Centrosome amplification and aneuploidy driven by the HIV-1-induced Vpr•VprBP•Plk4 complex in CD4+ T cells.

HIV-1 infection elevates the risk of developing various cancers, including T-cell lymphoma. Whether HIV-1-encoded proteins directly contribute to oncogenesis remains unknown. We observe that approximately 1-5% of CD4+ T cells from the blood of people living with HIV-1 exhibit over-duplicated centrioles, suggesting that centrosome amplification underlies the development of HIV-1-associated cancers by driving aneuploidy. Through affinity purification, biochemical, and cellular analyses, we discover that Vpr, an accessory protein of HIV-1, hijacks the centriole duplication machinery and induces centrosome amplification and aneuploidy. Mechanistically, Vpr forms a cooperative ternary complex with an E3 ligase subunit, VprBP, and polo-like kinase 4 (Plk4). Unexpectedly, however, the complex enhances Plk4's functionality by promoting its relocalization to the procentriole assembly and induces centrosome amplification. Loss of either Vpr's C-terminal 17 residues or VprBP acidic region, the two elements required for binding to Plk4 cryptic polo-box, abrogates Vpr's capacity to induce these events. Furthermore, HIV-1 WT, but not its Vpr mutant, induces multiple centrosomes and aneuploidy in human primary CD4+ T cells. We propose that the Vpr•VprBP•Plk4 complex serves as a molecular link that connects HIV-1 infection to oncogenesis and that inhibiting the Vpr C-terminal motif may reduce the occurrence of HIV-1-associated cancers.

Centrosome amplification and aneuploidy driven by the HIV-1-induced Vpr•VprBP•Plk4 complex in CD4+ T cells.

HIV-1 infection elevates the risk of developing various cancers, including T-cell lymphoma. Whether HIV-1-encoded proteins directly contribute to oncogenesis remains unknown. We observed that approximately 1-5% of CD4+ T cells from the blood of people living with HIV-1 exhibit over-duplicated centrioles, suggesting that centrosome amplification underlies the development of HIV-1-associated cancers by driving aneuploidy. Through affinity purification, biochemical, and cell biology analyses, we discovered that Vpr, an accessory protein of HIV-1, hijacks the centriole duplication machinery and induces centrosome amplification and aneuploidy. Mechanistically, Vpr formed a cooperative ternary complex with an E3 ligase subunit, VprBP, and polo-like kinase 4 (Plk4). Unexpectedly, however, the complex enhanced Plk4's functionality by promoting its relocalization to the procentriole assembly and induced centrosome amplification. Loss of either Vpr's C-terminal 17 residues or VprBP acidic region, the two elements required for binding to Plk4 cryptic polo-box, abrogated Vpr's capacity to induce all these events. Furthermore, HIV-1 WT, but not its Vpr mutant, induced multiple centrosomes and aneuploidy in primary CD4+ T cells. We propose that the Vpr•VprBP•Plk4 complex serves as a molecular link that connects HIV-1 infection to oncogenesis and that inhibiting the Vpr C-terminal motif may reduce the occurrence of HIV-1-associated cancers.

Aire drives steroid hormone biosynthesis by medullary thymic epithelial cells.

Thymic epithelial cells (TECs) produce glucocorticoids, which antagonize negative selection of autoreactive thymocytes and promote a competent T cell antigen-specific repertoire. To characterize their source, we generated a knock-in reporter mouse in which endogenous Cyp11b1, the final enzyme in de novo production of active glucocorticoids, was fluorescently tagged with mScarlet. Here, we find that Cyp11b1 is expressed in medullary TECs (mTECs) but not cortical TECs or other cells in the thymus. A distinct characteristic of mTECs is the presence of Aire, a transcription factor that drives expression of tissue-restricted antigens (TRAs) important for establishing immune tolerance. Cyp11b1 expression was highest in Aire+ mTECs, lower in post-Aire mTECs, and absent in mTECs of Aire-deficient mice. Transcriptomic analyses found that multiple enzymatic biosynthetic pathways are expressed specifically in mTECs and are also Aire dependent. In particular, we found that the thymus expresses messenger RNA for enzymes that catalyze production of many bioactive steroids and that glucocorticoids and sex steroids were secreted by cultured thymi. Expression of the transcripts for these genes and production of their final steroid products were markedly reduced in the absence of Aire. Thus, in addition to its well-established role in inducing TRAs that promote negative selection, Aire has an additional and contrary function of inducing glucocorticoids that antagonize negative selection, which together may expand and enhance the TCR repertoire. Furthermore, because Aire drives expression of multiple enzymes responsible for production of other non-gene-encoded bioactive molecules, it might have yet other roles in thymus development and function.

Tumors produce glucocorticoids by metabolite recycling, not synthesis, and activate Tregs to promote growth.

Glucocorticoids are steroid hormones with potent immunosuppressive properties. Their primary source is the adrenals, where they are generated via de novo synthesis from cholesterol. In addition, many tissues have a recycling pathway in which glucocorticoids are regenerated from inactive metabolites by the enzyme 11ß-hydroxysteroid dehydrogenase type 1 (11ß-HSD1, encoded by Hsd11b1). Here, we find that multiple tumor types express Hsd11b1 and produce active glucocorticoids. Genetic ablation of Hsd11b1 in such cells had no effect on in vitro growth, but reduced in vivo tumor progression, which corresponded with increased frequencies of CD8+ tumor-infiltrating lymphocytes (TILs) expressing activation markers and producing effector cytokines. Tumor-derived glucocorticoids were found to promote signatures of Treg activation and suppress signatures of conventional T cell activation in tumor-infiltrating Tregs. Indeed, CD8+ T cell activation was restored and tumor growth reduced in mice with Treg-specific glucocorticoid receptor deficiency. Importantly, pharmacologic inhibition of 11ß-HSD1 reduced tumor growth to the same degree as gene knockout and rendered immunotherapy-resistant tumors susceptible to PD-1 blockade. Given that HSD11B1 expression is upregulated in many human tumors and that inhibition of 11ß-HSD1 is well tolerated in clinical studies, these data suggest that targeting 11ß-HSD1 may be a beneficial adjunct in cancer therapy.

Effects of sex steroids on thymic epithelium and thymocyte development.

Sex steroid hormones have major effects on the thymus. Age-related increases in androgens and estrogens and pregnancy-induced increases in progestins all cause dramatic thymic atrophy. Atrophy can also be induced by treatment with exogenous sex steroids and reversed by ablation of endogenous sex steroids. Although these observations are frequently touted as evidence of steroid lymphotoxicity, they are often driven by steroid signaling in thymic epithelial cells (TEC), which are highly steroid responsive. Here, we outline the effects of sex steroids on the thymus and T cell development. We focus on studies that have examined steroid signaling in vivo, aiming to emphasize the actions of endogenous steroids which, via TEC, have remarkable programming effects on the TCR repertoire. Due to the dramatic effects of steroids on TEC, especially thymic involution, the direct effects of sex steroid signaling in thymocytes are less well understood. We outline studies that could be important in addressing these possibilities, and highlight suggestive findings of sex steroid generation within the thymus itself.

TNF plays a crucial role in inflammation by signaling via T cell TNFR2.

TNF, produced largely by T and innate immune cells, is potently proinflammatory, as are cytokines such as IFN-γ and IL-17 produced by Th1 and Th17 cells, respectively. Here, we asked if TNF is upstream of Th skewing toward inflammatory phenotypes. Exposure of mouse CD4+ T cells to TNF and TGF-ß generated Th17 cells that express low levels of IL-17 (ROR-γt+IL-17lo) and high levels of inflammatory markers independently of IL-6 and STAT3. This was mediated by the nondeath TNF receptor TNFR2, which also contributed to the generation of inflammatory Th1 cells. Single-cell RNA sequencing of central nervous system-infiltrating CD4+ T cells in mouse experimental autoimmune encephalomyelitis (EAE) found an inflammatory gene expression profile similar to cerebrospinal fluid-infiltrating CD4+ T cells from patients with multiple sclerosis. Notably, TNFR2-deficient CD4+ T cells produced fewer inflammatory mediators and were less pathogenic in EAE and colitis. IL-1ß, a Th17-skewing cytokine, induced TNF and proinflammatory granulocyte-macrophage colony-stimulating factor (GM-CSF) in T cells, which was inhibited by disruption of TNFR2 signaling, demonstrating IL-1ß can function indirectly via the production of TNF. Thus, TNF is not just an effector but also an initiator of inflammatory Th differentiation.

Calcineurin inhibitors suppress acute graft-versus-host disease via NFAT-independent inhibition of T cell receptor signaling.

Inhibitors of calcineurin phosphatase activity (CNIs) such as cyclosporin A (CsA) are widely used to treat tissue transplant rejection and acute graft-versus-host disease (aGVHD), for which inhibition of gene expression dependent on nuclear factor of activated T cells (NFAT) is the mechanistic paradigm. We recently reported that CNIs inhibit TCR-proximal signaling by preventing calcineurin-mediated dephosphorylation of LckS59, an inhibitory modification, raising the possibility of another mechanism by which CNIs suppress immune responses. Here we used T cells from mice that express LckS59A, which cannot accept a phosphate at residue 59, to initiate aGVHD. Although CsA inhibited NFAT-dependent gene upregulation in allo-aggressive T cells expressing either LckWT or LckS59A, it was ineffective in treating disease when the T cells expressed LckS59A. Two important NFAT-independent T cell functions were found to be CsA-resistant in LckS59A T cells: upregulation of the cytolytic protein perforin in tissue-infiltrating CD8+ T cells and antigen-specific T/DC adhesion and clustering in lymph nodes. These results demonstrate that effective treatment of aGVHD by CsA requires NFAT-independent inhibition of TCR signaling. Given that NFATs are widely expressed and off-target effects are a major limitation in CNI use, it is possible that targeting TCR-associated calcineurin directly may provide effective therapies with less toxicity.

Glucocorticoids in T cell development, differentiation and function.

Glucocorticoids (GCs) are small lipid hormones produced by the adrenals that maintain organismal homeostasis. Circadian and stress-induced changes in systemic GC levels regulate metabolism, cardiovascular and neural function, reproduction and immune activity. Our understanding of GC effects on immunity comes largely from administration of exogenous GCs to treat immune or inflammatory disorders. However, it is increasingly clear that endogenous GCs both promote and suppress T cell immunity. Examples include selecting an appropriate repertoire of T cell receptor (TCR) self-affinities in the thymus, regulating T cell trafficking between anatomical compartments, suppressing type 1 T helper (TH1) cell responses while permitting TH2 cell and, especially, IL-17-producing T helper cell responses, and promoting memory T cell differentiation and maintenance. Furthermore, in addition to functioning at a distance, extra-adrenal (local) production allows GCs to act as paracrine signals, specifically targeting activated T cells in various contexts in the thymus, mucosa and tumours. These pleiotropic effects on different T cell populations during development and immune responses provide a nuanced understanding of how GCs shape immunity.

Using Chromatin-Nuclear Receptor Interactions to Quantitate Endocrine, Paracrine, and Autocrine Signaling.

Hormone-activated nuclear receptors (NRs) control myriad cellular processes. The classical paradigm for hormone delivery is secretion from endocrine organs and blood-borne distribution to responding cells. However, many hormones can also be synthesized in the same tissues in which responding cells are found (paracrine signaling). In both endocrine and paracrine signaling, numerous factors affect hormone availability to target cell NRs, including hormone access to and sequestration by carrier proteins, transport across cell membranes, metabolism, and receptor availability. These factors can differ dramatically during development, between anatomical locations, and across cell types, and may cause highly variable responses to the same hormone signal. This has been difficult to study because current approaches are unable to quantify cell-intrinsic exposure to NR hormone ligands, precluding assessment of cell-specific hormone access and signaling. We have used the ligand-dependent interaction of the endogenous glucocorticoid (GC) receptor with chromatin as a biosensor that quantifies systemic access of GCs to cells within tissues at the single cell level, showing that tissues are buffered against circulating GCs. This approach also showed highly targeted paracrine GC signaling within the thymus, where GCs promote the positive selection of thymocytes with moderate affinity for self-antigens and the development of a safe and effective T-cell repertoire. We believe that this and complementary biosensor approaches will be useful to identify endocrine and paracrine target cells in situ and quantify their exposure to hormones regardless of the mode of delivery.

Glucocorticoids Oppose Thymocyte Negative Selection by Inhibiting Helios and Nur77.

Glucocorticoid (GC) signaling in thymocytes shapes the TCR repertoire by antagonizing thymocyte negative selection. The transcription factors Nur77 and Helios, which are upregulated in TCR-signaled thymocytes, have been implicated in negative selection. In this study, we found that GCs inhibited Helios and, to a lesser extent, Nur77 upregulation in TCR-stimulated mouse thymocytes. Inhibition was increased by GC preincubation, and reductions in mRNA were prevented by a protein synthesis inhibitor, suggesting that GCs suppress indirectly via an intermediary factor. Upregulation of Helios in TCR-stimulated thymocytes was unaffected by deletion of Nur77, indicating Nur77 and Helios are regulated independently. Whereas CD4+ thymocytes are positively selected in wild-type AND TCR-transgenic B6 mice, loss of GC receptor expression resulted in increased negative selection. Correspondingly, Helios and Nur77 levels were elevated in TCRhiCD4+CD8+ (TCR-signaled) thymocytes. Notably, deletion of Helios fully reversed this negative selection, whereas deletion of Nur77 had no effect on CD4+CD8+ cell numbers but reversed the loss of mature CD4+ thymocytes. Thus, Nur77 and Helios are GC targets that play nonredundant roles in setting the signaling threshold for thymocyte negative selection.

Single-Cell Resolution and Quantitation of Targeted Glucocorticoid Delivery in the Thymus.

Glucocorticoids are lipid-soluble hormones that signal via the glucocorticoid receptor (GR), a ligand-dependent transcription factor. Circulating glucocorticoids derive from the adrenals, but it is now apparent that paracrine glucocorticoid signaling occurs in multiple tissues. Effective local glucocorticoid concentrations and whether glucocorticoid delivery can be targeted to specific cell subsets are unknown. We use fluorescence detection of chromatin-associated GRs as biosensors of ligand binding and observe signals corresponding to steroid concentrations over physiological ranges in vitro and in vivo. In the thymus, where thymic epithelial cell (TEC)-synthesized glucocorticoids antagonize negative selection, we find that CD4+CD8+TCRhi cells, a small subset responding to self-antigens and undergoing selection, are specific targets of TEC-derived glucocorticoids and are exposed to 3-fold higher levels than other cells. These results demonstrate and quantitate targeted delivery of paracrine glucocorticoids. This approach may be used to assess in situ nuclear receptor signaling in a variety of physiological and pathological contexts.

Intensity and duration of TCR signaling is limited by p38 phosphorylation of ZAP-70T293 and destabilization of the signalosome.

ZAP-70 is a tyrosine kinase that is essential for initiation of T cell antigen receptor (TCR) signaling. We have found that T cell p38 MAP kinase (MAPK), which is directly phosphorylated and activated by ZAP-70 downstream of the TCR, in turn phosphorylates Thr-293 in the interdomain B region of ZAP-70. Mutant T cells expressing ZAP-70 with an alanine substitution at this residue (ZAP-70T293A) had enhanced TCR proximal signaling and increased effector responses. Lack of ZAP-70T293 phosphorylation increased association of ZAP-70 with the TCR and prolonged the existence of TCR signaling microclusters. These results identify a tight negative feedback loop in which ZAP-70-activated p38 reciprocally phosphorylates ZAP-70 and destabilizes the signaling complex.

Cutting Edge: De Novo Glucocorticoid Synthesis by Thymic Epithelial Cells Regulates Antigen-Specific Thymocyte Selection.

Glucocorticoid (GC) signaling in thymocytes counters negative selection and promotes the generation of a self-tolerant yet Ag-responsive T cell repertoire. Whereas circulating GC are derived from the adrenals, GC are also synthesized de novo in the thymus. The significance of this local production is unknown. In this study we deleted 11ß-hydroxylase, the enzyme that catalyzes the last step of GC biosynthesis, in thymic epithelial cells (TEC) or thymocytes. Like GC receptor-deficient T cells, T cells from mice lacking TEC-derived but not thymocyte-derived GC proliferated poorly to alloantigen, had a reduced antiviral response, and exhibited enhanced negative selection. Strikingly, basal expression of GC-responsive genes in thymocytes from mice lacking TEC-derived GC was reduced to the same degree as in GC receptor-deficient thymocytes, indicating that at steady-state the majority of biologically active GC are paracrine in origin. These findings demonstrate the importance of extra-adrenal GC even in the presence of circulating adrenal-derived GC.

Unique properties of TCR-activated p38 are necessary for NFAT-dependent T-cell activation.

Nuclear factor of activated T cells (NFAT) transcription factors are required for induction of T-cell cytokine production and effector function. Although it is known that activation via the T-cell antigen receptor (TCR) results in 2 critical steps, calcineurin-mediated NFAT1 dephosphorylation and NFAT2 up-regulation, the molecular mechanisms underlying each are poorly understood. Here we find that T cell p38, which is activated by an alternative pathway independent of the mitogen-activated protein (MAP) kinase cascade and with different substrate specificities, directly controls these events. First, alternatively (but not classically) activated p38 was required to induce the expression of the AP-1 component c-Fos, which was necessary for NFAT2 expression and cytokine production. Second, alternatively (but not classically) activated p38 phosphorylated NFAT1 on a heretofore unidentified site, S79, and in its absence NFAT1 was unable to interact with calcineurin or migrate to the nucleus. These results demonstrate that the acquisition of unique specificities by TCR-activated p38 orchestrates NFAT-dependent T-cell functions.

Host-Derived CD70 Suppresses Murine Graft-versus-Host Disease by Limiting Donor T Cell Expansion and Effector Function.

Allogeneic hematopoietic cell transplantation (allo-HCT) is a potentially curative treatment for hematologic and immunologic diseases. However, graft-versus-host disease (GVHD) may develop when donor-derived T cells recognize and damage genetically distinct normal host tissues. In addition to TCR signaling, costimulatory pathways are involved in T cell activation. CD27 is a TNFR family member expressed on T cells, and its ligand, CD70, is expressed on APCs. The CD27/CD70 costimulatory pathway was shown to be critical for T cell function and survival in viral infection models. However, the role of this pathway in allo-HCT is previously unknown. In this study, we have examined its contribution in GVHD pathogenesis. Surprisingly, Ab blockade of CD70 after allo-HCT significantly increases GVHD. Interestingly, whereas donor T cell- or bone marrow-derived CD70 plays no role in GVHD, host-derived CD70 inhibits GVHD as CD70-/- hosts show significantly increased GVHD. This is evidenced by reduced survival, more severe weight loss, and increased histopathologic damage compared with wild-type hosts. In addition, CD70-/- hosts have higher levels of proinflammatory cytokines TNF-α, IFN-γ, IL-2, and IL-17. Moreover, accumulation of donor CD4+ and CD8+ effector T cells is increased in CD70-/- versus wild-type hosts. Mechanistic analyses suggest that CD70 expressed by host hematopoietic cells is involved in the control of alloreactive T cell apoptosis and expansion. Together, our findings demonstrate that host CD70 serves as a unique negative regulator of allogeneic T cell response by contributing to donor T cell apoptosis and inhibiting expansion of donor effector T cells.

Recruitment of calcineurin to the TCR positively regulates T cell activation.

Calcineurin is a phosphatase whose primary targets in T cells are NFAT transcription factors, and inhibition of calcineurin activity by treatment with cyclosporin A (CsA) or FK506 is a cornerstone of immunosuppressive therapies. Here we found that calcineurin was recruited to the T cell antigen receptor (TCR) signaling complex, where it reversed inhibitory phosphorylation of the tyrosine kinase Lck on Ser59 (LckS59). Loss of calcineurin activity impaired phosphorylation of Tyr493 of the tyrosine kinase ZAP-70 (ZAP-70Y493), as well as some downstream pathways in a manner consistent with signaling in cells expressing LckS59A (Lck that cannot be phosphorylated) or LckS59E (a phosphomimetic mutant). Notably, CsA inhibited integrin-LFA-1-dependent and NFAT-independent adhesion of T cells to the intercellular adhesion molecule ICAM-1, with little effect on cells expressing mutant Lck. These results provide new understanding of how widely used immunosuppressive drugs interfere with essential processes in the immune response.

Systemic toxoplasma infection triggers a long-term defect in the generation and function of naive T lymphocytes.

Because antigen-stimulated naive T cells either die as effectors or enter the activated/memory pool, continuous egress of new T lymphocytes from thymus is essential for maintenance of peripheral immune homeostasis. Unexpectedly, we found that systemic infection with the protozoan Toxoplasma gondii triggers not only a transient increase in activated CD4+ Th1 cells but also a persistent decrease in the size of the naive CD4+ T lymphocyte pool. This immune defect is associated with decreased thymic output and parasite-induced destruction of the thymic epithelium, as well as disruption of the overall architecture of that primary lymphoid organ. Importantly, the resulting quantitative and qualitative deficiency in naive CD4+ T cells leads to an immunocompromised state that both promotes chronic toxoplasma infection and leads to decreased resistance to challenge with an unrelated pathogen. These findings reveal that systemic infectious agents, such as T. gondii, can induce long-term immune alterations associated with impaired thymic function. When accumulated during the lifetime of the host, such events, even when occurring at low magnitude, could be a contributing factor in immunological senescence.

Correction: Non-Canonical NF-κB Activation and Abnormal B Cell Accumulation in Mice Expressing Ubiquitin Protein Ligase-Inactive c-IAP2.

[This corrects the article DOI: 10.1371/journal.pbio.1000518.].