Phosphoinositide 3-kinase δ inhibition promotes antitumor responses but antagonizes checkpoint inhibitors.

Multiple modes of immunosuppression restrain immune function within tumors. We previously reported that phosphoinositide 3-kinase δ (PI3Kδ) inactivation in mice confers resistance to a range of tumor models by disrupting immunosuppression mediated by regulatory T cells (Tregs). The PI3Kδ inhibitor idelalisib has proven highly effective in the clinical treatment of chronic lymphocytic leukemia and the potential to extend the use of PI3Kδ inhibitors to nonhematological cancers is being evaluated. In this work, we demonstrate that the antitumor effect of PI3Kδ inactivation is primarily mediated through the disruption of Treg function, and correlates with tumor dependence on Treg immunosuppression. Compared with Treg-specific PI3Kδ deletion, systemic PI3Kδ inactivation is less effective at conferring resistance to tumors. We show that PI3Kδ deficiency impairs the maturation and reduces the capacity of CD8+ cytotoxic T lymphocytes (CTLs) to kill tumor cells in vitro, and to respond to tumor antigen-specific immunization in vivo. PI3Kδ inactivation antagonized the antitumor effects of tumor vaccines and checkpoint blockade therapies intended to boost the CD8+ T cell response. These findings provide insights into mechanisms by which PI3Kδ inhibition promotes antitumor immunity and demonstrate that the mechanism is distinct from that mediated by immune checkpoint blockade.

Macrophage responses to lipopolysaccharide are modulated by a feedback loop involving prostaglandin E2, dual specificity phosphatase 1 and tristetraprolin.

In many different cell types, pro-inflammatory agonists induce the expression of cyclooxygenase 2 (COX-2), an enzyme that catalyzes rate-limiting steps in the conversion of arachidonic acid to a variety of lipid signaling molecules, including prostaglandin E2 (PGE2). PGE2 has key roles in many early inflammatory events, such as the changes of vascular function that promote or facilitate leukocyte recruitment to sites of inflammation. Depending on context, it also exerts many important anti-inflammatory effects, for example increasing the expression of the anti-inflammatory cytokine interleukin 10 (IL-10), and decreasing that of the pro-inflammatory cytokine tumor necrosis factor (TNF). The tight control of both biosynthesis of, and cellular responses to, PGE2 are critical for the precise orchestration of the initiation and resolution of inflammatory responses. Here we describe evidence of a negative feedback loop, in which PGE2 augments the expression of dual specificity phosphatase 1, impairs the activity of mitogen-activated protein kinase p38, increases the activity of the mRNA-destabilizing factor tristetraprolin, and thereby inhibits the expression of COX-2. The same feedback mechanism contributes to PGE2-mediated suppression of TNF release. Engagement of the DUSP1-TTP regulatory axis by PGE2 is likely to contribute to the switch between initiation and resolution phases of inflammation.

Gain-of-Function Mutation of Tristetraprolin Impairs Negative Feedback Control of Macrophages In Vitro yet Has Overwhelmingly Anti-Inflammatory Consequences In Vivo.

The mRNA-destabilizing factor tristetraprolin (TTP) binds in a sequence-specific manner to the 3' untranslated regions of many proinflammatory mRNAs and recruits complexes of nucleases to promote rapid mRNA turnover. Mice lacking TTP develop a severe, spontaneous inflammatory syndrome characterized by the overexpression of tumor necrosis factor and other inflammatory mediators. However, TTP also employs the same mechanism to inhibit the expression of the potent anti-inflammatory cytokine interleukin 10 (IL-10). Perturbation of TTP function may therefore have mixed effects on inflammatory responses, either increasing or decreasing the expression of proinflammatory factors via direct or indirect mechanisms. We recently described a knock-in mouse strain in which the substitution of 2 amino acids of the endogenous TTP protein renders it constitutively active as an mRNA-destabilizing factor. Here we investigate the impact on the IL-10-mediated anti-inflammatory response. It is shown that the gain-of-function mutation of TTP impairs IL-10-mediated negative feedback control of macrophage function in vitro However, the in vivo effects of TTP mutation are uniformly anti-inflammatory despite the decreased expression of IL-10.

Beta Interferon Production Is Regulated by p38 Mitogen-Activated Protein Kinase in Macrophages via both MSK1/2- and Tristetraprolin-Dependent Pathways.

Autocrine or paracrine signaling by beta interferon (IFN-ß) is essential for many of the responses of macrophages to pathogen-associated molecular patterns. This feedback loop contributes to pathological responses to infectious agents and is therefore tightly regulated. We demonstrate here that macrophage expression of IFN-ß is negatively regulated by mitogen- and stress-activated kinases 1 and 2 (MSK1/2). Lipopolysaccharide (LPS)-induced expression of IFN-ß was elevated in both MSK1/2 knockout mice and macrophages. Although MSK1 and -2 promote the expression of the anti-inflammatory cytokine interleukin 10, it did not strongly contribute to the ability of MSKs to regulate IFN-ß expression. Instead, MSK1 and -2 inhibit IFN-ß expression via the induction of dual-specificity phosphatase 1 (DUSP1), which dephosphorylates and inactivates the mitogen-activated protein kinases p38 and Jun N-terminal protein kinase (JNK). Prolonged LPS-induced activation of p38 and JNK, phosphorylation of downstream transcription factors, and overexpression of IFN-ß mRNA and protein were similar in MSK1/2 and DUSP1 knockout macrophages. Two distinct mechanisms were implicated in the overexpression of IFN-ß: first, JNK-mediated activation of c-jun, which binds to the IFN-ß promoter, and second, p38-mediated inactivation of the mRNA-destabilizing factor tristetraprolin, which we show is able to target the IFN-ß mRNA.

Dual-Specificity Phosphatase 1 and Tristetraprolin Cooperate To Regulate Macrophage Responses to Lipopolysaccharide.

Dual-specificity phosphatase (DUSP) 1 dephosphorylates and inactivates members of the MAPK superfamily, in particular, JNKs, p38α, and p38ß MAPKs. It functions as an essential negative regulator of innate immune responses, hence disruption of the Dusp1 gene renders mice extremely sensitive to a wide variety of experimental inflammatory challenges. The principal mechanisms behind the overexpression of inflammatory mediators by Dusp1(-/-) cells are not known. In this study, we use a genetic approach to identify an important mechanism of action of DUSP1, involving the modulation of the activity of the mRNA-destabilizing protein tristetraprolin. This mechanism is key to the control of essential early mediators of inflammation, TNF, CXCL1, and CXCL2, as well as the anti-inflammatory cytokine IL-10. The same mechanism also contributes to the regulation of a large number of transcripts induced by treatment of macrophages with LPS. These findings demonstrate that modulation of the phosphorylation status of tristetraprolin is an important physiological mechanism by which innate immune responses can be controlled.

Dominant Suppression of Inflammation via Targeted Mutation of the mRNA Destabilizing Protein Tristetraprolin.

In myeloid cells, the mRNA-destabilizing protein tristetraprolin (TTP) is induced and extensively phosphorylated in response to LPS. To investigate the role of two specific phosphorylations, at serines 52 and 178, we created a mouse strain in which those residues were replaced by nonphosphorylatable alanine residues. The mutant form of TTP was constitutively degraded by the proteasome and therefore expressed at low levels, yet it functioned as a potent mRNA destabilizing factor and inhibitor of the expression of many inflammatory mediators. Mice expressing only the mutant form of TTP were healthy and fertile, and their systemic inflammatory responses to LPS were strongly attenuated. Adaptive immune responses and protection against infection by Salmonella typhimurium were spared. A single allele encoding the mutant form of TTP was sufficient for enhanced mRNA degradation and underexpression of inflammatory mediators. Therefore, the equilibrium between unphosphorylated and phosphorylated TTP is a critical determinant of the inflammatory response, and manipulation of this equilibrium may be a means of treating inflammatory pathologies.

Interferon-gamma induces Fas trafficking and sensitization to apoptosis in vascular smooth muscle cells via a PI3K- and Akt-dependent mechanism.

Vascular smooth muscle cell (VSMC) apoptosis occurs in advanced atherosclerotic plaques where it may contribute to plaque instability. VSMCs express the death receptor Fas but are relatively resistant to Fas-induced apoptosis due in part to the intracellular sequestration of Fas. Although inflammatory cytokines such as interferon (IFN)-gamma present in plaques can prime VSMCs to FasL-induced death, the mechanism of this effect is unclear. We examined Fas expression and FasL-induced apoptosis in human VSMCs in response to IFN-gamma. IFN-gamma induced Fas trafficking to the cell surface within 24 hours, an effect that required Jak2/Stat1 activity. IFN-gamma also stimulated Akt activity, and both Fas trafficking and Stat1 activation were inhibited by blocking PI3K, Akt, or Jak-2. IFN-gamma increased Fas-induced apoptosis in vitro by 46 +/- 8% (mean +/- SEM, P = 0.04), an event that could be abrogated by inhibition of PI3K, Akt, or Jak-2. IFN-gamma also increased Fas-induced apoptosis in vivo 7.5- to 15-fold (P < 0.05) in human arteries transplanted into immunodeficient mice, accompanied by increased Fas and phospho-Ser727-Stat1. We conclude that IFN-gamma primes VSMCs to Fas-induced apoptosis, in part by relocation of Fas to the cell surface, a process that involves PI3K, Akt, and Jak-2/Stat1. IFN-gamma present in plaques may co-operate with FasL to induce VSMC apoptosis in atherosclerosis.

Loss of function of a lupus-associated FcgammaRIIb polymorphism through exclusion from lipid rafts.

Dysfunction of receptors for IgG (FcgammaRs) has been thought to be involved in the pathogenesis of systemic lupus erythematosus (SLE). We show that a recently described SLE-associated polymorphism of FcgammaRIIb (FcgammaRIIbT(232)), encoding a single transmembrane amino acid substitution, is functionally impaired. FcgammaRIIbT(232) is unable to inhibit activatory receptors because it is excluded from sphingolipid rafts, resulting in the unopposed proinflammatory signaling thought to promote SLE.

Rapamycin inhibits human in stent restenosis vascular smooth muscle cells independently of pRB phosphorylation and p53.

OBJECTIVE: Drug-eluting stents containing the immunosuppressant rapamycin markedly inhibit in stent restenosis (ISR). However, the molecular mechanisms that underlie its effect on ISR-derived vascular smooth muscle cells (VSMCs), as opposed to normal VSMCs, are unknown. Specifically, as ISR-VSMCs have altered cell cycle regulation, rapamycin may arrest these cells via novel molecular pathways. METHODS: We isolated human VSMCs from sites of ISR, and examined the effect of rapamycin on cell proliferation using MTT assay, time lapse videomicroscopy and flow cytometry. Regulation of G(1)-S transition was examined using Western blotting, and cell size and protein synthesis examined using flow cytometry and collagen assay, respectively. The requirement for pRB and p53 was examined using ISR VSMCs expressing E1A and a dominant negative p53, respectively. RESULTS: ISR-VSMC proliferation was potently inhibited by rapamycin. Arrest was confined to G(1), as cell proliferation (but not cell size) of S/G(2)-arrested cells was unaffected by rapamycin. Moreover, ISR-VSMC lines generated with disrupted p53 or pRB function still arrested in the presence of rapamycin, suggesting that these genes are dispensable for rapamycin-induced arrest. Significantly, rapamycin completely inhibited the phosphorylation of p70(S6K), an mTOR-regulated kinase implicated in the control of proliferation, but had no effect on collagen or total protein synthesis. CONCLUSIONS: We demonstrate that rapamycin is a potent inhibitor of ISR VSMC proliferation during G(1). Rapamycin's action does not require p53 or pRB. We show that p70(S6K) is markedly inhibited in rapamycin-arrested ISR cells, suggesting that regulation of its upstream kinase, mTOR, is important for the control of proliferation in ISR cells.