LAD-1/variant syndrome is caused by mutations in FERMT3

Myosin light chain 6 (Myl6) interacts with kindlin-3 and is required to support integrin αIIbβ3 activation in platelets in mice

BACKGROUND

Kindlin-3 in platelets plays an essential role in supporting integrin αIIbβ3 activation, platelet spreading, aggregation, and clot retraction by binding to the integrin β3 cytoplasmic tail. However, the mechanism by which kindlin-3 mediates the crosstalk between integrin αIIbβ3 and myosin in platelets remains unknown.

OBJECTIVES

To examine the role of myosin light chain 6 (Myl6) in supporting integrin αIIbβ3 activation in platelets.

METHODS

Myl6fl/flPF4-Cre mice with a deficiency of Myl6 in the megakaryocyte lineage were generated, and integrin αIIbβ3 activation in Myl6-deficient platelets was analyzed.

RESULTS

We identified a novel kindlin-3 binding protein, Myl6, an essential light chain of myosin in platelets. Myl6fl/flPF4-Cre mice exhibited significant macrothrombocytopenia resulting from defective proplatelet formation. In the absence of Myl6, integrin αIIbβ3 activation in platelets was significantly suppressed, and platelet aggregation was substantially impaired. Interestingly, the deficiency of Myl6 in platelets preferentially affected the binding of a multivalent ligand compared to a monovalent ligand to integrin αIIbβ3 upon activation, indicating that Myl6 may contribute to the avidity modulation of integrin αIIbβ3 by binding to kindlin-3. Furthermore, blood coagulation ability was impaired in Myl6fl/flPF4-Cre mice, and consistently, these mice exhibited defects in both hemostatic and thrombotic functions.

CONCLUSION

In summary, these results suggest that Myl6, as a novel kindlin-3 binding partner, is required to support integrin αIIbβ3 activation in platelets, which plays an important role in both hemostasis and thrombosis.

Inactivation of kindlin-3 increases human melanoma aggressiveness through the collagen-activated tyrosine kinase receptor DDR1

The role of the focal adhesion protein kindlin-3 as a tumor suppressor and its interaction mechanisms with extracellular matrix constitute a major field of investigation to better decipher tumor progression. Besides the well-described role of kindlin-3 in integrin activation, evidence regarding modulatory functions between melanoma cells and tumor microenvironment are lacking and data are needed to understand mechanisms driven by kindlin-3 inactivation. Here, we show that kindlin-3 inactivation through knockdown or somatic mutations increases BRAFV600mut melanoma cells oncogenic properties via collagen-related signaling by decreasing cell adhesion and enhancing proliferation and migration in vitro, and by promoting tumor growth in mice. Mechanistic analysis reveals that kindlin-3 interacts with the collagen-activated tyrosine kinase receptor DDR1 (Discoidin domain receptor 1) modulating its expression and its interaction with β1-integrin. Kindlin-3 knockdown or mutational inactivation disrupt DDR1/β1-integrin complex in vitro and in vivo and its loss improves the anti-proliferative effect of DDR1 inhibition. In agreement, kindlin-3 downregulation is associated with DDR1 over-expression in situ and linked to worse melanoma prognosis. Our study reveals a unique mechanism of action of kindlin-3 in the regulation of tumorigenesis mediated by the collagen-activated tyrosine kinase receptor DDR1 thus paving the way for innovative therapeutic targeting approaches in melanoma.

Association of neutrophil defects with oral ulcers but undetermined role of neutrophils in recurrent aphthous stomatitis

Objective

Recurrent oral ulcers and severe periodontal diseases in patients with quantitative or qualitative neutrophil defects highlight the important role of neutrophils in maintaining oral mucosal barrier homeostasis. Recurrent aphthous stomatitis (RAS) is a common oral mucosal disease affecting up to 25% of the population, yet its etiopathogenesis remains unclear, and management is unsatisfactory. This review aims to gain insight into the pathogenesis of RAS.

Design

This narrative review examines the characteristics of oral and blood neutrophils, the associations between neutrophil defects and the occurrence of oral ulcers, and the evidence for the involvement of neutrophils in RAS. To conduct the review, relevant literature was searched in PubMed and Google Scholar, which was then thoroughly reviewed and critically appraised.

Results

Neutropenia, specifically a decrease in the number of oral neutrophils, impaired extravasation, and defective ROS production appear to be associated with oral ulcers, while defects in granule enzymes or NETosis are unlikely to have a link to oral ulcers. The review of the histopathology of RAS shows that neutrophils are concentrated in the denuded area but are latecomers to the scene and early leavers. However, the evidence for the involvement of neutrophils in the pathogenesis of RAS is inconsistent, leading to the proposal of two different scenarios involving either impaired or hyperactive neutrophils in the pathogenesis of RAS.

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.

A novel leukocyte adhesion deficiency type III mutation manifests functional importance of the compact FERM domain in kindlin-3

BACKGROUND

Leukocyte adhesion deficiency III (LAD-III) is a rare autosomal recessive syndrome characterized by functional deficiencies of platelets and leukocytes that occurs due to mutations in the FERMT3 gene encoding kindlin-3. Kindlin-3 is a FERM domain-containing adaptor protein that is essential in integrin activation. We have previously demonstrated that the FERM domain of kindlin-3 is structurally compact and plays an important role in supporting integrin activation in a mouse model. The impact of destabilizing the compact FERM domain in kindlin-3 on the development of LAD-III in humans remains uncertain.

OBJECTIVES

To use primary cells from a patient with LAD-III to validate the role of the compact FERM domain in kindlin-3 function in platelets and leukocytes.

METHODS

The patient is a 4-year-old girl who since infancy has displayed clinical features of LAD-III. Patient platelets and leukocytes were functionally analyzed, and structural analysis of the kindlin-3 variant was conducted.

RESULTS

We identified a novel homozygous missense mutation in the FERMT3 (c.412G>A, p.E138K) FERM domain. Substantially reduced levels of kindlin-3 were detected in the proband's platelets and leukocytes. Functional evaluation verified that integrin αIIbβ3-mediated platelet activation, spreading, and aggregation and β2-integrin-mediated neutrophil adhesion and spreading were significantly compromised. Structural analysis revealed that this newly identified E138K substitution in kindlin-3 destabilizes the compacted FERM domain, resulting in poor expression of kindlin-3 in blood cells and subsequent LAD-III.

CONCLUSION

We have identified a novel missense mutation and verified the functional significance of the compact kindlin-3 FERM domain in supporting integrin functions in platelets and leukocytes.

Platelet genetic testing by next-generation sequencing: A practical update

Inherited platelet disorders (IPDs) are a heterogeneous group of disorders characterized by normal or reduced platelet counts, bleeding diatheses of varying severities, and the presence (syndromic) or absence (non-syndromic) of involvement of other organs. Due to the lack of highly specific platelet function tests and overlapping clinical and laboratory features, diagnosing the underlying cause of IPDs remains challenging. In recent years, genetic testing via next-generation sequencing (NGS) technologies to rapidly analyze multiple genes has gradually emerged as an important part of the laboratory investigation of patients with IPDs. A systemic clinical and laboratory testing approach and thorough phenotype and genotype correlation studies of both patients and their family members are crucial for accurate diagnoses of IPDs.

Clinical and Osteopetrosis-Like Radiological Findings in Patients with Leukocyte Adhesion Deficiency Type III

BACKGROUND

Leukocyte and platelet integrin function defects are present in leukocyte adhesion deficiency type III (LAD-III) due to mutations in FERMT3. Additionally, osteoclast/osteoblast dysfunction develops in LAD-III.

AIM

To discuss the distinguishing clinical, radiological, and laboratory features of LAD-III.

METHODS

This study included the clinical, radiological, and laboratory characteristics of twelve LAD-III patients.

RESULTS

The male/female ratio was 8/4. The parental consanguinity ratio was 100%. Half of the patients had a family history of patients with similar findings. The median age at presentation and diagnosis was 18 (1-60) days and 6 (1-20) months, respectively. The median leukocyte count on admission was 43,150 (30,900-75,700)/μL. The absolute eosinophil count was tested in 8/12 patients, and eosinophilia was found in 6/8 (75%). All patients had a history of sepsis. Other severe infections were pneumonia (66.6%), omphalitis (25%), osteomyelitis (16.6%), gingivitis/periodontitis (16%), chorioretinitis (8.3%), otitis media (8.3%), diarrhea (8.3%), and palpebral conjunctiva infection (8.3%). Four patients (33.3%) received hematopoietic stem cell transplantation (HSCT) from HLA-matched-related donors, and one deceased after HSCT. At initial presentation, 4 (33.3%) patients were diagnosed with other hematologic disorders, three patients (P5, P7, and P8) with juvenile myelomonocytic leukemia (JMML), and one (P2) with myelodysplastic syndrome (MDS).

CONCLUSION

In LAD-III, leukocytosis, eosinophilia, and bone marrow findings may mimic pathologies such as JMML and MDS. In addition to non-purulent infection susceptibility, patients with LAD-III exhibit Glanzmann-type bleeding disorder. In LAD-III, absent integrin activation due to kindlin-3 deficiency disrupts osteoclast actin cytoskeleton organization. This results in defective bone resorption and osteopetrosis-like radiological changes. These are distinctive features compared to other LAD types.

Machine learning algorithms assisted identification of post-stroke depression associated biological features

Objectives

Post-stroke depression (PSD) is a common and serious psychiatric complication which hinders functional recovery and social participation of stroke patients. Stroke is characterized by dynamic changes in metabolism and hemodynamics, however, there is still a lack of metabolism-associated effective and reliable diagnostic markers and therapeutic targets for PSD. Our study was dedicated to the discovery of metabolism related diagnostic and therapeutic biomarkers for PSD.

Methods

Expression profiles of GSE140275, GSE122709, and GSE180470 were obtained from GEO database. Differentially expressed genes (DEGs) were detected in GSE140275 and GSE122709. Functional enrichment analysis was performed for DEGs in GSE140275. Weighted gene co-expression network analysis (WGCNA) was constructed in GSE122709 to identify key module genes. Moreover, correlation analysis was performed to obtain metabolism related genes. Interaction analysis of key module genes, metabolism related genes, and DEGs in GSE122709 was performed to obtain candidate hub genes. Two machine learning algorithms, least absolute shrinkage and selection operator (LASSO) and random forest, were used to identify signature genes. Expression of signature genes was validated in GSE140275, GSE122709, and GSE180470. Gene set enrichment analysis (GSEA) was applied on signature genes. Based on signature genes, a nomogram model was constructed in our PSD cohort (27 PSD patients vs. 54 controls). ROC curves were performed for the estimation of its diagnostic value. Finally, correlation analysis between expression of signature genes and several clinical traits was performed.

Results

Functional enrichment analysis indicated that DEGs in GSE140275 enriched in metabolism pathway. A total of 8,188 metabolism associated genes were identified by correlation analysis. WGCNA analysis was constructed to obtain 3,471 key module genes. A total of 557 candidate hub genes were identified by interaction analysis. Furthermore, two signature genes (SDHD and FERMT3) were selected using LASSO and random forest analysis. GSEA analysis found that two signature genes had major roles in depression. Subsequently, PSD cohort was collected for constructing a PSD diagnosis. Nomogram model showed good reliability and validity. AUC values of receiver operating characteristic (ROC) curve of SDHD and FERMT3 were 0.896 and 0.964. ROC curves showed that two signature genes played a significant role in diagnosis of PSD. Correlation analysis found that SDHD (r = 0.653, P < 0.001) and FERM3 (r = 0.728, P < 0.001) were positively related to the Hamilton Depression Rating Scale 17-item (HAMD) score.

Conclusion

A total of 557 metabolism associated candidate hub genes were obtained by interaction with DEGs in GSE122709, key modules genes, and metabolism related genes. Based on machine learning algorithms, two signature genes (SDHD and FERMT3) were identified, they were proved to be valuable therapeutic and diagnostic biomarkers for PSD. Early diagnosis and prevention of PSD were made possible by our findings.

Hematologically important mutations: Leukocyte adhesion deficiency (second update)

Leukocyte adhesion deficiency (LAD) is an immunodeficiency caused by defects in the adhesion of leukocytes (especially neutrophils) to the blood vessel wall. As a result, patients with LAD suffer from severe bacterial infections and impaired wound healing, accompanied by neutrophilia. In LAD-I, characterized directly after birth by delayed separation of the umbilical cord, mutations are found in ITGB2, the gene that encodes the β subunit (CD18) of the β₂ integrins. In the rare LAD-II disease, the fucosylation of selectin ligands is disturbed, caused by mutations in SLC35C1, the gene that encodes a GDP-fucose transporter of the Golgi system. LAD-II patients lack the H and Lewis Le^a and Le^b blood group antigens. Finally, in LAD-III, the conformational activation of the hematopoietically expressed β integrins is disturbed, leading to leukocyte and platelet dysfunction. This last syndrome is caused by mutations in FERMT3, encoding the kindlin-3 protein in all blood cells, involved in the regulation of β integrin conformation. This article contains an update of the mutations that we consider to be relevant for the various forms of LAD.

A Novel Deletion in FERMT3 Causes LAD-III in a Turkish Family

Leukocyte adhesion deficiency-III (LAD-III) is an extremely rare autosomal recessive syndrome caused by mutations in FERMT3, the gene encoding kindlin-3. The genetic alterations in this gene lead to abnormal expression or activity of kindlin-3 in leukocytes and platelets. Kindlin-3 acts as an important regulator of integrin activation. LAD-III has features of the bleeding syndrome of Glanzmann and also of leukocyte adhesion deficiency. In this study, we report on two families, one of Turkish and one of Syrian origin, with clinical features of LAD-III, loss of kindlin-3 protein expression, and a functional leukocyte defect. A novel, homozygous deletion in FERMT3 (c.921delC, p.Ser307Argfs*21) was found in the Turkish patient. The parents were carriers of the mutation, consistent with an autosomal recessive inheritance. A common c.1525C > T (p.Arg509*) mutation was found in the Syrian patient. In conclusion, beside the variant c.1525C > T in the FERMT3 gene, which was previously found in more than 15 patients in Anatolia, our study is the first to identify the novel homozygous variant c.921delC in the FERMT3 gene.

Neutrophils as immune effector cells in antibody therapy in cancer

Tumor-targeting monoclonal antibodies are available for a number of cancer cell types (over)expressing the corresponding tumor antigens. Such antibodies can limit tumor progression by different mechanisms, including direct growth inhibition and immune-mediated mechanisms, in particular complement-dependent cytotoxicity, antibody-dependent cellular phagocytosis, and antibody-dependent cellular cytotoxicity (ADCC). ADCC can be mediated by various types of immune cells, including neutrophils, the most abundant leukocyte in circulation. Neutrophils express a number of Fc receptors, including Fcγ- and Fcα-receptors, and can therefore kill tumor cells opsonized with either IgG or IgA antibodies. In recent years, important insights have been obtained with respect to the mechanism(s) by which neutrophils engage and kill antibody-opsonized cancer cells and these findings are reviewed here. In addition, we consider a number of additional ways in which neutrophils may affect cancer progression, in particular by regulating adaptive anti-cancer immunity.

Structural Basis of β2 Integrin Inside—Out Activation

β2 integrins are expressed on all leukocytes. Precise regulation of the β2 integrin is critical for leukocyte adhesion and trafficking. In neutrophils, β2 integrins participate in slow rolling. When activated by inside–out signaling, fully activated β2 integrins mediate rapid leukocyte arrest and adhesion. The two activation pathways, starting with selectin ligand engagement and chemokine receptor ligation, respectively, converge on phosphoinositide 3-kinase, talin-1, kindlin-3 and Rap1. Here, we focus on recent structural insights into autoinhibited talin-1 and autoinhibited trimeric kindlin-3. When activated, both talin-1 and kindlin-3 can bind the β2 cytoplasmic tail at separate but adjacent sites. We discuss possible pathways for talin-1 and kindlin-3 activation, recruitment to the plasma membrane, and their role in integrin activation. We propose new models of the final steps of integrin activation involving the complex of talin-1, kindlin-3, integrin and the plasma membrane.

In vivo genome-wide CRISPR screening in murine acute myeloid leukemia uncovers microenvironmental dependencies

In vivo CRISPR screens in AML define key interactors of the microenvironment, including integrins, immune modulators, and glycosylation.

Eight in vivo–specific hits are recurrently associated with adverse prognosis: BTBD6, FERMT3, ILK, SLC19A1, TAP2, TLN1, TPST2, and TRMT12.

Genome-wide CRISPR screens have been extremely useful in identifying therapeutic targets in diverse cancers by defining genes that are essential for malignant growth. However, most CRISPR screens were performed in vitro and thus cannot identify genes that are essential for interactions with the microenvironment in vivo. Here, we report genome-wide CRISPR screens in 2 in vivo murine models of acute myeloid leukemia (AML) driven by the KMT2A/MLLT3 fusion or by the constitutive coexpression of Hoxa9 and Meis1. Secondary validation using a focused library identified 72 genes specifically essential for leukemic growth in vivo, including components of the major histocompatibility complex class I complex, Cd47, complement receptor Cr1l, and the β-4-galactosylation pathway. Importantly, several of these in vivo–specific hits have a prognostic effect or are inferred to be master regulators of protein activity in human AML cases. For instance, we identified Fermt3, a master regulator of integrin signaling, as having in vivo–specific dependency with high prognostic relevance. Overall, we show an experimental and computational pipeline for genome-wide functional screens in vivo in AML and provide a genome-wide resource of essential drivers of leukemic growth in vivo.

Progressive skeletal defects caused by Kindlin3 deficiency, a model of autosomal recessive osteopetrosis in humans

The cellular and molecular mechanisms of bone development and homeostasis are clinically important, but not fully understood. Mutations in integrins and Kindlin3 in humans known as Leukocyte adhesion deficiencies (LAD) cause a wide spectrum of complications, including osteopetrosis. Yet, the rarity, frequent misdiagnosis, and lethality of LAD preclude mechanistic analysis of skeletal abnormalities in these patients. Here, using inducible and constitutive tissue-specific Kindlin3 knockout (K3KO) mice, we show that the constitutive lack of embryonic-Kindlin3 in myeloid lineage cells causes growth retardation, edentulism, and skull deformity indicative of hydrocephaly. Micro-CT analysis revealed craniosynostosis, choanal stenosis, and micrognathia along with other skeletal abnormalities characteristic of osteopetrosis. A marked progression of osteosclerosis occurs in mature to middle-aged adults, resulting in the narrowing of cranial nerve foramina and bone marrow cavities of long bones. However, postnatal-Kindlin3 is less critical for bone remodeling and architecture. Thus, myeloid Kindlin3 is essential for skeletal development and its deficiency leads to autosomal recessive osteopetrosis (ARO). The study will aid in the diagnosis, management, and treatment choices for patients with LAD-III and ARO.

Molecular mechanisms of leukocyte β2 integrin activation

Integrins are transmembrane receptors that mediate cell-cell and cell-extracellular matrix adhesion. Although all integrins can undergo activation (affinity change for ligands), the degree of activation is most spectacular for integrins on blood cells. The β2 integrins are exclusively expressed on the surface of all leukocytes including neutrophils, lymphocytes, and monocytes. They are essential for many leukocyte functions and are strictly required for neutrophil arrest from rolling. The inside-out integrin activation process receives input from chemokine receptors and adhesion molecules. The integrin activation pathway involves many cytoplasmic signaling molecules such as spleen tyrosine kinase, other kinases like Bruton's tyrosine kinase, phosphoinositide 3-kinases, phospholipases, Rap1 GTPases, and the Rap1-GTP-interacting adapter molecule. These signaling events ultimately converge on talin-1 and kindlin-3, which bind to the integrin β cytoplasmic domain and induce integrin conformational changes: extension and high affinity for ligand. Here, we review recent structural and functional insights into how talin-1 and kindlin-3 enable integrin activation, with a focus on the distal signaling components that trigger β2 integrin conformational changes and leukocyte adhesion under flow.

Paxillin binding to the PH domain of kindlin-3 in platelets is required to support integrin αIIbβ3 outside-in signaling

BACKGROUND

Kindlin-3 is essential for supporting the bidirectional signaling of integrin αIIbβ3 in platelets by bridging the crosstalk between integrin αIIbβ3 and the cytoplasmic signaling adaptors.

OBJECTIVE

In this study, we identified a previously unrecognized paxillin binding site in the pleckstrin homology (PH) domain of kindlin-3 and verified its functional significance.

METHODS

Structure-based approaches were employed to identify the paxillin binding site in the PH domain of kindlin-3. In addition, the bidirectional signaling of integrin αIIbβ3 were evaluated in both human and mouse platelets.

RESULTS

In brief, we found that a β1-β2 loop in the PH domain of kindlin-3, an important part of the canonical membrane phospholipid binding pocket, was also involved in mediating paxillin interaction. Interestingly, the binding sites of paxillin and membrane phospholipids in the PH domain of kindlin-3 were mutually exclusive. Specific disruption of paxillin binding to the PH domain by point mutations inhibited platelet spreading on immobilized fibrinogen while having no inhibition on soluble fibrinogen binding to stimulated platelets. In addition, a membrane-permeable peptide derived from the β1-β2 loop in the PH domain of kindlin-3 was capable of inhibiting platelet spreading and clot retraction, but it had no effect on soluble fibrinogen binding to platelets and platelet aggregation. Treatment with this peptide significantly reduced thrombus formation in mice.

CONCLUSION

Taken together, these findings suggest that interaction between paxillin and the PH domain of kindlin-3 plays an important role in supporting integrin αIIbβ3 outside-in signaling in platelets, thus providing a novel antithrombotic target.

Mesenchymal stem cell treatment improves outcome of COVID-19 patients via multiple immunomodulatory mechanisms

The infusion of coronavirus disease 2019 (COVID-19) patients with mesenchymal stem cells (MSCs) potentially improves clinical symptoms, but the underlying mechanism remains unclear. We conducted a randomized, single-blind, placebo-controlled (29 patients/group) phase II clinical trial to validate previous findings and explore the potential mechanisms. Patients treated with umbilical cord-derived MSCs exhibited a shorter hospital stay (P = 0.0198) and less time required for symptoms remission (P = 0.0194) than those who received placebo. Based on chest images, both severe and critical patients treated with MSCs showed improvement by day 7 (P = 0.0099) and day 21 (P = 0.0084). MSC-treated patients had fewer adverse events. MSC infusion reduced the levels of C-reactive protein, proinflammatory cytokines, and neutrophil extracellular traps (NETs) and promoted the maintenance of SARS-CoV-2-specific antibodies. To explore how MSCs modulate the immune system, we employed single-cell RNA sequencing analysis on peripheral blood. Our analysis identified a novel subpopulation of VNN2⁺ hematopoietic stem/progenitor-like (HSPC-like) cells expressing CSF3R and PTPRE that were mobilized following MSC infusion. Genes encoding chemotaxis factors - CX3CR1 and L-selectin - were upregulated in various immune cells. MSC treatment also regulated B cell subsets and increased the expression of costimulatory CD28 in T cells in vivo and in vitro. In addition, an in vivo mouse study confirmed that MSCs suppressed NET release and reduced venous thrombosis by upregulating kindlin-3 signaling. Together, our results underscore the role of MSCs in improving COVID-19 patient outcomes via maintenance of immune homeostasis.

Inherited Platelet Disorders

Bleeding disorders due to platelet dysfunction are a common hematologic complication affecting patients, and typically present with mucocutaneous bleeding or hemorrhage. An inherited platelet disorder should be suspected in individuals with a suggestive family history and no identified secondary causes of bleeding. Genetic defects have been described at all levels of platelet activation, including receptor binding, signaling, granule release, cytoskeletal remodeling, and platelet hematopoiesis. Management of these disorders is typically supportive, with an emphasis on awareness, patient education, and anticipatory guidance to prevent future episodes of bleeding.

Structure basis of the FERM domain of kindlin-3 in supporting integrin αIIbβ3 activation in platelets

Kindlin-3, a protein 4.1, ezrin, radixin, and moesin (FERM) domain-containing adaptor in hematopoietic cells, is essentially required for supporting the bidirectional integrin αIIbβ3 signaling in platelets by binding to the integrin β3 cytoplasmic tail. However, the structural details of kindlin-3's FERM domain remain unknown. In this study, we crystalized the kindlin-3's FERM domain protein and successfully solved its 3-dimensional structure. The structure shows that the 3 kindlin-3's FERM subdomains (F1, F2, and F3) compact together and form a cloverleaf-shaped conformation, which is stabilized by the binding interface between the F1 and F3 subdomains. Interestingly, the FERM domain of kindlin-3 exists as a monomer in both crystal and solution, which is different from its counterpart in kindlin-2 that is able to form a F2 subdomain-swapped dimer; nonetheless, dimerization is required for kindlin-3 to support integrin αIIbβ3 activation, indicating that kindlin-3 may use alternative mechanisms for formation of a functional dimer in cells. To evaluate the functional importance of the cloverleaf-like FERM structure in kindlin-3, structure-based mutations were introduced into kindlin-3 to disrupt the F1/F3 interface. The results show that integrin αIIbβ3 activation is significantly suppressed in platelets expressing the kindlin-3 mutant compared with those expressing wild-type kindlin-3. In addition, introduction of equivalent mutations into kindlin-1 and kindlin-2 also significantly compromises their ability to support integrin αIIbβ3 activation in CHO cells. Together, our findings suggest that the cloverleaf-like FERM domain in kindlins is structurally important for supporting integrin αIIbβ3 activation.

β2 Integrin Signaling Cascade in Neutrophils: More Than a Single Function

Neutrophils are the most prevalent leukocytes in the human body. They have a pivotal role in the innate immune response against invading bacterial and fungal pathogens, while recent emerging evidence also demonstrates their role in cancer progression and anti-tumor responses. The efficient execution of many neutrophil effector responses requires the presence of β2 integrins, in particular CD11a/CD18 or CD11b/CD18 heterodimers. Although extensively studied at the molecular level, the exact signaling cascades downstream of β2 integrins still remain to be fully elucidated. In this review, we focus mainly on inside-out and outside-in signaling of these two β2 integrin members expressed on neutrophils and describe differences between various neutrophil stimuli with respect to integrin activation, integrin ligand binding, and the pertinent differences between mouse and human studies. Last, we discuss how integrin signaling studies could be used to explore the therapeutic potential of targeting β2 integrins and the intracellular signaling cascade in neutrophils in several, among other, inflammatory conditions in which neutrophil activity should be dampened to mitigate disease.

Inherited platelet diseases with normal platelet count: phenotypes, genotypes and diagnostic strategy

Inherited platelet disorders resulting from platelet function defects and a normal platelet count cause a moderate or severe bleeding diathesis. Since the description of Glanzmann thrombasthenia resulting from defects of ITGA2B and ITGB3, new inherited platelet disorders have been discovered, facilitated by the use of high throughput sequencing and genomic analyses. Defects of RASGRP2 and FERMT3 responsible for severe bleeding syndromes and integrin activation have illustrated the critical role of signaling molecules. Important are mutations of P2RY12 encoding the major ADP receptor causal for an inherited platelet disorder with inheritance characteristics that depend on the variant identified. Interestingly, variants of GP6 encoding the major subunit of the collagen receptor GPVI/FcRγ associate only with mild bleeding. The numbers of genes involved in dense granule defects including Hermansky-Pudlak and Chediak Higashi syndromes continue to progress and are updated. The ANO6 gene encoding a Ca2+-activated ion channel required for phospholipid scrambling is responsible for the rare Scott syndrome and decreased procoagulant activity. A novel EPHB2 defect in a familial bleeding syndrome demonstrates a role for this tyrosine kinase receptor independent of the classical model of its interaction with ephrins. Such advances highlight the large diversity of variants affecting platelet function but not their production, despite the difficulties in establishing a clear phenotype when few families are affected. They have provided insights into essential pathways of platelet function and have been at the origin of new and improved therapies for ischemic disease. Nevertheless, many patients remain without a diagnosis and requiring new strategies that are now discussed.

Kindlin-3 recruitment to the plasma membrane precedes high-affinity β2-integrin and neutrophil arrest from rolling

Integrin-mediated neutrophil adhesion starts by arrest from rolling. Activation of integrins involves conformational changes from an inactive, bent conformation to an extended conformation (E+) with high affinity for ligand binding (H+). The cytoplasmic protein kindlin-3 is necessary for leukocyte adhesion; mutations of kindlin-3 cause leukocyte adhesion deficiency type 3. Kindlin-3 binds the β2-integrin cytoplasmic tail at a site distinct from talin-1, but the molecular mechanism by which kindlin-3 activates β2-integrins is unknown. In this study, we measured the spatiotemporal dynamics of kindlin-3 and β2-integrin conformation changes during neutrophil and HL-60 cell rolling and arrest under flow. Using high-resolution quantitative dynamic footprinting microscopy and kindlin-3-fluorescent protein (FP) fusion proteins, we found that kindlin-3 was recruited to the plasma membrane in response to interleukin-8 (IL-8) before induction of the H+ β2-integrin conformation. Intravital imaging revealed that EGFP-kindlin-3-reconstituted, kindlin-3-knockout neutrophils arrest in vivo in response to CXCL1. EGFP-kindlin-3 in primary mouse neutrophils was also recruited to the plasma membrane before arrest. Upon arrest, we found small clusters of high-affinity β2-integrin molecules within large areas of membrane-proximal kindlin-3 FP. Deletion of kindlin-3 or its pleckstrin homology (PH) domain in neutrophil-like HL-60 cells completely abolished H+ β2-integrin induction. IL-8 also triggered recruitment of the isolated kindlin-3 PH domain to the plasma membrane before arrest. In summary, we showed that the kindlin-3 PH domain is necessary for recruitment to the plasma membrane, where full-length kindlin-3 is indispensable for the induction of high-affinity β2-integrin.

Severe Bleeding Diathesis in Siblings with Platelet Dysfunction due to a Novel Nonsense RASGRP2 Mutation

Next-generation sequencing is increasingly applied during the diagnostic work-up of patients with bleeding diathesis and has facilitated the diagnosis of rare bleeding disorders such as inherited platelet function disorders. Mutations in RAS guanyl releasing protein 2 (RasGRP2), also known as calcium- and diacylglycerol-regulated guanine nucleotide exchange factor I (CalDAG-GEFI), underlie a recently described platelet signal transduction abnormality. Here we present the case of a consanguineous family originating from Afghanistan with two siblings affected by recurrent severe mucocutaneous bleedings. Platelet function testing demonstrated a marked reduction of aggregation induced by collagen and adenosine diphosphate. Whole exome sequencing revealed a novel homozygous nonsense RASGRP2 mutation segregating with the bleeding disorder in the family.

Kindlin3-Dependent CD11b/CD18-Integrin Activation Is Required for Potentiation of Neutrophil Cytotoxicity by CD47-SIRPα Checkpoint Disruption

The CD47-signal regulatory protein-alpha (SIRPα) immune checkpoint constitutes a therapeutic target in cancer, and initial clinical studies using inhibitors of CD47-SIRPα interactions in combination with tumor-targeting antibodies show promising results. Blockade of CD47-SIRPα interaction can promote neutrophil antibody-dependent cellular cytotoxicity (ADCC) toward antibody-opsonized targets. Neutrophils induce killing of antibody-opsonized tumor cells by a process identified as trogoptosis, a necrotic/lytic type of cancer cell death that involves trogocytosis, the antibody-mediated endocytic acquisition of cancer membrane fragments by neutrophils. Both trogocytosis and killing strictly depend on CD11b/CD18-(Mac-1)-mediated neutrophil-cancer cell conjugate formation, but the mechanism by which CD47-SIRPα checkpoint disruption promotes cytotoxicity has remained elusive. Here, by using neutrophils from patients with leukocyte adhesion deficiency type III carrying FERMT3 gene mutations, hence lacking the integrin-associated protein kindlin3, we demonstrated that CD47-SIRPα signaling controlled the inside-out activation of the neutrophil CD11b/CD18-integrin and cytotoxic synapse formation in a kindlin3-dependent fashion. Our findings also revealed a role for kindlin3 in trogocytosis and an absolute requirement in the killing process, which involved direct interactions between kindlin3 and CD18 integrin. Collectively, these results identified a dual role for kindlin3 in neutrophil ADCC and provide mechanistic insights into the way neutrophil cytotoxicity is governed by CD47-SIRPα interactions.

Clinical and Genetic Spectrum of a Large Cohort of Patients With Leukocyte Adhesion Deficiency Type 1 and 3: A Multicentric Study From India

Leukocyte adhesion deficiency (LAD) syndrome is a group of inborn errors of immunity characterized by a defect in the cascade of the activation and adhesion leading to the failure of leukocyte to migrate to the site of tissue injury. Three different types of LAD have been described. The most common subtype is LAD type 1 (LAD1) caused due to defects in the ITGβ2 gene. LAD type 2 (LAD2) is caused by mutations in the SLC35C1 gene leading to a generalized loss of expression of fucosylated glycans on the cell surface and LAD type 3 (LAD3) is caused by mutations in the FERMT3 gene resulting in platelet function defects along with immunodeficiency. There is a paucity of data available from India on LAD syndromes. The present study is a retrospective analysis of patients with LAD collated from 28 different centers across India. For LAD1, the diagnosis was based on clinical features and flow cytometric expression of CD18 on peripheral blood leukocytes and molecular confirmation by Sanger sequencing. For patients with LAD3 diagnosis was largely based on clinical manifestations and identification of the pathogenic mutation in the FERMT3 gene by next-generation Sequencing. Of the total 132 cases diagnosed with LAD, 127 were LAD1 and 5 were LAD3. The majority of our patients (83%) had CD18 expression less than 2% on neutrophils (LAD1°) and presented within the first three months of life with omphalitis, skin and soft tissue infections, delayed umbilical cord detachment, otitis media, and sepsis. The patients with CD18 expression of more than 30% (LAD1⁺) presented later in life with skin ulcers being the commonest manifestation. Bleeding manifestations were common in patients with LAD3. Persistent neutrophilic leukocytosis was the characteristic finding in all patients. 35 novel mutations were detected in the ITGβ2 gene, and 4 novel mutations were detected in the FERMT3 gene. The study thus presents one of the largest cohorts of patients from India with LAD, focusing on clinical features, immunological characteristics, and molecular spectrum.

Kindlin-3 loss curbs chronic myeloid leukemia in mice by mobilizing leukemic stem cells from protective bone marrow niches

Kindlin-3 (K3)-mediated integrin adhesion controls homing and bone marrow (BM) retention of normal hematopoietic cells. However, the role of K3 in leukemic stem cell (LSC) retention and growth in the remodeled tumor-promoting BM is unclear. We report that loss of K3 in a mouse model of chronic myeloid leukemia (CML) triggers the release of LSCs from the BM into the circulation and impairs their retention, proliferation, and survival in secondary organs, which curbs CML development, progression, and metastatic dissemination. We found de novo expression of cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) on CML-LSCs but not normal hematopoietic stem cells and this enabled us to specifically deplete K3 with a CTLA-4-binding RNA aptamer linked to a K3-siRNA (small interfering RNA) in CTLA-4⁺ LSCs in vivo, which mobilized LSCs in the BM, induced disease remission, and prolonged survival of mice with CML. Thus, disrupting interactions of LSCs with the BM environment is a promising strategy to halt the disease-inducing and relapse potential of LSCs.

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

An increasing number of primary immunodeficiencies (PIDs) have been identified over the last decade, which are caused by deleterious mutations in genes encoding for proteins involved in actin cytoskeleton regulation. These mutations primarily affect hematopoietic cells and lead to defective function of immune cells, such as impaired motility, signaling, proliferative capacity, and defective antimicrobial host defense. Here, we review several of these immunological "actinopathies" and cover both clinical aspects, as well as cellular mechanisms of these PIDs. We focus in particular on the effect of these mutations on human neutrophil function.

Replication in the Mononuclear Phagocyte System (MPS) as a Determinant of Hantavirus Pathogenicity

Members of different virus families including Hantaviridae cause viral hemorrhagic fevers (VHFs). The decisive determinants of hantavirus-associated pathogenicity are still enigmatic. Pathogenic hantavirus species, such as Puumala virus (PUUV), Hantaan virus (HTNV), Dobrava-Belgrade virus (DOBV), and Sin Nombre virus (SNV), are associated with significant case fatality rates. In contrast, Tula virus (TULV) only sporadically causes mild disease in immunocompetent humans and Prospect Hill virus (PHV) so far has not been associated with any symptoms. They are thus defined here as low pathogenic/apathogenic hantavirus species. We found that productive infection of cells of the mononuclear phagocyte system (MPS), such as monocytes and dendritic cells (DCs), correlated well with the pathogenicity of hantavirus species tested. HTNV (intermediate case fatality rates) replicated more efficiently than PUUV (low case fatality rates) in myeloid cells, whereas low pathogenic/apathogenic hantavirus species did not produce any detectable virus titers. Analysis of PHPUV, a reassortant hantavirus derived from a pathogenic (PUUV) and an apathogenic (PHV) hantavirus species, indicated that the viral glycoproteins are not decisive for replication in MPS cells. Moreover, blocking acidification of endosomes with chloroquine decreased the number of TULV genomes in myeloid cells suggesting a post-entry block for low pathogenic/apathogenic hantavirus species in myeloid cells. Intriguingly, pathogenic but not low pathogenic/apathogenic hantavirus species induced conversion of monocytes into inflammatory DCs. The proinflammatory programming of MPS cells by pathogenic hantavirus species required integrin signaling and viral replication. Our findings indicate that the capacity to replicate in MPS cells is a prominent feature of hantaviral pathogenicity.

The role of neutrophils in host defense and disease

Neutrophils, the most abundant circulating leukocyte, are critical for host defense. Granulopoiesis is under the control of transcriptional factors and culminates in mature neutrophils with a broad armamentarium of antimicrobial pathways. These pathways include nicotinamide adenine dinucleotide phosphate oxidase, which generates microbicidal reactive oxidants, and nonoxidant pathways that target microbes through several mechanisms. Activated neutrophils can cause or worsen tissue injury, underscoring the need for calibration of activation and resolution of inflammation when infection has been cleared. Acquired neutrophil disorders are typically caused by cytotoxic chemotherapy or immunosuppressive agents. Primary neutrophil disorders typically result from disabling mutations of individual genes that result in impaired neutrophil number or function, and provide insight into basic mechanisms of neutrophil biology. Neutrophils can also be activated by noninfectious causes, including trauma and cellular injury, and can have off-target effects in which pathways that typically defend against infection exacerbate injury and disease. These off-target effects include acute organ injury, autoimmunity, and variable effects on the tumor microenvironment that can limit or worsen tumor progression. A greater understanding of neutrophil plasticity in these conditions is likely to pave the way to new therapeutic approaches.

Glanzmann thrombasthenia: genetic basis and clinical correlates

Glanzmann thrombasthenia (GT) is an autosomal recessive disorder of platelet aggregation caused by quantitative or qualitative defects in integrins αIIb and β3. These integrins are encoded by the ITGA2B and ITGB3 genes and form platelet glycoprotein (GP)IIb/IIIa, which acts as the principal platelet receptor for fibrinogen. Although there is variability in the clinical phenotype, most patients present with severe mucocutaneous bleeding at an early age. A classic pattern of abnormal platelet aggregation, platelet glycoprotein expression and molecular studies confirm the diagnosis. Management of bleeding is based on a combination of hemostatic agents including recombinant activated factor VII with or without platelet transfusions and antifibrinolytic agents. Refractory bleeding and platelet alloimmunization are common complications. In addition, pregnant patients pose unique management challenges. This review highlights clinical and molecular aspects in the approach to patients with GT, with particular emphasis on the significance of multidisciplinary care.

Successful hematopoietic stem cell transplant in leukocyte adhesion deficiency type III presenting primarily as malignant infantile osteopetrosis

Leukocyte adhesion deficiency type III (LAD-III) is caused by mutations in FERMT3 that encodes Kindlin-3 which regulates integrins activation. LAD-III predisposes to infections and bleeding. Osteopetrosis was reported in some cases. We report three patients who presented as malignant infantile osteopetrosis. One had recurrent infections and none had bleeding. Exome sequencing revealed a novel homozygous mutation in FERMT3 c.1555C > T (p.Gln519Ter). Two patients underwent successful hematopoietic stem cell transplant (HSCT) from matched siblings with resolution of osteopetrosis. The third patient died secondary to sepsis prior to HSCT. Our results support early HSCT in LAD-III prior to the occurrence of life-threatening complications.

Site-specific phosphorylation regulates the functions of kindlin-3 in a variety of cells

Studies of isolated cells, mice, and humans have demonstrated the vital role of the FERM domain protein kindlin-3 in integrin activation in certain hematopoietic and non-hematopoietic cells, consequent to binding to integrin β-subunits. To explore regulatory mechanisms, we developed a monoclonal antibody that selectively recognizes the phosphorylated form of Ser⁴⁸⁴ (pS⁴⁸⁴) in kindlin-3. Activation of platelets, HEL megakaryocytic-like cells and BT549 breast cancer cells led to enhanced expression of pS⁴⁸⁴ as assessed by immunofluorescence or Western blotting. In platelets, pS⁴⁸⁴ rose rapidly and transiently upon stimulation. When a mutant form of kindlin-3, T⁴⁸²S⁴⁸⁴/AA kindlin-3, was transduced into mouse megakaryocytes, it failed to support activation of integrin α_IIbβ₃, whereas wild-type kindlin-3 did. In MDA-MB231 breast cancer cells, expression of T⁴⁸²S⁴⁸⁴/AA kindlin-3 suppressed cell spreading, migration, invasion, and VEGF production. Wild-type kindlin-3 expressing cells markedly increased tumor growth in vivo, whereas T⁴⁸²S⁴⁸⁴/AA kindlin-3 significantly blunted tumor progression. Thus, our data establish that a unique phosphorylation event in kindlin-3 regulates its cellular functions.

Novel variants in FERMT3 and RASGRP2-Genetic linkage in Glanzmann-like bleeding disorders

Defects of platelet intracellular signaling can result in severe platelet dysfunction. Several mutations in each of the linked genes FERMT3 and RASGRP2 on chromosome 11 causing a Glanzmann-like bleeding phenotype have been identified so far. We report on novel variants in two unrelated pediatric patients with severe bleeding diathesis-one with leukocyte adhesion deficiency type III due to a homozygous frameshift in FERMT3 and the other with homozygous variants in both, FERMT3 and RASGRP2. We focus on the challenging genetic and functional variant assessment and aim to accentuate the risk of obtaining misleading results due to the phenomenon of genetic linkage.

Activation and suppression of hematopoietic integrins in hemostasis and immunity

Nolte and Margadant review the current understanding of the activation and inactivation of integrin receptors expressed by hematopoietic cells and the role of these conformational changes in modulating platelet and leukocyte function.

The Integrin Activating Protein Kindlin-3 Is Cleaved in Human Platelets during ST-Elevation Myocardial Infarction

Kindlins are important proteins for integrin signaling and regulation of the cytoskeleton, but we know little about their precise function and regulation in platelets during acute ischemic events. In this work, we investigated kindlin-3 protein levels in platelets isolated from patients with ST-elevation myocardial infarction (STEMI) compared to patients with non-ischemic chest pain. Platelets from twelve patients with STEMI and twelve patients with non-ischemic chest pain were isolated and analyzed for kindlin-3 protein levels and intracellular localization by immunoblotting and two-dimensional gel electrophoresis. Platelet proteome analysis by two-dimensional gel electrophoresis and protein sequencing identified kindlin-3 as a protein that is cleaved in platelets from patients with myocardial infarction. Kindlin-3 full-length protein was significantly decreased in patients with STEMI compared to patients with non-ischemic chest pain (1.0 ± 0.2 versus 0.28 ± 0.2, p < 0.05) by immunoblotting. Kindlin-3 showed a differential distribution and was primarily cleaved in the cytosolic and membrane compartment of platelets in myocardial infarction. Platelet activation with thrombin alone did not affect kindlin-3 protein levels. The present study demonstrates that kindlin-3 protein levels become significantly reduced in platelets of patients with myocardial infarction compared to controls. The results suggest that kindlin-3 cleavage in platelets is associated with the ischemic event of myocardial infarction.

Kindlin-3 in platelets and myeloid cells differentially regulates deep vein thrombosis in mice

Platelets and myeloid cells cooperate to promote deep vein thrombosis (DVT). Here we evaluated the role of kindlin-3, a key integrin activator in these cells, in regulating stenosis-induced DVT in mice. DVT was significantly suppressed in mice that express a kindlin-3 mutant defective for integrin binding, showing that kindlin-3-mediated integrin signaling in blood cells is required for DVT. While platelet-specific deficiency of kindlin-3 in Kindlin-3^fl/flPF4-Cre mice significantly suppressed DVT, deficiency of kindlin-3 specifically in myeloid cells in Kindlin-3^fl/flLysM-Cre mice remarkably enhanced the early development of DVT, indicating that kindlin-3 in platelets and myeloid cells can play distinct roles in regulating DVT. Mechanistically, the levels of neutrophil extracellular traps (NETs) in plasma, a key DVT facilitator, were significantly elevated in Kindlin-3^fl/flLysM-Cre mice upon the IVC stenosis; and treatment with either DNase I or PAD4 inhibitor could effectively compromise the enhancement of DVT in these mice, suggesting that kindlin-3 in neutrophils may affect DVT via restraining NET release. In addition, we found that the kindlin-3-integrin αIIbβ3 signaling in platelets was required to promote NET release. Together, our studies reveal that kindlin-3 in platelets and myeloid cells can differentially regulate DVT through orchestrating NET release, thus providing further mechanistic insights into DVT.

A Novel Nonsense Mutation in FERMT3 Causes LAD-III in a Pakistani Family

Leukocyte adhesion deficiency-III (LAD3) is an extremely rare primary immunodeficiency disorder, transmitted with autosomal-recessive inheritance. It is caused by genetic alteration in the FERMT3 gene, which leads to abnormal expression of kindlin-3. This cytoplasmic protein is highly expressed in leukocytes and platelets, and acts as an important regulator of integrin activation. LAD3 has features like bleeding syndrome of Glanzmann-type and leukocyte adhesion deficiency. FERMT3 mutation(s) have not been well characterized in Pakistani patients with LAD3. In this study, an infant and his family of Pakistani origin, presenting with clinical features of LAD, were investigated to determine the underlying genetic defect. Targeted next generation sequencing (TGS) and Sanger sequencing were performed to identify and confirm the causative mutations, respectively, and their segregation within the family. A novel, homozygous FERMT3 nonsense mutation (c.286C > T, p.Q96^∗) was found in the proband, and its co-segregation with LAD3 phenotype within the family was consistent with an autosomal recessive inheritance. Both parents were carriers of the same mutation. This family was offered prenatal diagnosis during first trimester of the subsequent pregnancy; the fetus carried the variant. In conclusion, our study is the first report to identify the novel homozygous variant c.286C > T, p.Q96^∗in the FERMT3 gene, which might be the causative mutation for LAD3 patients of Pakistani origin.

Inherited Platelet Disorders: A Modern Approach to Evaluation and Treatment

The inherited platelet disorders are a heterogeneous group of disorders that can be pleotropic in their clinical presentations. They may present with variable platelet counts and bleeding, making their diagnosis difficult. New diagnostic tools range from flow cytometric platelet function assessments to next-generation sequencing. Several platelet disorders may now be treated with gene therapy or bone marrow transplant. Improved understanding of the molecular and biologic mechanisms of the inherited platelet disorders may lead to novel targeted therapies.

Inside-Out Control of Fc-Receptors

Receptors recognizing the Fc-part of immunoglobulins (FcR) are important in the engagement of phagocytes with opsonized micro-organisms, but they also play a major role in the pathogenesis of chronic inflammatory diseases. Different FcRs are specifically recognizing and binding the different classes of immunoglobulins, transmitting different signals into the cell. The function of IgG (FcγR's) and IgA (FcαR) recognizing receptors is controlled by cellular signals evoked by activation of heterologous receptors in a process generally referred to as inside-out control. This concept is clearly described for the regulation of integrin receptors. Inside-out control can be achieved at different levels by modulation of: (i) receptor affinity, (ii) receptor avidity/valency, (iii) interaction with signaling chains, (iv) interaction with other receptors and (v) localization in functionally different membrane domains. The inside-out control of FcRs is an interesting target for novel therapy by therapeutical antibodies as it can potentiate or decrease the functionality of the response to the antibodies depending on the mechanisms of the diseases they are applied for.

Platelet integrin αIIbβ3: signal transduction, regulation, and its therapeutic targeting

Integrins are a family of transmembrane glycoprotein signaling receptors that can transmit bioinformation bidirectionally across the plasma membrane. Integrin αIIbβ3 is expressed at a high level in platelets and their progenitors, where it plays a central role in platelet functions, hemostasis, and arterial thrombosis. Integrin αIIbβ3 also participates in cancer progression, such as tumor cell proliferation and metastasis. In resting platelets, integrin αIIbβ3 adopts an inactive conformation. Upon agonist stimulation, the transduction of inside-out signals leads integrin αIIbβ3 to switch from a low- to high-affinity state for fibrinogen and other ligands. Ligand binding causes integrin clustering and subsequently promotes outside-in signaling, which initiates and amplifies a range of cellular events to drive essential platelet functions such as spreading, aggregation, clot retraction, and thrombus consolidation. Regulation of the bidirectional signaling of integrin αIIbβ3 requires the involvement of numerous interacting proteins, which associate with the cytoplasmic tails of αIIbβ3 in particular. Integrin αIIbβ3 and its signaling pathways are considered promising targets for antithrombotic therapy. This review describes the bidirectional signal transduction of integrin αIIbβ3 in platelets, as well as the proteins responsible for its regulation and therapeutic agents that target integrin αIIbβ3 and its signaling pathways.

β-1,3-Glucan/CR3/SYK pathway-dependent LC3B-II accumulation enhanced the fungicidal activity in human neutrophils

Since molecular genotyping has been established for the Candida species, studies have found that a single Candida strain (endemic strain) can persist over a long period of time and results in the spread of nosocomial invasive candidiasis without general characteristics of horizontal transmissions. Our previous study also found the existence of endemic strains in a cancer center in Tianjin, China. In the current study, we performed further investigation on endemic and non-endemic Candida albicans strains, with the aim of explaining the higher morbidity of endemic strains. In an in vivo experiment, mice infected with endemic strains showed significantly shorter survival time and higher kidney fungal burdens compared to mice infected with non-endemic strains. In an in vitro experiment, the killing percentage of neutrophils to endemic strains was significantly lower than that to non-endemic strains, which is positively linked to the ratio of LC3B-II/I in neutrophils. An immunofluorescence assay showed more β-1,3-glucan exposure on the cell walls of non-endemic strains compared to endemic strains. After blocking the β-glucan receptor (CR3) or inhibiting downstream kinase (SYK) in neutrophils, the killing percent to C. albicans (regardless of endemic and non-endemic strains) and the ratio of LC3B-II/I of neutrophils were significantly decreased. These data suggested that the killing capability of neutrophils to C. albicans was monitored by β-1,3-glucan via CR3/SYK pathway-dependent LC3B-II accumulation and provided an explanation for the variable killing capability of neutrophils to different strains of C. albicans, which would be beneficial in improving infection control and therapeutic strategies for invasive candidiasis.

Leucocyte adhesion deficiency-A multicentre national experience

Leucocyte adhesion deficiency (LAD) is a rare, innate autosomal recessive immunodeficiency with three subtypes. Twenty-nine patients with LADs were diagnosed and treated in Israeli Medical Centers and in the Palestinian Authority. We discuss the phenotypic, genotypic and biochemical features of LAD-I, LAD-II and LAD-III diagnosed during the neonatal period and early infancy in 18, 6 and 5 patients, respectively. Consanguinity was frequent. Common features were severe infections of variable aetiology, excessive leukocytosis and delayed umbilical cord detachment. In LAD-I, the integrin CD18 expression varied from negligible to normal. However, CD11a expression was negligible in all tested patients, suggesting both CD11a and CD18 should be used to assess this subtype. LAD-II patients showed distinctive facial features, physical malformations, short stature and developmental delay. These patients show defective expression of SLeX (CD15a) on cell surface glycoproteins and lack of H antigen on erythroid cell surfaces resulting in Bombay blood group (hh). LAD-III showed intact but inactive β₂ integrins associated with severe infections and significant bleeding disorders caused by defective platelet aggregation and thrombocytopenia. We report four patients with two new unpublished mutations: two LAD-I patients with c.1099delG in ITGB2 and two LAD-III patients with c.1069C>T in FERMT3. LAD-I patients harbouring the c.119_128 deletion in ITGB2 seemed to have better outcomes as compared to other LAD-I patients. Eight patients with LAD-I and -III underwent successful haematopoietic stem cell transplantation. Cumulative survival was 75%, 50% and 40% for LAD-I, LAD-II and LAD-III, with a median follow-up of 4 (0.08-19), 3.25 (1-32) and 6 (0.08-8) years, respectively. Prenatal diagnosis is recommended in families with LAD syndromes.

Neutrophils: New insights and open questions

Neutrophils are the first line of defense against bacteria and fungi and help combat parasites and viruses. They are necessary for mammalian life, and their failure to recover after myeloablation is fatal. Neutrophils are short-lived, effective killing machines. Their life span is significantly extended under infectious and inflammatory conditions. Neutrophils take their cues directly from the infectious organism, from tissue macrophages and other elements of the immune system. Here, we review how neutrophils traffic to sites of infection or tissue injury, how they trap and kill bacteria, how they shape innate and adaptive immune responses, and the pathophysiology of monogenic neutrophil disorders.

Differential requirement of kindlin-3 for T cell progenitor homing to the non-vascularized and vascularized thymus

The role of integrin-mediated adhesion during T cell progenitor homing to and differentiation within the thymus is ill-defined, mainly due to functional overlap. To circumvent compensation, we disrupted the hematopoietic integrin regulator kindlin-3 in mice and found a progressive thymus atrophy that is primarily caused by an impaired homing capacity of T cell progenitors to the vascularized thymus. Notably, the low shear flow conditions in the vascular system at midgestation allow kindlin-3-deficient fetal liver-derived T cell progenitors to extravasate via pharyngeal vessels and colonize the avascular thymus primordium. Once in the thymus, kindlin-3 promotes intrathymic T cell proliferation by facilitating the integrin-dependent crosstalk with thymic antigen presenting cells, while intrathymic T cell migration, maturation into single positive CD4 and CD8 T cells and release into the circulation proceed without kindlin-3. Thus, kindlin-3 is dispensable for integrin-mediated T cell progenitor adhesion and signalling at low and indispensable at high shear forces.

Congenital Disorders of Platelet Function and Number

Mucocutaneous bleeding symptoms and/or persistent thrombocytopenia occur in individuals with congenital disorders of platelet function and number. Apart from bleeding, these disorders are often associated with additional hematologic and clinical manifestations, including auditory, immunologic, and oncologic disease. Autosomal recessive, dominant, and X-linked inheritance patterns have been demonstrated. Precise delineation of the molecular cause of the platelet disorder can aid the pediatrician in the detection and prevention of specific disorder-associated manifestations and guide appropriate treatment and anticipatory care for the patient and family.

Kindlin-3 negatively regulates the release of neutrophil extracellular traps

Neutrophils fight infections by generating reactive oxygen species (ROS) and extracellular traps (NETs). However, how neutrophils modulate ROS/NET generation is mechanistically unclear. Kindlin-3, an essential integrin activator expressed in hematopoietic cells, is required to support integrin-mediated neutrophil recruitment during inflammation. Here, we report a novel role of kindlin-3 in regulating ROS/NET generation in neutrophils. When overexpressing kindlin-3 in neutrophil-like differentiated HL-60 cells (HL-60N), ROS/NET generation from these cells were significantly suppressed. Interestingly, overexpression of a kindlin-3 mutant that is defective for interacting with integrins in HL-60N cells still inhibited ROS/NET generation, suggesting that the role of kindlin-3 in inhibiting ROS/NET signaling may be independent of its binding to integrins. Consistently, knockdown of kindlin-3 in HL-60N cells led to enhanced ROS/NET generation. In addition, bone marrow neutrophils isolated from kindlin-3-deficient mice showed elevated ROS/NET generation when compared with WT counterparts. As expected, overexpression of exogenous kindlin-3 in mouse neutrophils could suppress NET release ex vivo and in vivo. Collectively, these results demonstrate that kindlin-3 in neutrophils is involved in modulating the ROS/NET signaling, providing a novel mechanism for fine-tuning neutrophil behaviors during inflammation.

Kindlins: Roles in development and cancer progression

The Kindlins are FERM domain proteins comprising three members (Kindlin-1, -2 and -3) which are evolutionarily conserved. Kindlins bind with β-integrin cytoplasmic tails and execute broad biological functions including directed cell migration, proliferation, differentiation and survival. In light of more and more evidence point to the importance of Kindlin family members in normal development and human diseases especially in cancers, we aim to portrait the profile of Kindlins in the regulation of embryonic development and cancer progression. We first summarize all the known binding proteins for individual member of Kindlin family. We then outline the Kindlin-regulated signaling pathways including Wnt/β-catenin, TGFβ, EGFR, and Hedgehog signalings. Furthermore, we descript the pivotal role of Kindlins in embryonic development in detail with notions that Kindlin-1 is highly expressed in endo/ectodermal originated tissues, Kindlin-2 is highly expressed in mesoderm-derived tissues and Kindlin-3 is highly expressed in mesoderm- and ectoderm-derived tissues. Deregulation of Kindlins is generally reported in cancers from different organs. We also briefly descript the role of Kindlins in other diseases. Finally, we update the recent understanding of how Kindlins are regulated and modified as well as the degradation mechanism of Kindlins, respectively.