HMGB1, an alarmin promoting HIV dissemination and latency in dendritic cells

Abstract

Dendritic cells (DCs) initiate immune responses by transporting antigens and migrating to lymphoid tissues to initiate T-cell responses. DCs are located in the mucosal surfaces that are involved in human immunodeficiency virus (HIV) transmission and they are probably among the earliest targets of HIV-1 infection. DCs have an important role in viral transmission and dissemination, and HIV-1 has evolved different strategies to evade DC antiviral activity. High mobility group box 1 (HMGB1) is a DNA-binding nuclear protein that can act as an alarmin, a danger signal to alert the innate immune system for the initiation of host defense. It is the prototypic damage-associated molecular pattern molecule, and it can be secreted by innate cells, including DCs and natural killer (NK) cells. The fate of DCs is dependent on a cognate interaction with NK cells, which involves HMGB1 expressed at NK-DC synapse. HMGB1 is essential for DC maturation, migration to lymphoid tissues and functional type-1 polarization of naïve T cells. This review highlights the latest advances in our understanding of the impact of HIV on the interactions between HMGB1 and DCs, focusing on the mechanisms of HMGB1-dependent viral dissemination and persistence in DCs, and discussing the consequences on antiviral innate immunity, immune activation and HIV pathogenesis.

Figure 1

Structure of the HMGB1 protein. (a) HMGB1 is a 25-kDa conserved chromosomal protein of 215 amino acids. It is organized in three domains made up by two positively charged homologous DNA-binding structures (A and B box), and a negatively charged acidic tail composed of 30 glutamic and aspartic acids, exclusively. There are two positively charged nuclear-localization signals, including amino-acid sequence segments 28–44 for NLS1 and 179–185 for NLS2. Amino-acids 50–183 segment in the C-terminus of HMGB1 is responsible for RAGE binding. The extracellular cytokine activity resides within the B box, and it can be antagonized by truncated A box domain. The cysteine in position 106 in the B box is indispensable for HMGB1 to activate cytokine release, as shown by selective mutation. The C-terminal acidic tail is required for transcription stimulatory function of HMGB1. (b) The primary HMGB1 sequence is 98.5% identical in all mammals. Comparison of human HMGB1 to the murine counterpart shows two differences in the acidic tail and one amino-acid change in the B box, whereas bovine HMGB1 shows only one amino-acid change in the acidic tail, and canine HMGB1 shows 100% homology with human HMGB1 (adapted from Murua Escobar et al.)

Figure 2

HMGB1-mediated cross talk between DCs and NK cells is pivotal to DC maturation and further induction of adaptive immunity. The disruption of an epithelial barrier allows invasion of microbial pathogens, which elicit an innate response at the site of infection (1). Neutrophils and macrophages infiltrate the site of tissue infection and release alarmins, including HMGB1 (2). HMGB1 recruits iDCs, resulting in an increase in local mobilization of iDCs (3). Functional DC maturation requires a cross talk with NK cells, which involves HMGB1 expressed at NK–DC synapse (4). Immature to mDC conversion allows DCs to migrate to secondary lymphoid organs (5) and contributes to the enhanced uptake, processing and presentation of microbial antigens to naïve T cells, thus polarizing a Th1 response (6). This T-cell response involves IL-12 and IL-18 released by mDCs. Cognate interaction between NK cells and DCs may also lead to the selective killing of DCs that are not appropriate for antigen presentation to T cells (7). This editing process, which involves TRAIL, allows NK cells to control the quality of DCs, thus regulating adaptive immunity

Figure 3

Confocal microscopy analysis of NK–DC cross talk and HMGB1 expression. CD56⁺ NK cells sorted from PBMC of a healthy donor were cocultured for 24 h with iDCs generated from autologous monocytes (cultured for 5 days with IL-4 and GM-CSF). NK–DC cognate interaction was analyzed by confocal microscopy. (a) Stable NK–DC interaction. NK cells and DCs were stained with red and green Cell Tracker, respectively. (b) HMGB1 expression is detected with specific antibodies both in DCs, co-stained with DC-SIGN antibodies, and NK cells during NK–DC interaction. (c) Intracytoplasmatic HMGB1 expression in a DC costained with CD40- and HMGB1-specific antibodies. A brightfield picture of this DC is shown. (d) Sequential events from a video showing the killing of a DC following its contact with aNK cells (NK–DC ratio 5 : 1). NK cells and DCs were stained with red and green Cell Trackers, respectively. During the coculture, one NK cell interacted several times with the DC (pointed out with a star), leading to the killing of the DC. The DC died by apoptosis, as shown by the blebs (indicated with the yellow arrows). This editing process occurred very rapidly, within less than 1 min following the kiss of death by NK cells. (e) Mitochondria rearrangement at NK–DC synapse, detected with a green MitoTracker

Figure 4

Impact of HIV-1 infection on HMGB1-dependent DC maturation and Th1 polarization. Cognate interaction between resting NK cells and DCs is required for DC maturation (1). This bidirectional cross talk that involves NK receptors NKp30 and CD94/NKG2A, leads both to NK cell activation (2) and DC maturation (3), through the release of cytokines by both cells. In addition, HMGB1 produced both by NK cells and DCs, has a pivotal in NK-dependent DC maturation. During HIV-1 infection, NK–DC cross talk is impaired. NK cell activation is altered and HMGB1-dependent DC maturation is incomplete. Cognate interaction of infected iDCs with NK cells leads to their phenotypic maturation, as evidenced by the expression of maturation markers, including CD80, CD83 or CD86 (3), but not to their functional maturation shown by the inability of DCs to polarize a Th1 response in naïve T cells (4). This functional impairment is associated with the absence of release by DCs of IL-12 and IL-18 required for Th1 induction. Moreover, cognate NK–DC interaction leads to the increase of HIV-1 replication in infected DCs and to the accumulation of DCs with proviral DNA (5). The triggering of HIV-1 replication in DCs is mediated by HMGB1. Thus, HMGB1 contributes to HIV-1 dissemination and persistence

Figure 5

Impact of HIV-1 infection on NK cell-mediated editing of iDCs. NK cells are involved in the positive selection of DCs through the editing process, which is required to keep in check the quality of DCs prone to mature and further present the antigens to T cells. Thus, cognate interaction between aNK and iDC may lead to the killing of iDC (1). NK cell-dependent killing of iDCs involves the activating NKp30 receptor (1) and the DR4/TRAIL death receptor pathway (2). HIV-1 infection induces a defective editing process. NK cells show a decreased ability to kill infected DCs associated with impaired NKp30 expression and TRAIL secretion (3), and infected DCs become resistant to NK-mediated killing due to the upregulation of two anti-apoptotic molecules, c-FLIP and c-IAP2 (4). HMGB1 induces these two potent inhibitors of apoptosis in infected DCs, thus making them resistant to NK killing

Figure 6

Proposed contribution of HMGB1 to HIV persistence and dissemination. HIV disease progression is characterized by gut inflammation and consequently microbial translocation leading to the release of LPS in the bloodstream (1). Circulating LPS may induce active release of HMGB1 by innate cells, including macrophages and DCs (2). Necrotic and apoptotic cells that accumulate during chronic HIV infection may also be a constant source of HMGB1 (3). HMGB1 activates DCs by signaling through the receptor RAGE or TLR4 if cooperate with TLR4 ligand LPS, thus triggering NF-κB activation (4). This results in the release of HMGB1 (5) and proinflammatory cytokines (6), and the triggering of HIV replication in mDCs (7). Trans-infection of HIV from mDCs to CD4 T cells involves HIV capture by DC-SIGN followed by its recruitment at the site of T-cell interaction (8). This infectious synapse will lead to the productive infection of CD4 T cells (9). Trans-infection of HIV can also be mediated by exocytosis of the HIV-1 particles captured by DCs. After endocytosis, the captured HIV-1 particles are targeted to a multi-vesicular endosomal body (MVB) in DCs (10). Although some of the MVB-localized virus fraction is targeted to the lysosome and degraded to be further presented to TCR in the context of MHC molecules, fusion of MVB with the plasma membrane results in the release of virus particles along with exosomes (11). Virus produced by infected CD4 T cells, DCs and macrophages spread the infection to the draining lymph nodes and other lymphoid tissue