Jorg van Loosdregt

Juvenile Idiopathic Arthritis (JIA) is the most common rheumatic disease in children and is characterized by chronic joint inflammation driven by immune dysregulation. A disturbed regulatory T (Treg) cell-effector T cell axis has been demonstrated in these patients which contributes to a dysregulated immune tolerance in JIA. Therefore, strategies to enhance Treg numbers and suppressive capacity to restore immune homeostasis represent a promising therapeutic approach. Nicotinamide (NAM), a form of vitamin B₃ and precursor of nicotinamide adenine dinucleotide (NAD⁺), has emerged as an immunomodulator capable of promoting Treg cell function. Here, we investigate the effects of NAM on Treg percentages and suppressive capacity in mononuclear cells derived from peripheral blood (PBMCs) and synovial fluid of JIA patients (SFMCs). In an exploratory pilot phase II clinical study, NAM concentrations were measured in paired blood plasma and SF from 6 JIA patients after oral administration of daily 1.8 g/m² for three days. NAM treatment significantly increased the proportion of FOXP3-expressing CD4⁺ T cells in both PBMCs and SFMCs and Treg suppressive capacity in PBMCs from healthy controls in vitro. Comparable NAM concentrations were detected in plasma and SF following oral administration, demonstrating its ability to penetrate the inflamed joint environment. Given its established safety in both adults and children, NAM represents a promising future candidate for adjunctive therapy in JIA by restoring the balance between regulatory and effector T cells. Further clinical and mechanistic studies are warranted to define optimal dosing regimens and its place in the long-term treatment strategy of JIA. Assessing penetration of high dose nicotinamide (Vitamin B3) in synovial fluid. EudraCT Number: 2018-002245-11, start date Jan 16th, 2019.

Alternative splicing is a key mechanism for expanding transcriptomic and proteomic complexity, yet its role in innate immune activation remains incompletely understood. Here, we applied Oxford Nanopore native RNA-sequencing to generate an isoform-level transcriptome of primary human monocytes before and after activation with lipopolysaccharide. We identify over 24,000 expressed isoforms, including thousands of previously unannotated variants. Activation induced widespread isoform-specific expression changes, leading to extensive isoform switching events, which were validated using matched short-read RNA-Seq. These activation-induced shifts enhanced transcript immune-regulatory functions: activated monocytes preferentially express longer, coding-competent isoforms with complete open reading frames, fewer retained introns, and increased domain complexity. By integrating matched Ribo-seq and proteomic data, we demonstrate that these isoform modulations are associated with enhanced translation of immune effector proteins. Together, our findings position alternative splicing as a dynamic and functional regulator of monocyte activation, emphasizing the need for isoform-level resolution to fully understand immune cell function.

Human CD4 ⁺ T cells utilize nutrients, including lipids, to support their activation and polarization. Considering the pivotal role of lipoproteins in lipid transport, we reasoned that lipoprotein uptake and processing could effect CD4 ⁺ T cell function. Here, we demonstrate that activation of human CD4 ⁺ T cells induced expression of LDL receptor (LDLR) to facilitate LDLR-mediated endocytosis of LDL. Degradation of surface LDLR on CD4 ⁺ T cells with PCSK9 hampered activation and proliferation of the cells. Lipoprotein deprivation or blocking of lysosomal cholesterol egress impaired activation of mechanistic target of rapamycin complex 1 (mTORC1), affecting CD4 ⁺ T cell activation and proliferation. Furthermore, lipoprotein deprivation of cultured primary CD4 ⁺ T cells lead to reduced expression of c-MAF and FOXP3, key transcription factors for IL-10, accompanied by reduced IL-10 secretion. The pivotal role of LDLR-mediated lipoprotein uptake for mTORC1 activity, c-MAF and FOXP3 expression, and IL-10 secretion was confirmed using LDLR-dysfunctional CD4 ⁺ T cells from patients with homozygous familial hypercholesterolemia. Our study offers valuable insights into the lipoprotein metabolism of human CD4 ⁺ T cells and their reliance on the LDLR pathway for activation and polarization, a feature that may be leveraged to modulate CD4 ⁺ T cell function.

Alternative splicing is a key regulator of immune regulation by enabling rapid and context-specific responses. However, the role of splicing regulators such as CDC-like kinase 1 (CLK1) in monocyte biology remains poorly defined. Here, we identify and characterize distinct CLK1-splice isoforms in human CD14⁺ monocytes using long-read RNA sequencing. In resting monocytes, we observe predominant expression of a truncated isoform lacking exon 4 (CLK1Δ4), which undergoes nonsense-mediated decay resulting in minimal protein output. Lipopolysaccharide (LPS) stimulation induces a shift toward the full-length isoform (CLK1+4), associated with increased transcript stability and protein expression. This splicing switch was confirmed by RT-qPCR, short-read RNA sequencing, and Western blot analysis. Pharmacological inhibition of CLK1 selectively reduced TNFα production without affecting cell viability, implicating that the isoform shift enhances pro-inflammatory signaling. These findings uncover a stimulus-dependent splicing mechanism that modulates monocyte activation through differential CLK1 isoform expression and suggest a potential therapeutic avenue by targeting splicing regulators in immune-related disease with an established role of activated monocytes.

N6-methyladenosine (m6A) is a dynamic RNA modification that critically modulates gene expression in immune responses. While m6A regulators such as WTAP are implicated in inflammatory and autoimmune diseases, the mechanisms governing their expression during innate immune activation remain unclear. Here, we demonstrate that WTAP expression in human CD14⁺ monocytes is upregulated upon lipopolysaccharide (LPS) stimulation and is associated with alternative promoter usage leading to distinct mRNA isoforms. Bioinformatic analysis and pharmacological inhibition reveal that the transcription factor RELA (NF-κB pathway) directly contributes to this promoter-specific induction. Functional analyses show that both WTAP isoforms encode identical proteins, indicating transcriptional, rather than post-transcriptional, regulation. These findings uncover a novel NF-κB-dependent mechanism regulating WTAP isoform expression in activated monocytes, providing insight into the epitranscriptomic modulation of inflammation and potential dysregulation in autoimmune disease.

N⁶-methyladenosine (m⁶A) has recently emerged as a post-transcriptional modulator governing cell-specific gene expression in innate immune cells, particularly in monocytes. Disruptions in m⁶A homeostasis, manifested as the altered expression of m⁶A-related proteins and m⁶A levels, have been implicated in autoimmune disorders. Perturbations in m⁶A dynamics within total Peripheral blood mononuclear cells (PBMCs) have shown strong correlations with disease severity in rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE). It remains unclear in which specific cell type(s) m⁶A homeostasis is disturbed, and also whether other rheumatic diseases such as juvenile idiopathic arthritis (JIA) show similar features. Here, we assess the involvement of m⁶A and m⁶A-regulatory proteins in JIA monocytes. Notably, the diminished expression of m⁶A-eraser proteins FTO and ALKBH5 was observed in JIA monocytes extracted from the inflamed joint. This resulted in increased m⁶A-methylated transcripts in monocytes from these patients. Correspondingly, we observed that culturing monocytes in the presence of synovial fluid from JIA inflamed joints reduced the expression of both FTO and ALKBH5. The knock-out of FTO in human monocytes of healthy controls increased monocyte activation, indicating the relevance of FTO and m⁶A in the context of JIA. These findings underscore the potential of ALKBH5 and FTO expression as a biomarker in JIA and identify the m⁶A machinery as a potential therapeutic target for the treatment of JIA and possibly other autoimmune diseases in the future.

Chronic inflammation and autoimmune diseases are driven, in part, by the activation of (auto)reactive CD4+ T-cells, highlighting their potential as therapeutic targets for these diseases. Nicotinamide (NAM) has demonstrated anti-inflammatory properties in various disease models and has already demonstrated safety in several large clinical trials in humans. The mechanisms behind these observations, and especially their direct effects on CD4+ T-cells, remain poorly understood. Here, we address this gap by investigating how NAM influences CD4+ T-cell activation and function. We also describe that NAM treatment significantly suppresses CD4+ T-cell activation in vitro, as evidenced by impaired proliferation and reduced expression of surface activation markers. Additionally, NAM treatment resulted in reduced production of pro-inflammatory cytokines, IL-2, IFNy, and IL-17, further highlighting its anti-inflammatory potential. We found that NAM modulates key metabolic processes, including glycolysis and reactive oxygen species (ROS) production—both essential to T-cell activation. Taken together, our findings provide novel mechanistic insight into the regulation of T-cell activation by NAM, suggesting NAM as an attractive candidate for novel therapies targeting immune-related diseases.

Immune responses demand the rapid and precise regulation of gene protein expression. Splicing is a crucial step in this process; ~95% of protein-coding gene transcripts are spliced during mRNA maturation. Alternative splicing allows for distinct functional regulation, as it can affect transcript degradation and can lead to alternative functional protein isoforms. There is increasing evidence that splicing can directly regulate immune responses. For several genes, immune cells display dramatic changes in isoform-level transcript expression patterns upon activation. Recent advances in long-read RNA sequencing assays have enabled an unbiased and complete description of transcript isoform expression patterns. With an increasing amount of cell types and conditions that have been analyzed with such assays, thousands of novel transcript isoforms have been identified. Alternative splicing has been associated with autoimmune diseases, including arthritis. Here, GWASs revealed that SNPs associated with arthritis are enriched in splice sites. In this review, we will discuss how alternative splicing is involved in immune responses and how the dysregulation of alternative splicing can contribute to arthritis pathogenesis. In addition, we will discuss the therapeutic potential of modulating alternative splicing, which includes examples of spliceform-based biomarkers for disease severity or disease subtype, splicing manipulation using antisense oligonucleotides, and the targeting of specific immune-related spliceforms using antibodies.

T cell activation is critical for an effective immune response against pathogens. However, dysregulation contributes to the pathogenesis of autoimmune diseases, including Juvenile Idiopathic Arthritis (JIA). The molecular mechanisms underlying T cell activation are still incompletely understood. T cell activation promotes the acetylation of histone 3 at Lysine 27 (H3K27ac) at enhancer and promoter regions of proinflammatory cytokines, thereby increasing the expression of these genes which is essential for T cell function. Co-activators E1A binding protein P300 (P300) and CREB binding protein (CBP), collectively known as P300/CBP, are essential to facilitate H3K27 acetylation. Presently, the role of P300/CBP in human CD4+ T cells activation remains incompletely understood. To assess the function of P300/CBP in T cell activation and autoimmune disease, we utilized iCBP112, a selective inhibitor of P300/CBP, in T cells obtained from healthy controls and JIA patients. Treatment with iCBP112 suppressed T cell activation and cytokine signaling pathways, leading to reduced expression of many proinflammatory cytokines, including IL-2, IFN-γ, IL-4, and IL-17A. Moreover, P300/CBP inhibition in T cells derived from the inflamed synovium of JIA patients resulted in decreased expression of similar pathways and preferentially suppressed the expression of disease-associated genes. This study underscores the regulatory role of P300/CBP in regulating gene expression during T cell activation while offering potential insights into the pathogenesis of autoimmune diseases. Our findings indicate that P300/CBP inhibition could potentially be leveraged for the treatment of autoimmune diseases such as JIA in the future.

Respiratory syncytial virus (RSV) is the most prevalent cause of acute lower respiratory infection in young children. Currently, the first RSV vaccines are approved by the FDA. Recently, N6-methyladenosine (m⁶A) RNA methylation has been implicated in the regulation of the viral life cycle and replication of many viruses, including RSV. m⁶A methylation of RSV RNA has been demonstrated to promote replication and prevent anti-viral immune responses by the host. Whether m⁶A is also involved in viral entry and whether m⁶A can also affect RSV infection via different mechanisms than methylation of viral RNA is poorly understood. Here, we identify m⁶A reader YTH domain-containing protein 1 (YTHDC1) as a novel negative regulator of RSV infection. We demonstrate that YTHDC1 abrogates RSV infection by reducing the expression of RSV entry receptor CX3C motif chemokine receptor 1 (CX3CR1) on the cell surface of lung epithelial cells. Altogether, these data reveal a novel role for m⁶A methylation and YTHDC1 in the viral entry of RSV. These findings may contribute to the development of novel treatment options to control RSV infection.