A subset of HLA-DP molecules serve as ligands for the natural cytotoxicity receptor NKp44. Nature Immunology 20 (9) , pp.

O ne major obstacle toward an effective human immunodeficiency virus type 1 (HIV-1) cure is the establishment of a pool of long-lived latently infected cells early after infection (1). Using the rhesus macaque model of simian immunodeficiency virus (SIV) infection, it was recently shown that latent reservoirs could be seeded as early as 3 days after SIV exposure and before the detection of viremia in blood (2). The vast majority of cells infected with HIV-1 will die as a consequence of the infection or will be eliminated by the immune system. A minority of infected cells, however, turns into latently infected resting cells. These cells can either be reactivated and release de novo HIV-1 particles (3) or persist and homeostatically proliferate as long-lived memory T cells (4, 5). While current antiretroviral treatments (ARTs) can efficiently suppress HIV-1 replication and have dramatically improved the life expectancy and life quality of infected individuals, ART cannot eradicate the latent viral reservoir.Several different biological processes have been described to maintain latency in HIV-1-infected cells. Host transcription factors (TFs) such as nuclear factor kappa light-chain enhancer of activated B cells (NF-B) have multiple binding sites in the 5= long terminal repeat (LTR) of the HIV-1 genome, and their binding has been demonstrated to be necessary to initiate HIV-1 transcription (6). Sequestration of these TFs in the cytoplasm is one of the mechanisms enabling viral latency (7). Another described HIV-1 latency mechanism involves histone deacetylase (HDAC)-mediated epigenetic silencing (8). During latency establishment, HDAC molecules are recruited toward the 5= LTR of HIV-1 (9, 10) and therefore maintain the LTR in a repressed state (11). Several HDAC inhibitors (HDACis) targeting HDAC molecules have been tested for their ability to reactivate latently HIV-1-infected cells, including vorinostat, panobinostat, entinostat, and romidepsin (RMD). These HDACis proved to efficiently induce HIV-1 expression in latently infected resting CD4 ϩ T cells from HIV-1-infected individuals (8,12,13). RMD, a drug that has been used for the treatment of peripheral T-cell lymphoma,

Human immunodeficiency virus type 1 (HIV-1) has evolved elaborate ways to evade immune cell recognition, including downregulation of classical HLA class I (HLA-I) from the surfaces of infected cells. Recent evidence identified HLA-E, a nonclassical HLA-I, as an important part of the antiviral immune response to HIV-1. Changes in HLA-E surface levels and peptide presentation can prompt both CD8+T-cell and natural killer (NK) cell responses to viral infections. Previous studies reported unchanged or increased HLA-E levels on HIV-1-infected cells. Here, we examined HLA-E surface levels following infection of CD4+T cells with primary HIV-1 strains and observed that a subset downregulated HLA-E. Two primary strains of HIV-1 that induced the strongest reduction in surface HLA-E expression were chosen for further testing. Expression of single Nef or Vpu proteins in a T-cell line, as well as tail swap experiments exchanging the cytoplasmic tail of HLA-A2 with that of HLA-E, demonstrated that Nef modulated HLA-E surface levels and targeted the cytoplasmic tail of HLA-E. Furthermore, infection of primary CD4+T cells with HIV-1 mutants showed that a lack of functional Nef (and Vpu to some extent) impaired HLA-E downmodulation. Taken together, the results of this study demonstrate for the first time that HIV-1 can downregulate HLA-E surface levels on infected primary CD4+T cells, potentially rendering them less vulnerable to CD8+T-cell recognition but at increased risk of NKG2A+NK cell killing.IMPORTANCEFor almost two decades, it was thought that HIV-1 selectively downregulated the highly expressed HLA-I molecules HLA-A and HLA-B from the cell surface in order to evade cytotoxic-T-cell recognition, while leaving HLA-C and HLA-E molecules unaltered. It was stipulated that HIV-1 infection thereby maintained inhibition of NK cells via inhibitory receptors that bind HLA-C and HLA-E. This concept was recently revised when a study showed that primary HIV-1 strains reduce HLA-C surface levels, whereas the cell line-adapted HIV-1 strain NL4-3 lacks this ability. Here, we demonstrate that infection with distinct primary HIV-1 strains results in significant downregulation of surface HLA-E levels. Given the increasing evidence for HLA-E as an important modulator of CD8+T-cell and NKG2A+NK cell functions, this finding has substantial implications for future immunomodulatory approaches aimed at harnessing cytotoxic cellular immunity against HIV.

Natural Killer (NK) cells were initially described as part of the innate immune system and characterized by their ability to lyse malignant and virus-infected cells. The cytolytic function of NK cells is tightly controlled by activating and inhibitory receptors expressed on the cell surface. Ligands that interact with a variety of NK-cell receptors include the human leukocyte antigen (HLA) molecules, and the regulation of NK-cell function by HLA class I molecules is well-established. Earlier studies also suggested a role of HLA class II molecules in regulating NK cell activity; yet, interactions between HLA class II molecules and NK cell receptors have not been well-characterized. We recently identified a subset of HLA-DP molecules that can serve as ligands for the natural cytotoxicity receptor NKp44 and activate NK cells. This novel receptor-ligand interaction provides a potential mechanism to explain the strong associations of HLA-DP molecules with HBV infection outcomes, graft-vs.-host disease and inflammatory bowel disease. Furthermore, it adds a new mechanism for NK-cell crosstalk with immune cells expressing HLA class II molecules. In this perspective article, we discuss the potential implications of NK cell receptor interactions with HLA class II molecules for the regulation of immune responses.