Autophagy-independent functions of UVRAG are essential for peripheral naive T-cell homeostasis.

UV radiation resistance-associated gene (UVRAG) encodes a tumor suppressor with putative roles in autophagy, endocytic trafficking, and DNA damage repair but its in vivo role in T cells is unknown. Because conditional homozygous deletion of Uvrag in mice results in early embryonic lethality, we generated T-cell-specific UVRAG-deficient mice that lacked UVRAG expression specifically in T cells. This loss of UVRAG led to defects in peripheral homeostasis that could not be explained by the increased sensitivity to cell death and impaired proliferation observed for other autophagy-related gene knockout mice. Instead, UVRAG-deficient T-cells exhibited normal mitochondrial clearance and activation-induced autophagy, suggesting that UVRAG has an autophagy-independent role that is critical for peripheral naive T-cell homeostatic proliferation. In vivo, T-cell-specific loss of UVRAG dampened CD8(+) T-cell responses to LCMV infection in mice, delayed viral clearance, and impaired memory T-cell generation. Our data provide novel insights into the control of autophagy in T cells and identify UVRAG as a new regulator of naïve peripheral T-cell homeostasis.

Toso controls encephalitogenic immune responses by dendritic cells and regulatory T cells.

The ability to mount a strong immune response against pathogens is crucial for mammalian survival. However, excessive and uncontrolled immune reactions can lead to autoimmunity. Unraveling how the reactive versus tolerogenic state is controlled might point toward novel therapeutic strategies to treat autoimmune diseases. The surface receptor Toso/Faim3 has been linked to apoptosis, IgM binding, and innate immune responses. In this study, we used Toso-deficient mice to investigate the importance of Toso in tolerance and autoimmunity. We found that Toso(-/-) mice do not develop severe experimental autoimmune encephalomyelitis (EAE), a mouse model for the human disease multiple sclerosis. Toso(-/-) dendritic cells were less sensitive to Toll-like receptor stimulation and induced significantly lower levels of disease-associated inflammatory T-cell responses. Consistent with this observation, the transfer of Toso(-/-) dendritic cells did not induce autoimmune diabetes, indicating their tolerogenic potential. In Toso(-/-) mice subjected to EAE induction, we found increased numbers of regulatory T cells and decreased encephalitogenic cellular infiltrates in the brain. Finally, inhibition of Toso activity in vivo at either an early or late stage of EAE induction prevented further disease progression. Taken together, our data identify Toso as a unique regulator of inflammatory autoimmune responses and an attractive target for therapeutic intervention.

The contextual role of TNFR family members in CD8(+) T-cell control of viral infections.

Immunity to viruses must be tightly controlled to avoid pathology. Receptors and ligands of the tumor necrosis factor (TNF) family play important roles in controlling lymphocyte activation and survival during an immune response. The role of specific TNF receptor (TNFR) family members in antiviral immunity depends on the stage of the immune response and can vary with the virus type and its virulence. Here, we focus on five members of the TNFR family that are prominently expressed on CD8(+) T cells during viral infections, namely, 4-1BB (CD137), CD27, OX40 (CD134), GITR, and TNFR2. 4-1BB, CD27, OX40, and GITR have primarily prosurvival roles for CD8(+) T cells during viral infection, although under some circumstances 4-1BB, GITR, or CD27 signals can limit immunity. Although TNFR2 can be costimulatory under some circumstances, its main role in CD8(+) T-cell responses during viral infection appears to be in contraction of the response. Several TNF family ligands are being explored as adjuvants for viral vaccines, and agonistic antibodies to TNFR family members are being investigated for immunotherapy of chronic viral infection alone and in combination with checkpoint blockade. Such therapies will require thorough and specific optimization to avoid pathology induced by hyperstimulation of these pathways.

GITR-dependent regulation of 4-1BB expression: implications for T cell memory and anti-4-1BB-induced pathology.

The TNFR family member 4-1BB plays a key role in the survival of activated and memory CD8 T cells. However, the mechanisms that regulate 4-1BB re-expression on memory CD8 T cells after Ag clearance are unknown. In unimmunized mice, ∼10% of CD8 CD44(hi) memory T cells in the bone marrow (BM) and liver express 4-1BB, with minimal 4-1BB expression in spleen and lymph node. IL-2, IL-15, and IL-7 are collectively dispensable for 4-1BB expression on the memory CD8 T cells. Rather, T cell-intrinsic glucocorticoid-induced TNFR-related protein (GITR) contributes to 4-1BB expression on CD8 T cells upon their entry into the BM or liver. Consistent with its role in regulation of 4-1BB, GITR is required on memory CD8 T cells for their persistence in vivo. These findings reveal site-specific effects of the BM and liver microenvironment on CD8 memory T cells. Previous work has demonstrated that 4-1BB agonists given to unimmunized mice induce splenomegaly, hepatitis, and other immune system anomalies. Moreover, severe liver pathology has been observed in a subset of anti-4-1BB-treated melanoma patients. Remarkably, the absence of GITR in mice almost completely abrogates cellular expansions, splenomegaly, and liver inflammation associated with anti-4-1BB agonist treatment of unimmunized mice. In contrast, lack of CD8 T cells selectively improves liver pathology, but not splenomegaly in the mice. Thus, the regulation of 4-1BB expression by GITR on CD8 T cells, as well as on other cells, contributes to the pathological effects of anti-4-1BB in unimmunized mice.

T-cell intrinsic effects of GITR and 4-1BB during viral infection and cancer immunotherapy.

GITR [glucocorticoid inducible tumor necrosis factor receptor (TNFR)-related protein] and 4-1BB are costimulatory TNFR family members that are expressed on regulatory and effector T cells as well as on other cells of the immune system. Here we discuss the role of GITR and 4-1BB on T cells during viral infections and in cancer immunotherapy. Systemic treatment with agonistic anti-4-1BB antibody leads to a number of immune system abnormalities, and clinical trials of anti-4-1BB have been terminated. However, other modes of 4-1BB ligation may be less toxic. To date, similar toxicities have not been reported for anti-GITR treatment of mice, although anti-GITR antibodies can exacerbate mouse autoimmune models. Intrinsic effects of GITR and 4-1BB on effector T cells appear to predominate over their effects on other cell types in some models. Despite their similarities in enhancing T-cell survival, 4-1BB and GITR are clearly not redundant, and both pathways are required for maximal CD8(+) T-cell responses and mouse survival following severe respiratory influenza infection. GITR uses TNFR-associated factor (TRAF) 2 and TRAF5, whereas 4-1BB recruits TRAF1 and TRAF2 to mediate survival signaling in T cells. The differential use of signaling adapters combined with their differential expression may explain the non-redundant roles of GITR and 4-1BB in the immune system.

IL-15-dependent upregulation of GITR on CD8 memory phenotype T cells in the bone marrow relative to spleen and lymph node suggests the bone marrow as a site of superior bioavailability of IL-15.

CD8 memory T cells are enriched in the bone marrow, a site where these cells are thought to receive homeostatic signals. However, the primary site where CD8 memory T cells receive their cytokine-induced homeostatic signals has recently come under debate. In this study, we demonstrate that the bone marrow contains a fraction of CD8 memory phenotype T cells with elevated expression of glucocorticoid-induced TNFR-related protein (GITR). In contrast, splenic and lymph node memory phenotype T cells have GITR levels similar to those on naive T cells. The bone marrow GITR(hi) memory T cells have a phenotype indicative of cytokine activation, with higher CD122 and lower CD127 than do the GITR(basal) memory T cells. Remarkably, these bone marrow-specific GITR(hi) cells are almost completely ablated in the absence of IL-15, whereas TNFR2 and 4-1BB expression on the CD8 memory T cells are IL-15 independent. Furthermore, adoptively transferred splenic CD8 memory phenotype T cells show IL-15-dependent GITR upregulation upon entry into the bone marrow. This result implies that the selective appearance of GITR(hi) memory phenotype T cells in the bone marrow reflects the local microenvironment rather than a different subset of memory T cells. GITR(-/-) mice have a lower frequency of CD8 memory phenotype cells in the bone marrow, yet the GITR(-/-) cells hyperproliferate compared with those in wild-type mice. Taken together, these data suggest that GITR plays a role in the survival of CD8 memory phenotype T cells and that GITR upregulation represents a precise marker of cells that have responded to IL-15.

Contribution of 4-1BBL on radioresistant cells in providing survival signals through 4-1BB expressed on CD8⁺ memory T cells in the bone marrow.

The persistence of memory lymphocytes is a critical feature of adaptive immunity. The TNF family ligand 4-1BBL supports the antigen-independent survival of CD8⁺ memory T cells. Here, we show that mice lacking 4-1BB only on αß T cells show a similar defect in CD8⁺ T-cell recall responses, as previously shown in 4-1BBL-deficient mice. We show that 4-1BB is selectively expressed on BM CD8⁺ but not CD4⁺ memory T cells of unimmunized mice. Its ligand, 4-1BBL, is found on VCAM-1⁺ stromal cells, CD11c⁺ cells, and a Gr1(lo) myeloid population in unimmunized mice. Adoptive transfer of in vitro generated memory T cells into mice lacking 4-1BBL only on radioresistant cells recapitulates the defect in CD8⁺ T-cell survival seen in the complete knockout mice, with smaller effects of 4-1BBL on hematopoietic cells. In BM, adoptively transferred DsRed CD8⁺ memory T cells are most often found in proximity to VCAM-1⁺ cells or Gr1⁺ cells, followed by B220⁺ cells and to a much lesser extent near CD11c⁺ cells. Thus, a VCAM-1⁺CD45(-) stromal cell is a plausible candidate for the radioresistant cell that provides 4-1BBL to CD8⁺ memory T cells in the BM.

Immune regulation by 4-1BB and 4-1BBL: complexities and challenges.

SUMMARY: The tumor necrosis factor receptor family member 4-1BB plays a key role in the survival of activated and memory CD8(+) T cells. Depending on the disease model, 4-1BB can participate at different stages and influence different aspects of the immune response, likely due to the differential expression of receptor and ligand relative to other costimulatory molecules. Studies comparing mild versus severe influenza infection of mice suggest that the immune system uses inducible receptors such as 4-1BB to prolong the immune response when pathogens take longer to clear. The expression of 4-1BB on diverse cell types, evidence for bidirectional as well as receptor-independent signaling by 4-1BBL, the unexpected hyperproliferation of 4-1BB-deficient T cells, and complex effects of agonistic anti-4-1BB therapy have revealed additional roles for the 4-1BB/4-1BBL receptor/ligand pair in the immune system. In this review, we discuss these diverse roles of 4-1BB and its ligand in the immune response, exploring possible mechanisms for the observed complexities and implications for therapeutic applications of 4-1BB/4-1BBL.

Targeting CD47-SIRPa axis shows potent preclinical anti-tumor activity as monotherapy and synergizes with PARP inhibition.

The objective was to correlate CD47 gene expression with resistance to immune checkpoint inhibitors (ICI) in tumor tissue of gynecological cancer (GC). Further, we sought to assess the efficacy of targeting CD47 pathway alone and in combination in pre-clinical ovarian cancer (OC) models. We performed transcriptomic analyses in GC treated with ICI. Signaling pathway enrichment analysis was performed using Ingenuity Pathway Analysis. Immune cell abundance was estimated. CD47 expression was correlated with other pathways, objective response, and progression-free survival (PFS). Anti-tumor efficacy of anti-CD47 therapy alone and in combination was investigated both in-vitro and in-vivo using cell-line derived xenograft (CDX) and patient-derived xenograft (PDX) models. High CD47 expression associated with lower response to ICI and trended toward lower PFS in GC patients. Higher CD47 associated negatively with PDL1 and CTLA4 expression, as well as cytotoxic T-cells and dendritic cells but positively with TGF-ß, BRD4 and CXCR4/CXCL12 expression. Anti-CD47 significantly enhanced macrophage-mediated phagocytosis of OC cells in-vitro and exhibited potent anti-tumor activity in-vivo in OC CDX and PDX models. In-vitro treatment with PARPi increased CD47 expression. Anti-CD47 led to significantly enhanced in-vitro phagocytosis, enhanced STING pathway and synergized in-vivo when combined with PARP inhibitors in BRCA-deficient OC models. This study provides insight on the potential role of CD47 in mediating immunotherapy resistance and its association with higher TGF-ß, BRD4 and CXCR4/CXCL12 expression. Anti-CD47 showed potent anti-tumor activity and synergized with PARPi in OC models. These data support clinical development of anti-CD47 therapy with PARPi in OC.

In vivo accumulation of T cells in response to IL-2/anti-IL-2 mAb complexes is dependent in part on the TNF family ligand 4-1BBL.

Immune complexes combining IL-2 with particular anti-IL-2 antibodies can be used to selectively expand regulatory T cells or memory T cells. Combining IL-2 with anti-IL-2 (Clone S4B6) greatly enhances the biological potency of IL-2 in vivo leading to selective expansion of CD8 memory T cells and NK cells compared with regulatory T cells. Here we show that in vivo administration of IL-2/anti-IL-2 mAb (IL-2/mAb) complexes induces 4-1BB expression on both adoptively transferred antigen-specific memory CD8 T cells as well as on endogenous memory phenotype cells. Remarkably, the accumulation of adoptively transferred memory CD8 T cells following in vivo IL-2/mAb-complex treatment was found to be dependent in part on the presence of 4-1BBL in the host. These effects were independent of IL-2-induced cell division, suggesting that 4-1BBL-induced survival signals contribute to IL-2/mAb-complex-induced T-cell accumulation in vivo.

CD8 T cell-intrinsic GITR is required for T cell clonal expansion and mouse survival following severe influenza infection.

The regulation of T cell expansion by TNFR family members plays an important role in determining the magnitude of the immune response to pathogens. As several members of the TNFR family, including glucocorticoid-induced TNFR-related protein (GITR), are found on both regulatory and effector T cells, there is much interest in understanding how their effects on these opposing arms of the immune system affect disease outcome. Whereas much work has focused on the role of GITR on regulatory T cells, little is known about its intrinsic role on effector T cells in an infectious disease context. In this study, we demonstrate that GITR signaling on CD8 T cells leads to TNFR-associated factor (TRAF) 2/5-dependent, TRAF1-independent NF-κB induction, resulting in increased Bcl-x(L). In vivo, GITR on CD8 T cells has a profound effect on CD8 T cell expansion, via effects on T cell survival. Moreover, GITR is required on CD8 T cells for enhancement of influenza-specific CD8 T cell expansion upon administration of agonistic anti-GITR Ab, DTA-1. Remarkably, CD8 T cell-intrinsic GITR is essential for mouse survival during severe, but dispensable during mild respiratory influenza infection. These studies highlight the importance of GITR as a CD8 T cell costimulator during acute viral infection, and argue that despite the similarity among several TNFR family members in inducing T lymphocyte survival, they clearly have nonredundant functions in protection from severe infection.

Endogenous 4-1BB ligand plays a critical role in protection from influenza-induced disease.

A critical issue during severe respiratory infection is whether it is the virus or the host response that does the most damage. In this study, we show that endogenous 4-1BBL plays a critical role in protecting mice from severe effects of influenza disease. During mild respiratory influenza infection in which virus is rapidly cleared, the inducible costimulatory receptor 4-1BB is only transiently induced on lung T cells and 4-1BB ligand (4-1BBL) is completely dispensable for the initial CD8 T cell response and mouse survival. In contrast, during more severe respiratory influenza infection with prolonged viral load, 4-1BB expression on lung CD8 T cells is sustained, and 4-1BBL-deficient mice show decreased CD8 T cell accumulation in the lungs, decreased viral clearance, impaired lung function, and increased mortality. Transfer of an optimal number of naive Ag-specific T cells before infection protects wild-type but not 4-1BBL-deficient mice from an otherwise lethal dose of influenza virus. Transfer of T cells lacking the proapoptotic molecule Bim extends the lifespan of 4-1BBL-deficient mice by one to three days, suggesting that at least part of the role of 4-1BB/4-1BBL is to prolong effector cell survival long enough to clear virus. Intranasal delivery of 4-1BBL by recombinant adenovirus marginally improves survival of 4-1BBL-deficient mice at low dose, but exacerbates disease at high dose. These findings suggest a rationale for the evolutionary accumulation of inducible costimulatory molecules, thereby allowing the immune system to sustain the expression of molecules such as 4-1BB to a level commensurate with severity of infection.

Clinical Response to Anti-CD47 Immunotherapy Is Associated with Rapid Reduction of Exhausted Bystander CD4+ BTLA+ T Cells in Tumor Microenvironment of Mycosis Fungoides.

Cancer progression in mycosis fungoides, the most common form of cutaneous T-cell lymphoma, occurs in a predictable, sequential pattern that starts from patches and that evolves to plaques and later to tumors. Therefore, unlocking the relationship between the microarchitecture of mycosis fungoides and the clinical counterparts of that microstructure represents important steps for the design of targeted therapies. Using multispectral fluorescent imaging, we show that the progression of mycosis fungoides from plaque to tumor parallels the cutaneous expansion of the malignant CD4+ T cells that express TOX. The density of exhausted BTLA+ CD4+ T cells around malignant CD4+TOX+ cells was higher in tumors than it was in plaques, suggesting that undesired safeguards are in place within the tumor microenvironment that prevent immune activation and subsequent cancer eradication. Overriding the CD47 checkpoint with an intralesional SIRPαFc fusion decoy receptor induced the resolution of mycosis fungoides in patients that paralleled an amplified expansion of NK and CD8+ T cells in addition to a reduction of the exhausted BTLA+ CD4+ T cells that were engaged in promiscuous intercellular interactions. These therapeutic benefits of the CD47 blockade were further unleashed by adjuvant interferon-α, which stimulates cytotoxic cells, underscoring the importance of an inflamed microenvironment in facilitating the response to immunotherapy. Collectively, these findings support CD47 as a therapeutic target in treating mycosis fungoides and demonstrate a synergistic role of interferon-α in exploiting these clinical benefits.

Intralesional TTI-621, a novel biologic targeting the innate immune checkpoint CD47, in patients with relapsed or refractory mycosis fungoides or Sézary syndrome: a multicentre, phase 1 study.

BACKGROUND: Intravenous TTI-621 (SIRPα-IgG1 Fc) was previously shown to have activity in relapsed or refractory haematological malignancies. This phase 1 study evaluated the safety and activity of TTI-621 in patients with percutaneously accessible relapsed or refractory mycosis fungoides, Sézary syndrome, or solid tumours. Here we report the clinical and translational results among patients with mycosis fungoides or Sézary syndrome. METHODS: This multicentre, open-label, phase 1 study was conducted at five academic health-care and research centres in the USA. Eligible patients were aged 18 years or older; had injectable, histologically or cytologically confirmed relapsed or refractory cutaneous T-cell lymphoma (CTCL) or solid tumours; Eastern Cooperative Oncology Group performance status of 2 or less; and adequate haematological, renal, hepatic, and cardiac function. TTI-621 was injected intralesionally in a sequential dose escalation (cohorts 1-5; single 1 mg, 3 mg, or 10 mg injection or three 10 mg injections weekly for 1 or 2 weeks) and in expansion cohorts (cohorts 6-9; 2 week induction at the maximum tolerated dose; weekly continuation was allowed). In cohort 6, patients were injected with TTI-621 in a single lesion and in cohort 7, they were injected in multiple lesions. In cohort 8, TTI-621 was combined with pembrolizumab 200 mg injections per product labels. In cohort 9, TTI-621 was combined with the standard labelled dose of subcutaneous pegylated interferon alpha-2a 90 µg. The primary endpoint was the incidence and severity of adverse events. The study is registered with ClinicalTrials.gov, NCT02890368, and was closed by the sponsor to focus on intravenous studies with TTI-621. FINDINGS: Between Jan 30, 2017, and March 31, 2020, 66 patients with mycosis fungoides, Sézary syndrome, other CTCL, or solid tumours were screened, 35 of whom with mycosis fungoides or Sézary syndrome were enrolled and received intralesional TTI-621 (escalation, n=13; expansion, n=22). No dose-limiting toxicities occurred; the maximum tolerated dose was not established. In the dose expansion cohorts, the maximally assessed regimen (10 mg thrice weekly for 2 weeks) was used. 25 (71%) patients had treatment-related adverse events; the most common (occurring in ≥10% of patients) were chills (in ten [29%] patients), injection site pain (nine [26%]), and fatigue (eight [23%]). No treatment-related adverse events were grade 3 or more or serious. There were no treatment-related deaths. Rapid responses (median 45 days, IQR 17-66) occurred independently of disease stage or injection frequency. 26 (90%) of 29 evaluable patients had decreased Composite Assessment of Index Lesion Severity (CAILS) scores; ten (34%) had a decrease in CAILS score of 50% or more (CAILS response). CAILS score reductions occurred in adjacent non-injected lesions in eight (80%) of ten patients with paired assessments and in distal non-injected lesions in one additional patient. INTERPRETATION: Intralesional TTI-621 was well tolerated and had activity in adjacent or distal non-injected lesions in patients with relapsed or refractory mycosis fungoides or Sézary syndrome, suggesting it has systemic and locoregional abscopal effects and potential as an immunotherapy for these conditions. FUNDING: Trillium Therapeutics.

Choline acetyltransferase-expressing T cells are required to control chronic viral infection.

Although widely studied as a neurotransmitter, T cell-derived acetylcholine (ACh) has recently been reported to play an important role in regulating immunity. However, the role of lymphocyte-derived ACh in viral infection is unknown. Here, we show that the enzyme choline acetyltransferase (ChAT), which catalyzes the rate-limiting step of ACh production, is robustly induced in both CD4+ and CD8+ T cells during lymphocytic choriomeningitis virus (LCMV) infection in an IL-21-dependent manner. Deletion of Chat within the T cell compartment in mice ablated vasodilation in response to infection, impaired the migration of antiviral T cells into infected tissues, and ultimately compromised the control of chronic LCMV clone 13 infection. Our results reveal a genetic proof of function for ChAT in T cells during viral infection and identify a pathway of T cell migration that sustains antiviral immunity.

Constitutive interaction between 4-1BB and 4-1BBL on murine LPS-activated bone marrow dendritic cells masks detection of 4-1BBL by TKS-1 but not 19H3 antibody.

4-1BB is a TNFR family member associated with NF-κB mediated survival signaling. 4-1BB is widely expressed on activated cells of the immune system, including activated T cells, NK cells and dendritic cells. Its ligand, 4-1BBL, is transiently expressed on activated antigen presenting cells and at low levels on activated T cells. Although 4-1BBL-deficient mice clearly demonstrate a role for 4-1BBL in CD8 T cell responses to viruses such as influenza, 4-1BBL can be difficult to detect following infection of mice. Here we provide evidence for a constitutive interaction between endogenous 4-1BB and 4-1BBL on LPS activated bone marrow-derived murine dendritic cells that can mask its detection, with implications for measurement of 4-1BBL expression. The masking of 4-1BBL by its receptor results in loss of reactivity to the anti-4-1BBL antibody TKS-1, whereas the 19H3 antibody binds to 4-1BBL in the presence or absence of 4-1BB. Moreover, 4-1BB/4-1BBL interaction can occur in trans between 4-1BB+/+ and 4-1BB-/- dendritic cells in culture. These data suggest that 19H3 is the preferable antibody to use to detect 4-1BBL in the presence of its receptor.

TTI-621 (SIRPαFc), a CD47-blocking cancer immunotherapeutic, triggers phagocytosis of lymphoma cells by multiple polarized macrophage subsets.

Tumor-associated macrophages (TAMs) are heterogeneous and can adopt a spectrum of activation states between pro-inflammatory and pro-tumorigenic in response to the microenvironment. We have previously shown that TTI-621, a soluble SIRPαFc fusion protein that blocks the CD47 "do-not-eat" signal, promotes tumor cell phagocytosis by IFN-γ-primed macrophages. To assess the impact of CD47 blockade on diverse types of macrophages that are found within the tumor microenvironment, six different polarized human macrophage subsets (M(-), M(IFN-γ), M(IFN-γ+LPS), M(IL-4), M(HAGG+IL-1ß), M(IL-10 + TGFß)) with distinct cell surface markers and cytokine profiles were generated. Blockade of CD47 using TTI-621 significantly increased phagocytosis of lymphoma cells by all macrophage subsets, with M(IFN-γ), M(IFN-γ+LPS) and M(IL-10 + TGFß) macrophages having the highest phagocytic response. TTI-621-mediated phagocytosis involves macrophage expression of both the low- and high-affinity Fcγ receptors II (CD32) and I (CD64), respectively. Moreover, macrophages with lower phagocytic capabilities (M(-), M(IL-4), M(HAGG+IL-1ß)) could readily be re-polarized into highly phagocytic macrophages using various cytokines or TLR agonists. In line with the in vitro study, we further demonstrate that TTI-621 can trigger phagocytosis of tumor cells by diverse subsets of isolated mouse TAMs ex vivo. These data suggest that TTI-621 may be efficacious in triggering the destruction of cancer cells by a diverse population of TAMs found in vivo and support possible combination approaches to augment the activity of CD47 blockade.

K48-linked KLF4 ubiquitination by E3 ligase Mule controls T-cell proliferation and cell cycle progression.

T-cell proliferation is regulated by ubiquitination but the underlying molecular mechanism remains obscure. Here we report that Lys-48-linked ubiquitination of the transcription factor KLF4 mediated by the E3 ligase Mule promotes T-cell entry into S phase. Mule is elevated in T cells upon TCR engagement, and Mule deficiency in T cells blocks proliferation because KLF4 accumulates and drives upregulation of its transcriptional targets E2F2 and the cyclin-dependent kinase inhibitors p21 and p27. T-cell-specific Mule knockout (TMKO) mice develop exacerbated experimental autoimmune encephalomyelitis (EAE), show impaired generation of antigen-specific CD8+ T cells with reduced cytokine production, and fail to clear LCMV infections. Thus, Mule-mediated ubiquitination of the novel substrate KLF4 regulates T-cell proliferation, autoimmunity and antiviral immune responses in vivo.

TTI-621 (SIRPαFc): A CD47-Blocking Innate Immune Checkpoint Inhibitor with Broad Antitumor Activity and Minimal Erythrocyte Binding.

Purpose: The ubiquitously expressed transmembrane glycoprotein CD47 delivers an anti-phagocytic (do not eat) signal by binding signal-regulatory protein α (SIRPα) on macrophages. CD47 is overexpressed in cancer cells and its expression is associated with poor clinical outcomes. TTI-621 (SIRPαFc) is a fully human recombinant fusion protein that blocks the CD47-SIRPα axis by binding to human CD47 and enhancing phagocytosis of malignant cells. Blockade of this inhibitory axis using TTI-621 has emerged as a promising therapeutic strategy to promote tumor cell eradication.Experimental Design: The ability of TTI-621 to promote macrophage-mediated phagocytosis of human tumor cells was assessed using both confocal microscopy and flow cytometry. In vivo antitumor efficacy was evaluated in xenograft and syngeneic models and the role of the Fc region in antitumor activity was evaluated using SIRPαFc constructs with different Fc tails.Results: TTI-621 enhanced macrophage-mediated phagocytosis of both hematologic and solid tumor cells, while sparing normal cells. In vivo, TTI-621 effectively controlled the growth of aggressive AML and B lymphoma xenografts and was efficacious in a syngeneic B lymphoma model. The IgG1 Fc tail of TTI-621 plays a critical role in its antitumor activity, presumably by engaging activating Fcγ receptors on macrophages. Finally, TTI-621 exhibits minimal binding to human erythrocytes, thereby differentiating it from CD47 blocking antibodies.Conclusions: These data indicate that TTI-621 is active across a broad range of human tumors. These results further establish CD47 as a critical regulator of innate immune surveillance and form the basis for clinical development of TTI-621 in multiple oncology indications. Clin Cancer Res; 23(4); 1068-79. ©2016 AACR.