Prime, Shock, and Kill: Priming CD4 T Cells from HIV Patients with a BCL-2 Antagonist before HIV Reactivation Reduces HIV Reservoir Size

Abstract

Understanding how some HIV-infected cells resist the cytotoxicity of HIV replication is crucial to enabling HIV cure efforts. HIV killing of CD4 T cells that replicate HIV can involve HIV protease-mediated cleavage of procaspase 8 to generate a fragment (Casp8p41) that directly binds and activates the mitochondrial proapoptotic protein BAK. Here, we demonstrate that Casp8p41 also binds with nanomolar affinity to the antiapoptotic protein Bcl-2, which sequesters Casp8p41 and prevents apoptosis. Intense activity is focused on identifying a clinical intervention that results in a long term, drug free remission of HIV-1 infection. Latently infected CD4 T cells harbor transcriptionally silent, replication-competent HIV. Because these cells persist long term and are unaffected by current therapies, the cells represent an HIV reservoir that remains undiminished by current approaches. Pilot clinical trials have tested whether reactivation of HIV-1 from latency will decrease the number of cells containing HIV DNA due to viral cytopathic effect or immune-mediated clearance. The latency reversal agents vorinostat, panobinostat, and romidepsin result in HIV reactivation, as measured by increases in cell-associated HIV RNA, but no change in cell-associated HIV DNA, indicating that the reactivating cells do not die (1-4). Multiple ongoing studies are testing augmentation of the anti-HIV immune response in combination with viral reactivation as a strategy for HIV eradication.In an effort to inform these attempts to eradicate the HIV reservoir, we and others have been examining the mechanistic basis for HIV-induced killing of CD4 T cells under different circumstances of activation and HIV replication. Although numerous pathways may contribute to the decline of uninfected CD4 T cells during uncontrolled HIV infection (5), fewer pathways have been implicated in the demise of cells directly infected by HIV. After HIV attachment, at least three distinct pathways can initiate the death of infected cells: (i) RIG-I-mediated sensing of HIV RNA (6, 7), (ii) IFI-16 sensing of unintegrated HIV DNA (8, 9), and (iii) DNA-PK-sensing of HIV integrase nicking of host DNA (10). Once integrated into host DNA, HIV can remain in a latent state

Getting the “Kill” into “Shock and Kill”: Strategies to Eliminate Latent HIV

“…Sensitizing the cell towards an apoptotic state followed by the production of pro-apoptotic HIV proteins such as protease and HIV RNA could tip the balance towards apoptosis and specific killing of the cells that only produce these HIV proteins. This has been demonstrated with the pro-apoptotic drug Venetoclax (Cummins et al, 2016b) that when combined with LRAs led to the selective apoptosis and clearance of HIV infected cells. Additionally, the acitretin RIG-inducer that acts as an LRA as well as RIG-I inducer to drive apoptosis also leads to selective death of HIV-infected cells (Li et al, 2016).…”

“…In contrast, in the late stages of the viral life cycle, the HIV envelope (Env) and Vpu proteins can promote apoptosis of infected cells by a variety of mechanisms (Timilsina & Gaur, 2016). Expression of the viral protease later in viral replication can also lead to the generation of a pro-apoptotic Casp8p41 peptide that promotes apoptosis (Cummins et al, 2016b). HIV Tat, Vpr and Nef also transition to pro-apoptotic roles later in the viral lifecycle when Tat and Vpr expression increases (Timilsina & Gaur, 2016) (Figure 3).…”

“…Many of the Bcl-2 antagonists currently being investigated are BH3-mimetics (Lessene et al, 2008). Compounds such as Venetoclax and Navitoclax mimic the BH3 binding domain present in pro-apoptotic molecules and bind with higher affinity to anti-apoptotic Bcl-2 family proteins (Balakrishnan & Gandhi, 2013; Cummins et al, 2016b), blocking their anti-apoptotic function thereby favouring apoptosis. Venetoclax has recently been licensed for the treatment of chronic lymphocytic leukaemia (CLL) and is in advanced clinical development for non-Hodgkin’s lymphoma (NHL) (Souers et al, 2013).…”

“…During active HIV replication, the HIV protease results in cleavage of pro-caspase-8 to generate the Casp8p41 fragment containing a BH3-like domain that subsequently binds to and activates Bak, triggering apoptosis (Sainski et al, 2014). However, during reactivation of HIV latency in resting CD4+ T-cells, HIV protease-dependent apoptosis is prevented as the cells contain high levels of anti-apoptotic Bcl-2, which can sequester the pro-apoptotic Casp8p41 to prevent apoptosis (Cummins et al, 2016b). Therefore, inhibiting Bcl-2 may prevent the Bcl-2-mediated sequestration of pro-apoptotic Casp8p41, liberating Casp8p41 generated by HIV protease to induce apoptosis of these activated latently infected cells.…”

“…Encouragingly, pre-treatment of latently infected cells from individuals on ART with the Bcl-2 antagonist Venetoclax and subsequent reactivation with anti-CD3 plus anti-CD28 T-cell stimulation reduced the frequency of latently-infected T-cells in cultures from 8 of 11 individuals (Cummins et al, 2016b). Whether combinations of Bcl-2 inhibitors like Venetoclax with LRAs that do not induce maximal T-cell stimulation also lead to the death of latently infected cells remains unclear.…”