Intrinsic T- and B-cell defects impair T-cell-dependent antibody responses in mice lacking the actin-bundling protein L-plastin

LCP1 correlates with immune infiltration: a prognostic marker for triple-negative breast cancer

OBJECTIVE

Triple-Negative Breast Cancer (TNBC) is known for its aggressiveness and treatment challenges due to the absence of ER, PR, and HER2 receptors. Our work emphasizes the prognostic value of LCP1 (Lymphocyte cytosolic protein 1), which plays a crucial role in cell processes and immune cell activity, to predict outcomes and guide treatments in TNBC.

METHODS

We explored LCP1 as a potential biomarker in TNBC and investigated the mRNA and protein expression levels of LCP1. We investigated different databases, including GTEX, TCGA, GEO, cBioPortal and Kaplan-Meier Plotter. Immunohistochemistry on TNBC and benign tumor samples was performed to examine LCP1's relationship with patient clinical characteristics and macrophage markers. We also assessed survival rates, immune cell infiltration, and drug sensitivity related to LCP1 using various bioinformatics tools.

RESULTS

The results indicated that LCP1 expression was higher in TNBC tissues compared to adjacent normal tissues. However, high expression of LCP1 was significantly associated with favorable survival outcomes in patients with TNBC. Enrichment analysis revealed that genes co-expressed with LCP1 were significantly enriched in various immune processes. LCP1 showed a positive correlation with the infiltration of resting dendritic cells, M1 macrophages, and memory CD4 T cells, and a negative correlation with M2 macrophages. Further analysis suggested a link between high levels of LCP1 and increased survival outcomes in cancer patients receiving immunotherapy.

CONCLUSION

LCP1 may serve as a potential diagnostic and prognostic biomarker for TNBC, which was closely associated with immune cell infiltration, particularly M1 and M2 macrophages. Our findings may provide valuable insights into immunotherapeutic strategies for TNBC patients.

The anti-B7-H3 blocking antibody MJ18 does not recognize B7-H3 in murine tumor models

The immune checkpoint molecule B7-H3 is regarded as one of the most promising therapeutic targets for the treatment of human cancers. B7-H3 is highly expressed in many cancers and its expression has been associated to impaired antitumor immunity and poor patient prognosis. In immunocompetent mouse tumor models, genetic deletion of B7-H3 in tumor cells enhances antitumor immune response leading to tumor shrinkage. The underlying mechanisms of B7-H3 inhibitory function remain largely uncharacterized and the identity of potential cognate(s) receptor(s) of B7-H3 is still to be defined. To better understand B7-H3 function in vivo, several studies have employed MJ18, a monoclonal antibody reported to bind murine B7-H3 and blocks its immune-inhibitory function. In this brief research report, we show that 1) MJ18 does not bind B7-H3, 2) MJ18 binds the Fc receptor FcγRIIB on surface of murine splenocytes, and 3) MJ18 does not induce tumor regression in a mouse model responsive to B7-H3 knockout. Given the high profile of B7-H3 as therapeutic target for human cancers, our work emphasizes that murine B7-H3 studies using the MJ18 antibody should be interpreted with caution. Finally, we hope that our study will motivate the scientific community to establish much-needed validated research tools to study B7-H3 biology in mouse models.

Follow that cell: leukocyte migration in L-plastin mutant zebrafish

Actin assemblies are important in motile cells such as leukocytes which form dynamic plasma membrane extensions or podia. L-plastin (LCP1) is a leukocyte-specific calcium-dependent actin-bundling protein that, in mammals, is known to affect immune cell migration. Previously, we generated CRISPR/Cas9 engineered zebrafish lacking L-plastin (lcp1-/-) and reported that they had reduced survival to adulthood, suggesting that lack of this actin-bundler might negatively affect the immune system. To test this hypothesis, we examined the distribution and migration of neutrophils and macrophages in the transparent tail of early zebrafish larvae using cell-specific markers and an established wound-migration assay. Knockout larvae were similar to their heterozygous siblings in having equal body sizes and comparable numbers of neutrophils in caudal hematopoietic tissue at two days post-fertilization, indicating no gross defect in neutrophil production or developmental migration. When stimulated by a tail wound, all genotypes of neutrophils were equally migratory in a two-hour window. However for macrophages we observed both migration defects and morphological differences. L-plastin knockout macrophages (lcp1 -/-) still homed to wounds but were slower, less directional and had a star-like morphology with many leading and trailing projections. In contrast, heterozygous macrophages lcp1 (+/-) were faster, more directional, and had a streamlined, slug-like morphology. Overall, these findings show that in larval zebrafish L-plastin knockout primarily affects the macrophage response with possible consequences for organismal immunity. Consistent with our observations, we propose a model in which cytoplasmic L-plastin negatively regulates macrophage integrin adhesion by holding these transmembrane heterodimers in a 'clasped', inactive form and is a necessary part of establishing macrophage polarity during chemokine-induced motility. This article is protected by copyright. All rights reserved.

Efficient T Cell Migration and Activation Require L-Plastin

Rapid re-organization of the actin cytoskeleton supports T-cell trafficking towards immune sites and interaction with antigen presenting cells (APCs). F-actin rearrangement enables T-cell trafficking by stabilizing adhesion to vascular endothelial cells and promoting transendothelial migration. T-cell/APC immune synapse (IS) maturation also relies upon f-actin-anchored LFA-1:ICAM-1 ligation. Therefore, efficient T-cell responses require tight regulation of f-actin dynamics. In this review, we summarize how the actin-bundling protein L-plastin (LPL) regulates T-cell activation and migration. LPL enhances f-actin polymerization and also directly binds to the β2 chain of the integrin LFA-1 to support intercellular adhesion and IS formation in human and murine T cells. LPL- deficient T cells migrate slowly in response to chemo-attractants such as CXCL12, CCL19, and poorly polarize towards ICAM-1. Loss of LPL impairs thymic egress and intranodal motility. LPL is also required for T-cell IS maturation with APCs, and therefore for efficient cytokine production and proliferation. LPL^-/- mice are less susceptible to T-cell mediated pathologies, such as allograft rejection and experimental autoimmune encephalomyelitis (EAE). LPL activity is regulated by its N-terminal “headpiece”, which contains serine and threonine phosphorylation and calcium- and calmodulin-binding sites. LPL phosphorylation is required for lamellipodia formation during adhesion and migration, and also for LFA-1 clustering during IS formation. However, the precise molecular interactions by which LPL supports T-cell functional responses remain unclear. Future studies elucidating LPL-mediated regulation of T-cell migration and/or activation may illuminate pathways for therapeutic targeting in T-cell-mediated diseases.

Lymphocyte cytosolic protein 1 (L-plastin) I232F mutation impairs granulocytic proliferation and causes neutropenia

Neutrophils migrate into inflamed tissue, engage in phagocytosis, and clear pathogens or apoptotic cells. These processes require well-coordinated events involving the actin cytoskeleton. We describe a child with severe neutropenia and episodes of soft tissue infections and pneumonia. Bone marrow examination showed granulocytic hypoplasia with dysplasia. Whole-exome sequencing revealed a de novo heterozygous missense mutation in LCP1, which encodes the F-actin-binding protein Lymphocyte Cytosolic Protein 1. To determine its pathophysiological significance, we stably transduced cells with doxycycline-inducible wild-type LCP1 and LCP1 I232F lentiviral constructs. We observed dysplastic granulocytic 32D cells expressing LCP1 I232F cells. These cells showed decreased proliferation without a block in differentiation. In addition, expression of LCP1 I232F resulted in a cell cycle arrest at the G2/M phase, but it did not lead to increased levels of genes involved in apoptosis or the unfolded protein response. Both 32D and HeLa cells expressing mutant LCP1 displayed impaired cell motility and invasiveness. Flow cytometry showed increased F-actin. However, mutant LCP1-expressing 32D cells exhibited normal oxidative burst upon stimulation. Confocal imaging and subcellular fractionation revealed diffuse intracellular localization of LCP1, but only the mutant form was found in the nucleus. We conclude that LCP1 is a new gene involved in granulopoiesis, and the missense variant LCP1 I232F leads to neutropenia and granulocytic dysplasia with aberrant actin dynamics. Our work supports a model of neutropenia due to aberrant actin regulation.

L-plastin enhances NLRP3 inflammasome assembly and bleomycin-induced lung fibrosis

Macrophage adhesion and stretching have been shown to induce interleukin (IL)-1β production, but the mechanism of this mechanotransduction remains unclear. Here we specify the molecular link between mechanical tension on tissue-resident macrophages and activation of the NLRP3 inflammasome, which governs IL-1β production. NLRP3 activation enhances antimicrobial defense, but excessive NLRP3 activity causes inflammatory tissue damage in conditions such as pulmonary fibrosis and acute respiratory distress syndrome. We find that the actin-bundling protein L-plastin (LPL) significantly enhances NLRP3 assembly. Specifically, LPL enables apoptosis-associated speck-like protein containing a caspase activation and recruitment domain (ASC) oligomerization during NLRP3 assembly by stabilizing ASC interactions with the kinase Pyk2, a component of cell-surface adhesive structures called podosomes. Upon treatment with exogenous NLRP3 activators, lung-resident alveolar macrophages (AMs) lacking LPL exhibit reduced caspase-1 activity, IL-1β cleavage, and gasdermin-D processing. LPL^−/− mice display resistance to bleomycin-induced lung injury and fibrosis. These findings identify the LPL-Pyk2-ASC pathway as a target for modulation in NLRP3-mediated inflammatory conditions.

In this study, Joshi et al. identify a crucial modulator, L-plastin, in lung inflammation. L-plastin supports the macrophage inflammatory response to enhance lung fibrosis during lung injury by connecting inflammation and mechanical stimuli in a process called mechanotransduction. The findings from this study will help determine efficient targets for diagnosis and treatment of lung inflammatory diseases.

LCP1 is a prognostic biomarker correlated with immune infiltrates in gastric cancer

BACKGROUND

Previous studies have identified LCP1 as a diagnostic and prognostic marker in several cancers. However, the role of LCP1 in gastric cancer (GC) and its effect on tumor immune infiltration remain unclear.

OBJECTIVE

The aim was to explore the role of LCP1 in GC and its effect on tumor immune infiltration.

METHODS

We explored the expression of LCP1 relative to clinicopathology in GC patients by bioinformatics analysis and immunohistochemistry. Using cBioportal database, we analyzed the characteristic genetic variations of LCP1 in GC. In addition, we evaluated the correlation between LCP1 expression and tumor-infiltrating lymphocytes (TILs) using R software, TIMER and TISIDB databases. Finally, we analyzed the biological functions in which LCP1 may participate and the signaling pathways it may regulate.

RESULTS

Here, we showed that LCP1 expression is significantly correlated with tumor aggressiveness and poor prognosis in GC patients. Additionally, the results indicated that LCP1 was associated with TILs, including both immunosuppressive and immunosupportive cells, and was strongly correlated with various immune marker sets in GC. GSEA analysis demonstrated that LCP1 expression played an important role in lymphocyte formation and immune reaction.

CONCLUSIONS

LCP1 may be a potential prognostic biomarker for GC patients and a marker for tumor immunotherapy.

The actin-bundling protein L-plastin-A double-edged sword: Beneficial for the immune response, maleficent in cancer

The dynamic organization of the actin cytoskeleton into bundles and networks is orchestrated by a large variety of actin-binding proteins. Among them, the actin-bundling protein L-plastin is normally expressed in hematopoietic cells, where it is involved in the immune response. However, L-plastin is also often ectopically expressed in malignant cancer cells of non-hematopoietic origin and is even considered as a marker for cancer progression. Post-translational modification modulates L-plastin activity. In particular, L-plastin Ser5 phosphorylation has been shown to be important for the immune response in leukocytes as well as for invasion and metastasis formation of carcinoma cells. This chapter discusses the physiological and pathological role of L-plastin with a special focus on the importance of L-plastin Ser5 phosphorylation for the protein functions. The potential use of Ser5 phosphorylated L-plastin as a biomarker and/or therapeutic target will be evoked.

Neutrophil L-Plastin Controls Ocular Paucibacteriality and Susceptibility to Keratitis

Why ocular mucosa is paucibacterial is unknown. Many different mechanisms have been suggested but the comprehensive experimental studies are sparse. We found that a deficiency in L-plastin (LCP1), an actin bundling protein, resulted in an ocular commensal overgrowth, characterized with increased presence of conjunctival Streptococcal spp. The commensal overgrowth correlated with susceptibility to P. aeruginosa-induced keratitis. L-plastin knock-out (KO) mice displayed elevated bacterial burden in the P. aeruginosa-infected corneas, altered inflammatory responses, and compromised bactericidal activity. Mice with ablation of LPL under the LysM Cre (LysM. CreposLPLfl/fl ) and S100A8 Cre (S100A8.CreposLPLfl/fl ) promoters had a similar phenotype to the LPL KOs mice. In contrast, infected CD11c.CreposLPLfl/fl mice did not display elevated susceptibility to infection, implicating the myeloid L-plastin-sufficient cells (e.g., macrophages and neutrophils) in maintaining ocular homeostasis. Mechanistically, the elevated commensal burden and the susceptibility to infection were linked to defects in neutrophil frequencies at steady state and during infection and compromised bactericidal activities upon priming. Macrophage exposure to commensal organisms primed neutrophil responses to P. aeruginosa, augmenting PMN bactericidal capacity in an L-plastin dependent manner. Cumulatively, our data highlight the importance of neutrophils in controlling ocular paucibacteriality, reveal molecular and cellular events involved in the process, and suggest a link between commensal exposure and resistance to infection.

Integrated Functional Analysis of the Nuclear Proteome of Classically and Alternatively Activated Macrophages

Macrophages (Mφ) play a central role in coordinating host response to pathogens, cellular injury, and environmental stimuli. Herein, we report multidimensional, nuclear proteomic analyses of protein expression and posttranslational modifications (PTMs) that control biological processes during Mφ activation. For this, Mφ were incubated with IFN-γ/LPS and IL-4, and their differentiation to proinflammatory (M1) and anti-inflammatory (M2a, referred as M2 for simplicity throughtout the manuscript) phenotypes was confirmed by detection of CD64 and CD206 surface markers and TNF-α, arginase I, and iNOS-dependent nitrite levels. We used a sequential method of organellar enrichment and labeling of nuclear fractions with BODIPY FL-maleimide fluorescence dye followed by two-dimensional electrophoresis (2DE) to capture quantitative changes in abundance and S-nitrosylated (SNO) proteome signatures. Exact same gels were then labeled with Pro-Q Diamond to detect protein phosphorylation. MALDI-TOF/TOF MS analysis of the protein spots with fold change of ≥|1.5| in any of the groups yielded 229 identifications. We found that 145, 78, and 173 protein spots in M1 Mφ and 105, 81, and 164 protein spots in M2 Mφ were changed in abundance, S-nitrosylation, and phosphorylation, respectively, with respect to M0 controls (fold change: ≥|1.5|, p ≤ 0.05). Targeted analysis by immunoprecipitation and Western blotting was performed to verify the differential abundance and phosphorylation levels of two of the proteins in M1 and M2 (vs. M0) Mφ. Ingenuity Pathway Analysis of the nuclear proteome datasets showed that the abundance and posttranslational (SNO and Phosphor) modifications of the proteins predicted to be involved in cytoskeletal organization/cell movement, phagocytosis/endocytosis, and cell proliferation/cell death were differentially regulated with proinflammatory and anti-inflammatory activation of Mφ.

Targeted deletion of the zebrafish actin-bundling protein L-plastin (lcp1)

Regulation of the cytoskeleton is essential for cell migration in health and disease. Lymphocyte cytosolic protein 1 (lcp1, also called L-plastin) is a hematopoietic-specific actin-bundling protein that is highly conserved in zebrafish, mice and humans. In addition, L-plastin expression is documented as both a genetic marker and a cellular mechanism contributing to the invasiveness of tumors and transformed cell lines. Despite L-plastin’s role in both immunity and cancer, in zebrafish there are no direct studies of its function, and no mutant, knockout or reporter lines available. Using CRISPR-Cas9 genome editing, we generated null alleles of zebrafish lcp1 and examined the phenotypes of these fish throughout the life cycle. Our editing strategy used gRNA to target the second exon of lcp1, producing F0 mosaic fish that were outcrossed to wild types to confirm germline transmission. F1 heterozygotes were then sequenced to identify three unique null alleles, here called ‘Charlie’, ‘Foxtrot’ and ‘Lima’. In silico, each allele truncates the endogenous protein to less than 5% normal size and removes both essential actin-binding domains (ABD1 and ABD2). Although none of the null lines express detectable LCP1 protein, homozygous mutant zebrafish (-/-) can develop and reproduce normally, a finding consistent with that of the L-plastin null mouse (LPL -/-). However, such mice do have a profound immune defect when challenged by lung bacteria. Interestingly, we observed reduced long-term survival of zebrafish lcp1 -/- homozygotes (~30% below the expected numbers) in all three of our knockout lines, with greatest mortality corresponding to the period (4–6 weeks post-fertilization) when the innate immune system is functional, but the adaptive immune system is not yet mature. This suggests that null zebrafish may have reduced capacity to combat opportunistic infections, which are more easily transmissible in the aquatic environment. Overall, our novel mutant lines establish a sound genetic model and an enhanced platform for further studies of L-plastin gene function in hematopoiesis and cancer.

Plastins regulate ectoplasmic specialization via its actin bundling activity on microfilaments in the rat testis

Plastins are a family of actin binding proteins (ABPs) known to cross-link actin microfilaments in mammalian cells, creating actin microfilament bundles necessary to confer cell polarity and cell shape. Plastins also support cell movement in response to changes in environment, involved in cell/tissue growth and development. They also confer plasticity to cells and tissues in response to infection or other pathological conditions (e.g., inflammation). In the testis, the cell-cell anchoring junction unique to the testis that is found at the Sertoli cell-cell interface at the blood-testis barrier (BTB) and at the Sertoli-spermatid (e.g., 8–19 spermatids in the rat testis) is the basal and the apical ectoplasmic specialization (ES), respectively. The ES is an F-actin-rich anchoring junction constituted most notably by actin microfilament bundles. A recent report using RNAi that specifically knocks down plastin 3 has yielded some insightful information regarding the mechanism by which plastin 3 regulates the status of actin microfilament bundles at the ES via its intrinsic actin filament bundling activity. Herein, we provide a brief review on the role of plastins in the testis in light of this report, which together with recent findings in the field, we propose a likely model by which plastins regulate ES function during the epithelial cycle of spermatogenesis via their intrinsic activity on actin microfilament organization in the rat testis.

Integrated Cellular and Plasma Proteomics of Contrasting B-cell Cancers Reveals Common, Unique and Systemic Signatures

Approximately 800,000 leukemia and lymphoma cases are diagnosed worldwide each year. Burkitt's lymphoma (BL) and chronic lymphocytic leukemia (CLL) are examples of contrasting B-cell cancers; BL is a highly aggressive lymphoid tumor, frequently affecting children, whereas CLL typically presents as an indolent, slow-progressing leukemia affecting the elderly. The B-cell-specific overexpression of the myc and TCL1 oncogenes in mice induce spontaneous malignancies modeling BL and CLL, respectively. Quantitative mass spectrometry proteomics and isobaric labeling were employed to examine the biology underpinning contrasting Eμ-myc and Eμ-TCL1 B-cell tumors. Additionally, the plasma proteome was evaluated using subproteome enrichment to interrogate biomarker emergence and the systemic effects of tumor burden. Over 10,000 proteins were identified (q<0.01) of which 8270 cellular and 2095 plasma proteins were quantitatively profiled. A common B-cell tumor signature of 695 overexpressed proteins highlighted ribosome biogenesis, cell-cycle promotion and chromosome segregation. Eμ-myc tumors overexpressed several methylating enzymes and underexpressed many cytoskeletal components. Eμ-TCL1 tumors specifically overexpressed ER stress response proteins and signaling components in addition to both subunits of the interleukin-5 (IL5) receptor. IL5 treatment promoted Eμ-TCL1 tumor proliferation, suggesting an amplification of IL5-induced AKT signaling by TCL1. Tumor plasma contained a substantial tumor lysis signature, most prominent in Eμ-myc plasma, whereas Eμ-TCL1 plasma contained signatures of immune-response, inflammation and microenvironment interactions, with putative biomarkers in early-stage cancer. These findings provide a detailed characterization of contrasting B-cell tumor models, identifying common and specific tumor mechanisms. Integrated plasma proteomics allowed the dissection of a systemic response and a tumor lysis signature present in early- and late-stage cancers, respectively. Overall, this study suggests common B-cell cancer signatures exist and illustrates the potential of the further evaluation of B-cell cancer subtypes by integrative proteomics.

Alveolar macrophage development in mice requires L-plastin for cellular localization in alveoli

Alveolar macrophages are lung-resident sentinel cells that develop perinatally and protect against pulmonary infection. Molecular mechanisms controlling alveolar macrophage generation have not been fully defined. Here, we show that the actin-bundling protein L-plastin (LPL) is required for the perinatal development of alveolar macrophages. Mice expressing a conditional allele of LPL (CD11c.Cre^pos-LPL^fl/fl) exhibited significant reductions in alveolar macrophages and failed to effectively clear pulmonary pneumococcal infection, showing that immunodeficiency results from reduced alveolar macrophage numbers. We next identified the phase of alveolar macrophage development requiring LPL. In mice, fetal monocytes arrive in the lungs during a late fetal stage, maturing to alveolar macrophages through a prealveolar macrophage intermediate. LPL was required for the transition from prealveolar macrophages to mature alveolar macrophages. The transition from prealveolar macrophage to alveolar macrophage requires the upregulation of the transcription factor peroxisome proliferator-activated receptor-γ (PPAR-γ), which is induced by exposure to granulocyte-macrophage colony-stimulating factor (GM-CSF). Despite abundant lung GM-CSF and intact GM-CSF receptor signaling, PPAR-γ was not sufficiently upregulated in developing alveolar macrophages in LPL^-/- pups, suggesting that precursor cells were not correctly localized to the alveoli, where GM-CSF is produced. We found that LPL supports 2 actin-based processes essential for correct localization of alveolar macrophage precursors: (1) transmigration into the alveoli, and (2) engraftment in the alveoli. We thus identify a molecular pathway governing neonatal alveolar macrophage development and show that genetic disruption of alveolar macrophage development results in immunodeficiency.

Comprehensive Proteomic and Metabolomic Signatures of Nontypeable Haemophilus influenzae-Induced Acute Otitis Media Reveal Bacterial Aerobic Respiration in an Immunosuppressed Environment

A thorough understanding of the molecular details of the interactions between bacteria and host are critical to ultimately prevent disease. Recent technological advances allow simultaneous analysis of host and bacterial protein and metabolic profiles from a single small tissue sample to provide insight into pathogenesis. We used the chinchilla model of human otitis media to determine, for the first time, the most expansive delineation of global changes in protein and metabolite profiles during an experimentally induced disease. After 48 h of infection with nontypeable Haemophilus influenzae, middle ear tissue lysates were analyzed by high-resolution quantitative two-dimensional liquid chromatography-tandem mass spectrometry. Dynamic changes in 105 chinchilla proteins and 66 metabolites define the early proteomic and metabolomic signature of otitis media. Our studies indicate that establishment of disease coincides with actin morphogenesis, suppression of inflammatory mediators, and bacterial aerobic respiration. We validated the observed increase in the actin-remodeling complex, Arp2/3, and experimentally showed a role for Arp2/3 in nontypeable Haemophilus influenzae invasion. Direct inhibition of actin branch morphology altered bacterial invasion into host epithelial cells, and is supportive of our efforts to use the information gathered to modify outcomes of disease. The twenty-eight nontypeable Haemophilus influenzae proteins identified participate in carbohydrate and amino acid metabolism, redox homeostasis, and include cell wall-associated metabolic proteins. Quantitative characterization of the molecular signatures of infection will redefine our understanding of host response driven developmental changes during pathogenesis. These data represent the first comprehensive study of host protein and metabolite profiles in vivo in response to infection and show the feasibility of extensive characterization of host protein profiles during disease. Identification of novel protein targets and metabolic biomarkers will advance development of therapeutic and diagnostic options for treatment of disease.

Pathophysiologic and transcriptomic analyses of viscerotropic yellow fever in a rhesus macaque model

Infection with yellow fever virus (YFV), an explosively replicating flavivirus, results in viral hemorrhagic disease characterized by cardiovascular shock and multi-organ failure. Unvaccinated populations experience 20 to 50% fatality. Few studies have examined the pathophysiological changes that occur in humans during YFV infection due to the sporadic nature and remote locations of outbreaks. Rhesus macaques are highly susceptible to YFV infection, providing a robust animal model to investigate host-pathogen interactions. In this study, we characterized disease progression as well as alterations in immune system homeostasis, cytokine production and gene expression in rhesus macaques infected with the virulent YFV strain DakH1279 (YFV-DakH1279). Following infection, YFV-DakH1279 replicated to high titers resulting in viscerotropic disease with ∼72% mortality. Data presented in this manuscript demonstrate for the first time that lethal YFV infection results in profound lymphopenia that precedes the hallmark changes in liver enzymes and that although tissue damage was noted in liver, kidneys, and lymphoid tissues, viral antigen was only detected in the liver. These observations suggest that additional tissue damage could be due to indirect effects of viral replication. Indeed, circulating levels of several cytokines peaked shortly before euthanasia. Our study also includes the first description of YFV-DakH1279-induced changes in gene expression within peripheral blood mononuclear cells 3 days post-infection prior to any clinical signs. These data show that infection with wild type YFV-DakH1279 or live-attenuated vaccine strain YFV-17D, resulted in 765 and 46 differentially expressed genes (DEGs), respectively. DEGs detected after YFV-17D infection were mostly associated with innate immunity, whereas YFV-DakH1279 infection resulted in dysregulation of genes associated with the development of immune response, ion metabolism, and apoptosis. Therefore, WT-YFV infection is associated with significant changes in gene expression that are detectable before the onset of clinical symptoms and may influence disease progression and outcome of infection.

Yellow fever virus causes ∼200,000 infections and 30,000 deaths annually in Africa and South America. Although this is an important human pathogen, the basis of yellow fever disease severity remains poorly understood. Rhesus macaques are susceptible to yellow fever and develop similar symptoms as severe as those observed in humans. In this study, we characterized disease progression in this model and observed a profound loss of lymphocytes that preceded the appearance of serum markers of virus-induced liver pathology. This change might provide an early indicator of fatal yellow fever. In addition, we also identified significant changes in gene expression in white blood cells that occur before any measurable disease symptoms and these genetic signatures may provide future targets for antiviral therapeutics and better diagnostics.

Swiprosin-1/EFhd2 limits germinal center responses and humoral type 2 immunity

Activated B cells are selected for in germinal centers by regulation of their apoptosis. The Ca2+ -binding cytoskeletal adaptor protein Swiprosin-1/EFhd2 (EFhd2) can promote apoptosis in activated B cells. We therefore hypothesized that EFhd2 might limit humoral immunity by repressing both the germinal center reaction and the expected enhancement of immune responses in the absence of EFhd2. Here, we established EFhd2(-/-) mice on a C57BL/6 background, which revealed normal B- and T-cell development, basal Ab levels, and T-cell independent type 1, and T-cell independent type 2 responses. However, T cell-dependent immunization with sheep red blood cells and infection with the helminth Nippostrongylus brasiliensis (N.b) increased production of antibodies of multiple isotypes, as well as germinal center formation in EFhd2(-/-) mice. In addition, serum IgE levels and numbers of IgE+ plasma cells were strongly increased in EFhd2(-/-) mice, both after primary as well as after secondary N.b infection. Finally, mixed bone marrow chimeras unraveled an EFhd2-dependent B cell-intrinsic contribution to increased IgE plasma cell numbers in N.b-infected mice. Hence, we established a role for EFhd2 as a negative regulator of germinal center-dependent humoral type 2 immunity, with implications for the generation of IgE.

L-plastin is essential for alveolar macrophage production and control of pulmonary pneumococcal infection

We report that mice deficient for the hematopoietic-specific, actin-bundling protein L-plastin (LPL) succumb rapidly to intratracheal pneumococcal infection. The increased susceptibility of LPL(-/-) mice to pulmonary pneumococcal challenge correlated with reduced numbers of alveolar macrophages, consistent with a critical role for this cell type in the immediate response to pneumococcal infection. LPL(-/-) mice demonstrated a very early clearance defect, with an almost 10-fold-higher bacterial burden in the bronchoalveolar lavage fluid 3 h following infection. Clearance of pneumococci from the alveolar space in LPL(-/-) mice was defective compared to that in Rag1(-/-) mice, which lack all B and T lymphocytes, indicating that innate immunity is defective in LPL(-/-) mice. We did not identify defects in neutrophil or monocyte recruitment or in the production of inflammatory cytokines or chemokines that would explain the early clearance defect. However, efficient alveolar macrophage regeneration following irradiation required LPL. We thus identify LPL as being key to alveolar macrophage development and essential to an effective antipneumococcal response. Further analysis of LPL(-/-) mice will illuminate critical regulators of the generation of alveolar macrophages and, thus, effective pulmonary innate immunity.

The actin-bundling protein L-plastin supports T-cell motility and activation

Tight regulation of actin dynamics is essential for T-cell trafficking and activation. Recent studies in human and murine T cells reveal that T-cell motility and full T-cell activation require the hematopoietic-specific, actin-bundling protein L-plastin (LPL). T cells lacking LPL do not form fully mature synapses and thus demonstrate reduced cytokine production and proliferation. Reduction or loss of LPL expression also reduces the velocity of T cells and impairs thymic egress and intranodal motility. Whereas dispensable for proximal T-cell receptor and chemokine receptor signaling, LPL is critical to the later stages of synapse maturation and cellular polarization. Serine phosphorylation, calcium, and calmodulin binding regulate the bundling activity and localization of LPL following T-cell receptor and chemokine receptor engagement. However, the interaction between these regulatory domains and resulting changes in local control of actin cytoskeletal structures has not been fully elucidated. Circumstantial evidence suggests a function for LPL in either the formation or maintenance of integrin-associated adhesion structures. As LPL may be a target of the commonly used immunosuppressive agent dexamethasone, full elucidation of the regulation and function of LPL in T-cell biology may illuminate new pathways for clinically useful immunotherapeutics.