Integration is essential for HIV-1 replication, and the viral integrase (IN) protein is an important therapeutic target. Allosteric IN inhibitors (ALLINIs) that engage the IN dimer interface at the binding site for the host protein lens epithelium-derived growth factor (LEDGF)/ transcriptional coactivator p75 are an emerging class of small molecule antagonists. Consistent with the inhibition of a multivalent drug target, ALLINIs display steep antiviral dose-response curves ex vivo. ALLINIs multimerize IN protein and concordantly block its assembly with viral DNA in vitro, indicating that the disruption of two integration-associated functions, IN catalysis and the IN-LEDGF/p75 interaction, determines the multimode mechanism of ALLINI action. We now demonstrate that ALLINI potency is unexpectedly accounted for during the late phase of HIV-1 replication. The compounds promote virion IN multimerization and, reminiscent of class II IN mutations, block the formation of the electron-dense viral core and inhibit reverse transcription and integration in subsequently infected target cells. Mature virions are recalcitrant to ALLINI treatment, and compound potency during virus production is independent of the level of LEDGF/p75 expression. We conclude that cooperative multimerization of IN by ALLINIs together with the inability for LEDGF/p75 to effectively engage the virus during its egress from cells underscores the multimodal mechanism of ALLINI action. Our results highlight the versatile nature of allosteric inhibitors to primarily inhibit viral replication at a step that is distinct from the catalytic requirement for the target enzyme. The vulnerability of IN to small molecules during the late phase of HIV-1 replication unveils a pharmacological Achilles' heel for exploitation in clinical ALLINI development. AIDS | antiretroviral therapy

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SUMMARY While an essential role of HIV-1 integrase (IN) for integration of viral cDNA into human chromosome is established, studies with IN mutants and allosteric IN inhibitors (ALLINIs) have suggested that IN can also influence viral particle maturation. However, it has remained enigmatic as to how IN contributes to virion morphogenesis. Here we demonstrate that IN directly binds the viral RNA genome in virions. These interactions have specificity as IN exhibits distinct preference for select viral RNA structural elements. We show that IN substitutions that selectively impair its binding to viral RNA result in eccentric, non-infectious virions without affecting nucleocapsid-RNA interactions. Likewise, ALLINIs impair IN binding to viral RNA in virions of wild type but not escape mutant virus. These results reveal an unexpected biological role of IN binding to the viral RNA genome during virion morphogenesis and elucidate the mode of action of ALLINIs.

Pancreatic stellate cells (PSC) are a subset of pancreatic cancer-associated fibroblasts. These cells provide prosurvival signals to tumors; however, little is known regarding their interactions with immune cells within the tumor microenvironment. We hypothesized that factors produced by human PSC could enhance myeloid-derived suppressor cell (MDSC) differentiation and function, which promotes an immunosuppressive microenvironment. Primary PSC cell lines (n = 7) were generated from human specimens and phenotypically confirmed via expression of vimentin, α-smooth muscle actin (α-SMA), and glial fibrillary acidic protein (GFAP). Luminex analysis indicated that PSC but not human fetal primary pancreatic fibroblast cells (HPF; negative controls) produced MDSC-promoting cytokines [interleukin (IL-6), VEGF, macrophage colony-stimulating factor (M-CSF)] and chemokines (SDF-1, MCP-1). Culture of peripheral blood mononuclear cells [peripheral blood mononuclear cell (PBMC), n = 3 donors] with PSC supernatants or IL-6/granulocyte macrophage colony-stimulating factor (GM-CSF; positive control) for 7 days promoted PBMC differentiation into an MDSC (CD11b+CD33+) phenotype and a subpopulation of polymorphonuclear CD11b+CD33+CD15+ cells. The resulting CD11b+CD33+ cells functionally suppressed autologous T-lymphocyte proliferation. In contrast, supernatants from HPF did not induce an MDSC phenotype in PBMCs. Culture of normal PBMCs with PSC supernatants led to STAT3 but not STAT1 or STAT5 phosphorylation. IL-6 was an important mediator as its neutralization inhibited PSC supernatant-mediated STAT3 phosphorylation and MDSC differentiation. Finally, the FLLL32 STAT3 inhibitor abrogated PSC supernatant-mediated MDSC differentiation, PSC viability, and reduced autocrine IL-6 production indicating these processes are STAT3 dependent. These results identify a novel role for PSC in driving immune escape in pancreatic cancer and extend the evidence that STAT3 acts as a driver of stromal immunosuppression to enhance its interest as a therapeutic target.