Endocytic and exocytic regulation of CD4 expression and function

Roles for endocytosis in lentiviral replication

Endocytosis is essential for the entry of many viruses into cells. The primate lentiviruses [human immunodeficiency virus (HIV) 1 and 2, and the simian immunodeficiency viruses (SIVs)], however, use endocytosis in other aspects of their life cycles. Here, the authors describe the ways in which the endocytic pathway is used by HIV and SIV and discuss the mechanisms through which endocytosis may contribute to the pathogenic properties of these viruses.

Mechanisms of HIV-1 Nef function and intracellular signaling

Advances in the last several years have enhanced mechanistic understanding of Nef-induced CD4 and MHCI downregulation and have suggested a new paradigm for analyzing Nef function. In both of these cases, Nef acts by forming ternary complexes with significant contributions to stability imparted by non-canonical interactions. The mutational analyses and binding assays that have led to these conclusions are discussed. The recent progress has been dependent on conservative mutations and multi-protein binding assays. The poorly understood Nef functions of p21 activated protein kinase (PAK2) activation, enhancement of virion infectivity, and inhibition of immunoglobulin class switching are also likely to involve ternary complexes and non-canonical interactions. Hence, investigation of these latter Nef functions should benefit from a similar approach. Six historically used alanine substitutions for determining structure-function relationships of Nef are discussed. These are M20A, E62A/E63A/E64A/E65A (AAAA), P72A/P75A (AXXA), R106A, L164A/L165A, and D174A/D175A. Investigations of less-disruptive mutations in place of AAAA and AXXA have led to different interpretations of mechanism. Two recent examples of this alternate approach, F191I for studying PAK2 activation and D123E for the critical residue D123 are discussed. The implications of the new findings and the resulting new paradigm for Nef structure-function are discussed with respect to creating a map of Nef functions on the protein surface. We report the results of a PPI-Pred analysis for protein-protein interfaces. There are three predicted patches produced by the analysis which describe regions consistent with the currently known mutational analyses of Nef function.

Reduced monomeric CD4 is the preferred receptor for HIV

CD4 is a co-receptor for binding of T cells to antigen-presenting cells and the primary receptor for the human immunodeficiency virus type 1 (HIV). CD4 exists in three different forms on the cell surface defined by the state of the domain 2 cysteine residues: an oxidized monomer, a reduced monomer, and a covalent dimer linked through the domain 2 cysteines. The disulfide-linked dimer is the preferred immune co-receptor. The form of CD4 that is preferred by HIV was examined in this study. HIV entry and envelope-mediated cell-cell fusion were tested using cells expressing comparable levels of wild-type or disulfide bond mutant CD4 in which the domain 2 cysteines were mutated to alanine. Eliminating the domain 2 disulfide bond increased entry of HIV reporter viruses and enhanced HIV envelope-mediated cell-cell fusion 2-4-fold. These observations suggest that HIV enters susceptible cells preferably through monomeric reduced CD4, whereas dimeric CD4 is the preferred receptor for binding to antigen-presenting cells. Cleavage of the domain 2 disulfide bond is possibly involved in the conformational change in CD4 associated with fusion of the HIV and cell membranes.

Differential subcellular localization of the regulatory T-cell protein LAG-3 and the coreceptor CD4

CD4 binds to MHC class II molecules and enhances T-cell activation. The CD4-related transmembrane protein LAG-3 (lymphocyte activation gene-3, CD223) binds to the same ligand but inhibits T-cell proliferation. We have previously shown that LAG-3 cell surface expression is tightly regulated by extracellular cleavage in order to regulate its potent inhibitory activity. Given this observation and the contrasting functions of CD4 and LAG-3, we investigated the cell distribution, location and transport of these related cell surface molecules. As expected, the vast majority of CD4 is expressed at the cell surface with minimal intracellular localization, as determined by flow cytometry, immunoblotting and confocal microscopy. In contrast, nearly half the cellular content of LAG-3 is retained in intracellular compartments. This significant intracellular storage of LAG-3 appears to facilitate its rapid translocation to the cell surface following T-cell activation, which was much faster for LAG-3 than CD4. Increased vesicular pH inhibited translocation of both CD4 and LAG-3 to the plasma membrane. While some colocalization of the microtubule organizing center, early/recycling endosomes and secretory lysosomes was observed with CD4, significantly greater colocalization was observed with LAG-3. Analysis of CD4:LAG-3 chimeras suggested that multiple domains may contribute to intracellular retention of LAG-3. Thus, in contrast with CD4, the substantial intracellular storage of LAG-3 and its close association with the microtubule organizing center and recycling endosomes may facilitate its rapid translocation to the cell surface during T-cell activation and help to mitigate T-cell activation.

Regulated movement of CD4 in and out of the immunological synapse

The mechanism underlying the transient accumulation of CD4 at the immunological synapse (IS) and its significance for T cell activation are not understood. To investigate these issues, we mutated a serine phosphorylation site (S408) in the cytoplasmic tail of murine CD4. Preventing phosphorylation of S408 did not block CD4 recruitment to the IS; rather, it blocked the ability of CD4 to leave the IS. Surprisingly, enhanced and prolonged CD4 accumulation at the supramolecular activation cluster in the contact area had no functional consequence for T cell activation, cytokine production, or proliferation. Protein kinase C theta (PKCtheta)-deficient T cells also displayed enhanced and prolonged accumulation of wild-type CD4 at the IS, indicating that theta is the critical PKC isoform involved in CD4 movement. These findings suggest a model wherein recruitment of CD4 to the IS allows its phosphorylation by PKCtheta and subsequent removal from the IS. Thus, an important role for PKCtheta in T cell activation involves its recruitment to the IS, where it phosphorylates specific substrates that help to maintain the dynamism of protein turnover at the IS.

HIV-1 Nef: at the crossroads

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

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

Functional characterization of the human immunodeficiency virus type 1 Nef acidic domain

The human immunodeficiency virus type 1 (HIV-1) accessory protein Nef downregulates major histocompatibility complex class I (MHC-I) from the cell surface. It has been proposed that the direct interaction of the acidic cluster (AC) of Nef, (62)EEEE(65), with the furin binding region (fbr) of PACS-1 is crucial for this Nef function. Contrary to this proposal, evidence is presented here that the four glutamates in Nef do not functionally engage the PACS-1 fbr. (i) The binding of Nef to the PACS-1 fbr in vitro is much weaker than the binding of the canonical furin AC to the PACS-1 fbr. (ii) The mutation of two of the four glutamates in Nef's AC to alanines does not alter Nef's ability to downregulate MHC-I, and triply mutated Nefs exhibit 50% activity. (iii) The introduction of lysine into the AC has little effect on Nef function. (iv) The mutation of all four glutamates to alanine does debilitate Nef MHC-I downregulation, but this quadruple mutation also impairs the ability of Nef to regulate p21-activated protein kinase and enhance viral particle infectivity. (v) The replacement of the Nef AC with the bona fide AC from furin results in the loss of the expected regulatory properties of the furin AC. (vi) The insertion of the conformation-disrupting amino acid proline into the Nef AC does not disrupt MHC-I downregulation. Our results are consistent with an alternative model in which (62)EEEE(65) plays a stabilizing role in the formation of a ternary complex between Nef, the MHC-I cytoplasmic domain, and AP-1.

Receptor-mediated entry by equine infectious anemia virus utilizes a pH-dependent endocytic pathway

Previous studies of human and nonhuman primate lentiviral entry mechanisms indicate a predominant use of pH-independent pathways, although more recent studies of human immunodeficiency virus type 1 entry appear to reveal the use of a low-pH-dependent entry pathway in certain target cells. To expand the characterization of the specificity of lentiviral entry mechanisms, we have in the current study examined the entry pathway of equine infectious anemia virus (EIAV) during infection of its natural target, equine macrophages, permissive equine fibroblastic cell lines, and an engineered mouse cell line expressing the recently defined equine lentivirus receptor-1. The specificity of EIAV entry into these various cells was determined by assaying the effects of specific drug treatments on the level of virus entry as measured by quantitative real-time PCR assay of early reverse transcripts or by measurements of virion production. The results of these studies demonstrated that EIAV entry into all cell types was substantially inhibited in a dose-dependent manner by treatment with the vacuolar H+-ATPase inhibitors concanamycin A and bafilomycin A1 or the lysosomotropic weak base ammonium chloride. In contrast, treatments with sucrose to block clathrin-mediated endocytosis or with chloroquine to block organelle acidification failed to inhibit EIAV entry into the same target cells. The observed inhibition of EIAV entry was shown not to be related to cytotoxicity. Taken together, these experiments reveal for the first time that EIAV receptor-mediated entry into target cells is via a low-pH-dependent endocytic pathway.

p56Lck anchors CD4 to distinct microdomains on microvilli

Cell-surface microvilli play a central role in adhesion, fusion, and signaling processes. Some adhesion and signaling receptors segregate on microvilli but the determinants of this localization remain mostly unknown. In this study, we considered CD4, a receptor involved in immune response and HIV infection, and p56(Lck), a CD4-associated tyrosine kinase. Analysis of CD4 trafficking reveals that p56(Lck) binds tightly to CD4 independently of its activation state and inhibits CD4 internalization. Electron microscopy analysis established that p56(Lck) mediates CD4 association with microvilli whereas biochemical data indicate that p56(Lck) expression renders CD4 insoluble by the nonionic detergent Triton X-100. In addition, cytoskeleton-disrupting agent increased CD4 solubility, suggesting the involvement of cytoskeletal elements in CD4 anchoring to microvilli. This concept was supported further by the observation that the lateral mobility of CD4 within the plasma membrane was decreased in cells expressing p56(Lck). Finally, isolation of detergent-resistant membranes revealed that the complex CD4-p56(Lck) is enriched within these domains as opposed to conditions in which CD4 does not interact with p56(Lck). In conclusion, our results show that p56(Lck) targets CD4 to specialized lipid microdomains preferentially localized on microvilli. This localization, which prevents CD4 internalization, might facilitate CD4-mediated adhesion processes and could correspond to the signaling site of the receptor.

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.

Down regulation of CD4 expression following isolation and culture of human monocytes

The down regulation of CD4 by cultured monocytes has been observed by our group and by other investigators. Flow cytometric experiments were done to examine which factors might influence this phenomenon. The addition of lipopolysaccharide, granulocyte-macrophage colony-stimulating factor, macrophage colony-stimulating factor, or interleukin-10 to monocyte cultures failed to inhibit the decrease in monocyte CD4 expression routinely observed following overnight culture. The down regulation was an adherence-independent phenomenon and was not influenced by the type of anticoagulant into which the peripheral blood was collected or by the presence or absence of lymphocytes within the cultures. The avoidance of the use of Ficoll-Paque to isolate peripheral blood mononuclear cells did not prevent monocyte CD4 down regulation. Finally, by tagging monocyte CD4 with an anti-CD4 phycoerythrin-conjugated monoclonal antibody prior to culture, we were able to determine that the down regulation observed was the result of the internalization of the molecule. At this time, we conclude that the observed down regulation of monocyte CD4 is probably due to the differentiation of blood monocytes into tissue culture-derived macrophages rather than to some artifact of the isolation procedure.

Di-leucine signals mediate targeting of tyrosinase and synaptotagmin to synaptic-like microvesicles within PC12 cells

One pathway in forming synaptic-like microvesicles (SLMV) involves direct budding from the plasma membrane, requires adaptor protein 2 (AP2) and is brefeldin A (BFA) resistant. A second route leads from the plasma membrane to an endosomal intermediate from which SLMV bud in a BFA-sensitive, AP3-dependent manner. Because AP3 has been shown to bind to a di-leucine targeting signal in vitro, we have investigated whether this major class of targeting signals is capable of directing protein traffic to SLMV in vivo. We have found that a di-leucine signal within the cytoplasmic tail of human tyrosinase is responsible for the majority of the targeting of HRP-tyrosinase chimeras to SLMV in PC12 cells. Furthermore, we have discovered that a Met-Leu di-hydrophobic motif within the extreme C terminus of synaptotagmin I supports 20% of the SLMV targeting of a CD4-synaptotagmin chimera. All of the traffic to the SLMV mediated by either di-Leu or Met-Leu is BFA sensitive, strongly suggesting a role for AP3 and possibly for an endosomal intermediate in this process. The differential reduction in SLMV targeting for HRP-tyrosinase and CD4-synaptotagmin chimeras by di-alanine substitutions or BFA treatment implies that different proteins use the two routes to the SLMV to differing extents.

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.

Cluster of differentiation antigen 4 (CD4) endocytosis and adaptor complex binding require activation of the CD4 endocytosis signal by serine phosphorylation

Cluster of differentiation antigen 4 (CD4), the T lymphocyte antigen receptor component and human immunodeficiency virus coreceptor, is down-modulated when cells are activated by antigen or phorbol esters. During down-modulation CD4 dissociates from p56(lck), undergoes endocytosis through clathrin-coated pits, and is then sorted in early endosomes to late endocytic organelles where it is degraded. Previous studies have suggested that phosphorylation and a dileucine sequence are required for down-modulation. Using transfected HeLa cells, in which CD4 endocytosis can be studied in the absence of p56(lck), we show that the dileucine sequence in the cytoplasmic domain is essential for clathrin-mediated CD4 endocytosis. However, this sequence is only functional as an endocytosis signal when neighboring serine residues are phosphorylated. Phosphoserine is required for rapid endocytosis because CD4 molecules in which the cytoplasmic domain serine residues are substituted with glutamic acid residues are not internalized efficiently. Using surface plasmon resonance, we show that CD4 peptides containing the dileucine sequence bind weakly to clathrin adaptor protein complexes 2 and 1. The affinity of this interaction is increased 350- to 700-fold when the peptides also contain phosphoserine residues.

Lack of p56lck expression correlates with CD4 endocytosis in primary lymphoid and myeloid cells

In cell lines the endocytic properties of CD4 are regulated through its association with the src-family tyrosine kinase p56lck. In lymphoid cell lines expressing p56lck, CD4 is restricted to the cell surface and undergoes only limited internalization. Phosphorylation of the cytoplasmic domain of CD4 causes p56lck to dissociate and activates an endocytosis signal leading to the internalization of CD4 through clathrin-coated pits. In p56lck-negative transfected cell lines CD4 is constitutively internalized, but internalization is inhibited when p56lck is expressed in these cells. We now demonstrate that these endocytic properties of CD4 determined in transfected cell lines hold true for CD4 naturally expressed on myeloid cell lines (HL-60 and U937), as well as on primary lymphocytes, monocytes and macrophages isolated from human blood. CD4 showed limited internalization on p56lck-positive lymphocytes, but was rapidly internalized in p56lck-negative monocytes and macrophages. Surprisingly, rapid internalization of CD4 was seen with the lymphocytes from one unidentified donor. In these cells we failed to detect p56lck expression by Western blotting.

The SH3 domain-binding surface and an acidic motif in HIV-1 Nef regulate trafficking of class I MHC complexes

Nef, a regulatory protein of human and simian immunodeficiency viruses, downregulates cell surface expression of both class I MHC and CD4 molecules in T cells by accelerating their endocytosis. Fibroblasts were used to study alterations in the traffic of class I MHC complexes induced by Nef. We found that Nef downregulates class I MHC complexes by a novel mechanism involving the accumulation of endocytosed class I MHC in the trans-Golgi, where it colocalizes with the adaptor protein-1 complex (AP-1). This effect of Nef on class I MHC traffic requires the SH3 domain-binding surface and a cluster of acidic amino acid residues in Nef, both of which are also required for Nef to downregulate class I MHC surface expression and to alter signal transduction in T cells. Downregulation of class I MHC complexes from the surface of T cells also requires a tyrosine residue in the cytoplasmic domain of the class I MHC heavy chain molecule. The requirement of the same surfaces of the Nef molecule for downregulation of surface class I MHC complexes in T cells and for their accumulation in the trans-Golgi of fibroblasts indicates that the two effects of Nef involve similar interactions with the host cell machinery and involve a molecular mechanism regulating class I MHC traffic that is common for both of these cell types. Interestingly, the downregulation of class I MHC does not require the ability of Nef to colocalize with the adaptor protein-2 complex (AP-2). We showed previously that the ability of Nef to colocalize with AP-2 correlates with the ability of Nef to downregulate CD4 expression. Our observations indicate that Nef downregulates class I MHC and CD4 surface expression via different interactions with the protein sorting machinery, and link the sorting and signal transduction machineries in the regulation of class I MHC surface expression by Nef.

Co-localization of HIV-1 Nef with the AP-2 adaptor protein complex correlates with Nef-induced CD4 down-regulation

The nef gene of human and simian immunodeficiency viruses is critical for AIDS pathogenesis. Its function in vivo is unknown, but in vitro natural isolates of Nef down-regulate expression of the cell surface CD4 molecule, a component of the T cell antigen receptor and the viral receptor, by accelerating its endocytosis. We have used chimeric proteins comprised of the natural HIV-1 NA7 Nef fused to a strongly fluorescing mutant of green fluorescent protein (GFP) to correlate Nef function with intracellular localization in human CD4-positive Jurkat T cells. The NA7-GFP fusion protein co-localizes with components of the clathrin coat, including clathrin and the beta-subunit of the AP-2 adaptor protein complex, at discrete locations that are consistent with the normal cellular distribution of clathrin coats at the plasma membrane. The NA7-GFP protein is also found in the perinuclear region of the cell, which is likely to reflect the Golgi apparatus. Evidence from a CD4-negative fibroblast cell line indicates that co-localization of NA7-GFP with components of the clathrin coat does not require expression of the CD4 molecule. Analysis of a large panel of chimeric molecules containing mutant Nef moieties demonstrated that the N-terminal membrane targeting signal cooperates with additional element(s) in the disordered loops in the Nef molecule to co-localize the Nef protein with AP-2 adaptor complexes at the cell margin. This localization of NA7-GFP correlates with, but is not sufficient for, down-regulation of surface CD4 and at least one additional function of Nef is required. In T cells co-expressing CD4 and NA7-GFP, CD4 at the cell surface is redistributed into a discrete pattern that co-localizes with that of NA7-GFP. Our observations place NA7-GFP in physical proximity to AP-2-containing clathrin coat at the plasma membrane and imply that Nef interacts, either directly or indirectly, with a component of the AP-2-containing coat at this location. This evidence supports a model whereby Nef recruits CD4 to the endocytic machinery via AP-2-containing clathrin coats at the plasma membrane.