When Actin is Not Actin' Like It Should: A New Category of Distinct Primary Immunodeficiency Disorders

Structural and functional mechanisms of actin isoforms

Actin is a highly conserved and fundamental protein in eukaryotes and participates in a broad spectrum of cellular functions. Cells maintain a conserved ratio of actin isoforms, with muscle and non-muscle actins representing the main actin isoforms in muscle and non-muscle cells, respectively. Actin isoforms have specific and redundant functional roles and display different biochemistries, cellular localization, and interactions with myosins and actin-binding proteins. Understanding the specific roles of actin isoforms from the structural and functional perspective is crucial for elucidating the intricacies of cytoskeletal dynamics and regulation and their implications in health and disease. Here, we review how the structure contributes to the functional mechanisms of actin isoforms with a special emphasis on the questions of how post-translational modifications and disease-linked mutations affect actin isoforms biochemistry, function, and interaction with actin-binding proteins and myosin motors.

SKAP2 acts downstream of CD11b/CD18 and regulates neutrophil effector function

Background

The importance of CD11b/CD18 expression in neutrophil effector functions is well known. Beyond KINDLIN3 and TALIN1, which are involved in the induction of the high-affinity binding CD11b/CD18 conformation, the signaling pathways that orchestrate this response remain incompletely understood.

Method

We performed an unbiased screening method for protein selection by biotin identification (BioID) and investigated the KINDLIN3 interactome. We used liquid chromatography with tandem mass spectrometry as a powerful analytical tool. Generation of NB4 CD18, KINDLIN3, or SKAP2 knockout neutrophils was achieved using CRISPR-Cas9 technology, and the cells were examined for their effector function using flow cytometry, live cell imaging, microscopy, adhesion, or antibody-dependent cellular cytotoxicity (ADCC).

Results

Among the 325 proteins significantly enriched, we identified Src kinase-associated phosphoprotein 2 (SKAP2), a protein involved in actin polymerization and integrin-mediated outside-in signaling. CD18 immunoprecipitation in primary or NB4 neutrophils demonstrated the presence of SKAP2 in the CD11b/CD18 complex at a steady state. Under this condition, adhesion to plastic, ICAM-1, or fibronectin was observed in the absence of SKAP2, which could be abrogated by blocking the actin rearrangements with latrunculin B. Upon stimulation of NB4 SKAP2-deficient neutrophils, adhesion to fibronectin was enhanced whereas CD18 clustering was strongly reduced. This response corresponded with significantly impaired CD11b/CD18-dependent NADPH oxidase activity, phagocytosis, and cytotoxicity against tumor cells.

Conclusion

Our results suggest that SKAP2 has a dual role. It may restrict CD11b/CD18-mediated adhesion only under resting conditions, but its major contribution lies in the regulation of dynamic CD11b/CD18-mediated actin rearrangements and clustering as required for cellular effector functions of human neutrophils.

WAVE2 Regulates Actin-Dependent Processes Induced by the B Cell Antigen Receptor and Integrins

B cell antigen receptor (BCR) signaling induces actin cytoskeleton remodeling by stimulating actin severing, actin polymerization, and the nucleation of branched actin networks via the Arp2/3 complex. This enables B cells to spread on antigen-bearing surfaces in order to increase antigen encounters and to form an immune synapse (IS) when interacting with antigen-presenting cells (APCs). Although the WASp, N-WASp, and WAVE nucleation-promoting factors activate the Arp2/3 complex, the role of WAVE2 in B cells has not been directly assessed. We now show that both WAVE2 and the Arp2/3 complex localize to the peripheral ring of branched F-actin when B cells spread on immobilized anti-Ig antibodies. The siRNA-mediated depletion of WAVE2 reduced and delayed B cell spreading on immobilized anti-Ig, and this was associated with a thinner peripheral F-actin ring and reduced actin retrograde flow compared to control cells. Depleting WAVE2 also impaired integrin-mediated B cell spreading on fibronectin and the LFA-1-induced formation of actomyosin arcs. Actin retrograde flow amplifies BCR signaling at the IS, and we found that depleting WAVE2 reduced microcluster-based BCR signaling and signal amplification at the IS, as well as B cell activation in response to antigen-bearing cells. Hence, WAVE2 contributes to multiple actin-dependent processes in B lymphocytes.

Inherited ARPC5 mutations cause an actinopathy impairing cell motility and disrupting cytokine signaling

We describe the first cases of germline biallelic null mutations in ARPC5, part of the Arp2/3 actin nucleator complex, in two unrelated patients presenting with recurrent and severe infections, early-onset autoimmunity, inflammation, and dysmorphisms. This defect compromises multiple cell lineages and functions, and when protein expression is reestablished in-vitro, the Arp2/3 complex conformation and functions are rescued. As part of the pathophysiological evaluation, we also show that interleukin (IL)-6 signaling is distinctively impacted in this syndrome. Disruption of IL-6 classical but not trans-signaling highlights their differential roles in the disease and offers perspectives for therapeutic molecular targets.

An exome-wide study of renal operational tolerance

Background

Renal operational tolerance is a rare and beneficial state of prolonged renal allograft function in the absence of immunosuppression. The underlying mechanisms are unknown. We hypothesized that tolerance might be driven by inherited protein coding genetic variants with large effect, at least in some patients.

Methods

We set up a European survey of over 218,000 renal transplant recipients and collected DNAs from 40 transplant recipients who maintained good allograft function without immunosuppression for at least 1 year. We performed an exome-wide association study comparing the distribution of moderate to high impact variants in 36 tolerant patients, selected for genetic homogeneity using principal component analysis, and 192 controls, using an optimal sequence-kernel association test adjusted for small samples.

Results

We identified rare variants of HOMER2 (3/36, FDR 0.0387), IQCH (5/36, FDR 0.0362), and LCN2 (3/36, FDR 0.102) in 10 tolerant patients vs. 0 controls. One patient carried a variant in both HOMER2 and LCN2. Furthermore, the three genes showed an identical variant in two patients each. The three genes are expressed at the primary cilium, a key structure in immune responses.

Conclusion

Rare protein coding variants are associated with operational tolerance in a sizable portion of patients. Our findings have important implications for a better understanding of immune tolerance in transplantation and other fields of medicine.ClinicalTrials.gov, identifier: NCT05124444.

Not too little, not too much: the impact of mutation types in Wiskott-Aldrich syndrome and RAC2 patients

Primary immune deficiencies (PIDs) are genetic disorders impacting the appropriate development or functioning of any portion of the immune system. The broad adoption of high-throughput sequencing has driven discovery of new genes as well as expanded phenotypes associated with known genes. Beginning with the identification of WAS mutations in patients with severe Wiskott-Aldrich Syndrome, recognition of WAS mutations in additional patients has revealed phenotypes including isolated thrombocytopenia and X-linked neutropenia. Likewise RAC2 patients present with vastly different phenotypes depending on the mutation-ranging from reticular dysgenesis or severe neutrophil dysfunction with neonatal presentation to later onset common variable immune deficiency. This review examines genotype-phenotype correlations in patients with WAS (Wiskott-Aldrich Syndrome) and RAC2 mutations, highlighting functional protein domains, how mutations alter protein interactions, and how specific mutations can affect isolated functions of the protein leading to disparate phenotypes.

Understanding immunoactinopathies: A decade of research on WAS gene defects

Immunoactinopathies caused by mutations in actin-related proteins are a growing group of inborn errors of immunity (IEI). Immunoactinopathies are caused by a dysregulated actin cytoskeleton and affect hematopoietic cells especially because of their unique capacity to survey the body for invading pathogens and altered self, such as cancer cells. These cell motility and cell-to-cell interaction properties depend on the dynamic nature of the actin cytoskeleton. Wiskott-Aldrich syndrome (WAS) is the archetypical immunoactinopathy and the first described. WAS is caused by loss-of-function and gain-of-function mutations in the actin regulator WASp, uniquely expressed in hematopoietic cells. Mutations in WAS cause a profound disturbance of actin cytoskeleton regulation of hematopoietic cells. Studies during the last 10 years have shed light on the specific effects on different hematopoietic cells, revealing that they are not affected equally by mutations in the WAS gene. Moreover, the mechanistic understanding of how WASp controls nuclear and cytoplasmatic activities may help to find therapeutic alternatives according to the site of the mutation and clinical phenotypes. In this review, we summarize recent findings that have added to the complexity and increased our understanding of WAS-related diseases and immunoactinopathies.

Deciphering actin remodelling in immune cells through the prism of actin-related inborn errors of immunity

Actin cytoskeleton remodelling drives cell motility, cell to cell contacts, as well as membrane and organelle dynamics. Those cellular activities operate at a particularly high pace in immune cells since these cells migrate through various tissues, interact with multiple cellular partners, ingest microorganisms and secrete effector molecules. The central and multifaceted role of actin cytoskeleton remodelling in sustaining immune cell tasks in humans is highlighted by rare inborn errors of immunity due to mutations in genes encoding proximal and distal actin regulators. In line with the specificity of some of the actin-based processes at work in immune cells, the expression of some of the affected genes, such as WAS, ARPC1B and HEM1 is restricted to the hematopoietic compartment. Exploration of these natural deficiencies highlights the fact that the molecular control of actin remodelling is tuned distinctly in the various subsets of myeloid and lymphoid immune cells and sustains different networks associated with a vast array of specialized tasks. Furthermore, defects in individual actin remodelling proteins are usually associated with partial cellular impairments highlighting the plasticity of actin cytoskeleton remodelling. This review covers the roles of disease-associated actin regulators in promoting the actin-based processes of immune cells. It focuses on the specific molecular function of those regulators across various immune cell subsets and in response to different stimuli. Given the fact that numerous immune-related actin defects have only been characterized recently, we further discuss the challenges lying ahead to decipher the underlying patho-mechanisms.

Mechanisms regulating neutrophil responses in immunity, allergy, and autoimmunity

Neutrophil granulocytes, or neutrophils, are the most abundant circulating leukocytes in humans and indispensable for antimicrobial immunity, as exemplified in patients with inborn and acquired defects of neutrophils. Neutrophils were long regarded as the foot soldiers of the immune system, solely destined to execute a set of effector functions against invading pathogens before undergoing apoptosis, the latter of which was ascribed to their short life span. This simplistic understanding of neutrophils has now been revised on the basis of insights gained from the use of mouse models and single-cell high-throughput techniques, revealing tissue- and context-specific roles of neutrophils in guiding immune responses. These studies also demonstrated that neutrophil responses were controlled by sophisticated feedback mechanisms, including directed chemotaxis of neutrophils to tissue-draining lymph nodes resulting in modulation of antimicrobial immunity and inflammation. Moreover, findings in mice and humans showed that neutrophil responses adapted to different deterministic cytokine signals, which controlled their migration and effector function as well as, notably, their biologic clock by affecting the kinetics of their aging. These mechanistic insights have important implications for health and disease in humans, particularly, in allergic diseases, such as atopic dermatitis and allergic asthma bronchiale, as well as in autoinflammatory and autoimmune diseases. Hence, our improved understanding of neutrophils sheds light on novel therapeutic avenues, focusing on molecularly defined biologic agents.

Autoimmune and autoinflammatory manifestations in inborn errors of immunity

PURPOSE OF REVIEW

Autoimmune and inflammatory complications have been shown to arise in all age groups and across the spectrum of inborn errors of immunity (IEI). This review aims to highlight recent ground-breaking research and its impact on our understanding of IEI.

RECENT FINDINGS

Three registry-based studies of unprecedented size revealed the high prevalence of autoimmune, inflammatory and malignant complications in IEI. Two novel IEI were discovered: an autoinflammatory relopathy, cleavage-resistant RIPK1-induced autoinflammatory syndrome, as well as an inheritable phenocopy of PD-1 blockade-associated complication (as seen in cancer therapy) manifesting with multiorgan autoimmunity and Mycobacterium tuberculosis infection. A study examining patients with partial RAG deficiency pinpointed the specific defects leading to the failure of central and peripheral tolerance resulting in wide-ranging autoimmunity. A novel variant of Immunodeficiency Polyendocrinopathy Enteropathy X-linked syndrome was described, associated with preferential expression of a FOXP3 isoform lacking exon 2, linking exon-specific functions and the phenotypes corresponding to their absence. Lastly, we touch on recent findings pertaining actinopathies, the prototypical IEI with autoimmune, inflammatory and atopic complications.

SUMMARY

Dysregulated immunity has been associated with IEI since their discovery. Recently, large concerted efforts have shown how common these complications actually are while providing insight into normal and dysregulated molecular mechanisms, as well as describing novel diseases.

Proteomic Signatures of the Serological Response to Influenza Vaccination in a Large Human Cohort Study

The serological response to the influenza virus vaccine is highly heterogeneous for reasons that are not entirely clear. While the impact of demographic factors such as age, body mass index (BMI), sex, prior vaccination and titer levels are known to impact seroconversion, they only explain a fraction of the response. To identify signatures of the vaccine response, we analyzed 273 protein levels from 138 serum samples of influenza vaccine recipients (2019-2020 season). We found that levels of proteins functioning in cholesterol transport were positively associated with seroconversion, likely linking to the known impact of BMI. When adjusting seroconversion for the demographic factors, we identified additional, unexpected signatures: proteins regulating actin cytoskeleton dynamics were significantly elevated in participants with high adjusted seroconversion. Viral strain specific analysis showed that this trend was largely driven by the H3N2 strain. Further, we identified complex associations between adjusted seroconversion and other factors: levels of proteins of the complement system associated positively with adjusted seroconversion in younger participants, while they were associated negatively in the older population. We observed the opposite trends for proteins of high density lipoprotein remodeling, transcription, and hemostasis. In sum, careful integrative modeling can extract new signatures of seroconversion from highly variable data that suggest links between the humoral response as well as immune cell communication and migration.

Roles of the nucleus in leukocyte migration

Leukocytes patrol our bodies in search of pathogens and migrate to sites of injury in response to various stimuli. Rapid and directed leukocyte motility is therefore crucial to our immunity. The nucleus is the largest and stiffest cellular organelle and a mechanical obstacle for migration through constrictions. However, the nucleus is also essential for 3D cell migration. Here, we review the roles of the nucleus in leukocyte migration, focusing on how cells deform their nuclei to aid cell motility and the contributions of the nucleus to cell migration. We discuss the regulation of the nuclear biomechanics by the nuclear lamina and how it, together with the cytoskeleton, modulates the shapes of leukocyte nuclei. We then summarize the functions of nesprins and SUN proteins in leukocytes and discuss how forces are exerted on the nucleus. Finally, we examine the mechanical roles of the nucleus in cell migration, including its roles in regulating the direction of migration and path selection.

Radiosensitivity in patients affected by ARPC1B deficiency: a new disease trait?

Actin-related protein 2/3 complex subunit 1B (ARPC1B) deficiency is a recently described inborn error of immunity (IEI) presenting with combined immunodeficiency and characterized by recurrent infections and thrombocytopenia. Manifestations of immune dysregulation, including colitis, vasculitis, and severe dermatitis, associated with eosinophilia, hyper-IgA, and hyper-IgE are also described in ARPC1B-deficient patients. To date, hematopoietic stem cell transplantation seems to be the only curative option for patients. ARPC1B is part of the actin-related protein 2/3 complex (Arp2/3) and cooperates with the Wiskott–Aldrich syndrome protein (WASp) in the regulation of the actin cytoskeleton remodeling and in driving double-strand break clustering for homology-directed repair. In this study, we aimed to investigate radiosensitivity (RS) in ARPC1B-deficient patients to assess whether it can be considered an additional disease trait. First, we performed trio-based next-generation-sequencing studies to obtain the ARPC1B molecular diagnosis in our index case characterized by increased RS, and then we confirmed, using three different methods, an increment of radiosensitivity in all enrolled ARPC1B-deficient patients. In particular, higher levels of chromatid-type aberrations and γH2AX foci, with an increased number of cells arrested in the G2/M-phase of the cell cycle, were found in patients’ cells after ionizing radiation exposition and radiomimetic bleomycin treatment. Overall, our data suggest increased radiosensitivity as an additional trait in ARPC1B deficiency and support the necessity to investigate this feature in ARPC1B patients as well as in other IEI with cytoskeleton defects to address specific clinical follow-up and optimize therapeutic interventions.

HEM1 Actin Immunodysregulatory Disorder: Genotypes, Phenotypes, and Future Directions

Cells of the innate and adaptive immune systems depend on proper actin dynamics to control cell behavior for effective immune responses. Dysregulated actin networks are known to play a pathogenic role in an increasing number of inborn errors of immunity. The WAVE regulatory complex (WRC) mediates branched actin polymerization, a process required for key cellular functions including migration, phagocytosis, vesicular transport, and immune synapse formation. Recent reports of pathogenic variants in NCKAP1L, a hematopoietically restricted gene encoding the HEM1 protein component of the WRC, defined a novel disease involving recurrent bacterial and viral infections, autoimmunity, and excessive inflammation (OMIM 141180). This review summarizes the diverse clinical presentations and immunological phenotypes observed in HEM1-deficient patients. In addition, we integrate the pathophysiological mechanisms described in current literature and highlight the outstanding questions for diagnosis and management of the HEM1 actin immunodysregulatory disorder.

Immune Cell Plasticity in Inflammation: Insights into Description and Regulation of Immune Cell Phenotypes

Inflammation is a life-saving immune reaction occurring in response to invading pathogens. Nonetheless, inflammation can also occur in an uncontrolled, unrestricted manner, leading to chronic disease and organ damage. Mechanisms triggering an inflammatory response, hindering such a response, or leading to its resolution are well-studied but so far insufficiently elucidated with regard to precise therapeutic interventions. Notably, as an immune reaction evolves, requirements and environments for immune cells change, and thus cellular phenotypes adapt and shift, leading to the appearance of distinct cellular subpopulations with new functional features. In this article, we aim to highlight properties of, and overarching regulatory factors involved in, the occurrence of immune cell phenotypes with a special focus on neutrophils, macrophages and platelets. Additionally, we point out implications for both diagnostics and therapeutics in inflammation research.

Diagnosis and clinical management of Wiskott-Aldrich syndrome: current and emerging techniques

INTRODUCTION

Wiskott-Aldrich syndrome (WAS) serves as the prototype of how variants in a gene which encodes a protein central to actin cytoskeletal homeostasis can manifest clinically in a variety of ways including infection, atopy, autoimmunity, inflammation, bleeding, neutropenia, non-malignant lymphoproliferation, and malignancy. Despite the discovery of the WAS gene almost 30 years ago, our understanding of the pathophysiological mechanisms underlying WAS continues to unfold.

AREAS COVERED

This review will provide an overview of the approach to the diagnosis of WAS as well as the management of its associated complications. Advances in the use of allogeneic hematopoietic stem cell transplantation (HSCT) and gene therapy as well as the associated challenges unique to WAS will be discussed.

EXPERT OPINION

Basic research, combined with clinical research focusing on longitudinal analysis of WAS patients, will help clarify determinants that influence WAS pathogenesis as well as clinical complications and outcomes. Advances in curative approaches including the use of alternative donor HSCT for WAS continue to evolve. Gene therapy employing safer and more effective protocols ensuring full correction of WAS will provide life-saving benefit to WAS patients that are unable to undergo HSCT.

Neutrophils in acute inflammation: current concepts and translational implications

Modulation of neutrophil recruitment and function is crucial for targeting inflammatory cells to sites of infection to combat invading pathogens while, at the same time, limiting host tissue injury or autoimmunity. The underlying mechanisms regulating recruitment of neutrophils, 1 of the most abundant inflammatory cells, have gained increasing interest over the years. The previously described classical recruitment cascade of leukocytes has been extended to include capturing, rolling, adhesion, crawling, and transmigration, as well as a reverse-transmigration step that is crucial for balancing immune defense and control of remote organ endothelial leakage. Current developments in the field emphasize the importance of cellular interplay, tissue environmental cues, circadian rhythmicity, detection of neutrophil phenotypes, differential chemokine sensing, and contribution of distinct signaling components to receptor activation and integrin conformations. The use of therapeutics modulating neutrophil activation responses, as well as mutations causing dysfunctional neutrophil receptors and impaired signaling cascades, have been defined in translational animal models. Human correlates of such mutations result in increased susceptibility to infections or organ damage. This review focuses on current advances in the understanding of the regulation of neutrophil recruitment and functionality and translational implications of current discoveries in the field with a focus on acute inflammation and sepsis.

Formation of neutrophil extracellular traps requires actin cytoskeleton rearrangements

Neutrophils are important effector cells in the host defense against invading micro-organisms. One of the mechanisms they employ to eliminate pathogens is the release of neutrophil extracellular traps (NETs). Although NET release and subsequent cell death known as NETosis have been intensively studied, the cellular components and factors determining or facilitating the formation of NETs remain incompletely understood. Using various actin polymerization and myosin II modulators on neutrophils from healthy individuals, we show that intact F-actin dynamics and myosin II function are essential for NET formation when induced by different stimuli, i.e. phorbol 12-myristate 13-acetate, monosodium urate crystals and Candida albicans. The role of actin polymerization in NET formation could not be explained by the lack of reactive oxygen species production or granule release, which were normal or enhanced under the given conditions. Neutrophils from patients with very rare inherited actin polymerization defects by either ARPC1B- or MKL1-deficiency also failed to show NETosis. We found that upon inhibition of actin dynamics there is a lack of translocation of NE to the nucleus, which may well explain the impaired NET formation. Collectively, our data illustrate the essential requirement of an intact and active actin polymerization process, as well as active myosin II to enable the release of nuclear DNA by neutrophils during NET formation.

The ERM protein moesin regulates natural killer cell homeostasis in vivo

Moesin is a member of the ezrin-radixin-moesin (ERM) family of proteins that link plasma membrane proteins with actin filaments in the cell cortex. Hemizygous mutations in the X-linked moesin gene are associated with primary immunodeficiency with T and B cell lymphopenia, which also affects natural killer (NK) cells in most cases. We previously showed that moesin deficiency in mice substantially affects lymphocyte homeostasis, but its impact on NK cells remains unexplored. Here, we found that in moesin-deficient mice, NK cells were decreased in the peripheral blood and bone marrow but increased in the spleen. Analysis of female heterozygous mice showed a selective advantage for moesin-expressing NK cells in the blood. Moesin-deficient NK cells exhibited increased cell death and impaired signaling in response to IL-15, suggesting that moesin regulates NK cell survival through IL-15-mediated signaling. Our findings thus identify moesin as an NK cell homeostasis regulator in vivo.

The Actin-Disassembly Protein Glia Maturation Factor γ Enhances Actin Remodeling and B Cell Antigen Receptor Signaling at the Immune Synapse

Signaling by the B cell antigen receptor (BCR) initiates actin remodeling. The assembly of branched actin networks that are nucleated by the Arp2/3 complex exert outward force on the plasma membrane, allowing B cells to form membrane protrusions that can scan the surface of antigen-presenting cells (APCs). The resulting Arp2/3 complex-dependent actin retrograde flow promotes the centripetal movement and progressive coalescence of BCR microclusters, which amplifies BCR signaling. Glia maturation factor γ (GMFγ) is an actin disassembly-protein that releases Arp2/3 complex-nucleated actin filaments from actin networks. By doing so, GMFγ could either oppose the actions of the Arp2/3 complex or support Arp2/3 complex-nucleated actin polymerization by contributing to the recycling of actin monomers and Arp2/3 complexes. We now show that reducing the levels of GMFγ in human B cell lines via transfection with a specific siRNA impairs the ability of B cells to spread on antigen-coated surfaces, decreases the velocity of actin retrograde flow, diminishes the coalescence of BCR microclusters into a central cluster at the B cell-APC contact site, and decreases APC-induced BCR signaling. These effects of depleting GMFγ are similar to what occurs when the Arp2/3 complex is inhibited. This suggests that GMFγ cooperates with the Arp2/3 complex to support BCR-induced actin remodeling and amplify BCR signaling at the immune synapse.

Bidirectional feedback between BCR signaling and actin cytoskeletal dynamics

When B cells are exposed to antigens, they use their B-cell receptors (BCRs) to transduce this external signal into internal signaling cascades and uptake antigen, which activate transcriptional programs. Signaling activation requires complex cytoskeletal remodeling initiated by BCR signaling. The actin cytoskeletal remodeling drives B-cell morphological changes, such as spreading, protrusion, contraction, and endocytosis of antigen by mechanical forces, which in turn affect BCR signaling. Therefore, the relationship between the actin cytoskeleton and BCR signaling is a two-way feedback loop. These morphological changes represent the indirect ways by which the actin cytoskeleton regulates BCR signaling. Recent studies using high spatiotemporal resolution microscopy techniques have revealed that actin also can directly influence BCR signaling. Cortical actin networks directly affect BCR mobility, not only during the resting stage by serving as diffusion barriers, but also at the activation stage by altering BCR diffusivity through enhanced actin flow velocities. Furthermore, the actin cytoskeleton, along with myosin, enables B cells to sense the physical properties of its environment and generate and transmit forces through the BCR. Consequently, the actin cytoskeleton modulates the signaling threshold of BCR to antigenic stimulation. This review discusses the latest research on the relationship between BCR signaling and actin remodeling, and the research techniques. Exploration of the role of actin in BCR signaling will expand fundamental understanding of the relationship between cell signaling and the cytoskeleton and the mechanisms underlying cytoskeleton-related immune disorders and cancer.

The Wdr1-LIMK-Cofilin Axis Controls B Cell Antigen Receptor-Induced Actin Remodeling and Signaling at the Immune Synapse

When B cells encounter membrane-bound antigens, the formation and coalescence of B cell antigen receptor (BCR) microclusters amplifies BCR signaling. The ability of B cells to probe the surface of antigen-presenting cells (APCs) and respond to APC-bound antigens requires remodeling of the actin cytoskeleton. Initial BCR signaling stimulates actin-related protein (Arp) 2/3 complex-dependent actin polymerization, which drives B cell spreading as well as the centripetal movement and coalescence of BCR microclusters at the B cell-APC synapse. Sustained actin polymerization depends on concomitant actin filament depolymerization, which enables the recycling of actin monomers and Arp2/3 complexes. Cofilin-mediated severing of actin filaments is a rate-limiting step in the morphological changes that occur during immune synapse formation. Hence, regulators of cofilin activity such as WD repeat-containing protein 1 (Wdr1), LIM domain kinase (LIMK), and coactosin-like 1 (Cotl1) may also be essential for actin-dependent processes in B cells. Wdr1 enhances cofilin-mediated actin disassembly. Conversely, Cotl1 competes with cofilin for binding to actin and LIMK phosphorylates cofilin and prevents it from binding to actin filaments. We now show that Wdr1 and LIMK have distinct roles in BCR-induced assembly of the peripheral actin structures that drive B cell spreading, and that cofilin, Wdr1, and LIMK all contribute to the actin-dependent amplification of BCR signaling at the immune synapse. Depleting Cotl1 had no effect on these processes. Thus, the Wdr1-LIMK-cofilin axis is critical for BCR-induced actin remodeling and for B cell responses to APC-bound antigens.

Immunity and Genetics at the Revolving Doors of Diagnostics in Primary Immunodeficiencies

Primary immunodeficiencies (PIDs) are a large and growing group of disorders commonly associated with recurrent infections. However, nowadays, we know that PIDs often carry with them consequences related to organ or hematologic autoimmunity, autoinflammation, and lymphoproliferation in addition to simple susceptibility to pathogens. Alongside this conceptual development, there has been technical advancement, given by the new but already established diagnostic possibilities offered by new genetic testing (e.g., next-generation sequencing). Nevertheless, there is also the need to understand the large number of gene variants detected with these powerful methods. That means advancing beyond genetic results and resorting to the clinical phenotype and to immunological or alternative molecular tests that allow us to prove the causative role of a genetic variant of uncertain significance and/or better define the underlying pathophysiological mechanism. Furthermore, because of the rapid availability of results, laboratory immunoassays are still critical to diagnosing many PIDs, even in screening settings. Fundamental is the integration between different specialties and the development of multidisciplinary and flexible diagnostic workflows. This paper aims to tell these evolving aspects of immunodeficiencies, which are summarized in five key messages, through introducing and exemplifying five clinical cases, focusing on diseases that could benefit targeted therapy.

Molecular Mechanisms of Leukocyte Migration and Its Potential Targeting-Lessons Learned From MKL1/SRF-Related Primary Immunodeficiency Diseases

Megakaryoblastic leukemia 1 (MKL1) deficiency is one of the most recently discovered primary immunodeficiencies (PIDs) caused by cytoskeletal abnormalities. These immunological “actinopathies” primarily affect hematopoietic cells, resulting in defects in both the innate immune system (phagocyte defects) and adaptive immune system (T-cell and B-cell defects). MKL1 is a transcriptional coactivator that operates together with serum response factor (SRF) to regulate gene transcription. The MKL/SRF pathway has been originally described to have important functions in actin regulation in cells. Recent results indicate that MKL1 also has very important roles in immune cells, and that MKL1 deficiency results in an immunodeficiency affecting the migration and function of primarily myeloid cells such as neutrophils. Interestingly, several actinopathies are caused by mutations in genes which are recognized MKL(1/2)-dependent SRF-target genes, namely ACTB, WIPF1, WDR1, and MSN. Here we summarize these and related (ARPC1B) actinopathies and their effects on immune cell function, especially focusing on their effects on leukocyte adhesion and migration. Furthermore, we summarize recent therapeutic efforts targeting the MKL/SRF pathway in disease.

Case Report: DOCK8 Deficiency Without Hyper-IgE in a Child With a Large Deletion

Autosomal recessive (AR) DOCK8 deficiency is a well-known actinopathy, a combined primary immune deficiency with impaired actin polymerization that results in altered cell mobility and immune synapse. DOCK8-deficient patients present early in life with eczema, viral cutaneous infections, chronic mucocutaneous candidiasis, bacterial pneumonia, and abscesses, together with eosinophilia, thrombocytosis, lymphopenia, and variable dysgammaglobulinemia that usually includes Hyper-IgE. In fact, before its genetic etiology was known, patients were described as having a form of Hyper-IgE syndrome, a name now deprecated in favor of genetic defects. We describe a school-age male patient with a clinical picture suggestive of DOCK8 deficiency, except for high serum IgE or a family history: early onset, failure to thrive, eczema, warts, condyloma, bronchiolitis, pneumonia, recurrent otitis media, bronchiectasis, candidiasis, leukocytosis, eosinophilia, high IgA, low IgG, and low CD4+ T cells. We were able to confirm the diagnosis through protein expression and whole-exome sequencing. We review the clinical, laboratory, and genetic features of 200 DOCK8-deficient patients; at least 4 other patients have had no elevated IgE, and about 40% do not have Hyper-IgE (above 1,000 IU/mL). Despite this, the constellation of signs, symptoms, and findings allow the suspicion of DOCK8 deficiency and other actinopathies.