Intracellular immune sensing promotes inflammation via gasdermin D-driven release of a lectin alarmin

Inflammatory caspase sensing of cytosolic lipopolysaccharide (LPS) triggers pyroptosis and the concurrent release of damage-associated molecular patterns (DAMPs). Collectively, DAMPs are key determinants that shape the aftermath of inflammatory cell death. However, the identity and function of the individual DAMPs released are poorly defined. Our proteomics study revealed that cytosolic LPS sensing triggered the release of galectin-1, a β-galactoside-binding lectin. Galectin-1 release is a common feature of inflammatory cell death, including necroptosis. In vivo studies using galectin-1-deficient mice, recombinant galectin-1 and galectin-1-neutralizing antibody showed that galectin-1 promotes inflammation and plays a detrimental role in LPS-induced lethality. Mechanistically, galectin-1 inhibition of CD45 (Ptprc) underlies its unfavorable role in endotoxin shock. Finally, we found increased galectin-1 in sera from human patients with sepsis. Overall, we uncovered galectin-1 as a bona fide DAMP released as a consequence of cytosolic LPS sensing, identifying a new outcome of inflammatory cell death.

Cymerus™ iPSC-MSCs significantly prolong survival in a pre-clinical, humanized mouse model of Graft-vs-host disease

The immune-mediated tissue destruction of graft-vs-host disease (GvHD) remains a major barrier to greater use of hematopoietic stem cell transplantation (HSCT). Mesenchymal stem cells (MSCs) have intrinsic immunosuppressive qualities and are being actively investigated as a therapeutic strategy for treating GvHD. We characterized Cymerus™ MSCs, which are derived from adult, induced pluripotent stem cells (iPSCs), and show they display surface markers and tri-lineage differentiation consistent with MSCs isolated from bone marrow (BM). Administering iPSC-MSCs altered phosphorylation and cellular localization of the T cell-specific kinase, Protein Kinase C theta (PKCθ), attenuated disease severity, and prolonged survival in a humanized mouse model of GvHD. Finally, we evaluated a constellation of pro-inflammatory molecules on circulating PBMCs that correlated closely with disease progression and which may serve as biomarkers to monitor therapeutic response. Altogether, our data suggest Cymerus iPSC-MSCs offer the potential for an off-the-shelf, cell-based therapy to treat GvHD.

NOTCH1 Can Initiate NF-κB Activation via Cytosolic Interactions with Components of the T Cell Signalosome

T cell stimulation requires the input and integration of external signals. Signaling through the T cell receptor (TCR) is known to induce formation of the membrane-tethered CBM complex, comprising CARMA1, BCL10, and MALT1, which is required for TCR-mediated NF-κB activation. TCR signaling has been shown to activate NOTCH proteins, transmembrane receptors also implicated in NF-κB activation. However, the link between TCR-mediated NOTCH signaling and early events leading to induction of NF-κB activity remains unclear. In this report, we demonstrate a novel cytosolic function for NOTCH1 and show that it is essential to CBM complex formation. Using a model of skin allograft rejection, we show in vivo that NOTCH1 acts in the same functional pathway as PKCθ, a T cell-specific kinase important for CBM assembly and classical NF-κB activation. We further demonstrate in vitro NOTCH1 associates physically with PKCθ and CARMA1 in the cytosol. Unexpectedly, when NOTCH1 expression was abrogated using RNAi approaches, interactions between CARMA1, BCL10, and MALT1 were lost. This failure in CBM assembly reduced inhibitor of kappa B alpha phosphorylation and diminished NF-κB–DNA binding. Finally, using a luciferase gene reporter assay, we show the intracellular domain of NOTCH1 can initiate robust NF-κB activity in stimulated T cells, even when NOTCH1 is excluded from the nucleus through modifications that restrict it to the cytoplasm or hold it tethered to the membrane. Collectively, these observations provide evidence that NOTCH1 may facilitate early events during T cell activation by nucleating the CBM complex and initiating NF-κB signaling.

Immunity to Respiratory Infection Is Reinforced Through Early Proliferation of Lymphoid TRM Cells and Prompt Arrival of Effector CD8 T Cells in the Lungs

Cross-protection between serologically distinct strains of influenza A virus (IAV) is mediated by memory CD8 T cells that recognize epitopes from conserved viral proteins. Early viral control begins with activation of tissue-resident memory CD8 T cells (T_RM) cells at the site of viral replication. These CD8 T cells do not act in isolation, as protection against disseminated infection is reinforced by multiple waves of effector cells (T_EFF) that enter the lungs with different kinetics. To define how a protective CTL response evolves, we compared the functional properties of antiviral CD8 T cells in the respiratory tract and local lymphoid tissues. When analyzed 30 dpi, large numbers of antiviral CD8 T cells in the lungs and mediastinal lymph nodes (MLNs) expressed canonical markers of T_RM cells (CD69 and/or CD103). The check point inhibitor PD-1 was also highly expressed on NP-specific CD8 T cells in the lungs, while the ratios of CD8 T cells expressing CD69 and CD103 varied according to antigen specificity. We next used in vitro experiments to identify conditions that induce a canonical T_RM phenotype and found that that naïve and newly activated CD8 T cells maintain CD103 expression during culture with transforming growth factor-beta (TGFβ), while central memory CD8 T cells (T_CM) do not express CD103 under similar conditions. In vivo experiments showed that the distribution of antiviral CTLs in the MLN changed when immune mice were treated with reagents that block interactions with PD-L1. Importantly, the lymphoid T_RM cells were poised for early proliferation upon reinfection with a different strain of IAV and defenses in the lungs were augmented by a transient increase in numbers of T_EFF cells at the site of infection. As the interval between infections increased, lymphoid T_RM cells were replaced with T_CM cells which proliferated with delayed kinetics and contributed to an exaggerated inflammatory response in the lungs.

Notch1 primes CD4 T cells for T helper type I differentiation through its early effects on miR-29

The transmembrane receptor, Notch1 plays an important role during the differentiation of CD4 T cells into T helper (Th) subsets in the presence of appropriate cytokines, including differentiation into Th1 cells. MicroRNAs have also been shown to be important regulators of immune responses, including negatively regulating cytokine production by Th1 cells. The miR-29 family of microRNAs can act to inhibit tbx21 and ifng transcription, two important pro-inflammatory genes that are abundantly expressed in Th1 cells. Here we show that Notch1 may prime CD4 T cells to be responsive to Th1-polarizing cues through its early repressive effects on the miR-29 family of microRNAs. Using a combination of cell lines and primary cells, we demonstrate that Notch1 can repress miR-29a, miR-29b, and miR-29c transcription through a mechanism that is independent of NF-κB. We further show that this repression is mediated by canonical Notch signaling and requires active Mastermind like (MAML) 1, but this process is superseded by positive regulation of miR-29 in response to IFNγ at later stages of CD4 T cell activation and differentiation. Collectively, our data suggest an additional mechanism by which Notch1 signaling may fine-tune Th1 cell differentiation.

CTLs Get SMAD When Pathogens Tell Them Where to Go

Vaccines protect against infections by eliciting both Ab and T cell responses. Because the immunity wanes as protective epitopes get modified by accruing mutations, developing strategies for immunization against new variants is a major priority for vaccine development. CTLs eliminate cells that support viral replication and provide protection against new variants by targeting epitopes from internal viral proteins. This form of protection has received limited attention during vaccine development, partly because reliable methods for directing pathogen-specific memory CD8 T cells to vulnerable tissues are currently unavailable. In this review we examine how recent studies expand our knowledge of mechanisms that contribute to the functional diversity of CTLs as they respond to infection. We discuss the role of TGF-β and the SMAD signaling cascade during genetic programming of pathogen-specific CTLs and the pathways that promote formation of a newly identified subset of terminally differentiated memory CD8 T cells that localize in the vasculature.

SMAD4 and TGFβ are architects of inverse genetic programs during fate-determination of antiviral CTLs

Transforming growth factor β (TGFβ) is an important differentiation factor for cytotoxic T lymphocytes (CTLs) and alters the expression levels of several of homing receptors during infection. SMAD4 is part of the canonical signaling network used by members of the transforming growth factor family. For this study, genetically modified mice were used to determine how SMAD4 and TGFβ receptor II (TGFβRII) participate in transcriptional programming of pathogen-specific CTLs. We show that these molecules are essential components of opposing signaling mechanisms, and cooperatively regulate a collection of genes that determine whether specialized populations of pathogen-specific CTLs circulate around the body, or settle in peripheral tissues. TGFβ uses a canonical SMAD-dependent signaling pathway to downregulate Eomesodermin (EOMES), KLRG1, and CD62L, while CD103 is induced. Conversely, in vivo and in vitro data show that EOMES, KLRG1, CX₃CR1, and CD62L are positively regulated via SMAD4, while CD103 and Hobit are downregulated. Intravascular staining also shows that signaling via SMAD4 promotes formation of long-lived terminally differentiated CTLs that localize in the vasculature. Our data show that inflammatory molecules play a key role in lineage determination of pathogen-specific CTLs, and use SMAD-dependent signaling to alter the expression levels of multiple homing receptors and transcription factors with known functions during memory formation.

Smad signaling determines the fate of activated CTLs via multiple intersecting signaling pathways

Immune responses to an infection are controlled by various factors including antigens, co-stimulatory molecules and cytokines present in the tissue micro-environment. These factors modulate the differentiation of effector CD8 T cells and memory subsets. TGFβ plays a pivotal role in fate determination of activated CTLs. In TGFβ pathway, Smad proteins are critical for downstream signaling. Using transgenic animal models and pharmacological inhibitors, we investigate the role of TGFβ and Smad proteins in fate determination of activated CTLs. We identified that TGFβ and Smad proteins play important roles in regulating the expression of several homing receptors on effector CD8 T cells (KLRG1) and memory subsets (CD62L, S1PR1 and CD103). In addition, targeted-ablation of different Smad proteins alter the expression levels of different transcription factors that participate in fate determination, including Eomes and KLF2. We found that while Smad proteins regulate the gene expression in canonical TGFβ pathway, they also have TGFβ independent functions in determining the fate of activated CTLs. Particularly, we show that Smad4 is required for the expression of KLRG1 and acts as a suppressor of CD103, independent of TGFβ. This study reveals a novel role for Smad signaling cascade in guiding the fate decisions of activated CTLs and tissue localization via multiple intersecting signaling pathways.

Notch-1 regulation of microRNAs alter the expression of proinflammatory cytokines and Th1 response

Notch signaling is required for activation and differentiation of T cells. Upon its S3 cleavage by gamma secretase, Notch intracellular domain triggers the expression of various molecules depending on its binding partners. The data published from our lab states that Notch1 can induce Th1 immune response and regulates the expression of IFNγ by modulating the expression of the transcription factor T-bet. However the detailed mechanism behind Notch1 mediated Th1 differentiation is not clear. Here we report that Notch1 can regulate Th1 differentiation by targeting microRNAs. miR-29 is a family of micro RNAs (miR29a,b,c) that targets ifng and both its transcription factors tbx21 and eomes directly by binding to their respective 3′ UTRs. By using novel gamma secretase inhibitors (GSIs) we show that Notch can modulate the expression of miR-29. Overexpression of Notch1 in T cell hybridoma cell lines further validates Notch1- mediated mirR-29 regulation. These results indicate a novel regulatory mechanism of T cell differentiation through Notch1 mediated microRNA expression.

Autoregulated splicing of TRA2β programs T cell fate in response to antigen-receptor stimulation

T cell receptor (TCR) sensitivity to peptide-major histocompatibility complex (MHC) dictates T cell fate. Canonical models of TCR sensitivity cannot be fully explained by transcriptional regulation. In this work, we identify a posttranscriptional regulatory mechanism of TCR sensitivity that guides alternative splicing of TCR signaling transcripts through an evolutionarily ultraconserved poison exon (PE) in the RNA-binding protein (RBP) TRA2β in mouse and human. TRA2β-PE splicing, seen during cancer and infection, was required for TCR-induced effector T cell expansion and function. Tra2β-PE skipping enhanced T cell response to antigen by increasing TCR sensitivity. As antigen levels decreased, Tra2β-PE reinclusion allowed T cell survival. Finally, we found that TRA2β-PE was first included in the genome of jawed vertebrates that were capable of TCR gene rearrangements. We propose that TRA2β-PE splicing acts as a gatekeeper of TCR sensitivity to shape T cell fate.

Smad signaling determines the fate of activated CTLs via multiple intersecting signaling pathways

Immune responses to an infection are controlled by various factors including antigens, co-stimulatory molecules and cytokines present in the tissue micro-environment. TGFβ is a pleiotropic cytokine that modulates immune response and cellular differentiation usually through SMAD (Smad 2,3 & 4)-mediated signaling cascade. Interestingly, TGFβ and Smad4 have independent and opposing roles in determining the fate of activated CTLs. While TGFβ-receptor mediated signaling is required for resident memory (TRM) cells, Smad4 is required for effector (TEff) and central memory subsets (TCM). However, the mechanism behind TGFβ as well as Smad4 mediated differentiation is yet to be unraveled. Using transgenic animal models and pharmacological inhibitors, we investigate the role of TGFβ and multiple SMAD proteins in determining the fate of activated CTLs. We identified that while canonical TGFβ signaling, through Smad2 and Smad3, induces CD103 expression (TRM), Smad4 inhibits CD103 expression thus preventing TRM differentiation. In addition, targeted-ablation of Smad4 alters the expression of various transcription factors that participate in fate determination, including Eomes and KLF2. Although Eomes has previously been implicated in negative regulation of CD103, over-expression of Eomes does not affect CD103 expression in the absence of Smad4. This suggests that Smad4 is required for negative regulation of CD103. These results reveal a novel central role for Smad4 in guiding the fate decisions of activated CTLs and tissue localization via multiple intersecting signaling pathways.

Localization of terminal memory CD8 T cells in the lungs during respiratory infection

Cytotoxic CD8 T cells (CTLs) play an important role in recovery from respiratory virus infection and provide protective immunity by targeting epitopes in well-conserved viral proteins that are shared by different strains of the virus. To provide immunity, CTLs migrate to different anatomical compartments in the lungs and destroy infected cells by injecting lytic molecules into the cytoplasm. Several adhesion molecules aid migration to the infected tissues and cytolysis. These molecules include killer lectin like receptor G1 (KLRG1), which is a membrane bound adhesion molecule that is expressed on terminally differentiated CD8 T effector (Teff) cells. Recently, several studies identified some long lived “terminal memory CD8 T cells (TTM)” that express KLRG1. The goal of our study is to determine the location of these TTM cells within the different compartments of the lungs during a respiratory infection. Our previous work showed that SMAD4 regulates several homing receptors on activated CTLs including KLRG1. We used transfer experiments to examine the distribution of WT cells that express SMAD4 and mutant CTLs that lack SMAD4 in the lungs. At 30 days after intranasal infection with Listeria monocytogenes, there were larger numbers of mutant CTLs in the airways (BAL fluid), while WT CTLs were primarily located in the lung tissue. Interestingly, the localization of the WT cells correlated with the expression of KLRG1. We are using in situ imaging to visualize these transferred CTLs during respiratory infection to better understand their role in protective immunity. Intravascular staining will be used to determine whether these CTLs remain in the blood vessels or enter the tissues of the infected recipient mice.

This work was supported by NIH Grant AI123864.

The diverse effects of transforming growth factor-β and SMAD signaling pathways during the CTL response

Cytotoxic T lymphocytes (CTLs) play an important role in defense against infections with intracellular pathogens and anti-tumor immunity. Efficient migration is required to locate and destroy infected cells in different regions of the body. CTLs accomplish this task by differentiating into specialized subsets of effector and memory CD8 T cells that traffic to different tissues. Transforming growth factor-beta (TGFβ) belongs to a large family of growth factors that elicit diverse cellular responses via canonical and non-canonical signaling pathways. Canonical SMAD-dependent signaling pathways are required to coordinate changes in homing receptor expression as CTLs traffic between different tissues. In this review, we discuss the various ways that TGFβ and SMAD-dependent signaling pathways shape the cellular immune response and transcriptional programming of newly activated CTLs. As protective immunity requires access to the circulation, emphasis is placed on cellular processes that are required for cell-migration through the vasculature.