title: HIV-1 Tat protein induces viral internalization through Env-mediated interactions in dose-dependent manner.
abstract: OBJECTIVE
  To study the dose-dependent manner of HIV-1 Tat-induced effects on viral replication, internalization and spread, and to directly observe these effects on soluble Env immunogens and virus-like particles.
  
  
  METHODS
  In order to determine the manner through which Tat affects viral replication, we incubated cells, virions and soluble Env spikes with Tat at different concentrations, and directly visualized the effects of such incubation.
  
  
  METHODS
  Cell-based infectivity assays were carried out to assay Tat dose-dependency of viral infectivity. Transmission electron microscopy of virus-like particles and soluble gp140 immunogens incubated with Tat at various concentrations was performed to directly observe Tat-induced effects.
  
  
  RESULTS
  Treating virus with exogenous Tat increased infectivity in a dose-dependent manner. In the presence of anti-Tat antibodies, virus replication and spread were repressed, postulating Tat contributions to disease progression. When CXCR4 coreceptors were blocked, Tat treatment overcame the inhibition relative to absence of Tat treatment. Similarly, syncytium formation between chronically infected and uninfected target cells was also increased by exogenous Tat treatment. Inhibiting the CD4 receptor for virus entry abolished syncytium formation and Tat treatment was unable to overcome CD4 dependency. We show that Tat reduces virus infectivity at higher Tat concentrations through Env interactions resulting in viral aggregation.
  
  
  CONCLUSIONS
  Treating virions or chronically infected cells with exogenous Tat could enhance virus infectivity and spread through coreceptor tropism switch or through another undetermined mechanism. The aggregation potential of Tat suggests a mechanism of negative-feedback regulation of viral replication, providing another regulative function to control viral replication.
authors.affiliation: aDepartment of Laboratory Medicine, Division of Clinical Microbiology, Karolinska Institutet, Stockholm, Sweden bLaboratory of Structural Biochemistry, Genome Institute of Singapore, Singapore cDepartment of Molecular and Cellular Biology, University of California, Davis, California, USA.
authors.forename: Selina
authors.initials: S
authors.lastname: Poon
authors.forename: Carlos G
authors.initials: CG
authors.lastname: Moscoso
authors.forename: Onur M
authors.initials: OM
authors.lastname: Yenigun
authors.forename: Prasanna R
authors.initials: PR
authors.lastname: Kolatkar
authors.forename: R Holland
authors.initials: RH
authors.lastname: Cheng
authors.forename: Anders
authors.initials: A
authors.lastname: Vahlne
mesh_terms: D015704:CD4 Antigens / Q000378:metabolism; D015686:Gene Products, env / Q000276:immunology* / Q000378:metabolism; D015696:Gene Products, tat / Q000276:immunology* / Q000378:metabolism; D015726:Giant Cells / Q000378:metabolism; D006023:Glycoproteins / Q000378:metabolism; D015658:HIV Infections / Q000276:immunology* / Q000378:metabolism / Q000821:virology; D015497:HIV-1 / Q000276:immunology* / Q000378:metabolism; D006801:Humans; D066298:In Vitro Techniques; D046529:Microscopy, Electron, Transmission; D019713:Receptors, CCR5 / Q000378:metabolism; D019718:Receptors, CXCR4 / Q000378:metabolism; D014779:Virus Replication / Q000276:immunology*
chemical_list: C584933:CCR5 protein, human; D015704:CD4 Antigens; C527276:CXCR4 protein, human; C033904:GP 140; D015686:Gene Products, env; D015696:Gene Products, tat; D006023:Glycoproteins; D019713:Receptors, CCR5; D019718:Receptors, CXCR4