STAT3 protects hematopoietic stem cells by preventing activation of a deleterious autocrine type-I interferon response.

Hematopoietic stem and progenitor cells (HSPCs) maintain blood-forming and immune activity, yet intrinsic regulators of HSPCs remain elusive. STAT3 function in HSPCs has been difficult to dissect as Stat3-deficiency in the hematopoietic compartment induces systemic inflammation, which can impact HSPC activity. Here, we developed mixed bone marrow (BM) chimeric mice with inducible Stat3 deletion in 20% of the hematopoietic compartment to avoid systemic inflammation. Stat3-deficient HSPCs were significantly impaired in reconstitution ability following primary or secondary bone marrow transplantation, indicating hematopoietic stem cell (HSC) defects. Single-cell RNA sequencing of Lin-ckit+Sca1+ BM cells (LSKs) revealed aberrant activation of cell cycle, p53, and interferon (IFN) pathways in Stat3-deficient HSPCs. Stat3-deficient LSKs accumulated γH2AX and showed increased expression of DNA sensors and type-I IFN (IFN-I), while treatment with A151-ODN inhibited expression of IFN-I and IFN-responsive genes. Further, the blockade of IFN-I receptor signaling suppressed aberrant cell cycling, STAT1 activation, and nuclear p53 accumulation. Collectively, our results show that STAT3 inhibits a deleterious autocrine IFN response in HSCs to maintain long-term HSC function. These data signify the importance of ensuring therapeutic STAT3 inhibitors are targeted specifically to diseased cells to avoid off-target loss of healthy HSPCs.

STAT3 protects HSCs from intrinsic interferon signaling and loss of long-term blood-forming activity.

STAT3 function in hematopoietic stem and progenitor cells (HSPCs) has been difficult to discern as Stat3 deficiency in the hematopoietic system induces systemic inflammation, which can impact HSPC activity. To address this, we established mixed bone marrow (BM) chimeric mice with CreER-mediated Stat3 deletion in 20% of the hematopoietic compartment. Stat3-deficient HSPCs had impaired hematopoietic activity and failed to undergo expansion in BM in contrast to Stat3-sufficient (CreER) controls. Single-cell RNA sequencing of Lin-ckit+Sca1+ BM cells revealed altered transcriptional responses in Stat3-deficient hematopoietic stem cells (HSCs) and multipotent progenitors, including intrinsic activation of cell cycle, stress response, and interferon signaling pathways. Consistent with their deregulation, Stat3-deficient Lin-ckit+Sca1+ cells accumulated γH2AX over time. Following secondary BM transplantation, Stat3-deficient HSPCs failed to reconstitute peripheral blood effectively, indicating a severe functional defect in the HSC compartment. Our results reveal essential roles for STAT3 in HSCs and suggest the potential for using targeted synthetic lethal approaches with STAT3 inhibition to remove defective or diseased HSPCs.

Intestinal toxicity to CTLA-4 blockade driven by IL-6 and myeloid infiltration.

Immune checkpoint blockade (ICB) has revolutionized cancer treatment, yet quality of life and continuation of therapy can be constrained by immune-related adverse events (irAEs). Limited understanding of irAE mechanisms hampers development of approaches to mitigate their damage. To address this, we examined whether mice gained sensitivity to anti-CTLA-4 (αCTLA-4)-mediated toxicity upon disruption of gut homeostatic immunity. We found αCTLA-4 drove increased inflammation and colonic tissue damage in mice with genetic predisposition to intestinal inflammation, acute gastrointestinal infection, transplantation with a dysbiotic fecal microbiome, or dextran sodium sulfate administration. We identified an immune signature of αCTLA-4-mediated irAEs, including colonic neutrophil accumulation and systemic interleukin-6 (IL-6) release. IL-6 blockade combined with antibiotic treatment reduced intestinal damage and improved αCTLA-4 therapeutic efficacy in inflammation-prone mice. Intestinal immune signatures were validated in biopsies from patients with ICB colitis. Our work provides new preclinical models of αCTLA-4 intestinal irAEs, mechanistic insights into irAE development, and potential approaches to enhance ICB efficacy while mitigating irAEs.

STAT3 Inhibits Autocrine IFN Signaling in Type I Conventional Dendritic Cells.

Type I conventional dendritic cells (cDC1s) are an essential Ag-presenting population required for generating adaptive immunity against intracellular pathogens and tumors. While the transcriptional control of cDC1 development is well understood, the mechanisms by which extracellular stimuli regulate cDC1 function remain unclear. We previously demonstrated that the cytokine-responsive transcriptional regulator STAT3 inhibits polyinosinic:polycytidylic acid [poly(I:C)]-induced cDC1 maturation and cDC1-mediated antitumor immunity in murine breast cancer, indicating an intrinsic, suppressive role for STAT3 in cDC1s. To probe transcriptional mechanisms regulating cDC1 function, we generated novel RNA sequencing datasets representing poly(I:C)-, IL-10-, and STAT3-mediated gene expression responses in murine cDC1s. Bioinformatics analyses indicated that poly(I:C) stimulates multiple inflammatory pathways independent of STAT3, while IL-10-activated STAT3 uniquely inhibits the poly(I:C)-induced type I IFN (IFN-I) transcriptional response. We validated this mechanism using purified cDC1s deficient for STAT3 or IFN signaling. Our data reveal IL-10-activated STAT3 suppresses production of IFN-ß and IFN-γ, accrual of tyrosine phosphorylated STAT1, and IFN-stimulated gene expression in cDC1s after poly(I:C) exposure. Moreover, we found that maturation of cDC1s in response to poly(I:C) is dependent on the IFN-I receptor, but not the type II IFN receptor, or IFN-λ. Taken together, we elucidate an essential role for STAT3 in restraining autocrine IFN-I signaling in cDC1s elicited by poly(I:C) stimulation, and we provide novel RNA sequencing datasets that will aid in further delineating inflammatory and anti-inflammatory mechanisms in cDC1s.

Immune landscape of a genetically engineered murine model of glioma compared with human glioma.

Novel therapeutic strategies targeting glioblastoma (GBM) often fail in the clinic, partly because preclinical models in which hypotheses are being tested do not recapitulate human disease. To address this challenge, we took advantage of our previously developed spontaneous Qk/Trp53/Pten (QPP) triple-knockout model of human GBM, comparing the immune microenvironment of QPP mice with that of patient-derived tumors to determine whether this model provides opportunity for gaining insights into tumor physiopathology and preclinical evaluation of therapeutic agents. Immune profiling analyses and single-cell sequencing of implanted and spontaneous tumors from QPP mice and from patients with glioma revealed intratumoral immune components that were predominantly myeloid cells (e.g., monocytes, macrophages, and microglia), with minor populations of T, B, and NK cells. When comparing spontaneous and implanted mouse samples, we found more neutrophils and T and NK cells in the implanted model. Neutrophils and T and NK cells were increased in abundance in samples derived from human high-grade glioma compared with those derived from low-grade glioma. Overall, our data demonstrate that our implanted and spontaneous QPP models recapitulate the immunosuppressive myeloid-dominant nature of the tumor microenvironment of human gliomas. Our model provides a suitable tool for investigating the complex immune compartment of gliomas.

Regulation and function of Id2 in plasmacytoid dendritic cells.

Plasmacytoid dendritic cells (pDCs) are specialized type I interferon (IFN-I) producing cells that promote anti-viral immune responses and contribute to autoimmunity. Development of pDCs requires the transcriptional regulator E2-2 and is opposed by inhibitor of DNA binding 2 (Id2). Prior work indicates Id2 is induced in pDCs upon maturation and may affect pDC IFN-I production via suppression of E2-2, suggesting an important yet uncharacterized role in this lineage. We found TLR7 agonists stimulate Id2 mRNA and protein expression in pDCs. We further show that transcriptional activation of Id2 is dependent on the E2 ubiquitin-conjugating enzyme Ubc13, but independent of IFN-I signaling in response to TLR7 agonist stimulation. Nonetheless, conditional Id2 depletion in pDCs indicates Id2 is dispensable for TLR7 agonist-induced maturation and inhibition of E2-2 expression. Thus, we identify new mechanisms of Id2 regulation by Ubc13, which may be relevant for understanding Id2 gene regulation in other contexts, while ruling out major roles for Id2 in pDC responses to TLR7 agonists.

Dietary fiber and probiotics influence the gut microbiome and melanoma immunotherapy response.

Gut bacteria modulate the response to immune checkpoint blockade (ICB) treatment in cancer, but the effect of diet and supplements on this interaction is not well studied. We assessed fecal microbiota profiles, dietary habits, and commercially available probiotic supplement use in melanoma patients and performed parallel preclinical studies. Higher dietary fiber was associated with significantly improved progression-free survival in 128 patients on ICB, with the most pronounced benefit observed in patients with sufficient dietary fiber intake and no probiotic use. Findings were recapitulated in preclinical models, which demonstrated impaired treatment response to anti­programmed cell death 1 (anti­PD-1)­based therapy in mice receiving a low-fiber diet or probiotics, with a lower frequency of interferon-γ­positive cytotoxic T cells in the tumor microenvironment. Together, these data have clinical implications for patients receiving ICB for cancer.