Positive and negative selection shape the human naïve B cell repertoire

Molecular mimicry as a mechanism of viral immune evasion and autoimmunity

Mimicry of host protein structures, or 'molecular mimicry', is a common mechanism employed by viruses to evade the host's immune system. Short linear amino acid (AA) molecular mimics can elicit cross-reactive antibodies and T cells from the host, but the prevalence of such mimics throughout the human virome has not been fully explored. Here we evaluate 134 human-infecting viruses and find significant usage of linear mimicry across the virome, particularly those in the Herpesviridae and Poxviridae families. Furthermore, host proteins related to cellular replication and inflammation, autosomes, the X chromosome, and thymic cells are enriched as viral mimicry targets. Finally, we find that short linear mimicry from Epstein-Barr virus (EBV) is higher in auto-antibodies found in patients with multiple sclerosis than previously appreciated. Our results thus hint that human-infecting viruses leverage mimicry in the course of their infection, and that such mimicry may contribute to autoimmunity, thereby prompting potential targets for therapies.

B cell tolerance and autoimmunity: Lessons from repertoires

Adaptive immune cell function is regulated by a highly diverse receptor recombined from variable germline-encoded segments that can recognize an almost unlimited array of epitopes. While this diversity enables the recognition of any pathogen, it also poses a risk of self-recognition, leading to autoimmunity. Many layers of regulation are present during both the generation and activation of B cells to prevent this phenomenon, although they are evidently imperfect. In recent years, our ability to analyze immune repertoires at scale has drastically increased, both through advances in sequencing and single-cell analyses. Here, we review the current knowledge on B cell repertoire analyses, focusing on their implication for autoimmunity. These studies demonstrate that a failure of tolerance occurs at multiple independent checkpoints in different autoimmune contexts, particularly during B cell maturation, plasmablast differentiation, and within germinal centers. These failures are marked by distinct repertoire features that may be used to identify disease- or patient-specific therapeutic approaches.

Targeted therapy for multiple myeloma: an overview on CD138-based strategies

Multiple myeloma (MM) is an incurable hematological disease characterized by the uncontrolled growth of plasma cells primarily in the bone marrow. Although its treatment consists of the administration of combined therapy regimens mainly based on immunomodulators and proteosome inhibitors, MM remains incurable, and most patients suffer from relapsed/refractory disease with poor prognosis and survival. The robust results achieved by immunotherapy targeting MM-associated antigens CD38 and CD319 (also known as SLAMF7) have drawn attention to the development of new immune-based strategies and different innovative compounds in the treatment of MM, including new monoclonal antibodies, antibody-drug conjugates, recombinant proteins, synthetic peptides, and adaptive cellular therapies. In this context, Syndecan1 (CD138 or SDC1), a transmembrane heparan sulfate proteoglycan that is upregulated in malignant plasma cells, has gained increasing attention in the panorama of MM target antigens, since its key role in MM tumorigenesis, progression and aggressiveness has been largely reported. Here, our aim is to provide an overview of the most important aspects of MM disease and to investigate the molecular functions of CD138 in physiologic and malignant cell states. In addition, we will shed light on the CD138-based therapeutic approaches currently being tested in preclinical and/or clinical phases in MM and discuss their properties, mechanisms of action and clinical applications.

B cell development and antibody responses in human immune system mice: current status and future perspective

Humanized immune system (HIS) mice have been developed and used as a small surrogate model to study human immune function under normal or disease conditions. Although variations are found between models, most HIS mice show robust human T cell responses. However, there has been unsuccessful in constructing HIS mice that produce high-affinity human antibodies, primarily due to defects in terminal B cell differentiation, antibody affinity maturation, and development of primary follicles and germinal centers. In this review, we elaborate on the current knowledge about and previous attempts to improve human B cell development in HIS mice, and propose a potential strategy for constructing HIS mice with improved humoral immunity by transplantation of human follicular dendritic cells (FDCs) to facilitate the development of secondary follicles.

Targeting the B cell receptor signaling pathway in chronic lymphocytic leukemia

Aberrant signal transduction through the B cell receptor (BCR) plays a critical role in the pathogenesis of chronic lymphocytic leukemia (CLL)/small lymphocytic lymphoma (SLL). BCR-dependent signaling is necessary for the growth and survival of neoplastic cells, making inhibition of down-stream pathways a logical therapeutic strategy. Indeed, selective inhibitors against Bruton's tyrosine kinase (BTK) and phosphoinositide 3-kinase (PI3K) have been shown to induce high rates of response in CLL and other B cell lymphomas. In particular, the development of BTK inhibitors revolutionized the treatment approach to CLL, demonstrating long-term efficacy. While BTK inhibitors are widely used for multiple lines of treatment, PI3K inhibitors are much less commonly utilized, mainly due to toxicities. CLL remains an incurable disease and effective treatment options after relapse or development of TKI resistance are greatly needed. This review provides an overview of BCR signaling, a summary of the current therapeutic landscape, and a discussion of the ongoing trials targeting BCR-associated kinases.

Molecular Mimicry as a Mechanism of Viral Immune Evasion and Autoimmunity

Mimicry of host protein structures ("molecular mimicry") is a common mechanism employed by viruses to evade the host's immune system. To date, studies have primarily evaluated molecular mimicry in the context of full protein structural mimics. However, recent work has demonstrated that short linear amino acid (AA) molecular mimics can elicit cross-reactive antibodies and T-cells from the host, which may contribute to development and progression of autoimmunity. Despite this, the prevalence of molecular mimics throughout the human virome has not been fully explored. In this study, we evaluate 134 human infecting viruses and find significant usage of linear mimicry across the virome, particularly those in the herpesviridae and poxviridae families. Furthermore, we identify that proteins involved in cellular replication and inflammation, those expressed from autosomes, the X chromosome, and in thymic cells are over-enriched in viral mimicry. Finally, we demonstrate that short linear mimicry from Epstein-Barr virus (EBV) is significantly higher in auto-antibodies found in multiple sclerosis patients to a greater degree than previously appreciated. Our results demonstrate that human-infecting viruses frequently leverage mimicry in the course of their infection, point to substantial evolutionary pressure for mimicry, and highlight mimicry's important role in human autoimmunity. Clinically, our findings could translate to development of novel therapeutic strategies that target viral infections linked to autoimmunity, with the goal of eliminating disease-associated latent viruses and preventing their reactivation.

Anti-cytokine autoantibodies: mechanistic insights and disease associations

Anti-cytokine autoantibodies (ACAAs) are increasingly recognized as modulating disease severity in infection, inflammation and autoimmunity. By reducing or augmenting cytokine signalling pathways or by altering the half-life of cytokines in the circulation, ACAAs can be either pathogenic or disease ameliorating. The origins of ACAAs remain unclear. Here, we focus on the most common ACAAs in the context of disease groups with similar characteristics. We review the emerging genetic and environmental factors that are thought to drive their production. We also describe how the profiling of ACAAs should be considered for the early diagnosis, active monitoring, treatment or sub-phenotyping of diseases. Finally, we discuss how understanding the biology of naturally occurring ACAAs can guide therapeutic strategies.

FOXP3 deficiency, from the mechanisms of the disease to curative strategies

FOXP3 gene is a key transcription factor driving immune tolerance and its deficiency causes immune dysregulation, polyendocrinopathy, enteropathy X-linked syndrome (IPEX), a prototypic primary immune regulatory disorder (PIRD) with defective regulatory T (Treg) cells. Although life-threatening, the increased awareness and early diagnosis have contributed to improved control of the disease. IPEX currently comprises a broad spectrum of clinical autoimmune manifestations from severe early onset organ involvement to moderate, recurrent manifestations. This review focuses on the mechanistic advancements that, since the IPEX discovery in early 2000, have informed the role of the human FOXP3+ Treg cells in controlling peripheral tolerance and shaping the overall immune landscape of IPEX patients and carrier mothers, contributing to defining new treatments.

Antigen-presenting autoreactive B cells activate regulatory T cells and suppress autoimmune arthritis in mice

B cells undergo several rounds of selection to eliminate potentially pathogenic autoreactive clones, but in contrast to T cells, evidence of positive selection of autoreactive B cells remains moot. Using unique tetramers, we traced natural autoreactive B cells (C1-B) specific for a defined triple-helical epitope on collagen type-II (COL2), constituting a sizeable fraction of the physiological B cell repertoire in mice, rats, and humans. Adoptive transfer of C1-B suppressed arthritis independently of IL10, separating them from IL10-secreting regulatory B cells. Single-cell sequencing revealed an antigen processing and presentation signature, including induced expression of CD72 and CCR7 as surface markers. C1-B presented COL2 to T cells and induced the expansion of regulatory T cells in a contact-dependent manner. CD72 blockade impeded this effect suggesting a new downstream suppressor mechanism that regulates antigen-specific T cell tolerization. Thus, our results indicate that autoreactive antigen-specific naïve B cells tolerize infiltrating T cells against self-antigens to impede the development of tissue-specific autoimmune inflammation.

Complex regulatory effects of gut microbial short-chain fatty acids on immune tolerance and autoimmunity

Immune tolerance deletes or suppresses autoreactive lymphocytes and is established at multiple levels during the development, activation and effector phases of T and B cells. These mechanisms are cell-intrinsically programmed and critical in preventing autoimmune diseases. We have witnessed the existence of another type of immune tolerance mechanism that is shaped by lifestyle choices, such as diet, microbiome and microbial metabolites. Short-chain fatty acids (SCFAs) are the most abundant microbial metabolites in the colonic lumen and are mainly produced by the microbial fermentation of prebiotics, such as dietary fiber. This review focuses on the preventive and immunomodulatory effects of SCFAs on autoimmunity. The tissue- and disease-specific effects of dietary fiber, SCFAs and SCFA-producing microbes on major types of autoimmune diseases, including type I diabetes, multiple sclerosis, rheumatoid arthritis and lupus, are discussed. Additionally, their key regulatory mechanisms for lymphocyte development, tissue barrier function, host metabolism, immunity, autoantibody production, and inflammatory effector and regulatory lymphocytes are discussed. The shared and differential effects of SCFAs on different types and stages of autoimmune diseases are discussed.

Immune dysregulation

The understanding of immune dysregulation in many different diseases continues to grow. There is increasing evidence that altered microbiome and gut barrier dysfunction contribute to systemic inflammation in patients with primary immunodeficiency and in patients with rheumatic disease. Recent research provides insight into the process of induction and maturation of pathogenic age-associated B cells and highlights the role of age-associated B cells in creating tissue inflammation. T follicular regulatory cells are shown to help maintain B-cell tolerance, and therapeutic approaches to increase or promote T follicular regulatory cells may help prevent or decrease immune dysregulation. Meanwhile, novel studies of systemic-onset juvenile idiopathic arthritis reveal a strong HLA association with interstitial lung disease and identify key aspects of the pathogenesis of macrophage activation syndrome. Studies of hyperinflammatory syndromes, including the recently described multisystem inflammatory syndrome of children, characterize similarities and differences in cytokine profiles and T-cell activation. This review focuses on recent advances in the understanding of immune dysregulation and describes potential key factors that may function as biomarkers for disease or targets for therapeutic interventions. Future trials are necessary to address the many remaining questions with regards to pathogenesis, diagnosis, and treatment of autoimmune, inflammatory, and immunodeficiency syndromes.

Restriction of the Global IgM Repertoire in Antiphospholipid Syndrome

The typical anti-phospholipid antibodies (APLA) in the anti-phospholipid syndrome (APS) are reactive with the phospholipid-binding protein β2GPI as well as a growing list of other protein targets. The relation of APLA to natural antibodies and the fuzzy set of autoantigens involved provoked us to study the changes in the IgM repertoire in APS. To this end, peptides selected by serum IgM from a 7-residue linear peptide phage display library (PDL) were deep sequenced. The analysis was aided by a novel formal representation of the Igome (the mimotope set reflecting the IgM specificities) in the form of a sequence graph. The study involved women with APLA and habitual abortions (n=24) compared to age-matched clinically healthy pregnant women (n=20). Their pooled Igomes (297 028 mimotope sequences) were compared also to the global public repertoire Igome of pooled donor plasma IgM (n=2 796 484) and a set of 7-mer sequences found in the J regions of human immunoglobulins (n=4 433 252). The pooled Igome was represented as a graph connecting the sequences as similar as the mimotopes of the same monoclonal antibody. The criterion was based on previously published data. In the resulting graph, identifiable clusters of vertices were considered related to the footprints of overlapping antibody cross-reactivities. A subgraph based on the clusters with a significant differential expression of APS patients' mimotopes contained predominantly specificities underrepresented in APS. The differentially expressed IgM footprints showed also an increased cross-reactivity with immunoglobulin J regions. The specificities underexpressed in APS had a higher correlation with public specificities than those overexpressed. The APS associated specificities were strongly related also to the human peptidome with 1 072 mimotope sequences found in 7 519 human proteins. These regions were characterized by low complexity. Thus, the IgM repertoire of the APS patients was found to be characterized by a significant reduction of certain public specificities found in the healthy controls with targets representing low complexity linear self-epitopes homologous to human antibody J regions.