It has been recently reported that the endogenous expression of HIV-1 Nef in human monocyte/macrophages induces the release of chemokines and other as yet unidentified soluble factors leading to multiple effects of pathogenic significance, such as the recruitment and activation of quiescent lymphocytes. However, the description of underlying molecular mechanisms remained elusive. We recently demonstrated that human monocyte-derived macrophages (MDM) efficiently internalize soluble rNef, thereby inducing effects largely resembling those observed in cells endogenously expressing Nef. By exploiting the rNef/MDM model, we sought to gain more insights on the molecular mechanisms underlying the response of MDM to Nef. Array analysis for the detection of transcripts from a large number of monokines, chemokines, cytokines, and receptors thereof showed that MDM promptly responded to rNef treatment by increasing the transcription of genes for several inflammatory factors. Analysis of supernatants revealed that rNef treatment induced the release of macrophage inflammatory proteins 1α and 1β, IL-1β, IL-6, and TNF-α. Conversely, rNefs mutated in domains critical for the interaction with the endocytotic machinery (i.e., EE155-156QQ, and DD174-175AA) were ineffective. Interestingly, we found that the Nef-dependent release of inflammatory factors correlated with the activation of the NF-κB transcription factor, mainly in its p50/p50 homodimeric form, and in a de novo protein synthesis-independent manner. Our data add new hints supporting the idea that the presence of Nef is per se heavily detrimental for monocyte/macrophages and relative cross-talking cell types.

IntroductionThe pathology of acquired immunodeficiency syndrome (AIDS) should be considered as the sum of effects generated by human immunodeficiency virus (HIV) replication, with direct T-cell destruction, and by a deep alteration in the pattern of soluble factors. The latter largely depends on the extent of viral replication, because it is hindered by effective antiretroviral therapy. HIV/ simian immunodeficiency virus (SIV) Nef, a multifunctional 27-to 34-kd protein expressed early in viral replication, was demonstrated to be a strong candidate for many of the pathogenic effects of HIV/SIV. This was first highlighted by the evidence that monkeys failed to develop the disease on infection with nef-deleted SIV, 1 and later confirmed by the observation that nef transgenic mice developed a syndrome strictly related to AIDS. 2 Furthermore, Nef perturbs the pattern of secreted factors in different cell types. As an example, the engagement of Nef with the chain of T-cell receptor leads to release of Fas-ligand from T cells in an antigenindependent manner. 3 In addition, the expression of Nef in monocytes/macrophages leads to a massive release of macrophage inflammatory protein 1␣/␤ (MIP-1␣/␤) chemokines and of still unknown factor-activating lymphocytes. 4 These soluble factors possibly promote recruitment and activation of T lymphocytes, which become susceptible to HIV replication. Nef is also able to alter some cellular functions, that is, CD4 internalization/ recycling, 5-7 major histocompatibility complex class I surface expression, 8 and src tyrosine kinases signaling (reviewed in Herna Remkema 9 ) through intracellular mechanisms.We were interested in investigating whether the expression of Nef influences the activation of signal tranducers and activators of transcription (STAT) molecules (reviewed in Schindler and Darnell, 10 Leaman et al, 11 Darnell,12 Stark et al, 13 and Bromberg and Darnell 14 ). STAT-governed pathways were first described by Darnell and colleagues by studying the interferon (IFN)-induced intracellular signal transduction. 15 Seven different STATs have been characterized so far. Activation of STATs is involved in the response of a wide number of cytokines, growth factors, and hormones. Typically, binding of cytokines with specific receptors lacking intrinsic kinase activity in its cytoplasmic tail induces receptor aggregation and recruitment of members of Janus kinases. These become activated by phosphorylating themselves and tyrosine residues of the receptor cytoplasmic tails. The receptor phosphotyrosines serve as docking sites for the binding of inactive STAT through the Src-homology 2 (SH2) domains. STAT monomers become phosphorylated at a constant tyrosine residue and dimerize. The activated dimers translocate to the nucleus thereby binding to specific DNA response elements, ultimately influencing gene expression programs. Hence, alterations in the STAT pathways could substantially influence cellular homeostasis. Few reports regarding the effects of HIV-1 infection on STAT activation/...

Exosomes are 50-150 nm sized nanovesicles released by all eukaryotic cells. The authors very recently described a method to engineer exosomes in vivo with the E7 protein of Human Papilloma Virus (HPV). This technique consists in the intramuscular injection of a DNA vector expressing HPV-E7 fused at the C-terminus of an exosome-anchoring protein, that is, Nef , the authors previously characterized for its high levels of incorporation in exosomes. In this configuration, the ≈11 kDa E7 protein elicited a both strong and effective antigen-specific cytotoxic T lymphocyte (CTL) immunity. Attempting to establish whether this method could have general applicability, the authors expanded the immunogenicity studies toward an array of viral products of various origin and size including Ebola Virus VP24, VP40 and NP, Influenza Virus NP, Crimean-Congo Hemorrhagic Fever NP, West Nile Virus NS3, and Hepatitis C Virus NS3. All antigens appeared stable upon fusion with Nef , and are uploaded in exosomes at levels comparable to Nef . When injected in mice, DNA vectors expressing the diverse fusion products elicited a well detectable antigen-specific CD8 T cell response associating with a cytotoxic activity potent enough to kill peptide-loaded and/or antigen-expressing syngeneic cells. These data definitely proven both effectiveness and flexibility of this innovative CTL vaccine platform.

We recently proved that exosomes engineered in vitro to deliver high amounts of HPV E7 upon fusion with the Nef mut exosome-anchoring protein elicit an efficient anti-E7 cytotoxic T lymphocyte immune response. However, in view of a potential clinic application of this finding, our exosome-based immunization strategy was faced with possible technical difficulties including industrial manufacturing, cost of production, and storage. To overcome these hurdles, we designed an as yet unproven exosome-based immunization strategy relying on delivery by intramuscular inoculation of a DNA vector expressing Nef mut fused with HPV E7. In this way, we predicted that the expression of the Nef mut /E7 vector in muscle cells would result in a continuous source of endogenous (ie, produced by the inoculated host) engineered exosomes able to induce an E7-specific immune response. To assess this hypothesis, we first demonstrated that the injection of a Nef mut /green fluorescent protein-expressing vector led to the release of fluorescent exosomes, as detected in plasma of inoculated mice. Then, we observed that mice inoculated intramuscularly with a vector expressing Nef mut /E7 developed a CD8 + T-cell immune response against both Nef and E7. Conversely, no CD8 + T-cell responses were detected upon injection of vectors expressing either the wild-type Nef isoform of E7 alone, most likely a consequence of their inefficient exosome incorporation. The production of immunogenic exosomes in the DNA-injected mice was formally demonstrated by the E7-specific CD8 + T-cell immune response we detected in mice inoculated with exosomes isolated from plasma of mice inoculated with the Nef mut /E7 vector. Finally, we provide evidence that the injection of Nef mut /E7 DNA led to the generation of effective antigen-specific cytotoxic T lymphocytes whose activity was likely part of the potent, therapeutic antitumor effect we observed in mice implanted with TC-1 tumor cells. In summary, we established a novel method to generate immunogenic exosomes in vivo by the intramuscular inoculation of DNA vectors expressing the exosome-anchoring protein Nef mut and its derivatives.

BackgroundCompletion of HIV life cycle in CD4+ T lymphocytes needs cell activation. We recently reported that treatment of resting CD4+ T lymphocytes with exosomes produced by HIV-1 infected cells induces cell activation and susceptibility to HIV replication. Here, we present data regarding the effects of these exosomes on cells latently infected with HIV-1.ResultsHIV-1 latently infecting U937-derived U1 cells was activated upon challenge with exosomes purified from the supernatant of U937 cells chronically infected with HIV-1. This effect was no more detectable when exosomes from cells infected with HIV-1 strains either nef-deleted or expressing a functionally defective Nef were used, indicating that Nef is the viral determinant of exosome-induced HIV-1 activation. Treatment with either TAPI-2, i.e., a specific inhibitor of the pro-TNFα-processing ADAM17 enzyme, or anti-TNFα Abs abolished HIV-1 activation. Hence, similar to what previously demonstrated for the exosome-mediated activation of uninfected CD4+ T lymphocytes, the Nef-ADAM17-TNFα axis is part of the mechanism of latent HIV-1 activation. It is noteworthy that these observations have been reproduced using: (1) primary CD4+ T lymphocytes latently infected with HIV-1; (2) exosomes from both primary CD4+ T lymphocytes and macrophages acutely infected with HIV-1; (3) co-cultures of HIV-1 acutely infected CD4+ T lymphocytes and autologous lymphocytes latently infected with HIV-1, and (4) exosomes from cells expressing a defective HIV-1.ConclusionsOur results strongly suggest that latent HIV-1 can be activated by TNFα released by cells upon ingestion of exosomes released by infected cells, and that this effect depends on the activity of exosome-associated ADAM17. These pieces of evidence shed new light on the mechanism of HIV reactivation in latent reservoirs, and might also be relevant to design new therapeutic interventions focused on HIV eradication.