Contributions of Wiskott-Aldrich syndrome family cytoskeletal regulatory adapters to immune regulation

Inborn Error of WAS Presenting with SARS-CoV-2-Related Multisystem Inflammatory Syndrome in Children

Multisystem inflammatory syndrome in children (MIS-C) has been reported in patients with inborn errors of immunity (IEI), providing insights into disease pathogenesis. Here, we present the first case of MIS-C in a child affected by Wiskott-Aldrich syndrome (WAS) gene mutation, elucidating underlying predisposing factors and the involved inflammatory pathways. Genetic analysis revealed a frameshift truncating variant in the WAS gene, resulting in WAS protein expression between mild and severe forms, despite a clinical phenotype resembling X-linked thrombocytopenia (XLT). IL-1β secretion by LPS-stimulated peripheral blood mononuclear cells from patient during MIS-C was lower compared to healthy subjects but increased during follow-up. Conversely, the percentage of ASC (apoptosis-associated speck-like protein containing a CARD) specks in the patient's circulating monocytes during the acute phase was higher than in healthy subjects. The type I interferon (IFN) signature during MIS-C was normal, in contrast to the raised IFN signature measured far from the acute event. This case supports the association of IEI with MIS-C, potentially linked to delayed immune responses to SARS-CoV-2. The XLT phenotype underlies a subclinical immunodysregulation involving the NLRP3 inflammasome and the type-I IFN response.

Transcriptomic insights into the role of the spleen in a mouse model of Wiskott‑Aldrich syndrome

Wiskott-Aldrich syndrome (WAS) is a rare X-linked primary immunodeficiency characterized by microthrombocytopenia, eczema, recurrent infection and increased incidence of autoimmune disorders and malignancy. WAS is caused by mutations in the was gene, which is expressed exclusively in hematopoietic cells; the spleen serves an important role in hematopoiesis and red blood cell clearance. However, to the best of our knowledge, detailed comparative analysis of the spleen between WASp-knockout (WAS-KO) and wild-type (WT) mice, particularly at the transcriptomic level, have not been reported. The present study investigated the differences in the transcriptomes of spleen tissue of 10-week-old WAS-KO mice. Comparison of the gene expression profiles of WAS-KO and WT mice revealed 1,964 differentially expressed genes (DEGs). Among these genes, 996 DEGs were upregulated and 968 were downregulated in WAS-KO mice. To determine the functions of DEGs, Gene Ontology and Kyoto Encyclopedia of Genes and Genomes enrichment analyses were performed for significantly upregulated and downregulated DEGs. The results showed that the levels of cell senescence and apoptosis-associated genes were increased, antigen processing and presentation mechanisms involved in the immune response were damaged and signal transduction processes were impaired in the spleen of WAS-KO mice. Thus, was gene deletion may lead to anemia and hemolysis-associated disease, primarily due to increased osmotic fragility of red blood cells, low hemoglobin and increased bilirubin levels and serum ferritin. These results indicated that senescence and apoptosis of blood cells also play an important role in the occurrence of WAS. Therefore, the present findings provide a theoretical basis for further study to improve the treatment of WAS.

WASp Deficiency Selectively Affects the TCR Diversity of Different Memory T Cell Subsets in WAS Chimeric Mice

Background

The T cell receptor (TCR) diversity is essential for effective T cell immunity. Previous studies showed that TCR diversity in Wiskott–Aldrich Syndrome (WAS) patients was severely impaired, especially in the memory T cell populations. Whether this defect was caused by intrinsic WASp deficiency or extrinsic reasons is still unclear.

Methods

We sorted different T cell subsets from the bone marrow chimeric mice model using both magnetic beads and flow cytometry. TCR repertoires of memory T cells, especially CD4⁺ effector memory T (TEM) cells and CD8⁺ central memory T (TCM) cells, were analyzed using the UMI quantitative high-throughput sequencing (HTS).

Results

An average of 5.51 million sequencing reads of 32 samples was obtained from the Illumina sequencing platform. Bioinformatic analyses showed that compared with wild type (WT), WAS knock out (KO)-CD4⁺ TEM cells exhibited increased Simpson index and decreased D50 index (P <0.05); The rank abundance curve of KO-CD4⁺ TEM cells was shorter and steeper than that of WT, and the angle of ^qD and q in KO-CD4⁺ TEM cells was lower than that of WT, while these indexes showed few changes between WT and KO chimeric mice in the CD8⁺TCM population. Therefore, it indicated that the restriction on the TCRVβ repertoires is majorly in KO-CD4⁺ TEM cells but not KO- CD8⁺ TCM cells. Principal Component Analysis (PCA), a comprehensive parameter for TCRVβ diversity, successfully segregated CD4⁺ TEM cells from WT and KO, but failed in CD8⁺ TCM cells. Among the total sequences of TRB, the usage of TRBV12.2, TRBV30, TRBV31, TRBV4, TRBD1, TRBD2, TRBJ1.1, and TRBJ1.4 showed a significant difference between WT-CD4⁺ TEM cells and KO-CD4⁺ TEM cells (P <0.05), while in CD8⁺ TCM cells, only the usage of TRBV12.2 and TRBV20 showed a substantial difference between WT and KO (P <0.05). No significant differences in the hydrophobicity and sequence length of TCRVβ were found between the WT and KO groups.

Conclusion

WASp deficiency selectively affected the TCR diversity of different memory T cell subsets, and it had more impact on the TCRVβ diversity of CD4⁺ TEM cells than CD8⁺ TCM cells. Moreover, the limitation of TCRVβ diversity of CD4⁺ TEM cells and CD8⁺ TCM cells in WAS was not severe but intrinsic.

Wiskott-Aldrich Syndrome Protein: Roles in Signal Transduction in T Cells

Signal transduction regulates the proper function of T cells in an immune response. Upon binding to its specific ligand associated with major histocompatibility complex (MHC) molecules on an antigen presenting cell, the T cell receptor (TCR) initiates intracellular signaling that leads to extensive actin polymerization. Wiskott-Aldrich syndrome protein (WASp) is one of the actin nucleation factors that is recruited to TCR microclusters, where it is activated and regulates actin network formation. Here we highlight the research that has focused on WASp-deficient T cells from both human and mice in TCR-mediated signal transduction. We discuss the role of WASp in proximal TCR signaling as well as in the Ras/Rac-MAPK (mitogen-activated protein kinase), PKC (protein kinase C) and Ca2+-mediated signaling pathways.

Reduced Fluorescence versus Forward Scatter Time-of-Flight and Increased Peak versus Integral Fluorescence Ratios Indicate Receptor Clustering in Flow Cytometry

Clustering of surface receptors is often required to initiate signal transduction, receptor internalization, and cellular activation. To study the kinetics of clustering, we developed an economic high-throughput method using flow cytometry. The quantification of receptor clustering by flow cytometry is based on the following two observations: first, the fluorescence signal length (FL time-of-flight [ToF]) decreases relative to the forward scatter signal length (FSc-ToF), and second, the peak FL (FL-peak) increases relative to the integral FL (FL-integral) upon clustering of FL-labeled surface receptors. Receptor macroclustering can therefore be quantified using the ratios FL-ToF/FSc-ToF (method ToF) or FL-peak/FL-integral (method Peak). We have used these methods to analyze clustering of two immune receptors known to undergo different conformational and oligomeric states: the BCR and the complement receptor 3 (CR3), on murine splenocytes, purified B cells, and human neutrophils. Engagement of both the BCR and CR3, on immortalized as well as primary murine B cells and human neutrophil, respectively, resulted in decreased FL-ToF/FSc-ToF and increased FL-peak/FL-integral ratios. Manipulation of the actin-myosin cytoskeleton altered BCR clustering which could be measured using the established parameters. To confirm clustering of CR3 on neutrophils, we applied imaging flow cytometry. Because receptor engagement is as a biological process dependent on cell viability, energy metabolism, and temperature, receptor clustering can only be quantified by gating on viable cells under physiological conditions. In summary, with this novel method, receptor clustering on nonadherent cells can easily be monitored by high-throughput conventional flow cytometry.

Phagocytosis: receptors, signal integration, and the cytoskeleton

Phagocytosis is a remarkably complex and versatile process: it contributes to innate immunity through the ingestion and elimination of pathogens, while also being central to tissue homeostasis and remodeling by clearing effete cells. The ability of phagocytes to perform such diverse functions rests, in large part, on their vast repertoire of receptors. In this review, we address the various receptor types, their mobility in the plane of the membrane, and two modes of receptor crosstalk: priming and synergy. A major section is devoted to the actin cytoskeleton, which not only governs receptor mobility and clustering but also is instrumental in particle engulfment. Four stages of the actin remodeling process are identified and discussed: (i) the 'resting' stage that precedes receptor engagement, (ii) the disruption of the cortical actin prior to formation of the phagocytic cup, (iii) the actin polymerization that propels pseudopod extension, and (iv) the termination of polymerization and removal of preassembled actin that are required for focal delivery of endomembranes and phagosomal sealing. These topics are viewed in the larger context of the differentiation and polarization of the phagocytic cells.

T cell antigen receptor activation and actin cytoskeleton remodeling

T cells constitute a crucial arm of the adaptive immune system and their optimal function is required for a healthy immune response. After the initial step of T cell-receptor (TCR) triggering by antigenic peptide complexes on antigen presenting cell (APC), the T cell exhibits extensive cytoskeletal remodeling. This cytoskeletal remodeling leads to the formation of an "immunological synapse" [1] characterized by regulated clustering, segregation and movement of receptors at the interface. Synapse formation regulates T cell activation and response to antigenic peptides and proceeds via feedback between actin cytoskeleton and TCR signaling. Actin polymerization participates in various events during the synapse formation, maturation, and eventually its disassembly. There is increasing knowledge about the actin effectors that couple TCR activation to actin rearrangements [2,3], and how defects in these effectors translate into impairment of T cell activation. In this review we aim to summarize and integrate parts of what is currently known about this feedback process. In addition, in light of recent advancements in our understanding of TCR triggering and translocation at the synapse, we speculate on the organizational and functional diversity of microfilament architecture in the T cell. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.

WASH knockout T cells demonstrate defective receptor trafficking, proliferation, and effector function

WASH is an Arp2/3 activator of the Wiskott-Aldrich syndrome protein superfamily that functions during endosomal trafficking processes in collaboration with the retromer and sorting nexins, but its in vivo function has not been examined. To elucidate the physiological role of WASH in T cells, we generated a WASH conditional knockout (WASHout) mouse model. Using CD4(Cre) deletion, we found that thymocyte development and naive T cell activation are unaltered in the absence of WASH. Surprisingly, despite normal T cell receptor (TCR) signaling and interleukin-2 production, WASHout T cells demonstrate significantly reduced proliferative potential and fail to effectively induce experimental autoimmune encephalomyelitis. Interestingly, after activation, WASHout T cells fail to maintain surface levels of TCR, CD28, and LFA-1. Moreover, the levels of the glucose transporter, GLUT1, are also reduced compared to wild-type T cells. We further demonstrate that the loss of surface expression of these receptors in WASHout cells results from aberrant accumulation within the collapsed endosomal compartment, ultimately leading to degradation within the lysosome. Subsequently, activated WASHout T cells experience reduced glucose uptake and metabolic output. Thus, we found that WASH is a newly recognized regulator of TCR, CD28, LFA-1, and GLUT1 endosome-to-membrane recycling. Aberrant trafficking of these key T cell proteins may potentially lead to attenuated proliferation and effector function.

Cdc42 regulates neutrophil migration via crosstalk between WASp, CD11b, and microtubules

Chemotaxis promotes neutrophil participation in cellular defense by enabling neutrophil migration to infected tissue and is controlled by persistent cell polarization. One long-standing question of neutrophil polarity has been how the pseudopod and the uropod are coordinated. In our previous report, we suggested that Rho GTPase Cdc42 controls neutrophil polarity through CD11b signaling at the uropod, albeit through an unknown mechanism. Here, we show that Cdc42 controls polarity, unexpectedly, via its effector WASp. Cdc42 controls WASp activation and its distant localization to the uropod. At the uropod, WASp regulates the reorganization of CD11b integrin into detergent resistant membrane domains; in turn, CD11b recruits the microtubule end binding protein EB1 to capture and stabilize microtubules at the uropod. This organization is necessary to maintain neutrophil polarity during migration and is critical for neutrophil emigration into inflamed lungs. These results suggest unrecognized mechanism of neutrophil polarity in which WASp mediates long-distance control of the uropod by Cdc42 to maintain a proper balance between the pseudopod and the uropod. Our study reveals a new function for WASp in the control of neutrophil polarity via crosstalk between CD11b and microtubules.

A method for large-scale identification of protein arginine methylation

The lack of methods for proteome-scale detection of arginine methylation restricts our knowledge of its relevance in physiological and pathological processes. Here we show that most tryptic peptides containing methylated arginine(s) are highly basic and hydrophilic. Consequently, they could be considerably enriched from total cell extracts by simple protocols using either one of strong cation exchange chromatography, isoelectric focusing, or hydrophilic interaction liquid chromatography, the latter being by far the most effective of all. These methods, coupled with heavy methyl-stable isotope labeling by amino acids in cell culture and mass spectrometry, enabled in T cells the identification of 249 arginine methylation sites in 131 proteins, including 190 new sites and 93 proteins not previously known to be arginine methylated. By extending considerably the number of known arginine methylation sites, our data reveal a novel proline-rich consensus motif and identify for the first time arginine methylation in proteins involved in cytoskeleton rearrangement at the immunological synapse and in endosomal trafficking.

Hem-1: putting the "WAVE" into actin polymerization during an immune response

Most active processes by immune cells including adhesion, migration, and phagocytosis require the coordinated polymerization and depolymerization of filamentous actin (F-actin), which is an essential component of the actin cytoskeleton. This review focuses on a newly characterized hematopoietic cell-specific actin regulatory protein called hematopoietic protein-1 [Hem-1, also known as Nck-associated protein 1-like (Nckap1l or Nap1l)]. Hem-1 is a component of the "WAVE [WASP (Wiskott-Aldrich syndrome protein)-family verprolin homologous protein]" complex, which signals downstream of activated Rac to stimulate F-actin polymerization in response to immuno-receptor signaling. Genetic studies in cell lines and in mice suggest that Hem-1 regulates F-actin polymerization in hematopoietic cells, and may be essential for most active processes dependent on reorganization of the actin cytoskeleton in immune cells.

Multiple pathways leading from the T-cell antigen receptor to the actin cytoskeleton network

Dynamic rearrangements of the actin cytoskeleton, following T-cell antigen receptor (TCR) engagement, provide the structural matrix and flexibility to enable intracellular signal transduction, cellular and subcellular remodeling, and driving effector functions. Recently developed cutting-edge imaging technologies have facilitated the study of TCR signaling and its role in actin-dependent processes. In this review, we describe how TCR signaling cascades induce the activation of actin regulatory proteins and the formation of actin networks, and how actin dynamics is important for T-cell homeostasis, activation, migration, and other effector functions.

Elucidation of the integrin LFA-1-mediated signaling pathway of actin polarization in natural killer cells

The leukocyte integrin LFA-1 is critical for natural killer (NK) cell cytotoxicity as it mediates NK-cell adhesion to target cells and generates activating signals that lead to polarization of the actin cytoskeleton. However, the LFA-1-mediated signaling pathway is not fully understood. Here, we examined the subcellular localization of actin-associated proteins in wild-type, talin-deficient, and Wiskott-Aldrich Syndrome protein (WASP)-deficient NK cells bound to beads coated with the LFA-1 ligand intercellular adhesion molecule-1 (ICAM-1). In addition, we carried out coimmunoprecipitation analyses and also used a pharmacologic reagent to reduce the level of phosphatidylinositol-4,5-bisphosphate (PIP(2)). The results revealed the following signaling pathways. Upon ICAM-1 binding to LFA-1, talin redistributes to the site of LFA-1 ligation and initiates 2 signaling pathways. First, talin recruits the actin nucleating protein complex Arp2/3 via constitutive association of vinculin with talin and Arp2/3. Second, talin also associates with type I phosphatidylinositol 4-phosphate 5-kinase (PIPKI) and binding of LFA-1 to ICAM-1 results in localized increase in PIP(2). This increase in PIP(2) recruits WASP to the site of LFA-1 ligation where WASP promotes Arp2/3-mediated actin polymerization. These processes are critical for the initiation of NK cell-mediated cytotoxicity.