The HIV-1 accessory protein Nef increases surface expression of the checkpoint receptor Tim-3 in infected CD4(+) T cells

Prolonged immune activation drives the upregulation of multiple checkpoint receptors on the surface of virus-specific T cells, inducing their exhaustion. Reversing HIV-1-induced T cell exhaustion is imperative for efficient virus clearance; however, viral mediators of checkpoint receptor upregulation remain largely unknown. The enrichment of checkpoint receptors on T cells upon HIV-1 infection severely constrains the generation of an efficient immune response. Herein, we examined the role of HIV-1 Nef in mediating the upregulation of checkpoint receptors on peripheral blood mononuclear cells. We demonstrate that the HIV-1 accessory protein Nef upregulates cell surface levels of the checkpoint receptor T-cell immunoglobulin mucin domain-3 (Tim-3) and that this is dependent on Nef's dileucine motif LL164/165. Furthermore, we used a bimolecular fluorescence complementation assay to demonstrate that Nef and Tim-3 form a complex within cells that is abrogated upon mutation of the Nef dileucine motif. We also provide evidence that Nef moderately promotes Tim-3 shedding from the cell surface in a dileucine motif- dependent manner. Treating HIV-1-infected CD4(+) T cells with a matrix metalloprotease inhibitor enhanced cell surface Tim-3 levels and reduced Tim-3 shedding. Finally, Tim-3 expressing CD4(+) T cells displayed a higher propensity to release the proinflammatory cytokine interferon-gamma. Collectively, our findings uncover a novel mechanism by which HIV-1 directly increases the levels of a checkpoint receptor on the surface of infected CD4(+) T cells.

Automated summary

The study reveals a novel mechanism by which HIV-1 directly increases the levels of a checkpoint receptor, Tim-3, on the surface of infected CD4(+) T cells through the Nef protein. Nef upregulates cell surface levels of Tim-3 dependent on its dileucine motif, and forms a complex with Tim-3 within cells. Additionally, Nef moderately promotes Tim-3 shedding from the cell surface in a dileucine motif-dependent manner.