The cationic region from HIV tat enhances the cell-surface expression of epitope/MHC class I complexes

Abstract

The potential of genetic immunization has been acknowledged for almost a decade, but disappointing immunogenicity in humans has delayed its introduction into the clinical arena. To try to increase the potency of genetic immunization, we and others have evaluated 'translocatory' proteins, which are thought to exit living cells by an uncharacterized pathway, and enter neighboring cells in an energy-independent manner. Several laboratories, including our own, have begun to question these remarkable properties. Our previous studies showed that the ability of an epitope to induce major histocompatibility complex (MHC) class I restricted CD8 þ T cells was, indeed, enhanced by its being attached to the proposed translocatory sequence of the HIV-1 tat protein. However, we found little evidence that the increased immunogenicity resulted from transfer of the fusion peptide between living cells, and we proposed that it resulted instead from an increased epitope/MHC expression on the surface of transfected cells. Here, we directly test this hypothesis. We show that cells cotransfected with plasmids encoding an epitope, and the relevant MHC class I allele, can stimulate epitope-specific T cells, and that attachment of the epitope to a putative translocatory sequence -which we term herein an 'integral cationic region' (ICR) -leads to a marked increase in stimulatory activity. This elevated stimulatory capacity does not result from a nonspecific increase in MHC class I expression. We use a high-affinity T-cell receptor (TcR) specific for the epitope/MHC combination to quantitate directly the cell-surface expression of the immunogenic complex, and we show that the attachment of the tat ICR to an epitope results in a substantial enhancement of its cellsurface presentation. These data suggest an alternative explanation for the immune enhancement seen with ICRs.

HIV-1 Tat Suppresses gp120-Specific T Cell Response in IL-10-Dependent Manner

“…This increases the up-regulation of MHC class I and costimulatory molecules CD80, CD86, and LFA on the surface. Tat contains a cationic domain which may play a role in up-regulation of epitope-MHC class I complexes on the cell surface (50). However, in the present study, the down-regulation of the immune response toward coimmunized Ag is due to up-regulation of IL-10 (immunosuppressive cytokine) and down-regulation of IFN-␥, leading to an overall suppression of the cellular immune response.…”

The HIV-1 Tat Protein Induces the Activation of CD8+ T Cells and Affects In Vivo the Magnitude and Kinetics of Antiviral Responses

“…Tat is known to affect all these signalling pathways. Indeed, Tat modulates antigen processing and improves the expression of certain epitope/MHC-I complexes on the surface of APCs leading to an increased stimulation of epitope-specific CD8 + T cells (Signal 1) [15,16,20,29,56]. Moreover, it has been demonstrated that Tat activates NF-κB [19,57,58], a transcription factor critical for the stimulation of T lymphocytes [59].…”

HIV-1 Tat-Based Vaccines: An Overview and Perspectives in the Field of HIV/AIDS Vaccine Development

“…Consistently, Tat increases the capability of MDDCs to present both allogeneic and recall antigens, hence potentiating T cell responses against heterologous antigens [95,96]. In addition to the RGD domain, Tat contains in its sequence a short and highly basic region (aa 47-59) termed "protein transduction domain" (PTD), a feature shared with very few other proteins called "penetratins" [201,202], which enables Tat-or PTDconjugated proteins to efficiently enter the cells in an energy-and receptor-independent fashion [203,204]. Thereby, the PTD peptide or the Tat protein have been largely used to deliver conjugated antigens to the MHC class I restricted-antigen presentation pathway, and to increase CTL responses to the conjugated antigen [187,[205][206][207][208][209][210].…”

“…Thereby, the PTD peptide or the Tat protein have been largely used to deliver conjugated antigens to the MHC class I restricted-antigen presentation pathway, and to increase CTL responses to the conjugated antigen [187,[205][206][207][208][209][210]. It has also been suggested that the PTD domain itself, in addition to its antigen-delivery capability, exerts an adjuvant effect on the conjugated antigen, in a fashion similar to poly-L-arginine peptides [211], leading to increased expression of specific MHC-I/peptide complexes and activation of T cell responses [202]. In this context, it was demonstrated that a biologically active Tat protein (86 aa), intracellularly expressed or given extracellularly, modifies the catalytic subunit composition of the immunoproteasome in B and T cell cultures [98,212].…”