PRMT5 Promotes T follicular helper Cell Differentiation and Germinal Center Responses during Influenza Virus Infection

Protein arginine methyltransferases (PRMTs) modify diverse protein targets and regulate numerous cellular processes; yet, their contributions to individual effector T cell responses during infections are incompletely understood. In this study, we identify PRMT5 as a critical regulator of CD4+ T follicular helper cell (Tfh) responses during influenza virus infection in mice. Conditional PRMT5 deletion in murine T cells results in an almost complete ablation of both Tfh and T follicular regulatory populations and, consequently, reduced B cell activation and influenza-specific Ab production. Supporting a potential mechanism, we observe elevated surface expression of IL-2Rα on non-T regulatory effector PRMT5-deficient T cells. Notably, IL-2 signaling is known to negatively impact Tfh differentiation. Collectively, our findings identify PRMT5 as a prominent regulator of Tfh programming, with potential causal links to IL-2 signaling.

CD8+ T cells contribute to diet-induced memory deficits in aged male rats

We have previously shown that short-term (3-day) high fat diet (HFD) consumption induces a neuroinflammatory response and subsequent impairment of long-term memory in aged, but not young adult, male rats. However, the immune cell phenotypes driving this proinflammatory response are not well understood. Previously, we showed that microglia isolated from young and aged rats fed a HFD express similar levels of priming and proinflammatory transcripts, suggesting that additional factors may drive the exaggerated neuroinflammatory response selectively observed in aged HFD-fed rats. It is established that T cells infiltrate both the young and especially the aged central nervous system (CNS) and contribute to immune surveillance of the parenchyma. Thus, we investigated the modulating role of short-term HFD on T cell presence in the CNS in aged rats using bulk RNA sequencing and flow cytometry. RNA sequencing results indicate that aging and HFD altered the expression of genes and signaling pathways associated with T cell signaling, immune cell trafficking, and neuroinflammation. Moreover, flow cytometry data showed that aging alone increased CD4+ and CD8+ T cell presence in the brain and that CD8+, but not CD4+, T cells were further increased in aged rats fed a HFD. Based on these data, we selectively depleted circulating CD8+ T cells via an intravenous injection of an anti-CD8 antibody in aged rats prior to 3 days of HFD to infer the functional role these cells may be playing in long-term memory and neuroinflammation. Results indicate that peripheral depletion of CD8+ T cells lowered hippocampal cytokine levels and prevented the HFD-induced i) increase in brain CD8+ T cells, ii) memory impairment, and iii) alterations in pre- and post-synaptic structures in the hippocampus and amygdala. Together, these data indicate a substantial role for CD8+ T cells in mediating diet-induced memory impairments in aged male rats.

Sterols and immune mechanisms in asthma

The field of sterol and oxysterol biology in lung disease has recently gained attention, revealing a unique need for sterol uptake and metabolism in the lung. The presence of cholesterol transport, biosynthesis, and sterol/oxysterol-mediated signaling in immune cells suggests a role in immune regulation. In support of this idea, statin drugs that inhibit the cholesterol biosynthesis rate-limiting step enzyme, hydroxymethyl glutaryl coenzyme A reductase, show immunomodulatory activity in several models of inflammation. Studies in human asthma reveal contradicting results, whereas promising retrospective studies suggest benefits of statins in severe asthma. Here, we provide a timely review by discussing the role of sterols in immune responses in asthma, analytical tools to evaluate the role of sterols in disease, and potential mechanistic pathways and targets relevant to asthma. Our review reveals the importance of sterols in immune processes and highlights the need for further research to solve critical gaps in the field.

Sterols in asthma

While sterols regulate immune processes key to the pathogenesis of asthma, inhibition of sterols with statin drugs has shown conflicting results in human asthma. Here, a novel understanding of the impact of sterols on type 17 immune responses and asthma lead us to hypothesize that sterols and statins may be relevant to severe asthma endotypes with neutrophil infiltration.

Akt isoforms in the immune system

Akt is a PI3K-activated serine-threonine kinase that exists in three distinct isoforms. Akt's expression in most immune cells, either at baseline or upon activation, reflects its importance in the immune system. While Akt is most highly expressed in innate immune cells, it plays crucial roles in both innate and adaptive immune cell development and/or effector functions. In this review, we explore what's known about the role of Akt in innate and adaptive immune cells. Wherever possible, we discuss the overlapping and distinct role of the three Akt isoforms, namely Akt1, Akt2, and Akt3, in immune cells.

PRMT5 in T Cells Drives Th17 Responses, Mixed Granulocytic Inflammation, and Severe Allergic Airway Inflammation

Severe asthma is characterized by steroid insensitivity and poor symptom control and is responsible for most asthma-related hospital costs. Therapeutic options remain limited, in part due to limited understanding of mechanisms driving severe asthma. Increased arginine methylation, catalyzed by protein arginine methyltransferases (PRMTs), is increased in human asthmatic lungs. In this study, we show that PRMT5 drives allergic airway inflammation in a mouse model reproducing multiple aspects of human severe asthma. We find that PRMT5 is required in CD4⁺ T cells for chronic steroid-insensitive severe lung inflammation, with selective T cell deletion of PRMT5 robustly suppressing eosinophilic and neutrophilic lung inflammation, pathology, airway remodeling, and hyperresponsiveness. Mechanistically, we observed high pulmonary sterol metabolic activity, retinoic acid-related orphan receptor γt (RORγt), and Th17 responses, with PRMT5-dependent increases in RORγt's agonist desmosterol. Our work demonstrates that T cell PRMT5 drives severe allergic lung inflammation and has potential implications for the pathogenesis and therapeutic targeting of severe asthma.

The Minority Scientists' Experience: Challenging and Overcoming Barriers to Enhancing Diversity and Career Advancement

Minority groups face barriers in accessing quality health care, professional advancement, and representation in immunology research efforts as a result of institutional racism that if unaddressed can perpetuate a lack of diversity. In 2021, the AAI Minority Affairs Committee convened a cross section of academic and industry scientists from underrepresented groups at various stages of their professions to discuss how best to address the toll racism takes on study design and scientific careers. Panelists drew directly from their own experiences as scientists to share perspectives and strategies for countering a lack of representation in clinical research, responding to microaggressions, navigating academic advancement, and providing effective mentorship. The session reinforced the need for minority scientists to take an active role in advocating for diversity, engaging mentors, and taking responsibility to face rather than avoid institutional obstacles. Overall, increased dialogue and institutional awareness of the experience of scientists from underrepresented groups in research remain the best tools to ensure a health equity mindset and advancement of their careers.

Corticosteroid insensitivity persists in the absence of STAT1 signaling in severe allergic airway inflammation

Corticosteroid insensitivity in asthma limits the ability to effectively manage severe asthma, which is characterized by persistent airway inflammation, airway hyperresponsiveness (AHR), and airflow obstruction despite corticosteroid treatment. Recent reports indicate that corticosteroid insensitivity is associated with increased interferon-γ (IFN-γ) levels and T-helper (Th) 1 lymphocyte infiltration in severe asthma. Signal transducer and activator of transcription 1 (STAT1) activation by IFN-γ is a key signaling pathway in Th1 inflammation; however, its role in the context of severe allergic airway inflammation and corticosteroid sensitivity remains unclear. In this study, we challenged wild-type (WT) and Stat1^-/- mice with mixed allergens (MA) augmented with c-di-GMP [bis-(3'-5')-cyclic dimeric guanosine monophosphate], an inducer of Th1 cell infiltration with increased eosinophils, neutrophils, Th1, Th2, and Th17 cells. Compared with WT mice, Stat1^-/- had reduced neutrophils, Th1, and Th17 cell infiltration. To evaluate corticosteroid sensitivity, mice were treated with either vehicle, 1 or 3 mg/kg fluticasone propionate (FP). Corticosteroids significantly reduced eosinophil infiltration and cytokine levels in both c-di-GMP + MA-challenged WT and Stat1^-/- mice. However, histological and functional analyses show that corticosteroids did not reduce airway inflammation, epithelial mucous cell abundance, airway smooth muscle mass, and AHR in c-di-GMP + MA-challenged WT or Stat1^-/- mice. Collectively, our data suggest that increased Th1 inflammation is associated with a decrease in corticosteroid sensitivity. However, increased airway pathology and AHR persist in the absence of STAT1 indicate corticosteroid insensitivity in structural airway cells is a STAT1 independent process.

PRMT5 Promotes Symmetric Dimethylation of RNA Processing Proteins and Modulates Activated T Cell Alternative Splicing and Ca2+/NFAT Signaling

Protein arginine methyltransferase (PRMT) 5 is the type 2 methyltransferase catalyzing symmetric dimethylation of arginine. PRMT5 inhibition or deletion in CD4 Th cells reduces TCR engagement-induced IL-2 production and Th cell expansion and confers protection against experimental autoimmune encephalomyelitis, the animal model of multiple sclerosis. However, the mechanisms by which PRMT5 modulates Th cell proliferation are still not completely understood, and neither are the methylation targets in T cells. In this manuscript, we uncover the role of PRMT5 on alternative splicing in activated mouse T cells and identify several targets of PRMT5 symmetric dimethylation involved in splicing. In addition, we find a possible link between PRMT5-mediated alternative splicing of transient receptor potential cation channel subfamily M member 4 (Trpm4) and TCR/NFAT signaling/IL-2 production. This understanding may guide development of drugs targeting these processes to benefit patients with T cell-mediated diseases.

Dexamethasone and Fumaric Acid Ester Conjugate Synergistically Inhibits Inflammation and NF-κB in Macrophages

Macrophage-mediated inflammation drives autoimmune and chronic inflammatory diseases. Treatment with anti-inflammatory agents can be an effective strategy to reduce this inflammation; however, high concentrations of these agents can have immune-dampening and other serious side effects. Synergistic combination of anti-inflammatory agents can mitigate dosing by requiring less drug. Multiple anti-inflammatory agents were evaluated in combination for synergistic inhibition of macrophage inflammation. The most potent synergy was observed between dexamethasone (DXM) and fumaric acid esters (e.g., monomethyl fumarate (MMF)). Furthermore, this combination was found to synergistically inhibit inflammatory nuclear factor κB (NF-κB) transcription factor activity. The optimal ratio for synergy was determined to be 1:1, and DXM and MMF were conjugated by esterification at this molar ratio. The DXM-MMF conjugate displayed improved inhibition of inflammation over the unconjugated combination in both murine and human macrophages. In the treatment of human donor monocyte-derived macrophages, the combination of DXM and MMF significantly inhibited inflammatory gene expression downstream of NF-κB and overall performed better than either agent alone. Further, the DXM-MMF conjugate significantly inhibited expression of NOD-, LRR-, and pyrin domain-containing protein 3 (NLRP3) inflammasome-associated genes. The potent anti-inflammatory activity of the DXM-MMF conjugate in human macrophages indicates that it may have benefits in the treatment of autoimmune and inflammatory diseases.

PRMT5 Promotes Cyclin E1 and Cell Cycle Progression in CD4 Th1 Cells and Correlates With EAE Severity

Multiple Sclerosis (MS) is a debilitating central nervous system disorder associated with inflammatory T cells. Activation and expansion of inflammatory T cells is thought to be behind MS relapses and influence disease severity. Protein arginine N-methyltransferase 5 (PRMT5) is a T cell activation-induced enzyme that symmetrically dimethylates proteins and promotes T cell proliferation. However, the mechanism behind PRMT5-mediated control of T cell proliferation and whether PRMT5 contributes to diseases severity is unclear. Here, we evaluated the role of PRMT5 on cyclin/cdk pairs and cell cycle progression, as well as PRMT5's link to disease severity in an animal model of relapsing-remitting MS. Treatment of T helper 1 (mTh1) cells with the selective PRMT5 inhibitor, HLCL65, arrested activation-induced T cell proliferation at the G1 stage of the cell cycle, suggesting PRMT5 promotes cell cycle progression in CD4+ T cells. The Cyclin E1/Cdk2 pair promoting G1/S progression was also decreased after PRMT5 inhibition, as was the phosphorylation of retinoblastoma. In the SJL mouse relapsing-remitting model of MS, the highest PRMT5 expression in central nervous system-infiltrating cells corresponded to peak and relapse timepoints. PRMT5 expression also positively correlated with increasing CD4 Th cell composition, disease severity and Cyclin E1 expression. These data indicate that PRMT5 promotes G1/S cell cycle progression and suggest that this effect influences disease severity and/or progression in the animal model of MS. Modulating PRMT5 levels may be useful for controlling T cell expansion in T cell-mediated diseases including MS.

CD38: An Immunomodulatory Molecule in Inflammation and Autoimmunity

CD38 is a molecule that can act as an enzyme, with NAD-depleting and intracellular signaling activity, or as a receptor with adhesive functions. CD38 can be found expressed either on the cell surface, where it may face the extracellular milieu or the cytosol, or in intracellular compartments, such as endoplasmic reticulum, nuclear membrane, and mitochondria. The main expression of CD38 is observed in hematopoietic cells, with some cell-type specific differences between mouse and human. The role of CD38 in immune cells ranges from modulating cell differentiation to effector functions during inflammation, where CD38 may regulate cell recruitment, cytokine release, and NAD availability. In line with a role in inflammation, CD38 appears to also play a critical role in inflammatory processes during autoimmunity, although whether CD38 has pathogenic or regulatory effects varies depending on the disease, immune cell, or animal model analyzed. Given the complexity of the physiology of CD38 it has been difficult to completely understand the biology of this molecule during autoimmune inflammation. In this review, we analyze current knowledge and controversies regarding the role of CD38 during inflammation and autoimmunity and novel molecular tools that may clarify current gaps in the field.

Inosine is an alternative carbon source for CD8+-T-cell function under glucose restriction

T cells undergo metabolic rewiring to meet their bioenergetic, biosynthetic and redox demands following antigen stimulation. To fulfil these needs, effector T cells must adapt to fluctuations in environmental nutrient levels at sites of infection and inflammation. Here, we show that effector T cells can utilize inosine, as an alternative substrate, to support cell growth and function in the absence of glucose in vitro. T cells metabolize inosine into hypoxanthine and phosphorylated ribose by purine nucleoside phosphorylase. We demonstrate that the ribose subunit of inosine can enter into central metabolic pathways to provide ATP and biosynthetic precursors, and that cancer cells display diverse capacities to utilize inosine as a carbon source. Moreover, the supplementation with inosine enhances the anti-tumour efficacy of immune checkpoint blockade and adoptive T-cell transfer in solid tumours that are defective in metabolizing inosine, reflecting the capability of inosine to relieve tumour-imposed metabolic restrictions on T cells.

PRMT5 controls pathogenicity and metabolic genes during Th17 polarization

PRMT5 catalyzes symmetric dimethylation (SDM) on arginine residues. PRMT5 methylates histone H3 and H4 producing remodeling of chromatin and regulation of gene transcription. PRMT5 is induced after T cell activation. Furthermore, both PRMT5 inhibitors and PRMT5 knockdown impair T cell proliferation and activation. However, the role of PRMT5 in T cell differentiation has not been analyzed. T cells undergo metabolic reprograming during activation which is critical for polarization. Differentiating T cells increase bioenergetics and anabolic pathways enhancing glycolysis, catabolism, uptake of fatty acids and oxidative phosphorylation. Whether PRMT5 has an impact on Th17 differentiation and metabolic reprograming is unknown. To address this question, we performed Th17 differentiation and RNA sequencing analyses in T cells from PRMT5fl/fl and iCD4-PRMT5Δ/Δ mice. Th17 differentiation was sharply suppressed in PRMT5 KO T cells, in which 25–30% of genes in the glycolysis, lactate or TCA pathways were reduced. To evaluate the functional effects of PRMT5 on energy metabolism, extracellular acidification rate (ECAR) and Oxygen consumption rate (OCR) were analyzed in Jurkat T cells transduced with control or PRMT5 shRNA and lactate secretion was measured on supernatants of Th1 and Th17 cells from PRMT5fl/fl and iCD4-PRMT5Δ/Δ mice. Knock down of PRMT5 decreased ECAR in Jurkat cells and lactate production in Th1 and Th17 cells in absence of PRMT5. In conclusion, PRMT5 promotes glycolytic energy metabolism and Th17 differentiation in T cells.

PRMT5 has roles in thymic/peripheral T cell development and EAE

Protein Arginine Methyltransferase (PRMT) 5 is a type-II methyltransferase involved in oncogenesis, embryonic and hematopoietic development. However, the role of PRMT5 T-cell biology is only now coming into focus. To evaluate PRMT5’s functions in T-cells, we have developed a conditional knock-out (KO) model that allow for a deletion of PRMT5 during thymic development, by knocking-out PRMT5 in all peripheral T cells (T-PRMT5D/D). Using this model that deletes both protein-coding isoforms of Prmt5, we investigated the role of the PRMT5 gene during T-cell development, proliferation and experimental autoimmune encephalomyelitis (EAE). We saw reduced numbers of CD4+ T-cells, Tregs and iNK T-cells in the thymus. CD4+, Treg and iNK T-cell number defects were seen in the peripheral lymphoid organs such as spleen and lymph nodes. Peripheral analyses also showed reduced CD8+ T-cells, naive, effector and central memory CD4+ T-cells. We also induced PRMT5 deletion specifically in peripheral CD4+ T cells (iCD4-PRMT5D/D). We utilize this model to induce PRMT5 deletion specifically in peripheral CD4+ T cells while ensuring that the mice develop normal numbers of peripheral T cells. Using this model, we showed that EAE was completely abolished in the mice. Total CNS infiltrating T-cell numbers were substantially reduced, and significant losses were observed in the Th1 and Th17 cytokine production from these cells. These data substantively demonstrate a driver role for PRMT5 in thymic CD4 Th cell development, peripheral CD4+, iNKT and CD8+ T-cell maintenance and Th cell-mediated EAE autoimmunity.

Role of PRMT5 in cholesterol metabolism and Th17 pathogenicity

Protein Arginine Methyltransferase (PRMT) 5 is an enzyme catalyzing symmetric dimethylation (SDM) of arginine. This post-translational modification is known to be involved in oncogenesis and embryonic development. Our previous work has shown that PRMT5 is induced during T cell activation. However, the role and mechanisms by which PRMT5 modulates T helper (Th) cell polarization and autoimmune disease have not yet been elucidated. To study the impact of PRMT5 on gene expression programs induced in activated T cells, we developed a conditional PRMT5 knockout (KO) mouse model in which PRMT5 deletion can be induced in CD4+ T cells after thymic development and performed RNA sequencing (RNAseq) analyses in 3-day activated naïve CD4 T cells. We found that PRMT5 promoted expression of cholesterol biosynthetic pathway enzymes that produce Retinoid-Related Orphan Receptor (ROR) agonists that activate ROR-gt and promote Th17 differentiation. Indeed, Th17 differentiation was blunted in PRMT5 KO T cells. Finally, we identify the cholesterol biosynthesis pathway regulator SREBP-1 as a target of PRMT5 SDM. This work shows that PRMT5 expression in activated T cells is needed for the cholesterol biosynthesis gene expression program, generating ROR-gt agonistic activity and promoting Th17 differentiation. These results point to T cell PRMT5 and the downstream cholesterol biosynthesis pathway as promising therapeutic targets in Th17 cell-based diseases.

Protein arginine methyltransferase 5 promotes cholesterol biosynthesis-mediated Th17 responses and autoimmunity

Protein arginine methyltransferase 5 (PRMT5) catalyzes symmetric dimethylation (SDM) of arginine, a posttranslational modification involved in oncogenesis and embryonic development. However, the role and mechanisms by which PRMT5 modulates Th cell polarization and autoimmune disease have not yet been elucidated. Here, we found that PRMT5 promoted SREBP1 SDM and the induction of cholesterol biosynthetic pathway enzymes that produce retinoid-related orphan receptor (ROR) agonists that activate RORγt. Specific loss of PRMT5 in the CD4+ Th cell compartment suppressed Th17 differentiation and protected mice from developing experimental autoimmune encephalomyelitis (EAE). We also found that PRMT5 controlled thymic and peripheral homeostasis in the CD4+ Th cell life cycle and invariant NK (iNK) T cell development and CD8+ T cell maintenance. This work demonstrates that PRMT5 expression in recently activated T cells is necessary for the cholesterol biosynthesis metabolic gene expression program that generates RORγt agonistic activity and promotes Th17 differentiation and EAE. These results point to Th PRMT5 and its downstream cholesterol biosynthesis pathway as promising therapeutic targets in Th17-mediated diseases.

Pharmacological inhibition of Notch signaling regresses pre-established abdominal aortic aneurysm

Abdominal aortic aneurysm (AAA) is characterized by transmural infiltration of myeloid cells at the vascular injury site. Previously, we reported preventive effects of Notch deficiency on the development of AAA by reduction of infiltrating myeloid cells. In this study, we examined if Notch inhibition attenuates the progression of pre-established AAA and potential implications. Pharmacological Notch inhibitor (N-[N-(3,5-difluorophenacetyl)-L-alanyl]-(S)-phenylglycine t-butyl ester; DAPT) was administered subcutaneously three times a week starting at day 28 of angiotensin II (AngII) infusion. Progressive increase in pulse wave velocity (PWV), maximal intra-luminal diameter (MILD) and maximal external aortic diameter (MEAD) were observed at day 56 of the AngII. DAPT prevented such increase in MILD, PWV and MEAD (P < 0.01). Histologically, the aortae of DAPT-treated Apoe-/- mice had significant reduction in inflammatory response and elastin fragmentation. Naked collagen microfibrils and weaker banded structure observed in the aortae of Apoe-/- mice in response to AngII, were substantially diminished by DAPT. A significant decrease in the proteolytic activity in the aneurysmal tissues and vascular smooth muscle cells (vSMCs) was observed with DAPT (P < 0.01). In human and mouse AAA tissues, increased immunoreactivity of activated Notch signaling correlated strongly with CD38 expression (R2 = 0.61). Collectively, we propose inhibition of Notch signaling as a potential therapeutic target for AAA progression.

NF-κB/mTOR/MYC Axis Drives PRMT5 Protein Induction After T Cell Activation via Transcriptional and Non-transcriptional Mechanisms

Multiple sclerosis is an autoimmune disease of the central nervous system (CNS) mediated by CD4⁺ T cells and modeled via experimental autoimmune encephalomyelitis (EAE). Inhibition of PRMT5, the major Type II arginine methyltransferase, suppresses pathogenic T cell responses and EAE. PRMT5 is transiently induced in proliferating memory inflammatory Th1 cells and during EAE. However, the mechanisms driving PRMT5 protein induction and repression as T cells expand and return to resting is currently unknown. Here, we used naive mouse and memory mouse and human Th1/Th2 cells as models to identify mechanisms controlling PRMT5 protein expression in initial and recall T cell activation. Initial activation of naive mouse T cells resulted in NF-κB-dependent transient Prmt5 transcription and NF-κB, mTOR and MYC-dependent PRMT5 protein induction. In murine memory Th cells, transcription and miRNA loss supported PRMT5 induction to a lesser extent than in naive T cells. In contrast, NF-κB/MYC/mTOR-dependent non-transcriptional PRMT5 induction played a major role. These results highlight the importance of the NF-κB/mTOR/MYC axis in PRMT5-driven pathogenic T cell expansion and may guide targeted therapeutic strategies for MS.

CD38 Is Robustly Induced in Human Macrophages and Monocytes in Inflammatory Conditions

Macrophages and their monocyte precursors mediate innate immune responses and can promote a spectrum of phenotypes from pro-inflammatory to pro-resolving. Currently, there are few markers that allow for robust dissection of macrophage phenotype. We recently identified CD38 as a marker of inflammatory macrophages in murine in vitro and in vivo models. However, it is unknown whether CD38 plays a similar marker and/or functional role in human macrophages and inflammatory diseases. Here, we establish that CD38 transcript and protein are robustly induced in human macrophages exposed to LPS (±IFN-γ) inflammatory stimuli, but not with the alternative stimulus, IL-4. Pharmacologic and/or genetic CD38 loss-of-function significantly reduced the secretion of inflammatory cytokines IL-6 and IL-12p40 and glycolytic activity in human primary macrophages. Finally, monocyte analyses in systemic lupus erythematosus patients revealed that, while all monocytes express CD38, high CD38 expression in the non-classical monocyte subpopulation is associated with disease. These data are consistent with an inflammatory marker role for CD38 in human macrophages and monocytes.

Salient type 1 interleukin 1 receptor expression in peripheral non-immune cells

Interleukin 1 is a pleiotropic cytokine that mediates diverse functions through its receptor, type I interleukin 1 receptor (IL-1R1). Most previous studies have focused on the expression and function of IL-1R1 in immune cells. Here we performed a comprehensive mapping of IL-1R1 distribution in multiple peripheral tissues using our IL-1R1 reporter (IL-1R1^GR/GR) mice. This method yielded the highest sensitivity of in situ detection of IL-1R1 mRNA and protein. Besides validating previously reported IL-1R1 expression in the endocrine tissues including pituitary and pancreas, our results refuted previously reported exclusive IL-1R1 expression in neurons of the spinal cord dorsal horn and dorsal root ganglia (DRG). Instead, IL-1R1 expression was detected in endothelial cells within DRG, spinal cord, pancreas, colon, muscles and many immune organs. In addition, gp38⁺ fibroblastic reticular cells (FRCs), rather than tissue macrophages or other immune cells, were found to express high levels of IL-1R1 in colon and many immune organs. A functional test of spleen FRCs showed that they responded rapidly to systemic IL-1β stimulation in vivo. Taken together, this study provides a rigorous re-examination of IL-1R1 expression in peripheral tissues and reveals tissue FRCs as a previously unappreciated novel high IL-1R1-expressing cell type in peripheral IL-1 signaling.

Molecular Mechanisms Modulating the Phenotype of Macrophages and Microglia

Macrophages and microglia play crucial roles during central nervous system development, homeostasis and acute events such as infection or injury. The diverse functions of tissue macrophages and microglia are mirrored by equally diverse phenotypes. A model of inflammatory/M1 versus a resolution phase/M2 macrophages has been widely used. However, the complexity of macrophage function can only be achieved by the existence of varied, plastic and tridimensional macrophage phenotypes. Understanding how tissue macrophages integrate environmental signals via molecular programs to define pathogen/injury inflammatory responses provides an opportunity to better understand the multilayered nature of macrophages, as well as target and modulate cellular programs to control excessive inflammation. This is particularly important in MS and other neuroinflammatory diseases, where chronic inflammatory macrophage and microglial responses may contribute to pathology. Here, we perform a comprehensive review of our current understanding of how molecular pathways modulate tissue macrophage phenotype, covering both classic pathways and the emerging role of microRNAs, receptor-tyrosine kinases and metabolism in macrophage phenotype. In addition, we discuss pathway parallels in microglia, novel markers helpful in the identification of peripheral macrophages versus microglia and markers linked to their phenotype.

Helminth-induced Ly6Chi monocyte-derived alternatively activated macrophages suppress experimental autoimmune encephalomyelitis

Helminths cause chronic infections and affect the immune response to unrelated inflammatory diseases. Although helminths have been used therapeutically to ameliorate inflammatory conditions, their anti-inflammatory properties are poorly understood. Alternatively activated macrophages (AAMϕs) have been suggested as the anti-inflammatory effector cells during helminth infections. Here, we define the origin of AAMϕs during infection with Taenia crassiceps, and their disease-modulating activity on the Experimental Autoimmune Encephalomyelitis (EAE). Our data show two distinct populations of AAMϕs, based on the expression of PD-L1 and PD-L2 molecules, resulting upon T. crassiceps infection. Adoptive transfer of Ly6C+ monocytes gave rise to PD-L1+/PD-L2+, but not PD-L1+/PD-L2- cells in T. crassiceps-infected mice, demonstrating that the PD-L1+/PD-L2+ subpopulation of AAMϕs originates from blood monocytes. Furthermore, adoptive transfer of PD-L1+/PD-L2+ AAMϕs into EAE induced mice reduced disease incidence, delayed disease onset, and diminished the clinical disability, indicating the critical role of these cells in the regulation of autoimmune disorders.

PRMT5-Selective Inhibitors Suppress Inflammatory T Cell Responses and Experimental Autoimmune Encephalomyelitis

In the autoimmune disease multiple sclerosis and its animal model, experimental autoimmune encephalomyelitis (EAE), expansion of pathogenic, myelin-specific Th1 cell populations drives active disease; selectively targeting this process may be the basis for a new therapeutic approach. Previous studies have hinted at a role for protein arginine methylation in immune responses, including T cell-mediated autoimmunity and EAE. However, a conclusive role for the protein arginine methyltransferase (PRMT) enzymes that catalyze these reactions has been lacking. PRMT5 is the main PRMT responsible for symmetric dimethylation of arginine residues of histones and other proteins. PRMT5 drives embryonic development and cancer, but its role in T cells, if any, has not been investigated. In this article, we show that PRMT5 is an important modulator of CD4⁺ T cell expansion. PRMT5 was transiently upregulated during maximal proliferation of mouse and human memory Th cells. PRMT5 expression was regulated upstream by the NF-κB pathway, and it promoted IL-2 production and proliferation. Blocking PRMT5 with novel, highly selective small molecule PRMT5 inhibitors severely blunted memory Th expansion, with preferential suppression of Th1 cells over Th2 cells. In vivo, PRMT5 blockade efficiently suppressed recall T cell responses and reduced inflammation in delayed-type hypersensitivity and clinical disease in EAE mouse models. These data implicate PRMT5 in the regulation of adaptive memory Th cell responses and suggest that PRMT5 inhibitors may be a novel therapeutic approach for T cell-mediated inflammatory disease.

Estrogen-regulated STAT1 activation promotes TLR8 expression to facilitate signaling via microRNA-21 in systemic lupus erythematosus

Recent studies implicate innate immunity to systemic lupus erythematosus (SLE) pathogenesis. Toll-like receptor (TLR)8 is estrogen-regulated and binds viral ssRNA to stimulate innate immune responses, but recent work indicates that microRNA (miR)-21 within extracellular vesicles (EVs) can also trigger this receptor. Our objective was to examine TLR8 expression/activation to better understand sex-biased responses involving TLR8 in SLE. Our data identify an estrogen response element that promotes STAT1 expression and demonstrate STAT1-dependent transcriptional activation of TLR8 with estrogen stimulation. In lieu of viral ssRNA activation, we explored EV-encapsulated miR-21 as an endogenous ligand and observed induction of both TLR8 and cytokine expression in vitro. Moreover, extracellular miR detection was found predominantly within EVs. Thus, just as a cytokine or chemokine, EV-encapsulated miR-21 can act as an inflammatory signaling molecule, or miRokine, by virtue of being an endogenous ligand of TLR8. Collectively, our data elucidates a novel innate inflammatory pathway in SLE.

MicroRNAs targeting TGFβ signalling underlie the regulatory T cell defect in multiple sclerosis

Transforming growth factor beta (TGFβ) signalling is critical for regulatory T cell development and function, and regulatory T cell dysregulation is a common observation in autoimmune diseases, including multiple sclerosis. In a comprehensive miRNA profiling study of patients with multiple sclerosis naïve CD4 T cells, 19 differentially expressed miRNAs predicted to target the TGFβ signalling pathway were identified, leading to the hypothesis that miRNAs may be responsible for the regulatory T cell defect observed in patients with multiple sclerosis. Patients with multiple sclerosis had reduced levels of TGFβ signalling components in their naïve CD4 T cells. The differentially expressed miRNAs negatively regulated the TGFβ pathway, resulting in a reduced capacity of naïve CD4 T cells to differentiate into regulatory T cells. Interestingly, the limited number of regulatory T cells, that did develop when these TGFβ-targeting miRNAs were overexpressed, were capable of suppressing effector T cells. As it has previously been demonstrated that compromising TGFβ signalling results in a reduced regulatory T cell repertoire insufficient to control autoimmunity, and patients with multiple sclerosis have a reduced regulatory T cell repertoire, these data indicate that the elevated expression of multiple TGFβ-targeting miRNAs in naïve CD4 T cells of patients with multiple sclerosis impairs TGFβ signalling, and dampens regulatory T cell development, thereby enhancing susceptibility to developing multiple sclerosis.

Analysis of miRNA in Normal Appearing White Matter to Identify Altered CNS Pathways in Multiple Sclerosis

Genetic studies suggest that the immune system is the greatest genetic contributor to multiple sclerosis (MS) susceptibility. Yet, these immune-related genes do not explain why inflammation is limited to the CNS in MS. We hypothesize that there is an underlying dysregulation in the CNS of MS patients that makes them more vulnerable to CNS inflammation. The sparsity of CNS-related genes associated with MS suggests that epigenetic changes in the CNS may play a role. Thus, a miRNA profiling study was performed in NAWM of MS patients and control subjects to determine if specific CNS pathways can be identified that may be altered due to miRNA-mediated post-transcriptional dysregulation. There were 15 differentially expressed miRNAs found in the MS patients' NAWM. Pathway analysis indicated that the MAPK pathway and pathways associated with the blood-brain barrier were predicted to be significantly affected by these miRNAs. Using target predication and mRNA analysis, an inverse relationship was found between miR-191 and BDNF, SOX4, FZD5 and WSB1. The pathway and target analysis of the MS-associated miRNAs suggests that MS patients' CNS is more prone to inflammation and less capable of repair, yet enriched in neuroprotective mechanisms.

miRNA modulation of M1/M2 macrophage phenotype (CAM5P.231)

After spinal cord injury (SCI), “M1” (pro-inflammatory) and “M2” (anti-inflammatory) macrophages populate the site of injury. M1 macrophages are neurotoxic and hamper neuroregeneration while M2 macrophages have a pro-regenerative, anti-inflammatory phenotype. Therefore, therapeutic strategies that suppress M1 and enhance M2 macrophages after SCI are actively sought. Micro RNAs (miRNA) are small, non-coding RNAs, approximately 19-24 nucleotides in length, which regulate gene expression at the transcriptional level. miRNA are known to regulate inflammation in autoimmune disease and inflammatory disorders. We therefore hypothesized that miRNA play a key role in differentiation process of macrophages into the pro-inflammatory M1 phenotype. The relative expression of several miRNA was analyzed in vitro in bone marrow-derived macrophages undergoing differentiation into M1 versus M2 phenotypes. We found that M1 macrophages expressed significantly higher levels of miR-155 than M2 macrophages. In addition, miR-155 expression was increased in the injured spinal cord mirroring the increase in M1 macrophages at the site of injury. miR-155 deficiency or silencing resulted in blunted iNOS, IL-1β and TNFα up-regulation in M1 macrophages, among others. In contrast, Arginase-1 expression remained stable in M2 macrophages. These results indicate that miR-155 plays an important role in the differentiation of macrophages into an M1 phenotype.

Continuous retinoic acid induces the differentiation of mature regulatory monocytes but fails to induce regulatory dendritic cells

BACKGROUND

Myeloid cells (MC) have potent immunoregulatory abilities that can be therapeutically useful to treat inflammatory disease. However, the factors which promote regulatory myeloid cell differentiation remain poorly understood. We have previously shown that estriol (E3) induces mature regulatory dendritic cells in vivo. To determine whether additional steroid hormones could induce mature regulatory myeloid cells, we investigated the effects of retinoic acid (RA) on MCs. Retinoic acid is a steroid hormone important in regulating mucosal immunity in the gut and promoting myeloid differentiation. We hypothesized that the presence of RA during differentiation would promote the formation of mature regulatory myeloid cells (MCregs).

METHODS

To determine RA's ability to induce regulatory myeloid cells, we differentiated bone marrow progenitor cells with granulocytic-macrophage colony-stimulating factor (GM-CSF) under the influence of RA. We found that day 7 MCs differentiated in the presence of RA had an increase in the percent positive and relative expression levels of both maturation (CD80, CD86, and MHCII) and inhibitory (PD-L1 and PD-L2) markers compared to control cells. Functionally, these day 7 RA MCs expressed increased intracellular IL-10, induced regulatory T cells in vitro compared to controls and suppressed the proliferation of responder immune cells even after inflammatory challenge with LPS.

CONCLUSION

RA induced mature regulatory myeloid cells that were suppressive and had a CD11b+ CD11c-Ly6C low/intermediate monocyte phenotype. Surprisingly, RA CD11c+ dendritic cells were not suppressive and could contribute to enhanced proliferation. These results suggest that continuous RA has unique effects on different myeloid populations during monopoeisis and dendropoiesis and promotes a population of regulatory monocytes.

Treatment of experimental autoimmune encephalomyelitis by codelivery of disease associated Peptide and dexamethasone in acetalated dextran microparticles

Multiple sclerosis (MS) is an autoimmune, demyelinating disease of the central nervous system that can cause loss of motor function and is thought to result, in part, from chronic inflammation due to an antigen-specific T cell immune response. Current treatments suppress the immune system without antigen specificity, increasing the risks of cancer, chronic infection, and other long-term side effects. In this study, we show treatment of experimental autoimmune encephalomyelitis (EAE), a model of MS, by coencapsulating the immunodominant peptide of myelin oligodendrocyte glycoprotein (MOG) with dexamethasone (DXM) into acetalated dextran (Ac-DEX) microparticles (DXM/MOG/MPs) and administering the microparticles subcutaneously. The clinical score of the mice was reduced from 3.4 to 1.6 after 3 injections 3 days apart with the coencapsulated microparticulate formulation (MOG 17.6 μg and DXM 8 μg). This change in clinical score was significantly greater than observed with phosphate-buffered saline (PBS), empty MPs, free DXM and MOG, DXM/MPs, and MOG/MPs. Additionally, treatment with DXM/MOG/MPs significantly inhibited disease-associated cytokine (e.g., IL-17, GM-CSF) expression in splenocytes isolated in treated mice. Here we show a promising approach for the therapeutic treatment of MS using a polymer-based microparticle delivery platform.

T cell miRNA in multiple sclerosis pathogenesis (P5209)

Multiple sclerosis (MS) is an immune-mediated disease that results in destruction of the myelin sheath and axonal degeneration. Interestingly, while myelin-specific T cells are present in peripheral blood mononuclear cells (PBMC) of both healthy controls and MS patients, they only become pathogenic in MS patients. This suggests that some regulatory checkpoint is bypassed in MS T cells, unleashing their autoimmune potential. By regulating protein-coding mRNAs, microRNAs (miRNAs) control important cellular processes. Specifically, we have previously shown that dysregulated miRNAs in MS patient’s naïve T cells promote pro-inflammatory Th1 immune responses. In this study, we explored whether differences in miRNA expression in memory T cells of MS patients further contribute to the pathogenesis of MS. Memory CD4 T cells were purified by magnetic bead sorting to 95% purity and subjected to a comprehensive Nanostring technology miRNA profiling. In a study on healthy (n=17) and seven MS patients (n=21), a number of miRs were found to be significantly different in MS compared to healthy controls. Among the identified miRs, many target genes known to be involved in T cell proliferation and apoptosis, activation, adhesion and regulation. These results identify miRNAs in memory T cells as MS biomarkers of biological significance and potential clinical value, and reveal potential new therapeutic targets in MS.

miR-29ab1 deficiency identifies a negative feedback loop controlling Th1 bias that is dysregulated in multiple sclerosis

Th cell programming and function is tightly regulated by complex biological networks to prevent excessive inflammatory responses and autoimmune disease. The importance of microRNAs (miRNAs) in this process is highlighted by the preferential Th1 polarization of Dicer-deficient T cells that lack miRNAs. Using genetic knockouts, we demonstrate that loss of endogenous miR-29, derived from the miR-29ab1 genomic cluster, results in unrestrained T-bet expression and IFN-γ production. miR-29b regulates T-bet and IFN-γ via a direct interaction with the 3' untranslated regions, and IFN-γ itself enhances miR-29b expression, establishing a novel regulatory feedback loop. miR-29b is increased in memory CD4(+) T cells from multiple sclerosis (MS) patients, which may reflect chronic Th1 inflammation. However, miR-29b levels decrease significantly upon T cell activation in MS patients, suggesting that this feedback loop is dysregulated in MS patients and may contribute to chronic inflammation. miR-29 thus serves as a novel regulator of Th1 differentiation, adding to the understanding of T cell-intrinsic regulatory mechanisms that maintain a balance between protective immunity and autoimmunity.

Dimethyl fumarate inhibits dendritic cell maturation via nuclear factor κB (NF-κB) and extracellular signal-regulated kinase 1 and 2 (ERK1/2) and mitogen stress-activated kinase 1 (MSK1) signaling

Dimethyl fumarate (DMF) is an effective novel treatment for multiple sclerosis in clinical trials. A reduction of IFN-γ-producing CD4(+) T cells is observed in DMF-treated patients and may contribute to its clinical efficacy. However, the cellular and molecular mechanisms behind this clinical observation are unclear. In this study, we investigated the effects of DMF on dendritic cell (DC) maturation and subsequent DC-mediated T cell responses. We show that DMF inhibits DC maturation by reducing inflammatory cytokine production (IL-12 and IL-6) and the expression of MHC class II, CD80, and CD86. Importantly, this immature DC phenotype generated fewer activated T cells that were characterized by decreased IFN-γ and IL-17 production. Further molecular studies demonstrated that DMF impaired nuclear factor κB (NF-κB) signaling via reduced p65 nuclear translocalization and phosphorylation. NF-κB signaling was further decreased by DMF-mediated suppression of extracellular signal-regulated kinase 1 and 2 (ERK1/2) and its downstream kinase mitogen stress-activated kinase 1 (MSK1). MSK1 suppression resulted in decreased p65 phosphorylation at serine 276 and reduced histone phosphorylation at serine 10. As a consequence, DMF appears to reduce p65 transcriptional activity both directly and indirectly by promoting a silent chromatin environment. Finally, treatment of DCs with the MSK1 inhibitor H89 partially mimicked the effects of DMF on the DC signaling pathway and impaired DC maturation. Taken together, these studies indicate that by suppression of both NF-κB and ERK1/2-MSK1 signaling, DMF inhibits maturation of DCs and subsequently Th1 and Th17 cell differentiation.

Micro-RNA dysregulation in multiple sclerosis favours pro-inflammatory T-cell-mediated autoimmunity

Pro-inflammatory T cells mediate autoimmune demyelination in multiple sclerosis. However, the factors driving their development and multiple sclerosis susceptibility are incompletely understood. We investigated how micro-RNAs, newly described as post-transcriptional regulators of gene expression, contribute to pathogenic T-cell differentiation in multiple sclerosis. miR-128 and miR-27b were increased in naïve and miR-340 in memory CD4(+) T cells from patients with multiple sclerosis, inhibiting Th2 cell development and favouring pro-inflammatory Th1 responses. These effects were mediated by direct suppression of B lymphoma Mo-MLV insertion region 1 homolog (BMI1) and interleukin-4 (IL4) expression, resulting in decreased GATA3 levels, and a Th2 to Th1 cytokine shift. Gain-of-function experiments with these micro-RNAs enhanced the encephalitogenic potential of myelin-specific T cells in experimental autoimmune encephalomyelitis. In addition, treatment of multiple sclerosis patient T cells with oligonucleotide micro-RNA inhibitors led to the restoration of Th2 responses. These data illustrate the biological significance and therapeutic potential of these micro-RNAs in regulating T-cell phenotypes in multiple sclerosis.

miRNA profiling for biomarker discovery in multiple sclerosis: from microarray to deep sequencing

MicroRNAs (miRNAs) are small non-coding RNAs that regulate gene expression at the post-transcriptional level. miRNAs are highly expressed in cells of the immune and nervous system, attesting to their importance in Neuroimmunology. Besides their involvement in modulation of physiological and pathological processes, miRNAs hold high promise as disease biomarkers, therapeutic agents and/or drug targets. Several studies have recently explored the involvement of miRNAs in Multiple Sclerosis (MS) using a variety of miRNA profiling techniques. In this review, we discuss basic miRNA biology and nomenclature, the techniques available for miRNA profiling research and recent miRNA profiling studies in Multiple Sclerosis.

TGF-β signaling via Smad4 drives IL-10 production in effector Th1 cells and reduces T-cell trafficking in EAE

Effector Th1 cells perpetuate inflammatory damage in a number of autoimmune diseases, including MS and its animal model EAE. Recently, a self-regulatory mechanism was described in which effector Th1 cells produce the immunomodulatory cytokine IL-10 to dampen the inflammatory response in both normal and autoimmune inflammation. While the presence of TGF-β has been suggested to enhance and stabilize an IFN-γ(+) IL-10(+) phenotype, the molecular mechanism is poorly understood. Additionally, in the context of adoptive transfer EAE, it is unclear whether IL-10 acts on the transferred Th1 cells or on endogenous host cells. In the present study, using myelin-specific TCR-Tg mice, we show that repetitive Ag stimulation of effector Th1 cells in the presence of TGF-β increases the population of IFN-γ(+) IL-10(+) cells, which correlates with a decrease in EAE severity. Additionally, TGF-β signaling causes binding of Smad4 to the IL-10 promoter, providing molecular evidence for TGF-β-mediated IL-10 production from Th1 effector cells. Finally, this study demonstrates that IL-10 not only reduces encephalitogenic markers such as IFN-γ and T-bet on Th1 effector cells expressing the IL-10R but also prevents recruitment of both transferred and host-derived inflammatory T cells. These data establish a regulatory mechanism by which highly activated Th1 effector cells modulate their pathogenicity through the induction of IL-10.

miRNA modulation of T cell phenotype in Multiple Sclerosis (167.3)

Pro-inflammatory T cells mediate autoimmune demyelination in Multiple Sclerosis (MS). However, the factors driving their development and MS susceptibility are obscure. We investigated how miRNAs, newly described as post-transcriptional regulators of gene expression, contribute to pathogenic T cell differentiation in MS. We profiled miRNA expression in naïve CD4+ T cells of healthy (n=8) and untreated MS (n=22) donors. miR-128, miR-27a/b and miR-340 were increased in naïve CD4+ T cells from MS patients. miR-128, -27a/b and -340 directly suppressed the pro-Th2 factor Bmi1, resulting in decreased GATA-3 expression. miR-340 additionally suppressed the Th2 cytokine IL-4. Overall, these miRNAs suppressed Th2 and enhanced pro-inflammatory Th1 responses. In addition, transfection of myelin-specific T cells with these miRNAs worsened EAE while treatment of MS patient cells with miRNA inhibitors led to the restoration of Th2 responses. These findings link miR-128, -27ab and -340 overexpression in MS patients’ naïve CD4+ T cells to the pro-inflammatory T cell differentiation observed in MS and illustrate the biological significance and therapeutic potential of these miRNAs.

MicroRNA-29 controls Th1 bias of CD4+ T cells in multiple sclerosis (167.5)

Multiple sclerosis (MS) is an inflammatory disease of the central nervous system initiated by autoreactive CD4+ T cells of a T-helper (Th)1 and Th17 phenotype. Identifying novel factors that control Th differentiation will lead to a better understanding of MS pathogenesis and the development of new therapeutic strategies. MicroRNAs (miRs) are small non-coding RNAs that negatively regulate post-transcriptional gene expression. Dicer-deficient CD4+ T cells that lack miRs produce more of the Th1-associated cytokine IFN-γ; however, the specific miR(s) responsible for controlling Th1 programming have not been defined. Using in silico target prediction algorithms, we identified miR-29 as a factor that could potentially regulate multiple genes in the Th1 pathway. We confirmed that miR-29 directly targets T-bet and IFN-γ, and gain- and loss-of-function experiments demonstrated the ability of miR-29 to modulate Th1 polarization. Importantly, we found that naïve CD4+ T cells from MS patients express lower basal levels of miR-29 than controls. We further established an inverse relationship between basal expression of miR-29 and expression of T-bet/IFN-γ following activation of MS patient T cells. These results identify miR-29 as a novel regulator of Th1 differentiation and function, and implicate low miR-29 as an MS susceptibility factor. Restoring miR-29 may therefore be a powerful therapeutic modality in MS and other Th1-driven autoimmune diseases.

MicroRNAs: modulators of T cell phenotype in multiple sclerosis (143.57)

Multiple sclerosis (MS) is an inflammatory disease of the central nervous system mediated by autoreactive CD4+ T cells of a Th1 and Th17 phenotype. Identifying novel factors that control Th differentiation will lead to a better understanding of disease pathogenesis and the development of new therapeutic strategies. MicroRNAs (miRs) are a class of small non-coding RNAs that negatively regulate post-transcriptional gene expression. MiRs have an established role in immune system homeostasis, and profiling studies have found differential expression of miRs in autoimmune diseases, including MS. Dicer-deficient CD4+ T cells that lack miRs produce more cytokine upon stimulation. Together these data indicate the miR(s) are responsible for controlling cytokine expression and act as negative regulators of inflammation. However, the specific miR(s) responsible for modulating inflammatory cytokine production and Th programming have not been defined. Using target prediction algorithms, we identified miRs predicted to target genes that mediate Th1 specification and function. We have validated these miR-target pairs using gain- and loss-of-function experiments in human Th1-polarized cells. We have also profiled the expression of these miRs in specific CD4+ T cell subsets from MS patients. Our results identify miRs that modulate Th developmental programming and effector function and whose expression is altered in MS. These miRs may be useful immunomodulatory treatments or biomarkers in MS.

HLA-DR alleles determine responsiveness to Borrelia burgdorferi antigens in a mouse model of self-perpetuating arthritis

OBJECTIVE

Arthritis is a prominent manifestation of Lyme disease, which is caused by infection with Borrelia burgdorferi (Bb). Chronic Lyme arthritis persisting even after antibiotic treatment is linked to HLA-DRB1*0401 (DR4) and related alleles. In contrast, patients whose Lyme arthritis resolves within 3 months postinfection show an increased frequency of HLA-DRB1*1101 (DR11). The aim of this study was to analyze the underlying mechanism by which HLA-DR alleles confer genetic susceptibility or resistance to antibiotic-refractory Lyme arthritis.

METHODS

We generated DR11-transgenic (DR11-Tg) mice on a murine MHCII-/- background and compared their immune response to Bb antigens with the response of DR4-Tg mice after immunization with Bb outer surface protein A (OspA) or infection with live Bb.

RESULTS

T cells from OspA-immunized and Bb-infected DR11-Tg mice had defective production of interferon-gamma as compared with those from DR4-Tg mice. In contrast, DR11-Tg mice developed higher titers of anti-OspA and anti-Bb antibodies, respectively, than did DR4-Tg mice. Consistent with this observation, we found that the Bb-infected DR11-Tg mice had a decreased spirochetal burden as compared with the DR4-Tg mice, as measured by quantitative polymerase chain reaction.

CONCLUSION

This study provides direct evidence that in the presence of HLA-DR11, the immune response against Bb antigens is directed toward a protective antibody response. In contrast, an inflammatory Th1 response is induced in the presence of DR4. These observations offer an explanation for the differential genetic susceptibility of DR4+ and DR11+ individuals to the development of chronic Lyme arthritis and, eventually, the progression to antibiotic-refractory Lyme arthritis.

Neonatal tolerance revisited: a perinatal window for Aire control of autoimmunity

There has long been conceptual and experimental support for, but also challenges to, the notion that the initial period of the immune system's development is particularly important for the establishment of tolerance to self. The display of self-antigens by thymic epithelial cells is key to inducing tolerance in the T lymphocyte compartment, a process enhanced by the Aire transcription factor. Using a doxycycline-regulated transgene to target Aire expression to the thymic epithelium, complementing the Aire knockout in a temporally controlled manner, we find that Aire is essential in the perinatal period to prevent the multiorgan autoimmunity that is typical of Aire deficiency. Surprisingly, Aire could be shut down soon thereafter and remain off for long periods, with few deleterious consequences. The lymphopenic state present in neonates was a factor in this dichotomy because inducing lymphopenia during Aire turnoff in adults recreated the disease, which, conversely, could be ameliorated by supplementing neonates with adult lymphocytes. In short, Aire expression during the perinatal period is both necessary and sufficient to induce long-lasting tolerance and avoid autoimmunity. Aire-controlled mechanisms of central tolerance are largely dispensable in the adult, as a previously tolerized T cell pool can buffer newly generated autoreactive T cells that might emerge.

Transcriptional impact of Aire varies with cell type

Aire promotes T cell tolerance by inducing the expression of a broad swath of genes encoding peripheral tissue antigens (PTAs) in medullary epithelial cells (MECs) of the thymus. The exact mechanism used in inducing this ectopic transcription remains obscure. To address this issue, we generated transgenic mice expressing Aire in pancreatic islet beta cells. Gene-expression profiling of such islets revealed that Aire can have a significant impact on transcription in these cells, mainly inducing, but also repressing, transcript levels in a manner comparable with its influence on MECs. The exact transcripts affected differed in MECs and beta cells, with limited overlap between the two sets of Aire-modulated genes. We propose that Aire promotes ectopic gene expression by a generic mechanism that does not depend on any particular characteristics or transcription mechanisms operating in MECs, whereas the cellular environment does govern which genes are actually susceptible to Aire regulation.

Aggravated Lyme carditis in CD11a-/- and CD11c-/- mice

CD18 hypomorph mice expressing reduced levels of the common beta2 integrin chain develop aggravated Lyme carditis, compared to that developed by wild-type (WT) mice, upon infection with the spirochete Borrelia burgdorferi. The enhancement of Lyme carditis in these mice is characterized by increased macrophage infiltration, correlating with augmented expression of the monocyte/macrophage chemoattractant protein 1 (MCP-1). The lack of CD18 results in the deficiency of all beta2 integrins, i.e., CD11a/CD18 (LFA-1), CD11b/CD18 (Mac-1/CR3), CD11c/CD18 (p150,95/CR4), and CD11d/CD18. To determine the roles of the various beta2 integrins in controlling the development of aggravated Lyme carditis, disease induction was analyzed in CD11a-/-, CD11b-/-, and CD11c-/- mice. CD11a-/- and CD11c-/- mice, but not CD11b-/- mice, developed aggravated Lyme carditis after exposure to B. burgdorferi. Similarly to CD18 hypomorph mice, CD11c-/- mice expressed higher levels of MCP-1, compared to both WT and CD11a-/- mice, as determined by in vitro analysis of MCP-1 secretion by bone marrow-derived dendritic cells and in vivo analysis of MCP-1 mRNA expression in B. burgdorferi-infected hearts. On the other hand, CD11a deficiency was associated with heightened heart B. burgdorferi burden relative to that of WT mice. Overall, our results suggest that the increased severity of Lyme carditis in CD18 hypomorph mice is caused by deficiency in CD11a or CD11c, possibly via different mechanisms.

Beta2 integrins control the severity of murine Lyme carditis

Infection of C57BL/6 (B6) mice with the Lyme disease spirochete Borrelia burgdorferi can result in development of arthritis and carditis. B. burgdorferi induces expression of beta2/CD18 integrins, adhesion molecules that mediate the firm adhesion of leukocytes to the endothelium necessary for cellular extravasation during inflammation. The important role of beta2/CD18 integrins during extravasation suggests that these molecules play a role in the development of Lyme arthritis and carditis. The dependency of these inflammatory processes on the beta2 integrins was investigated in CD18 hypomorph mice, which express low levels of CD18. The results indicate that CD18 deficiency did not abrogate development of Lyme arthritis or carditis. Moreover, it resulted in increased severity of Lyme carditis. B. burgdorferi-infected CD18 hypomorph mice showed an increased macrophage infiltration of the heart, while they produced lower levels of borreliacidal anti-B. burgdorferi antibodies compared to wild-type mice. In accordance with these results, we demonstrate that dendritic cells from CD18 hypomorph mice secrete higher levels of monocyte/macrophage chemoattractant protein 1 (MCP-1/CCL2) in response to B. burgdorferi. Similarly, we show by real-time PCR that B. burgdorferi-infected hearts from CD18 hypomorph mice express increased levels of MCP-1 RNA compared to wild-type mice. Overall, our results indicate that beta2 integrin deficiency does not abrogate B. burgdorferi-induced inflammation; rather, it results in increased recruitment of macrophages into the B. burgdorferi-infected heart, likely due to the increased expression of MCP-1 in this tissue. Thus, beta2 integrins may play a regulatory role in B. burgdorferi-induced inflammation beyond mediating adhesion of leukocytes to the endothelium.

Chemokines and Toll-like receptors in Lyme disease pathogenesis

Lyme disease is a tick-transmitted inflammatory disorder, caused by the spirochete Borrelia burgdorferi (Bb). Recent discoveries cast new light on Bb dissemination and the ensuing pathogenesis of inflammation. Although the strong proinflammatory Bb lipoproteins have been implicated in the induction of inflammation, they do not seem to act exclusively through Toll-like receptor (TLR) engagement. In fact, mice that are deficient for MyD88, a component of the TLR signaling pathway, manifest similar or increased recruitment of cells into Bb-infected tissues. By contrast, the absence of the chemokine receptor CXCR2 results in reduced inflammation. Overall, these findings highlight the complexity of Lyme disease pathogenesis and identify chemokine pathways as novel therapeutic targets for the control of Bb-induced inflammation.

Development of autoimmunity in Lyme arthritis

Treatment-resistant Lyme arthritis (TRLA) develops in 10% of Lyme arthritis patients and is characterized by continuous joint inflammation that does not resolve with antibiotic therapy. TRLA is associated with HLA-DRB1*0401 and related alleles, as well as with an immune response to the Borrelia burgdorferi (Bb) outer surface protein A (OspA). The immunodominant epitope of OspA in the context of HLA-DRB1*0401 corresponds to amino acids 165-173 (OspA165-173). The human Lymphocyte Function Antigen-1 (hLFA1alpha) contains a peptide with homology to OspA165-173. Treatment-resistant Lyme arthritis patients' T cells, cloned based on their ability to bind OspA165-173-loaded HLA-DRB1*0401 tetramers, respond to OspA and hLFA1alpha with a different cytokine profile, suggesting that hLFA1alpha acts as a partial agonist with a potential role in the perpetuation of joint inflammation.