nef/SFK recruits the ZAP-70 homolog Syk to downregulate MHC-I, implicating this PI3K pathway in multiple HIV-1 reservoirs. Isoform-specific PI3K inhibitors repress MHC-I downregulation, identifying them as potential therapeutic agents to combat HIV-1. The discovery of this Nef-SFK-ZAP-70/Syk-PI3K signaling pathway explains the hierarchal role of the Nef motifs in effecting immunoevasion.     

Decreased expression of DNAM-1 on NK cells from acute myeloid leukemia patients

This study tested the hypothesis that the expression of CD112 and CD155 (DNAM-1 ligands) on leukemic blasts induces a decreased expression of the activating receptor DNAM-1 on natural killer (NK) cells from acute myeloid leukemia (AML) patients. DNAM-1 is a co-receptor involved in the activation of NK cell cytotoxicity after its interaction with its ligands CD112 and CD155 on target cells. Here we study the expression of DNAM-1 on NK cells and DNAM-1 ligands on blasts from AML patients stratified by age. The results demonstrate that NK cells from AML patients younger than 65 years have a reduced expression of DNAM-1 compared with age-matched controls. The analysis of DNAM-1 ligands showed a high expression of CD112 and CD155 on leukemic blasts. An inverse correlation between CD112 expression on leukemic blasts and DNAM-1 expression on NK cells was found. Furthermore, downregulation of DNAM-1 was induced on healthy donors' NK cells after in vitro culture with leukemic blasts expressing DNAM-1 ligands. In conclusion, these results support the hypothesis that receptor-ligand crosslinking downregulates DNAM-1 expression on NK cells from patients <65 years of age. Considering the relevance of DNAM-1 in NK recognition and killing of leukemic cells, the reduced expression of this receptor on NK cells from AML patients can represent an additional mechanism of tumor escape.