Cold agglutinin-associated B-cell lymphoproliferative disease shows highly recurrent gains of chromosome 3 and 12 or 18

Autoimmune haemolytic anaemias

Adult autoimmune haemolytic anaemias (AIHAs) include different subtypes of a rare autoimmune disease in which autoantibodies targeting autoantigens expressed on the membrane of autologous red blood cells (RBCs) are produced, leading to their accelerated destruction. In the presence of haemolytic anaemia, the direct antiglobulin test is the cornerstone of AIHA diagnosis. AIHAs are classified according to the isotype and the thermal optimum of the autoantibody into warm (wAIHAs), cold and mixed AIHAs. wAIHAs, the most frequent type of AIHAs, are associated with underlying conditions in ~50% of cases. In wAIHA, IgG autoantibody reacts with autologous RBCs at 37 °C, leading to antibody-dependent cell-mediated cytotoxicity and increased phagocytosis of RBCs in the spleen. Cold AIHAs include cold agglutinin disease (CAD) and cold agglutinin syndrome (CAS) when there is an underlying condition. CAD and cold agglutinin syndrome are IgM cold antibody-driven AIHAs characterized by classical complement pathway-mediated haemolysis. The management of wAIHAs has long been based around corticosteroids and splenectomy and on symptomatic measures and non-specific cytotoxic agents for CAD. Rituximab and the development of complement inhibitors, such as the anti-C1s antibody sutimlimab, have changed the therapeutic landscape of AIHAs, and new promising targeted therapies are under investigation.

Cold-antibody Autoimmune Hemolytic Anemia: its Association with Neoplastic Disease and Impact on Therapy

PURPOSE OF REVIEW

Cold-antibody mediated autoimmune hemolytic anemia (cAIHA) is subclassified as cold agglutinin disease (CAD), secondary cold agglutinin syndrome (CAS), and paroxysmal cold hemoglobinuria (PCH). This review aims to address the occurrence of neoplastic disorders with these three entities and analyze the impact of such neoplasias on treatment for cAIHA.

RECENT FINDINGS

"Primary" CAD is a distinct clonal B-cell lymphoproliferative disorder in probably all cases, although not classified as a malignant lymphoma. CAS is secondary to malignant lymphoma in a minority of cases. Recent findings allow a further clarification of these differential diagnoses and the therapeutic consequences of specific neoplastic entities. Appropriate diagnostic workup is critical for therapy in cAIHA. Patients with CAD should be treated if they have symptomatic anemia, significant fatigue, or bothersome circulatory symptoms. The distinction between CAD and CAS and the presence of any underlying malignancy in CAS have essential therapeutic implications.

The impact of individual clinical features in cold agglutinin disease: hemolytic versus non-hemolytic symptoms

INTRODUCTION

During the last decades, the pathogenesis of cold agglutinin disease (CAD) has been well elucidated and shown to be complex. Several documented or investigational therapies have been made available. This development has resulted in major therapeutic advances, but also in challenges in choice of therapy.

AREAS COVERED

In this review, we address each step in pathogenesis: bone marrow clonal lymphoproliferation, composition and effects of monoclonal cold agglutinin, non-complement mediated erythrocyte agglutination, complement-dependent hemolysis, and other effects of complement activation. We also discuss the heterogeneous clinical features and their relation to specific steps in pathogenesis, in particular with respect to the impact of complement involvement. CAD can be classified into three clinical phenotypes with consequences for established treatments as well as development of new therapies. Some promising future treatment approaches - beyond chemoimmunotherapy and complement inhibition - are reviewed.

EXPERT OPINION

The patient's individual clinical profile regarding complement involvement and hemolytic versus non-hemolytic features is important for the choice of treatment. Further development of treatment approaches is encouraged, and some candidate drugs are promising irrespective of clinical phenotype. Patients with CAD requiring therapy should be considered for inclusion in clinical trials.

New classifications of B-cell neoplasms: a comparison of 5th WHO and International Consensus classifications

In 2024, the World Health Organization (WHO) launched a new classification of lymphoid neoplasms, a revision of the previously used Revised 4th Edition of their classification (WHO-4R). However, this means that two classifications are now in simultaneous use: the 5th Edition of the WHO classification (WHO-5) and the International Consensus Classification (ICC). Instead of a comprehensive review of each disease entity, as already described elsewhere, this review focuses on revisions made in both the WHO-5 and ICC from WHO-4R and discrepancies between them regarding B-cell neoplasms. Similarities include cutaneous marginal zone lymphoma, cold agglutinin disease, non-primary effusion lymphoma-type effusion-based lymphoma, and gray zone lymphoma. Differences include plasma cell neoplasms, high-grade B-cell lymphoma (double hit lymphoma), follicular lymphoma, LPD with immune deficiency and dysregulation, extranodal large B-cell lymphoma, transformations of indolent B-cell lymphomas, and diffuse large B-cell lymphoma, not otherwise specified. Understanding the similarities and differences between the two latest classifications will aid daily diagnostic practice and future research on lymphoid neoplasms.

The 5th edition of the World Health Organization Classification of mature lymphoid and stromal tumors - an overview and update

The purpose of this review is to give an overview on the conceptual framework and major developments of the upcoming 5th edition of the World Health Organization (WHO) Classification of Haematolymphoid tumours (WHO-HAEM5) and to highlight the most significant changes made in WHO-HAEM5 compared with the revised 4th edition (WHO-HAEM4R) of lymphoid and stromal neoplasms. The changes from the revised 4th edition include the reorganization of entities by means of a hierarchical system that is realized throughout the 5th edition of the WHO classification of tumors of all organ systems, a modification of nomenclature for some entities, the refinement of diagnostic criteria or subtypes, deletion of certain entities, and introduction of new entities. For the first time, tumor-like lesions, mesenchymal lesions specific to lymph node and spleen, and germline predisposition syndromes associated with the lymphoid neoplasms are included in the classification.

Fifth Edition of the World Health Classification of Tumors of the Hematopoietic and Lymphoid Tissues: B-cell Neoplasms

We review B-cell neoplasms in the 5th edition of the World Health Organization classification of hematolymphoid tumors (WHO-HEM5). The revised classification is based on a multidisciplinary approach including input from pathologists, clinicians, and other experts. The WHO-HEM5 follows a hierarchical structure allowing the use of family (class)-level definitions when defining diagnostic criteria are partially met or a complete investigational workup is not possible. Disease types and subtypes have expanded compared with the WHO revised 4th edition (WHO-HEM4R), mainly because of the expansion in genomic knowledge of these diseases. In this review, we focus on highlighting changes and updates in the classification of B-cell lymphomas, providing a comparison with WHO-HEM4R, and offering guidance on how the new classification can be applied to the diagnosis of B-cell lymphomas in routine practice.

Gene expression analysis revealed downregulation of complement receptor 1 in clonal B cells in cold agglutinin disease

Cold agglutinin disease (CAD) is a rare B-cell lymphoproliferative disorder of the bone marrow, manifested by autoimmune hemolytic anemia caused by binding of monoclonal IgM autoantibodies to the I antigen. Underlying genetic changes have previously been reported, but their impact on gene expression profile has been unknown. Here, we define differentially expressed genes in CAD B cells. To unravel downstream alteration in cellular pathways, gene expression by RNA sequencing was undertaken. Clonal B-cell samples from 12 CAD patients and IgM-expressing memory B cells from 4 healthy individuals were analyzed. Differential expression analysis and filtering resulted in 93 genes with significant differential expression. Top upregulated genes included SLC4A1, SPTA1, YBX3, TESC, HBD, AHSP, TRAF1, HBA2, RHAG, CA1, SPTB, IL10, UBASH3B, ALAS2, HBA1, CRYM, RGCC, KANK2, and IGHV4-34. They were upregulated at least 8-fold, while complement receptor 1 (CR1/CD35) was downregulated 11-fold in clonal CAD B cells compared to control B cells. Flow cytometry analyses further confirmed reduced CR1 (CD35) protein expression by clonal CAD IgM+ B cells compared to IgM+ memory B cells in controls. CR1 (CD35) is an important negative regulator of B-cell activation and differentiation. Therefore, reduced CR1 (CD35) expression may increase activation, proliferation, and antibody production in CAD-associated clonal B cells.

IgM monoclonal gammopathy of undetermined significance: clinicopathologic features with and without IgM-related disorders

A subset of patients with immunoglobulin M (IgM) monoclonal gammopathy of undetermined significance (MGUS) develop IgM-related disorders (IgM-RD) including peripheral neuropathy, cryoglobulinemia and/or cold agglutinin disease (CAD). We examined the clinical and bone marrow pathologic findings in 191 IgM MGUS patients (2016 World Health Oragnization criteria). Clonal plasma cells were identified in 41 of 171 (24%) cases by immunohistochemistry (IHC) and clonal B cells in 43 of 157 (27%). IgM-RD was identified in 82 (43%) cases, including peripheral neuropathy (n=67, 35%), cryoglobulinemia (n=21, 11%), and CAD (n=10, 5%). Cases of CAD showed distinctive features including lack of MYD88 mutations (P=0.048), supporting the concept of primary CAD as a distinct clinicopathologic disorder. Following exclusion of CAD, comparison of the remaining cases with (n=72) or without (n=109) IgM-RD showed IgM-RD to be more frequent in men than women (P=0.02) and to be more highly associated with MYD88 L265P (P=0.011). Cases with and without IgM-RD otherwise showed similar features including serum IgM concentrations, presence of lymphoid aggregates, clonal B cells by flow cytometry or clonal plasma cells by IHC. No differences were observed in overall survival between cases with and without IgM-RD. No cases in this series met criteria for plasma cell type IgM MGUS as defined in the 2022 International Consensus Classification of lymphoid neoplasms. These results show IgM-RD to be common in patients with IgM MGUS. While CAD shows distinctive features, the remaining cases of IgM-RD largely show pathologic findings similar to IgM MGUS without IgM-RD.

The Histopathology of Cold Agglutinin Disease-Associated B-Cell Lymphoproliferative Disease

OBJECTIVES

Primary cold agglutinin disease is a type of autoimmune hemolytic anemia caused by circulating antibodies against I antigen, a carbohydrate expressed on most cells, including red blood cells. The underlying disease has been characterized in recent years as a distinct B-cell lymphoproliferative disease of the bone marrow, occurring mostly in the elderly. The disease has been now been included as a separate entity in the most recent classifications of mature B-cell neoplasms.

METHODS

A review of the characteristics of cold agglutinin disease is provided, with an emphasis on the pathology features.

RESULTS

A detailed description of the histopathology, immunophenotype, and genetics of cold agglutinin disease is provided and compared to other B-cell lymphoproliferative diseases in the bone marrow with similar features.

CONCLUSIONS

Recognition of the pathology features of cold agglutinin disease allows to distinguish it from other diseases, especially lymphoplasmacytic lymphoma and marginal zone lymphoma.

Monoclonal antibodies for treatment of cold agglutinin disease

INTRODUCTION

Cold agglutinin disease (CAD) is a difficult-to-treat autoimmune hemolytic anemia and B cell lymphoproliferative disorder associated with fatigue, acrocyanosis and a risk of thromboembolic events. Cold-induced binding of autoantibodies agglutinates red blood cells and triggers the classical complement pathway, leading to predominantly extravascular hemolysis.

AREAS COVERED

This review summarizes clinical and experimental antibody-based treatments for CAD and analyzes the risks and benefits of B cell and complement directed therapies, and discusses potential future treatments for CAD.

EXPERT OPINION

Conventional treatment of CAD includes a B cell targeted treatment approach with rituximab, yielding only limited treatment success. Addition of a cytotoxic agent (e.g. bendamustine) increases efficacy but this is accompanied by an increased risk of neutropenia and infection. Novel complement-directed therapies have emerged and were shown to have a good efficacy against hemolysis and safety profile but are expensive and unable to address circulatory symptoms. Complement inhibition with sutimlimab may be used as a bridging strategy until B cell directed therapy with rituximab takes effect or continued indefinitely if needed. Future antibody-based treatment approaches for CAD involve the further development of complement-directed antibodies, combination of rituximab and bortezomib, and daratumumab. Non-antibody based prospective treatments may include the use of Bruton tyrosine kinase inhibitors.

Sutimlimab for the Treatment of Cold Agglutinin Disease

Cold agglutinin disease (CAD) is a rare autoimmune hemolytic anemia and a bone marrow clonal lymphoproliferative disorder. Hemolysis in CAD is complement-dependent and mediated by the classical activation pathway. Patients also frequently suffer from fatigue and cold-induced circulatory symptoms. Although not all patients need treatment, the symptom burden has previously been underestimated. Effective therapies target the clonal lymphoproliferation or the complement activation. Sutimlimab, a humanized monoclonal IgG4 antibody that binds and inactivates complement protein C1s, is the most extensively investigated complement inhibitor for the treatment of CAD. This review addresses the preclinical studies of sutimlimab and the studies of pharmacokinetics and pharmacodynamics. We then describe and discuss the prospective clinical trials that established sutimlimab as a rapidly acting, highly efficacious, and low-toxic therapeutic agent. This complement inhibitor does not improve the cold-induced circulatory symptoms, which are not complement-mediated. Sutimlimab is approved for the treatment of CAD in the US, Japan, and the European Union. A tentative therapeutic algorithm is presented. The choice of therapy for CAD should be based on an individual assessment, and patients requiring therapy should be considered for inclusion in clinical trials.

Plasma cell neoplasms and related entities-evolution in diagnosis and classification

Plasma cell neoplasms including multiple myeloma (MM) and related terminally differentiated B-cell neoplasms are characterized by secretion of monoclonal immunoglobulin and stepwise development from a preneoplastic clonal B and/or plasma cell proliferation called monoclonal gammopathy of undetermined significance (MGUS). Diagnosis of these disorders requires integration of clinical, laboratory, and morphological features. While their classification mostly remains unchanged compared to the revised 2016 WHO classification and the 2014 International Myeloma Working Group consensus, some changes in criteria and terminology were proposed in the 2022 International Consensus Classification (ICC) of mature lymphoid neoplasms. MGUS of IgM type is now divided into IgM MGUS of plasma cell type, precursor to the rare IgM MM and characterized by MM-type cytogenetics, lack of clonal B-cells and absence of MYD88 mutation, and IgM MGUS, NOS including the remaining cases. Primary cold agglutinin disease is recognized as a new entity. MM is now formally subdivided into cytogenetic groups, recognizing the importance of genetics for clinical features and prognosis. MM with recurrent genetic abnormalities includes MM with CCND family translocations, MM with MAF family translocations, MM with NSD2 translocation, and MM with hyperdiploidy, with the remaining cases classified as MM, NOS. For diagnosis of localized plasma cell tumors, solitary plasmacytoma of bone, and primary extraosseous plasmacytoma, the importance of excluding minimal bone marrow infiltration by flow cytometry is emphasized. Primary systemic amyloidosis is renamed immunoglobulin light chain amyloidosis (AL), and a localized AL amyloidosis is recognized as a distinct entity. This review summarizes the updates on plasma cell neoplasms and related entities proposed in the 2022 ICC. KEY POINTS: • Lymphoplasmacytic lymphoma can be diagnosed with lymphoplasmacytic aggregates in trephine biopsies < 10% of cellularity and evidence of clonal B-cells and plasma cells. • IgM MGUS is subdivided into a plasma cell type and a not otherwise specified (NOS) type. • Primary cold agglutinin disease is recognized as a new entity. • The term "multiple myeloma" replaces the term "plasma cell myeloma" used in the 2016 WHO classification. • Multiple myeloma is subdivided into 4 mutually exclusive cytogenetic groups and MM NOS. • Minimal bone marrow infiltration detected by flow cytometry is of major prognostic importance for solitary plasmacytoma of bone and to a lesser extent for primary extraosseous plasmacytoma. • Localized IG light chain amyloidosis is recognized as a separate entity, distinct from systemic immunoglobulin light chain (AL) amyloidosis.

A comparison of the International Consensus and 5th World Health Organization classifications of mature B-cell lymphomas

Several editions of the World Health Organization (WHO) classifications of lympho-hemopoietic neoplasms in 2001, 2008 and 2017 served as the international standard for diagnosis. Since the 4th WHO edition, here referred as WHO-HAEM4, significant clinico-pathological, immunophenotypic and molecular advances have been made in the field of lymphomas, contributing to refining diagnostic criteria of several diseases, to upgrade entities previously defined as provisional and to identify new entities. This process has resulted in two recent classifying proposals of lymphoid neoplasms, the International Consensus Classification (ICC) and the 5th edition of the WHO classification (WHO-HAEM5). In this paper, we review and compare the two classifications in terms of diagnostic criteria and entity definition, with focus on mature B-cell neoplasms. The main aim is to provide a tool to facilitate the work of pathologists, hematologists and researchers involved in the diagnosis and treatment of lymphomas.

The International Consensus Classification of Mature Lymphoid Neoplasms: a report from the Clinical Advisory Committee

Since the publication of the Revised European-American Classification of Lymphoid Neoplasms in 1994, subsequent updates of the classification of lymphoid neoplasms have been generated through iterative international efforts to achieve broad consensus among hematopathologists, geneticists, molecular scientists, and clinicians. Significant progress has recently been made in the characterization of malignancies of the immune system, with many new insights provided by genomic studies. They have led to this proposal. We have followed the same process that was successfully used for the third and fourth editions of the World Health Organization Classification of Hematologic Neoplasms. The definition, recommended studies, and criteria for the diagnosis of many entities have been extensively refined. Some categories considered provisional have now been upgraded to definite entities. Terminology for some diseases has been revised to adapt nomenclature to the current knowledge of their biology, but these modifications have been restricted to well-justified situations. Major findings from recent genomic studies have impacted the conceptual framework and diagnostic criteria for many disease entities. These changes will have an impact on optimal clinical management. The conclusions of this work are summarized in this report as the proposed International Consensus Classification of mature lymphoid, histiocytic, and dendritic cell tumors.

Sutimlimab for treatment of cold agglutinin disease: why, how and for whom?

Therapies for cold agglutinin disease have been directed at the pathogenic B-cell clone. Sutimlimab, a monoclonal antibody that targets C1s, is the first complement inhibitor to be extensively studied in cold agglutinin disease. Sutimlimab selectively blocks the classical activation pathway and leaves the alternative and lectin pathways intact. Trials have documented high response rates with rapid improvement in hemolysis, hemoglobin levels and fatigue scores and low toxicity. Sutimlimab was recently approved in the USA. This drug appears to be particularly useful in severely anemic patients who require a rapid response, in acute exacerbations that do not resolve spontaneously and in patients in whom chemoimmunotherapy is contraindicated or has failed. The choice of therapy in cold agglutinin disease should be individualized.

Autoimmune hemolytic anemia: causes and consequences

INTRODUCTION

Autoimmune hemolytic anemia (AIHA) is classified according to the direct antiglobulin test (DAT) and thermal characteristics of the autoantibody into warm and cold forms, and in primary versus secondary depending on the presence of associated conditions.

AREAS COVERED

AIHA displays a multifactorial pathogenesis, including genetic (association with congenital conditions and certain mutations), environmental (drugs, infections, including SARS-CoV-2, pollution, etc.), and miscellaneous factors (solid/hematologic neoplasms, systemic autoimmune diseases, etc.) contributing to tolerance breakdown. Several mechanisms, such as autoantibody production, complement activation, monocyte/macrophage phagocytosis, and bone marrow compensation are implicated in extra-/intravascular hemolysis. Treatment should be differentiated and sequenced according to AIHA type (i.e. steroids followed by rituximab for warm, rituximab alone or in association with bendamustine or fludarabine for cold forms). Several new drugs targeting B-cells/plasma cells, complement, and phagocytosis are in clinical trials. Finally, thrombosis and infections may complicate disease course burdening quality of life and increasing mortality.

EXPERT OPINION

Beyond warm and cold AIHA, a gray-zone still exists including mixed and DAT negative forms representing an unmet need. AIHA management is rapidly changing through an increasing knowledge of the pathogenic mechanisms, the refinement of diagnostic tools, and the development of novel targeted and combination therapies.

How I treat cold agglutinin disease

The last decades have seen great progress in the treatment of cold agglutinin disease (CAD). Comparative trials are lacking, and recommendations must be based mainly on nonrandomized trials and will be influenced by personal experience. Herein, current treatment options are reviewed and linked to 3 cases, each addressing specific aspects of therapy. Two major steps in CAD pathogenesis are identified, clonal B-cell lymphoproliferation and complement-mediated hemolysis, each of which constitutes a target of therapy. Although drug treatment is not always indicated, patients with symptomatic anemia or other bothersome symptoms should be treated. The importance of avoiding ineffective therapies is underscored. Corticosteroids should not be used to treat CAD. Studies on safety and efficacy of relevant drugs and combinations are briefly described. The author recommends that B cell-directed approaches remain the first choice in most patients requiring treatment. The 4-cycle bendamustine plus rituximab combination is highly efficacious and sufficiently safe and induces durable responses in most patients, but the time to response can be many months. Rituximab monotherapy should be preferred in frail patients. The complement C1s inhibitor sutimlimab is an emerging option in the second line and may also find its place in the first line in specific situations.

Rituximab Use in Warm and Cold Autoimmune Hemolytic Anemia

Autoimmune hemolytic anemia is a rare condition characterized by destruction of red blood cells with and without involvement of complement. It is associated with significant morbidity and mortality. In warm autoimmune hemolytic anemia, less than 50% of patients remain in long-term remission following initial steroid therapy and subsequent therapies are required. Cold agglutinin disease is a clonal hematologic disorder that requires therapy in the majority of patients and responds poorly to steroids and alkylators. Rituximab has a favorable toxicity profile and has demonstrated efficacy in autoimmune hemolytic anemia in first-line as well as relapsed settings. Rituximab is the preferred therapy for steroid refractory warm autoimmune hemolytic anemia (wAIHA) and as part of the first- and second-line treatment of cold agglutinin disease. This article reviews the mechanism of action of rituximab and the current literature on its role in the management of primary and secondary warm autoimmune hemolytic anemia and cold agglutinin disease.

Autoimmune hemolytic anemia: current knowledge and perspectives

Autoimmune hemolytic anemia (AIHA) is an acquired, heterogeneous group of diseases which includes warm AIHA, cold agglutinin disease (CAD), mixed AIHA, paroxysmal cold hemoglobinuria and atypical AIHA. Currently CAD is defined as a chronic, clonal lymphoproliferative disorder, while the presence of cold agglutinins underlying other diseases is known as cold agglutinin syndrome. AIHA is mediated by autoantibodies directed against red blood cells (RBCs) causing premature erythrocyte destruction. The pathogenesis of AIHA is complex and still not fully understood. Recent studies indicate the involvement of T and B cell dysregulation, reduced CD4+ and CD25+ Tregs, increased clonal expansions of CD8 + T cells, imbalance of Th17/Tregs and Tfh/Tfr, and impaired lymphocyte apoptosis. Changes in some RBC membrane structures, under the influence of mechanical stimuli or oxidative stress, may promote autohemolysis. The clinical presentation and treatment of AIHA are influenced by many factors, including the type of AIHA, degree of hemolysis, underlying diseases, presence of concomitant comorbidities, bone marrow compensatory abilities and the presence of fibrosis and dyserthropoiesis. The main treatment for AIHA is based on the inhibition of autoantibody production by mono- or combination therapy using GKS and/or rituximab and, rarely, immunosuppressive drugs or immunomodulators. Reduction of erythrocyte destruction via splenectomy is currently the third line of treatment for warm AIHA. Supportive treatment including vitamin supplementation, recombinant erythropoietin, thrombosis prophylaxis and the prevention and treatment of infections is essential. New groups of drugs that inhibit immune responses at various levels are being developed intensively, including inhibition of antibody-mediated RBCs phagocytosis, inhibition of B cell and plasma cell frequency and activity, inhibition of IgG recycling, immunomodulation of T lymphocytes function, and complement cascade inhibition. Recent studies have brought about changes in classification and progress in understanding the pathogenesis and treatment of AIHA, although there are still many issues to be resolved, particularly concerning the impact of age-associated changes to immunity.

New Insights in the Pathogenesis and Therapy of Cold Agglutinin-Mediated Autoimmune Hemolytic Anemia

Autoimmune hemolytic anemias mediated by cold agglutinins can be divided into cold agglutinin disease (CAD), which is a well-defined clinicopathologic entity and a clonal lymphoproliferative disorder, and secondary cold agglutinin syndrome (CAS), in which a similar picture of cold-hemolytic anemia occurs secondary to another distinct clinical disease. Thus, the pathogenesis in CAD is quite different from that of polyclonal autoimmune diseases such as warm-antibody AIHA. In both CAD and CAS, hemolysis is mediated by the classical complement pathway and therefore can result in generation of anaphylotoxins, such as complement split product 3a (C3a) and, to some extent, C5a. On the other hand, infection and inflammation can act as triggers and drivers of hemolysis, exemplified by exacerbation of CAD in situations with acute phase reaction and the role of specific infections (particularly Mycoplasma pneumoniae and Epstein-Barr virus) as causes of CAS. In this review, the putative mechanisms behind these phenomena will be explained along with other recent achievements in the understanding of pathogenesis in these disorders. Therapeutic approaches have been directed against the clonal lymphoproliferation in CAD or the underlying disease in CAS. Currently, novel targeted treatments, in particular complement-directed therapies, are also being rapidly developed and will be reviewed.