Tomek Swigut

Mechanism for down-regulation of CD28 by Nef

SIV and HIV Nef proteins disrupt T‐cell receptor machinery by down‐modulating cell surface expression of CD4 and expression or signaling of CD3‐TCR. Nef also down‐modulates class I major histocompatibility complex (MHC) surface expression. We show that SIV and HIV‐1 Nefs down‐modulate CD28, a major co‐stimulatory receptor that mediates effective T‐cell activation, by accelerating CD28 endocytosis. The effects of Nef on CD28, CD4, CD3 and class I MHC expression are all genetically separable, indicating that all are selected independently. In cells expressing a Nef‐green fluorescent protein (GFP) fusion, CD28 co‐localizes with the AP‐2 clathrin adaptor and Nef‐GFP. Mutations that disrupt Nef interaction with AP‐2 disrupt CD28 down‐regulation. Furthermore, HIV and SIV Nefs use overlapping but distinct target sites in the membrane‐proximal region of the CD28 cytoplasmic domain. Thus, Nef probably induces CD28 endocytosis via the AP‐2 pathway, and this involves a ternary complex containing Nef, AP‐2 and CD28. The likely consequence of the concerted down‐regulation of CD28, CD4 and/or CD3 by Nef is disruption of antigen‐specific signaling machineries in infected T cells following a productive antigen recognition event.

HIV-1 Nef Binds the DOCK2–ELMO1 Complex to Activate Rac and Inhibit Lymphocyte Chemotaxis

The infectious cycle of primate lentiviruses is intimately linked to interactions between cells of the immune system. Nef, a potent virulence factor, alters cellular environments to increase lentiviral replication in the host, yet the mechanisms underlying these effects have remained elusive. Since Nef likely functions as an adaptor protein, we exploited a proteomic approach to directly identify molecules that Nef targets to subvert the signaling machinery in T cells. We purified to near homogeneity a major Nef-associated protein complex from T cells and identified by mass spectroscopy its subunits as DOCK2–ELMO1, a key activator of Rac in antigen- and chemokine-initiated signaling pathways, and Rac. We show that Nef activates Rac in T cell lines and in primary T cells following infection with HIV-1 in the absence of antigenic stimuli. Nef activates Rac by binding the DOCK2–ELMO1 complex, and this interaction is linked to the abilities of Nef to inhibit chemotaxis and promote T cell activation. Our data indicate that Nef targets a critical switch that regulates Rac GTPases downstream of chemokine- and antigen-initiated signaling pathways. This interaction enables Nef to influence multiple aspects of T cell function and thus provides an important mechanism by which Nef impacts pathogenesis by primate lentiviruses.

Two elements target SIV Nef to the AP-2 clathrin adaptor complex, but only one is required for the induction of CD4 endocytosis.

The simian immunodeficiency virus (SIV) and human immunodeficiency virus type 1 (HIV‐1) Nef proteins induce the endocytosis of CD4 and class I MHC molecules. Here we show that SIV Nef interacts with the AP‐2 adaptor complex via two elements located in the N‐terminal region of the Nef molecule, but only the N‐distal element is required to induce CD4 endocytosis. This N‐distal AP‐2 targeting element contains no canonical endocytic signals and probably contacts the AP‐2 complex via a novel interaction surface. The data support a model where SIV Nef induces CD4 endocytosis by promoting the normal interactions between the di‐leucine sorting signal in the CD4 cytoplasmic domain and AP‐2, but does not substitute for the CD4–AP‐2 adaptor interaction. Neither element is important for the induction of class I MHC endocytosis, thus indicating that different mechanisms underlie the induction of class I MHC and CD4 endocytosis by Nef. In contrast to SIV Nef, HIV‐1 Nef interacts with AP‐2 via a surface containing a di‐leucine endocytosis signal in the C‐terminal disordered loop of Nef. The fact that genetic selection maintains similar molecular interactions via different surfaces in SIV and HIV‐1 Nef proteins indicates that these interactions have critical roles for the viral life cycle in vivo.

Simian and Human Immunodeficiency Virus Nef Proteins Use Different Surfaces To Downregulate Class I Major Histocompatibility Complex Antigen Expression

ABSTRACT 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.

Disrupting Surfaces of Nef Required for Downregulation of CD4 and for Enhancement of Virion Infectivity Attenuates Simian Immunodeficiency Virus Replication In Vivo

ABSTRACT The multifunctional simian and human immunodeficiency virus (SIV and HIV) Nef proteins are important for virulence. We studied the importance of selected Nef functions using an SIV Nef with mutations in two regions that are required for CD4 downregulation. This Nef mutant is defective for downregulating CD4 and, in addition, for enhancing SIV infectivity and induction of SIV replication from infected quiescent peripheral blood mononuclear cells, but not for other known functions, including downregulation of class I major histocompatibility complex (MHC) cell surface expression. Replication of SIV containing this Nef variant in rhesus monkeys was attenuated early during infection. Subsequent increases in viral load coincided with selection of reversions and second-site compensatory changes in Nef. Our results indicate that the surfaces of Nef that mediate CD4 downregulation and the enhancement of virion infectivity are critical for SIV replication in vivo. Furthermore, these findings indicate that class I MHC downregulation by Nef is not sufficient for SIV virulence early in infection.

Cooperative Interactions of Simian Immunodeficiency Virus Nef, AP-2, and CD3-ζ Mediate the Selective Induction of T-Cell Receptor-CD3 Endocytosis

ABSTRACT The Nef proteins of human immunodeficiency virus and simian immunodeficiency virus (SIV) bind the AP-1 and AP-2 clathrin adaptors to downmodulate the expression of CD4 and CD28 by recruiting them to sites of AP-2 clathrin-dependent endocytosis. Additionally, SIV Nef directly binds the CD3-ζ subunit of the CD3 complex and downmodulates the T-cell receptor (TCR)-CD3 complex. We report here that SIV mac239 Nef induces the endocytosis of TCR-CD3 in Jurkat T cells. SIV Nef also induces the endocytosis of a chimeric CD8-CD3-ζ protein containing only the CD3-ζ cytoplasmic domain (8-ζ), in the absence of other CD3 subunits. Thus, the interaction of SIV Nef with CD3-ζ likely mediates the induction of TCR-CD3 endocytosis. In cells expressing SIV Nef and 8-ζ, both proteins colocalize with AP-2, indicating that Nef induces 8-ζ internalization via this pathway. Surprisingly, deletion of constitutively strong AP-2 binding determinants (CAIDs) in SIV Nef had little effect on its ability to induce TCR-CD3, or 8-ζ endocytosis, even though these determinants are required for the induction of CD4 and CD28 endocytosis via this pathway. Fluorescent microscopic analyses revealed that while neither the mutant SIV Nef protein nor 8-ζ colocalized with AP-2 when expressed independently, both proteins colocalized with AP-2 when coexpressed. In vitro binding studies using recombinant SIV Nef proteins lacking CAIDs and recombinant CD3-ζ cytoplasmic domain demonstrated that SIV Nef and CD3-ζ cooperate to bind AP-2 via a novel interaction. The fact that Nef uses distinct AP-2 interaction surfaces to recruit specific membrane receptors demonstrates how Nef independently selects distinct types of target receptors and recruits them to AP-2 for endocytosis.

T-Cell Receptor:CD3 Down-Regulation Is a Selected In Vivo Function of Simian Immunodeficiency Virus Nef but Is Not Sufficient for Effective Viral Replication in Rhesus Macaques

ABSTRACT We investigated the function of severely truncated simian immunodeficiency virus (SIV) Nef proteins (tNef) in vitro and in vivo. These variants emerged in rhesus monkeys infected with SIVmac239 containing a 152-bp deletion in the nef-unique region and have been suggested to enhance SIV virulence (E. T. Sawai, M. S. Hamza, M. Ye, K. E. Shaw, and P. A. Luciw, J. Virol. 74:2038-2045, 2000). We found that the tNef proteins were unable to down-regulate the cell surface expression of major histocompatibility complex class I proteins, CD4, and CD28 and neither stimulated SIV replication nor enhanced virion infectivity. The tNef proteins did efficiently down-regulate T-cell receptor (TCR):CD3 cell surface expression. Nevertheless, the SIVmac239 tnef variants were strongly attenuated in six infected juvenile rhesus macaques. Thus, while the ability of SIV Nef to down-modulate TCR:CD3 cell surface expression apparently confers a selective advantage in vivo, it is insufficient for efficient viral replication in infected macaques. Additional mutations elsewhere in SIVmac239 tnef genomes are required for a virulent phenotype.

Importance of the N-Distal AP-2 Binding Element in Nef for Simian Immunodeficiency Virus Replication and Pathogenicity in Rhesus Macaques

ABSTRACT Point mutations in SIVmac239 Nef disrupting CD4 downmodulation and enhancement of virion infectivity attenuate viral replication in acutely infected rhesus macaques, but changes selected later in infection fully restore Nef function (A. J. Iafrate et al., J. Virol. 74:9836-9844, 2000). To further evaluate the relevance of these Nef functions for viral persistence and disease progression, we analyzed an SIVmac239 Nef mutant containing a deletion of amino acids Q64 to N67 (Δ64-67Nef). This mutation inactivates the N-distal AP-2 clathrin adaptor binding element and disrupts the abilities of Nef to downregulate CD4, CD28 and CXCR4 and to stimulate viral replication in vitro. However, it does not impair the downmodulation of CD3 and class I major histocompatibility complex (MHC-I) or MHC-II and the upregulation of the MHC-II-associated invariant chain, and it has only a moderate effect on the enhancement of virion infectivity. Replication of the Δ64-67Nef variant in acutely infected macaques was intermediate between grossly nef-deleted and wild-type SIVmac239. Subsequently, three of six macaques developed moderate to high viral loads and developed disease, whereas the remaining animals efficiently controlled SIV replication and showed a more attenuated clinical course of infection. Sequence analysis revealed that the deletion in nef was not repaired in any of these animals. However, some changes that slightly enhanced the ability of Nef to downmodulate CD4 and moderately increased Nef-mediated enhancement of viral replication and infectivity in vitro were observed in macaques developing high viral loads. Our results imply that both the Nef functions that were disrupted by the Δ64-67 mutation and the activities that remained intact contribute to viral pathogenicity.

Erratum: Disrupting surfaces of Nef required for downregulation of CD4 and for enhancement of virion infectivity attenuates simian immunodeficiency virus replication in vivo (Journal of Virology (2000) 74; 21(9836-9844))

We thank Marion Hamacher, Mandy Krumbiegel, Nadim Shohdy, Pat Burfeind, and Maria Coronesi for excellent technical assistance; members of the Skowronski laboratory, Winship Herr, and Klaus Uberla for critical reading of the manuscript; and Bernhard Fleckenstein for support. We also thank Julie Overbaugh and Bryce Chackerian for sMAGI cells. This work was supported by the Sander-Stiftung, BMBF grant 01Ki9478 (F.K), Sonderforschungsbereich 466 (F.K.), PHS AI-42561 (J.S.), and The Council for Tobacco Research USA grant 4688 (J.S.).