Conserved stromal-immune cell circuits secure B cell homeostasis and function.

B cell zone reticular cells (BRCs) form stable microenvironments that direct efficient humoral immunity with B cell priming and memory maintenance being orchestrated across lymphoid organs. However, a comprehensive understanding of systemic humoral immunity is hampered by the lack of knowledge of global BRC sustenance, function and major pathways controlling BRC-immune cell interactions. Here we dissected the BRC landscape and immune cell interactome in human and murine lymphoid organs. In addition to the major BRC subsets underpinning the follicle, including follicular dendritic cells, PI16+ RCs were present across organs and species. As well as BRC-produced niche factors, immune cell-driven BRC differentiation and activation programs governed the convergence of shared BRC subsets, overwriting tissue-specific gene signatures. Our data reveal that a canonical set of immune cell-provided cues enforce bidirectional signaling programs that sustain functional BRC niches across lymphoid organs and species, thereby securing efficient humoral immunity.

PI16+ reticular cells in human palatine tonsils govern T cell activity in distinct subepithelial niches.

Fibroblastic reticular cells (FRCs) direct the interaction and activation of immune cells in discrete microenvironments of lymphoid organs. Despite their important role in steering innate and adaptive immunity, the age- and inflammation-associated changes in the molecular identity and functional properties of human FRCs have remained largely unknown. Here, we show that human tonsillar FRCs undergo dynamic reprogramming during life and respond vigorously to inflammatory perturbation in comparison to other stromal cell types. The peptidase inhibitor 16 (PI16)-expressing reticular cell (PI16+ RC) subset of adult tonsils exhibited the strongest inflammation-associated structural remodeling. Interactome analysis combined with ex vivo and in vitro validation revealed that T cell activity within subepithelial niches is controlled by distinct molecular pathways during PI16+ RC-lymphocyte interaction. In sum, the topological and molecular definition of the human tonsillar stromal cell landscape reveals PI16+ RCs as a specialized FRC niche at the core of mucosal immune responses in the oropharynx.

An Innate Checkpoint Determines Immune Dysregulation and Immunopathology during Pulmonary Murine Coronavirus Infection.

Hallmarks of life-threatening, coronavirus-induced disease include dysregulated antiviral immunity and immunopathological tissue injury. Nevertheless, the sampling of symptomatic patients overlooks the initial inflammatory sequela culminating in severe coronavirus-induced disease, leaving a fundamental gap in our understanding of the early mechanisms regulating anticoronavirus immunity and preservation of tissue integrity. In this study, we delineate the innate regulators controlling pulmonary infection using a natural mouse coronavirus. Within hours of infection, the cellular landscape of the lung was transcriptionally remodeled altering host metabolism, protein synthesis, and macrophage maturation. Genetic perturbation revealed that these transcriptional programs were type I IFN dependent and critically controlled both host cell survival and viral spread. Unrestricted viral replication overshooting protective IFN responses culminated in increased IL-1ß and alarmin production and triggered compensatory neutrophilia, interstitial inflammation, and vascular injury. Thus, type I IFNs critically regulate early viral burden, which serves as an innate checkpoint determining the trajectory of coronavirus dissemination and immunopathology.

Postvaccination anti-S IgG levels predict anti-SARS-CoV-2 neutralising activity over 24 weeks in patients with RA.

OBJECTIVES: To correlate immune responses following a two-dose regimen of mRNA anti-SARS-CoV-2 vaccines in patients with rheumatoid arthritis (RA) to the development of a potent neutralising antiviral activity. METHODS: The RECOVER study was a prospective, monocentric study including patients with RA and healthy controls (HCs). Assessments were performed before, and 3, 6, 12 and 24 weeks, after the first vaccine dose, respectively, and included IgG, IgA and IgM responses (against receptor binding domain, S1, S2, N), IFN-γ ELISpots as well as neutralisation assays. RESULTS: In patients with RA, IgG responses developed slower with lower peak titres compared with HC. Potent neutralising activity assessed by a SARS-CoV-2 pseudovirus neutralisation assay after 12 weeks was observed in all 21 HCs, and in 60.3% of 73 patients with RA. A significant correlation between peak anti-S IgG levels 2 weeks after the second vaccine dose and potent neutralising activity against SARS-CoV-2 was observed at weeks 12 and 24. The analysis of IgG, IgA and IgM isotype responses to different viral proteins demonstrated a delay in IgG but not in IgA and IgM responses. T cell responses were comparable in HC and patients with RA but declined earlier in patients with RA. CONCLUSION: In patients with RA, vaccine-induced IgG antibody levels were diminished, while IgA and IgM responses persisted, indicating a delayed isotype switch. Anti-S IgG levels 2 weeks after the second vaccine dose correlate with the development of a potent neutralising activity after 12 and 24 weeks and may allow to identify patients who might benefit from additional vaccine doses or prophylactic regimen.

Differentiation and activation of fibroblastic reticular cells.

Secondary lymphoid organs (SLO) are underpinned by fibroblastic reticular cells (FRC) that form dedicated microenvironmental niches to secure induction and regulation of innate and adaptive immunity. Distinct FRC subsets are strategically positioned in SLOs to provide niche factors and govern efficient immune cell interaction. In recent years, the use of specialized mouse models in combination with single-cell transcriptomics has facilitated the elaboration of the molecular FRC landscape at an unprecedented resolution. While single-cell RNA-sequencing has advanced the resolution of FRC subset characterization and function, the high dimensionality of the generated data necessitates careful analysis and validation. Here, we reviewed novel findings from high-resolution transcriptomic analyses that refine our understanding of FRC differentiation and activation processes in the context of infection and inflammation. We further discuss concepts, strategies, and limitations for the analysis of single-cell transcriptome data from FRCs and the wide-ranging implications for our understanding of stromal cell biology.

Insights into coronavirus immunity taught by the murine coronavirus.

Coronaviruses (CoVs) represent enveloped, ss RNA viruses with the ability to infect a range of vertebrates causing mainly lung, CNS, enteric, and hepatic disease. While the infection with human CoV is commonly associated with mild respiratory symptoms, the emergence of SARS-CoV, MERS-CoV, and SARS-CoV-2 highlights the potential for CoVs to cause severe respiratory and systemic disease. The devastating global health burden caused by SARS-CoV-2 has spawned countless studies seeking clinical correlates of disease severity and host susceptibility factors, revealing a complex network of antiviral immune circuits. The mouse hepatitis virus (MHV) is, like SARS-CoV-2, a beta-CoV and is endemic in wild mice. Laboratory MHV strains have been extensively studied to reveal coronavirus virulence factors and elucidate host mechanisms of antiviral immunity. These are reviewed here with the aim to identify translational insights for SARS-CoV-2 learned from murine CoVs.

Development and Immunological Function of Lymph Node Stromal Cells.

Stromal cells have for a long time been viewed as structural cells that support distinct compartments within lymphoid tissues and little more. Instead, an active cross-talk between endothelial and fibroblastic stromal cells drives the maturation of lymphoid niches, a relationship that is recapitulated during lymph node organogenesis, steady-state conditions, and following inflammation. In this review, we go over recent advances in genetic models and high-resolution transcriptomic analyses that have propelled the finer resolution of the stromal cell infrastructure of lymph nodes, revealing that the distinct subsets are strategically positioned to deliver a catered mixture of niche factors to interacting immune cell populations. Moreover, we discuss how changes in the activation state of poised stromal cell-underpinned niches rather than on-demand differentiation of new stromal cell subsets govern the efficient interaction of Ag, APC, and cognate B and T lymphocytes during adaptive immune responses.

B cell zone reticular cell microenvironments shape CXCL13 gradient formation.

Through the formation of concentration gradients, morphogens drive graded responses to extracellular signals, thereby fine-tuning cell behaviors in complex tissues. Here we show that the chemokine CXCL13 forms both soluble and immobilized gradients. Specifically, CXCL13+ follicular reticular cells form a small-world network of guidance structures, with computer simulations and optimization analysis predicting that immobilized gradients created by this network promote B cell trafficking. Consistent with this prediction, imaging analysis show that CXCL13 binds to extracellular matrix components in situ, constraining its diffusion. CXCL13 solubilization requires the protease cathepsin B that cleaves CXCL13 into a stable product. Mice lacking cathepsin B display aberrant follicular architecture, a phenotype associated with effective B cell homing to but not within lymph nodes. Our data thus suggest that reticular cells of the B cell zone generate microenvironments that shape both immobilized and soluble CXCL13 gradients.

Remodeling of light and dark zone follicular dendritic cells governs germinal center responses.

Efficient generation of germinal center (GC) responses requires directed movement of B cells between distinct microenvironments underpinned by specialized B cell-interacting reticular cells (BRCs). How BRCs are reprogrammed to cater to the developing GC remains unclear, and studying this process is largely hindered by incomplete resolution of the cellular composition of the B cell follicle. Here we used genetic targeting of Cxcl13-expressing cells to define the molecular identity of the BRC landscape. Single-cell transcriptomic analysis revealed that BRC subset specification was predetermined in the primary B cell follicle. Further topological remodeling of light and dark zone follicular dendritic cells required CXCL12-dependent crosstalk with B cells and dictated GC output by retaining B cells in the follicle and steering their interaction with follicular helper T cells. Together, our results reveal that poised BRC-defined microenvironments establish a feed-forward system that determines the efficacy of the GC reaction.

Siponimod therapy implicates Th17 cells in a preclinical model of subpial cortical injury.

Subpial demyelination is a specific hallmark of multiple sclerosis and a correlate of disease progression. Although the mechanism(s) that mediate pathogenesis in the subpial compartment remain unclear, it has been speculated that inflammation in the overlying meninges may be associated with subpial injury. Here we show that adoptive transfer of proteolipid protein-primed Th17 cells into SJL/J recipient mice induces subpial demyelination associated with microglial/macrophage activation, disruption of the glial limitans, and evidence of an oxidative stress response. This pathology was topologically associated with foci of immune cells in the meninges and occurred in the absence of measurable anti-myelin oligodendrocyte glycoprotein IgM or IgG antibodies. To test the role of brain-infiltrating leukocytes on subpial injury, we modulated sphingosine 1-phosphate (S1P) receptor1,5 activity with BAF312 (siponimod) treatment. Administration of BAF312, even after adoptively transferred T cells had entered the brain, significantly ameliorated clinical experimental autoimmune encephalomyelitis and diminished subpial pathology, concomitant with a selective reduction in the capacity of transferred T cells to make Th17 cytokines. We conclude that sustained subpial cortical injury is associated with the capacity for brain-resident T cells to produce Th17 cytokines, and this pathological process occurs in an S1P receptor1,5-dependent manner.

Type I Interferon Signaling Disrupts the Hepatic Urea Cycle and Alters Systemic Metabolism to Suppress T Cell Function.

Infections induce complex host responses linked to antiviral defense, inflammation, and tissue damage and repair. We hypothesized that the liver, as a central metabolic hub, may orchestrate systemic metabolic changes during infection. We infected mice with chronic lymphocytic choriomeningitis virus (LCMV), performed RNA sequencing and proteomics of liver tissue, and integrated these data with serum metabolomics at different infection phases. Widespread reprogramming of liver metabolism occurred early after infection, correlating with type I interferon (IFN-I) responses. Viral infection induced metabolic alterations of the liver that depended on the interferon alpha/beta receptor (IFNAR1). Hepatocyte-intrinsic IFNAR1 repressed the transcription of metabolic genes, including Otc and Ass1, which encode urea cycle enzymes. This led to decreased arginine and increased ornithine concentrations in the circulation, resulting in suppressed virus-specific CD8+ T cell responses and ameliorated liver pathology. These findings establish IFN-I-induced modulation of hepatic metabolism and the urea cycle as an endogenous mechanism of immunoregulation. VIDEO ABSTRACT.

Origin and differentiation trajectories of fibroblastic reticular cells in the splenic white pulp.

The splenic white pulp is underpinned by poorly characterized stromal cells that demarcate distinct immune cell microenvironments. Here we establish fibroblastic reticular cell (FRC)-specific fate-mapping in mice to define their embryonic origin and differentiation trajectories. Our data show that all reticular cell subsets descend from multipotent progenitors emerging at embryonic day 19.5 from periarterial progenitors. Commitment of FRC progenitors is concluded during the first week of postnatal life through occupation of niches along developing central arterioles. Single cell transcriptomic analysis facilitated deconvolution of FRC differentiation trajectories and indicated that perivascular reticular cells function both as adult lymphoid organizer cells and mural cell progenitors. The lymphotoxin-ß receptor-independent sustenance of postnatal progenitor stemness unveils that systemic immune surveillance in the splenic white pulp is governed through subset specification of reticular cells from a multipotent periarterial progenitor cell. In sum, the finding that discrete signaling events in perivascular niches determine the differentiation trajectories of reticular cell networks explains the development of distinct microenvironmental niches in secondary and tertiary lymphoid tissues that are crucial for the induction and regulation of innate and adaptive immune processes.

Association of Checkpoint Inhibitor-Induced Toxic Effects With Shared Cancer and Tissue Antigens in Non-Small Cell Lung Cancer.

IMPORTANCE: Immunotherapy with checkpoint inhibitors targeting the PD-1 (programmed cell death 1) axis has brought notable progress in patients with non-small cell lung cancer (NSCLC) and other cancers. However, autoimmune toxic effects are frequent and poorly understood, making it important to understand the pathophysiologic processes of autoimmune adverse effects induced by checkpoint inhibitor therapy. OBJECTIVE: To gain mechanistic insight into autoimmune skin toxic effects induced by anti-PD-1 treatment in patients with non-small cell lung cancer. DESIGN, SETTING, AND PARTICIPANTS: This prospective cohort study was conducted from July 1, 2016, to December 31, 2018. Patients (n = 73) with non-small cell lung cancer who received anti-PD-1 therapy (nivolumab or pembrolizumab) were recruited from 4 different centers in Switzerland (Kantonsspital St Gallen, Spital Grabs, Spital Wil, and Spital Flawil). Peripheral blood mononuclear cells, tumor biopsy specimens and biopsies from sites of autoimmune skin toxic effects were collected over a 2-year period, with patient follow-up after 1 year. MAIN OUTCOMES AND MEASURES: Response to treatment, overall survival, progression-free survival, and development of autoimmune toxic effects (based on standard laboratory values and clinical examinations). RESULTS: Of the cohort of 73 patients with NSCLC (mean [SD] age, 68.1 [8.9] years; 44 [60%] men), 25 (34.2% [95% CI, 24.4%-45.7%]) developed autoimmune skin toxic effects, which were more frequent in patients with complete remission or partial remission (68.2% [95% CI, 47.3%-83.6%]) than those with progressive or stable disease (19.6% [95% CI, 11.0%-32.5%]) (χ2 = 14.02, P < .001). Nine T-cell antigens shared between tumor tissue and skin were identified. These antigens were able to stimulate CD8+ and CD4+ T cells in vitro. Several of the antigen-specific T cells found in blood samples were also present in autoimmune skin lesions and lung tumors of patients who responded to anti-PD-1 therapy. CONCLUSIONS AND RELEVANCE: These findings highlight a potential mechanism of checkpoint inhibitor-mediated autoimmune toxic effects and describe the association between toxic effects and response to therapy; such an understanding will help in controlling adverse effects, deciphering new cancer antigens, and further improving immunotherapy.

CCL19-producing fibroblastic stromal cells restrain lung carcinoma growth by promoting local antitumor T-cell responses.

BACKGROUND: A particular characteristic of non-small cell lung cancer is the composition of the tumor microenvironment with a very high proportion of fibroblastic stromal cells (FSCs). OBJECTIVE: Lapses in our basic knowledge of fibroblast phenotype and function in the tumor microenvironment make it difficult to define whether FSC subsets exist that exhibit either tumor-promoting or tumor-suppressive properties. METHODS: We used gene expression profiling of lung versus tumor FSCs from patients with non-small cell lung cancer. Moreover, CCL19-expressing FSCs were studied in transgenic mouse models by using a lung cancer metastasis model. RESULTS: CCL19 mRNA expression in human tumor FSCs correlates with immune cell infiltration and intratumoral accumulation of CD8+ T cells. Mechanistic dissection in murine lung carcinoma models revealed that CCL19-expressing FSCs form perivascular niches to promote accumulation of CD8+ T cells in the tumor. Targeted ablation of CCL19-expressing tumor FSCs reduced immune cell recruitment and resulted in unleashed tumor growth. CONCLUSION: These data suggest that a distinct population of CCL19-producing FSCs fosters the development of an immune-stimulating intratumoral niche for immune cells to control cancer growth.

Lymphatic Endothelial Cells Control Initiation of Lymph Node Organogenesis.

Lymph nodes (LNs) are strategically situated throughout the body at junctures of the blood vascular and lymphatic systems to direct immune responses against antigens draining from peripheral tissues. The current paradigm describes LN development as a programmed process that is governed through the interaction between mesenchymal lymphoid tissue organizer (LTo) cells and hematopoietic lymphoid tissue inducer (LTi) cells. Using cell-type-specific ablation of key molecules involved in lymphoid organogenesis, we found that initiation of LN development is dependent on LTi-cell-mediated activation of lymphatic endothelial cells (LECs) and that engagement of mesenchymal stromal cells is a succeeding event. LEC activation was mediated mainly by signaling through receptor activator of NF-κB (RANK) and the non-canonical NF-κB pathway and was steered by sphingosine-1-phosphate-receptor-dependent retention of LTi cells in the LN anlage. Finally, the finding that pharmacologically enforced interaction between LTi cells and LECs promotes ectopic LN formation underscores the central LTo function of LECs.

Stromal Cell Niches in the Inflamed Central Nervous System.

Inflammation in the CNS must be tightly regulated to respond efficiently to infection with neurotropic pathogens. Access of immune cells to the CNS and their positioning within the tissue are controlled by stromal cells that construct the barriers of the CNS. Although the role of the endothelium in regulating the passage of leukocytes and small molecules into the CNS has been studied extensively, the contribution of fibroblastic stromal cells as portals of entry into the CNS was only recently uncovered. We review the critical immune-stimulating role of meningeal fibroblasts in promoting recruitment and retention of lymphocytes during CNS inflammation. Activated meningeal fibroblastic stromal cells have the capacity to rapidly elaborate an immune-competent niche that sustains protective immune cells entering the CNS from the draining cervical lymph node. Such stromal cell niches can ultimately foster the establishment of tertiary lymphoid tissues during chronic neuroinflammatory conditions.

Integration of Th17- and Lymphotoxin-Derived Signals Initiates Meningeal-Resident Stromal Cell Remodeling to Propagate Neuroinflammation.

Tertiary lymphoid tissues (TLTs) have been observed in the meninges of multiple sclerosis (MS) patients, but the stromal cells and molecular signals that support TLTs remain unclear. Here, we show that T helper 17 (Th17) cells induced robust TLTs within the brain meninges that were associated with local demyelination during experimental autoimmune encephalitis (EAE). Th17-cell-induced TLTs were underpinned by a network of stromal cells producing extracellular matrix proteins and chemokines, enabling leukocytes to reside within, rather than simply transit through, the meninges. Within the CNS, interactions between lymphotoxin αß (LTαß) on Th17 cells and LTßR on meningeal radio-resistant cells were necessary for the propagation of de novo interleukin-17 responses, and activated T cells from MS patients expressed elevated levels of LTßR ligands. Therefore, input from both Th17 cells and the lymphotoxin pathway induce the formation of an immune-competent stromal cell niche in the meninges.

B Cells in the Multiple Sclerosis Central Nervous System: Trafficking and Contribution to CNS-Compartmentalized Inflammation.

Clinical trial results of peripheral B cell depletion indicate abnormal proinflammatory B cell properties, and particularly antibody-independent functions, contribute to relapsing MS disease activity. However, potential roles of B cells in progressive forms of disease continue to be debated. Prior work indicates that presence of B cells is fostered within the inflamed MS central nervous system (CNS) environment, and that B cell-rich immune cell collections may be present within the meninges of patients. A potential association is reported between such meningeal immune cell collections and the subpial pattern of cortical injury that is now considered important in progressive disease. Elucidating the characteristics of B cells that populate the MS CNS, how they traffic into the CNS and how they may contribute to progressive forms of the disease has become of considerable interest. Here, we will review characteristics of human B cells identified within distinct CNS subcompartments of patients with MS, including the cerebrospinal fluid, parenchymal lesions, and meninges, as well as the relationship between B cell populations identified in these subcompartments and the periphery. We will further describe the different barriers of the CNS and the possible mechanisms of migration of B cells across these barriers. Finally, we will consider the range of human B cell responses (including potential for antibody production, cytokine secretion, and antigen presentation) that may contribute to propagating inflammation and injury cascades thought to underlie MS progression.

Meningeal Tertiary Lymphoid Tissues and Multiple Sclerosis: A Gathering Place for Diverse Types of Immune Cells during CNS Autoimmunity.

Collections of leukocytes in the meningeal space have been documented in Multiple Sclerosis (MS). These meningeal aggregates, which in the context of other autoimmune diseases have often been termed tertiary lymphoid tissues (TLT), have been associated with sub-pial cortical damage and disease progression. However, the key molecular and cellular signals required for their formation and maintenance remain unclear. Herein, we review TLT structures in other disease states in order to provide a framework for understanding these structures in the MS meninges. We then assess the evidence that the meningeal compartment serves as an important nexus for immune cells as well as a location for drainage of antigen into cervical lymph nodes. Extrapolating what is known about the molecular and cellular cues that initiate the formation of leukocyte aggregates in non-lymphoid tissues, we speculate on what signals lead to the formation and maintenance of meningeal TLT structures. Referring to the animal model of MS [experimental autoimmune encephalomyelitis (EAE)], we also explore what is known about these structures in supporting B cell and T cell responses during neuroinflammation. Last, we examine the evidence that connects these structures to ongoing neuropathology. Collectively, our review points to the meningeal compartment as an important player in neuroinflammatory processes. Moreover, we hypothesize that in order to gain insights into pro- and anti-inflammatory properties of lymphocytes in MS, one must understand the cellular scaffolds that support lymphocyte retention within the meninges, thus highlighting the importance of non-immune cells (stromal cells) in the neuroinflammatory process.

Secondary B cell receptor diversification is necessary for T cell mediated neuro-inflammation during experimental autoimmune encephalomyelitis.

BACKGROUND: Clinical studies of B cell depletion in Multiple Sclerosis (MS) have revealed that B Lymphocytes are involved in the neuro-inflammatory process, yet it remains unclear how B cells can exert pro- and anti-inflammatory functions during MS. Experimental Autoimmune Encephalomyelitis (EAE) is an animal model of MS whereby myelin-specific T cells become activated and subsequently migrate to the Central Nervous System (CNS) where they perform pro-inflammatory functions such as cytokine secretion. Typically EAE is induced by immunization of mice of a susceptible genetic background with peptide antigen emulsified in Complete Freund's Adjuvant. However, novel roles for B-lymphocytes in EAE may also be explored by immunization with full-length myelin oligodendrocyte glycoprotein (MOG) that contains the B cell conformational epitope. Here we show that full length MOG immunization promotes a chronic disease in mice that depends on antigen-driven secondary diversification of the B cell receptor. METHODS: Activation-Induced Deaminase (AID) is an enzyme that is essential for antigen-driven secondary diversification of the B cell receptor. We immunized AID(-/-) mice with the extracellular domain (amino acids 1-120) of recombinant human MOG protein (rhMOG) and examined the incidence and severity of disease in AID(-/-) versus wild type mice. Corresponding with these clinical measurements, we also evaluated parameters of T cell activation in the periphery and the CNS as well as the generation of anti-MOG antibodies (Ab). CONCLUSIONS: AID(-/-) mice exhibit reduced severity and incidence of EAE. This suggests that the secondary diversification of the B cell receptor is required for B cells to exert their full encephalogenic potential during rhMOG-induced EAE, and possibly also during MS.