Pentose Phosphate Pathway Regulates Tolerogenic Apoptotic Cell Clearance and Immune Tolerance

The efficient removal of apoptotic cells (ACs), a process termed as efferocytosis, is essential for immune homeostasis. While recent work has established an important interplay between efferocytosis and cellular metabolic changing, underlying mechanisms remain poorly known. Here, we discovered that pentose phosphate pathway (PPP) regulates tolerogenic ACs clearance and immune tolerance. ACs decreased levels of PPP-related genes and metabolites in macrophages. AG1, the agonist of PPP, increased the activity of PPP but greatly reduced macrophage phagocytosis of ACs and enhanced the inflammatory response during efferocytosis. miR-323-5p regulated the expression of PPP-related genes and its levels increased during efferocytosis. miR-323-5p inhibitor greatly promoted levels of PPP-related genes, reduced the macrophage phagocytosis of ACs, and increased inflammatory response during efferocytosis, suggesting that miR-323-5p was essential in regulating PPP activity and ACs clearance in macrophages. Correspondingly, the PPP agonist AG1 exacerbated the lupus-like symptoms in the AC-induced systemic lupus erythematosus (SLE) model. Our study reveals that regulating PPP-dependent metabolic reprogramming is critical for tolerogenic ACs phagocytosis and immune tolerance.

SOX2 Regulates Neuronal Differentiation of the Suprachiasmatic Nucleus

In mammals, the hypothalamic suprachiasmatic nucleus (SCN) functions as the central circadian pacemaker, orchestrating behavioral and physiological rhythms in alignment to the environmental light/dark cycle. The neurons that comprise the SCN are anatomically and functionally heterogeneous, but despite their physiological importance, little is known about the pathways that guide their specification and differentiation. Here, we report that the stem/progenitor cell transcription factor, Sex determining region Y-box 2 (Sox2), is required in the embryonic SCN to control the expression of SCN-enriched neuropeptides and transcription factors. Ablation of Sox2 in the developing SCN leads to downregulation of circadian neuropeptides as early as embryonic day (E) 15.5, followed by a decrease in the expression of two transcription factors involved in SCN development, Lhx1 and Six6, in neonates. Thymidine analog-retention assays revealed that Sox2 deficiency contributed to reduced survival of SCN neurons during the postnatal period of cell clearance, but did not affect progenitor cell proliferation or SCN specification. Our results identify SOX2 as an essential transcription factor for the proper differentiation and survival of neurons within the developing SCN.

A New Gene Set Identifies Senescent Cells and Predicts Senescence-Associated Pathways Across Tissues

Although cellular senescence is increasingly recognized as driving multiple age-related co-morbidities through the senescence-associated secretory phenotype (SASP), in vivo senescent cell identification, particularly in bulk or single cell RNA-sequencing (scRNA-seq) data remains challenging. Here, we generated a novel gene set (SenMayo) and first validated its enrichment in bone biopsies from two aged human cohorts. SenMayo also identified senescent cells in aged murine brain tissue, demonstrating applicability across tissues and species. For direct validation, we demonstrated significant reductions in SenMayo in bone following genetic clearance of senescent cells in mice, with similar findings in adipose tissue from humans in a pilot study of pharmacological senescent cell clearance. In direct comparisons, SenMayo outperformed all six existing senescence/SASP gene sets in identifying senescent cells across tissues and in demonstrating responses to senescent cell clearance. We next used SenMayo to identify senescent hematopoietic or mesenchymal cells at the single cell level from publicly available human and murine bone marrow/bone scRNA-seq data and identified monocytic and osteolineage cells, respectively, as showing the highest levels of senescence/SASP genes. Using pseudotime and cellular communication patterns, we found senescent hematopoietic and mesenchymal cells communicated with other cells through common pathways, including the Macrophage Migration Inhibitory Factor (MIF) pathway, which has been implicated not only in inflammation but also in immune evasion, an important property of senescent cells. Thus, SenMayo identifies senescent cells across tissues and species with high fidelity. Moreover, using this senescence panel, we were able to characterize senescent cells at the single cell level and identify key intercellular signaling pathways associated with these cells, which may be particularly useful for evolving efforts to map senescent cells (e.g., SenNet). In addition, SenMayo represents a potentially clinically applicable panel for monitoring senescent cell burden with aging and other conditions as well as in studies of senolytic drugs.

A New Gene Set Identifies Senescent Cells and Predicts Senescence-Associated Pathways Across Tissues

Although cellular senescence is increasingly recognized as driving multiple age-related co-morbidities through the senescence-associated secretory phenotype (SASP), in vivo senescent cell identification, particularly in bulk or single cell RNA-sequencing (scRNA-seq) data remains challenging. Here, we generated a novel gene set (SenMayo) and first validated its enrichment in bone biopsies from two aged human cohorts. SenMayo also identified senescent cells in aged murine brain tissue, demonstrating applicability across tissues and species. For direct validation, we demonstrated significant reductions in SenMayo in bone following genetic clearance of senescent cells in mice, with similar findings in adipose tissue from humans in a pilot study of pharmacological senescent cell clearance. In direct comparisons, SenMayo outperformed all six existing senescence/SASP gene sets in identifying senescent cells across tissues and in demonstrating responses to senescent cell clearance. We next used SenMayo to identify senescent hematopoietic or mesenchymal cells at the single cell level from publicly available human and murine bone marrow/bone scRNA-seq data and identified monocytic and osteolineage cells, respectively, as showing the highest levels of senescence/SASP genes. Using pseudotime and cellular communication patterns, we found senescent hematopoietic and mesenchymal cells communicated with other cells through common pathways, including the Macrophage Migration Inhibitory Factor (MIF) pathway, which has been implicated not only in inflammation but also in immune evasion, an important property of senescent cells. Thus, SenMayo identifies senescent cells across tissues and species with high fidelity. Moreover, using this senescence panel, we were able to characterize senescent cells at the single cell level and identify key intercellular signaling pathways associated with these cells, which may be particularly useful for evolving efforts to map senescent cells (e.g., SenNet). In addition, SenMayo represents a potentially clinically applicable panel for monitoring senescent cell burden with aging and other conditions as well as in studies of senolytic drugs.

C4d Deposition after Allogeneic Renal Transplantation in Rats Is Involved in Initial Apoptotic Cell Clearance

In the context of transplantation, complement activation is associated with poor prognosis and outcome. While complement activation in antibody-mediated rejection is well-known, less is known about complement activation in acute T cell-mediated rejection (TCMR). There is increasing evidence that complement contributes to the clearance of apoptotic debris and tissue repair. In this regard, we have analysed published human kidney biopsy transcriptome data clearly showing upregulated expression of complement factors in TCMR. To clarify whether and how the complement system is activated early during acute TCMR, experimental syngeneic and allogeneic renal transplantations were performed. Using an allogeneic rat renal transplant model, we also observed upregulation of complement factors in TCMR in contrast to healthy kidneys and isograft controls. While staining for C4d was positive, staining with a C3d antibody showed no C3d deposition. FACS analysis of blood showed the absence of alloantibodies that could have explained the C4d deposition. Gene expression pathway analysis showed upregulation of pro-apoptotic factors in TCMR, and apoptotic endothelial cells were detected by ultrastructural analysis. Monocytes/macrophages were found to bind to and phagocytise these apoptotic cells. Therefore, we conclude that early C4d deposition in TCMR may be relevant to the clearance of apoptotic cells.

Lysophospholipid Mediators in Health and Disease

Lysophospholipids, exemplified by lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P), are produced by the metabolism and perturbation of biological membranes. Both molecules are established extracellular lipid mediators that signal via specific G protein–coupled receptors in vertebrates. This widespread signaling axis regulates the development, physiological functions, and pathological processes of all organ systems. Indeed, recent research into LPA and S1P has revealed their important roles in cellular stress signaling, inflammation, resolution, and host defense responses. In this review, we focus on how LPA regulates fibrosis, neuropathic pain, abnormal angiogenesis, endometriosis, and disorders of neuroectodermal development such as hydrocephalus and alopecia. In addition, we discuss how S1P controls collective behavior, apoptotic cell clearance, and immunosurveillance of cancers. Advances in lysophospholipid research have led to new therapeutics in autoimmune diseases, with many more in earlier stages of development for a wide variety of diseases, such as fibrotic disorders, vascular diseases, and cancer. Expected final online publication date for the Annual Review of Pathology: Mechanisms of Disease, Volume 17 is January 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Gas6 Ameliorates Inflammatory Response and Apoptosis in Bleomycin-Induced Acute Lung Injury

Acute lung injury (ALI) is characterized by alveolar damage, lung edema, and exacerbated inflammatory response. Growth arrest-specific protein 6 (Gas6) mediates many different functions, including cell survival, proliferation, inflammatory signaling, and apoptotic cell clearance (efferocytosis). The role of Gas6 in bleomycin (BLM)-induced ALI is unknown. We investigated whether exogenous administration of mouse recombinant Gas6 (rGas6) has anti-inflammatory and anti-apoptotic effects on BLM-induced ALI. Compared to mice treated with only BLM, the administration of rGas6 reduced the secretion of proinflammatory cytokines, including tumor necrosis factor-α, interleukin-1β, and macrophage inflammatory protein-2, and increased the secretion of hepatocyte growth factor in bronchoalveolar lavage (BAL) fluid. rGas6 administration also reduced BLM-induced inflammation and apoptosis as evidenced by reduced neutrophil recruitment into the lungs, total protein levels in BAL fluid, caspase-3 activity, and TUNEL-positive lung cells in lung tissue. Apoptotic cell clearance by alveolar macrophages was also enhanced in mice treated with both BLM and rGas6 compared with mice treated with only BLM. rGas6 also had pro-resolving and anti-apoptotic effects in mouse bone marrow-derived macrophages and alveolar epithelial cell lines stimulated with BLM in vitro. These findings indicate that rGas6 may play a protective role in BLM-induced ALI.

Low Dose Cyclophosphamide in Combination with Elotuzumab - a Novel Immunotherapeutic Strategy for Multiple Myeloma

Introduction Cyclophosphamide (CTX) is a widely used anti-neoplastic, performing as an alkylating agent at high doses and immunomodulatory agent at low doses 1.. Combining CTX with monoclonal antibody (mAb) therapy has proven beneficial in potentiating relapsed and/or refractory multiple myeloma (RRMM) therapies, with daratumumab-directed MM cell death enhanced in the presence of CTX 2,3.. Elotuzumab (ELO), the second mAb approved for treating RRMM, promotes MM cell clearance by enhancing macrophage-mediated phagocytosis and CD16- and SLAMF7-directed NK cell cytotoxicity. ELO has been approved for use alongside dexamethasone and lenalidomide 4 or pomalidomide (POM) 5.. However, potential therapeutic benefits of ELO in combination with immunomodulatory drugs such as CTX and POM have yet to be examined. Our research investigates, the efficacy of combining low-dose CTX, alone or in combination with POM, and ELO in enhancing macrophage and NK cell infiltration and function in the MM tumour microenvironment. Materials and Methods Multiple myeloma cells (MM1S and H929) were treated with low-dose CTX and/or POM for 24hrs, washed to remove residual drug and resuspended in fresh media for tumour cell secretome (TCS) generation. Direct effects of CTX and/or POM on surface expression of checkpoint proteins (PD-1 and CD47) on MM cells was assessed by mean fluorescent intensity (MFI) flow cytometry. CD32/CD64 receptor expression on THP-1 macrophages, NKG2D, CD2, DNAM-1, CD96 and KIR2DL1 receptors on KHYG1 and primary NK cells, were measured using flow cytometry as a measure of activation. Migration of serum-starved, CFSE-labelled macrophages and NK cells towards CTX and/or POM TCS was assessed after 4hrs, with total number of migrated cells quantified using the Accuri flow cytometer. Immune cell function following indirect conditioning of macrophages/NK cells with MM cell TCS was measured by quantifying antibody-directed cellular phagocytosis (ADCP) or antibody-directed cellular cytotoxicity (ADCC), respectively. Conditioned immune cells were co-cultured with MM cells in a 2:1 effector to target ratio for 4hrs in the absence/presence of mAbs (ELO, nivolumab and anti-CD47), after which MM cell clearance was quantified by flow cytometry and presented as relative uptake (ADCP) and cytotoxicity (ADCC). One-way ANOVA statistical analysis was performed, followed by Tukey post hoc tests, with significance recognized at p<0.05. Results Direct treatment of MM cells with CTX increased surface expression of immune evading checkpoint proteins PD-1 and CD47 (p<0.05,n=3). POM monotherapy did not alter PD-1/CD47 expression, however dual therapy of CTX and POM supported the CTX-driven effect (p<0.001,n=3). Expression of CD32/CD64 macrophage activation markers was significantly increased on THP-1 cells following CTX-TCS conditioning (p<0.001,n=3). POM altered CD32, but not CD64, however dual treatment with CTX and POM significantly increased expression of both CD32 and CD64 (p<0.001, n=3). Migration of macrophages towards CTX-TCS was enhanced in a dose-dependent manner (p<0.01,n=3). CTX and POM dual therapy supported this CTX driven effect (p<0.001,n=3). Migration trends of both primary and KHYG1 NK cells were also increased towards the secretome from CTX treated MM cells. ADCP and ADCC were increased by CTX alone or in combination with POM (p<0.05, n=3). Effects of CTX on ADCP were not significantly enhanced by ELO, however ELO did significantly augment ADCC by CTX-conditioned primary NK cells (p<0.05,n=3). Given the increased expression of PD-1 and CD47, we investigated if the inclusion of nivolumab and anti-CD47 mAbs potentiated ADCC. Although ADCC was increased in all combinations, there was no significant difference between ELO alone versus ELO in combination with either nivolumab or anti-CD47. Conclusions Low-dose CTX and POM potentiated the immunomodulatory effects of ELO, with NK-directed cytotoxicity of MM cells enhanced in the presence of this mAb. Our data therefore indicates that the inclusion of low-dose CTX and or POM in combination with ELO could be a novel immunotherapeutic strategy for treating RRMM. References 1. Swan et al., Hemasphere. 2020;4(2). 2. Pallasch et al., Cell. 2014; 156(3):590-602. 3. Naicker et al., Oncoimmunology. 2021; 10(1):1859263 4. Dimopoulos et al., Blood Cancer Journal. 2020 10:91 5. Hose et al., Journal of Cancer Research and Clinical Oncology. 2021; 147:205-212 Figure 1 Figure 1. Disclosures No relevant conflicts of interest to declare.

Microenvironmental IL-6 inhibits anti-cancer immune responses generated by cytotoxic chemotherapy

Cytotoxic chemotherapeutics primarily function through DNA damage-induced tumor cell apoptosis, although the inflammation provoked by these agents can stimulate anti-cancer immune responses. The mechanisms that control these distinct effects and limit immunogenic responses to DNA-damage mediated cell death in vivo are currently unclear. Using a mouse model of BCR-ABL+ B-cell acute lymphoblastic leukemia, we show that chemotherapy-induced anti-cancer immunity is suppressed by the tumor microenvironment through production of the cytokine IL-6. The chemotherapeutic doxorubicin is curative in IL-6-deficient mice through the induction of CD8+ T-cell-mediated anti-cancer responses, while moderately extending lifespan in wild type tumor-bearing mice. We also show that IL-6 suppresses the effectiveness of immune-checkpoint inhibition with anti-PD-L1 blockade. Our results suggest that IL-6 is a key regulator of anti-cancer immune responses induced by genotoxic stress and that its inhibition can switch cancer cell clearance from primarily apoptotic to immunogenic, promoting and maintaining durable anti-tumor immune responses.