Escape of HIV-1-Infected Dendritic Cells from TRAIL-Mediated NK Cell Cytotoxicity during NK-DC Cross-Talk—A Pivotal Role of HMGB1

Abstract

Early stages of Human Immunodeficiency Virus-1 (HIV-1) infection are associated with local recruitment and activation of important effectors of innate immunity, i.e. natural killer (NK) cells and dendritic cells (DCs). Immature DCs (iDCs) capture HIV-1 through specific receptors and can disseminate the infection to lymphoid tissues following their migration, which is associated to a maturation process. This process is dependent on NK cells, whose role is to keep in check the quality and the quantity of DCs undergoing maturation. If DC maturation is inappropriate, NK cells will kill them (“editing process”) at sites of tissue inflammation, thus optimizing the adaptive immunity. In the context of a viral infection, NK-dependent killing of infected-DCs is a crucial event required for early elimination of infected target cells. Here, we report that NK-mediated editing of iDCs is impaired if DCs are infected with HIV-1. We first addressed the question of the mechanisms involved in iDC editing, and we show that cognate NK-iDC interaction triggers apoptosis via the TNF-related apoptosis-inducing ligand (TRAIL)-Death Receptor 4 (DR4) pathway and not via the perforin pathway. Nevertheless, once infected with HIV-1, DC _HIV become resistant to NK-induced TRAIL-mediated apoptosis. This resistance occurs despite normal amounts of TRAIL released by NK cells and comparable DR4 expression on DC _HIV. The escape of DC _HIV from NK killing is due to the upregulation of two anti-apoptotic molecules, the cellular-Flice like inhibitory protein (c-FLIP) and the cellular inhibitor of apoptosis 2 (c-IAP2), induced by NK-DC _HIV cognate interaction. High-mobility group box 1 (HMGB1), an alarmin and a key mediator of NK-DC cross-talk, was found to play a pivotal role in NK-dependent upregulation of c-FLIP and c-IAP2 in DC _HIV. Finally, we demonstrate that restoration of DC _HIV susceptibility to NK-induced TRAIL killing can be obtained either by silencing c-FLIP and c-IAP2 by specific siRNA, or by inhibiting HMGB1 with blocking antibodies or glycyrrhizin, arguing for a key role of HMGB1 in TRAIL resistance and DC _HIV survival. These findings provide evidence for a new strategy developed by HIV to escape immune attack, they challenge the question of the involvement of HMGB1 in the establishment of viral reservoirs in DCs, and they identify potential therapeutic targets to eliminate infected DCs.

Author Summary

Dendritic cells (DCs), the professional antigen presenting cells, are critical for host immunity by inducing specific immune responses against a broad variety of pathogens. Human Immunodeficiency Virus-1 (HIV-1) has evolved ways to exploit DCs, thereby facilitating viral dissemination and allowing evasion of antiviral immunity. In particular, infected DCs may function as cellular reservoirs for HIV-1, thus contributing to viral persistence in lymphoid tissues. The mechanisms involved in the constitution of HIV reservoirs in DCs are poorly understood. In this study, we reveal that DCs infected with HIV-1 (DC _HIV) become resistant to killing by natural killer (NK) cells, early effectors of innate immunity involved in the destruction of virus infected cells or cancer cells. This protection of DC _HIV from NK cytotoxicity is induced through a cross-talk between NK cells and DC _HIV, which induces the upregulation in DC _HIV of two inhibitors of cell death, i.e. cellular-Flice like inhibitory protein (c-FLIP) and cellular inhibitor of apoptosis 2 (c-IAP2). The molecule responsible for the induction of these inhibitors is High-mobility group box 1 (HMGB1), an alarmin involved in the functional maturation of DCs. Blocking HMGB1 restores DC _HIV susceptibility to NK cell killing, arguing for a key role of HMGB1 in the persistence of DC _HIV. These findings provide evidence of the crucial role of NK-DC cross-talk in promoting viral persistence, and they identify potential therapeutic targets to eliminate infected DCs.