HIV-1 Nef-Induced Secretion of the Proinflammatory Protease TACE into Extracellular Vesicles Is Mediated by Raf-1 and Can Be Suppressed by Clinical Protein Kinase Inhibitors

Abstract

Chronic immune activation contributes to the immunopathogenesis of human immunodeficiency virus type 1 (HIV-1) infection and is associated with poor recovery of the immune system despite potent antiretroviral therapy, which is observed in 10% to 40% drug-treated patients depending on the definition of immune reconstitution. We have previously shown that the HIV pathogenicity factor Nef can promote loading of the proinflammatory protease TACE into extracellular vesicles (EVs), and the levels of such TACE-containing EVs circulating in the blood correlate with low CD4 lymphocyte counts in HIV patients receiving antiretroviral therapy. ABSTRACT Chronic immune activation is an important driver of human immunodeficiency virus type 1 (HIV-1) pathogenesis and has been associated with the presence of tumor necrosis factor-α converting enzyme (TACE) in extracellular vesicles (EVs) circulating in infected individuals. We have recently shown that activation of the Src-family tyrosine kinase hematopoietic cell kinase (Hck) by HIV-1 Nef can trigger the packaging of TACE into EVs via an unconventional protein secretion pathway. Using a panel of HIV-1 Nef mutants and natural HIV-2 and simian immunodeficiency virus (SIV) Nef alleles, we now show that the capacity to promote TACE secretion depends on the superior ability of HIV-1-like Nef alleles to induce Hck kinase activity, whereas other Nef effector functions are dispensable. Strikingly, among the numerous Src-family downstream effectors, serine/threonine kinase Raf-1 was found to be necessary and alone sufficient to trigger the secretion of TACE into EVs. These data reveal the involvement of Raf-1 in regulation of unconventional protein secretion and highlight the importance of Raf-1 as a cellular effector of Nef, thereby suggesting a novel rationale for testing pharmacological inhibitors of the Raf-MAPK pathway to treat HIV-associated immune activation. IMPORTANCE Chronic immune activation contributes to the immunopathogenesis of human immunodeficiency virus type 1 (HIV-1) infection and is associated with poor recovery of the immune system despite potent antiretroviral therapy, which is observed in 10% to 40% drug-treated patients depending on the definition of immune reconstitution. We have previously shown that the HIV pathogenicity factor Nef can promote loading of the proinflammatory protease TACE into extracellular vesicles (EVs), and the levels of such TACE-containing EVs circulating in the blood correlate with low CD4 lymphocyte counts in HIV patients receiving antiretroviral therapy. Here, we show that Nef promotes uploading of TACE into EVs by triggering unconventional secretion via activation of the Hck/Raf/mitogen-activated protein kinase (MAPK) cascade. We find that several pharmaceutical inhibitors of these kinases that are currently in clinical use for other diseases can potently suppress this pathogenic deregulation and could thus provide a novel strategy for treating HIV-associated immune activation.