LIME, a novel transmembrane adaptor protein, associates with p56lck and mediates T cell activation

Single-cell RNA sequencing of chronic idiopathic erythroderma defines disease-specific markers

BACKGROUND

Chronic erythroderma is a potentially life-threatening condition that can be caused by various diseases, but approximately 30% of cases remain idiopathic, often with insufficient treatment options.

OBJECTIVE

We sought to establish a molecular disease map of chronic idiopathic erythroderma (CIE).

METHODS

We performed single-cell RNA sequencing combined with T-cell receptor sequencing of blood and skin from 5 patients with CIE and compared results with 8 cases of erythrodermic cutaneous T-cell lymphoma (eCTCL), 15 cases of moderate to severe atopic dermatitis, 10 cases of psoriasis, and 20 healthy control individuals.

RESULTS

In eCTCL, we found strong expansion of CD4+ malignant clones with a CCR7+SELL+ central memory phenotype. In contrast, CIE exhibited a pattern of low-level, but consistent, expansion of CD8A+KLRK1+ T-cell clones, both in blood and in skin. KLRK1 was also expressed by CCR10+FUT7+ skin-homing CIE blood T cells that had increased proliferation rates and were absent in all other conditions. While patients with CIE and eCTCL lacked the strong type 2 or type 17 immune skewing typically found in atopic dermatitis or psoriasis, respectively, they were characterized by upregulation of MHC II genes (HLA-DRB1, HLA-DRA, and CD74) in keratinocytes and fibroblasts, most likely in an IFN-γ-dependent fashion. Overall, we found the strongest upregulation of type 1 immune mediators in CIE samples, both in the expanded CD8A+ clones and in the tissue microenvironment.

CONCLUSIONS

Despite the notion that CIE might be a mere bundle of various yet uncharacterized disease processes, we found specific pathogenic signatures in these patients, which were different from other forms of erythroderma. These data might help to improve our pathogenic understanding of the blood and skin compartments of CIE, aiding in discovery of future treatment targets.

Tumor infiltration of inactive CD8 + T cells was associated with poor prognosis in Gastric Cancer

BACKGROUND

Gastric cancer (GC) shows limited response to immune checkpoint inhibitors due to its complex tumor immune microenvironment (TIME). This study explores the functions of various immune cells in the complex TIME in GC.

METHODS

We assessed CD8 + T-cell infiltration of GC tissues by immunohistochemistry, and performed single-cell RNA sequencing (scRNA-seq) of tumor and normal tissues from 34 patients with GC.

RESULTS

We categorized 157 GC patients into LOW, MID, and HIGH groups based on their CD8 + T-cell infiltration. Overall survival was notably lower for the HIGH and LOW groups compared with the MID group. Our scRNA-seq data analysis showed that CD8 + T-cell activity markers in the HIGH group were expressed at lower levels than in normal tissue, but the T-cell-attracting chemokine CCL5 was expressed at a higher level. Notably, CD8 + T-cells in the HIGH group displayed lower PD1 expression and higher CTLA4 expression. TCR repertoire analysis using only Epstein-Barr virus-negative cases showed that CD8 + T-cell receptor clonality was lower in the HIGH group than in the MID group. Furthermore, in the HIGH group, the antigen-presenting capacity of type 1 conventional dendritic cells was lower, the immunosuppressive capacity of myeloid-derived suppressor cells was higher, and the expression of CTLA4 in regulatory T-cells was higher.

CONCLUSION

The present data suggest that the infiltration of inactive CD8 + T-cells with low clonality is induced by chemotaxis in the HIGH group, possibly leading to a poor prognosis for patients with GC.

High-resolution profile of neoantigen-specific TCR activation links moderate stimulation to increased resilience of engineered TCR-T cells

Neoantigen-specific T cell receptors (neoTCRs) promise safe, personalized anti-tumor immunotherapy. However, detailed assessment of neoTCR-characteristics affecting therapeutic efficacy is mostly missing. Previously, we identified diverse neoTCRs restricted to different neoantigens in a melanoma patient. In this work, we now combine single-cell TCR-sequencing and RNA-sequencing after neoantigen-specific restimulation of peripheral blood-derived CD8+ T cells of this patient. We detect neoTCRs with specificity for the previously detected neoantigens and perform fine-characterization of neoTCR-transgenic (tg) T cells in vitro and in vivo. We describe a heterogeneous spectrum of TCR-intrinsic activation patterns in response to a shared neoepitope ranging from previously detected more highly frequent neoTCRs with moderate activation to rare ones with initially stronger activation. Experimental restimulation of adoptively transferred neoTCR-tg T cells in a xenogeneic rechallenge tumor model demonstrates superior anti-tumor responses of moderate neoTCR-tg T cells upon repeated tumor contact. These insights have significant implications for the selection of TCRs for therapeutic engineering of TCR-tg T cells.

Phosphorus Metabolism-Related Genes Serve as Novel Biomarkers for Predicting Prognosis in Bladder Cancer: A Bioinformatics Analysis

Background

Phosphorus metabolism might be associated with tumor initiation and progression. We aimed to screen out the phosphorus metabolism genes related to bladder cancer and construct a clinical prognosis model.

Methods

The dataset used for the analysis was obtained from TCGA database. GO and KEGG enrichment analyses were subsequently applied to differentially expressed genes. Consensus clustering was utilized, and different clusters of the tumor immune microenvironment and other features were compared. The phosphorus metabolism-related genes involved in prognosis were screened out by univariate Cox regression, LASSO regression and multivariate Cox regression analysis, and a nomogram was constructed. The performance of the nomogram was validated using TCGA dataset and the GEO dataset, respectively.

Results

Overall, 405 phosphorus metabolism-related differentially expressed genes from TCGA database were identified, which were associated with phosphorylation, cell proliferation, leukocyte activation, and signaling pathways. Two clusters were obtained by consistent clustering. After tumor immune microenvironment analysis, significant differences in immune cell infiltration between cluster 1 and cluster 2 were found. Four phosphorus metabolism-related genes (LIME1, LRP8, SPDYA, and MST1R) were associated with the prognosis of bladder cancer (BLCA) patients. We built a prognostic model and visualized the model as a nomogram. Calibration curves demonstrated the performance of this nomogram, in agreement with that shown by the ROC curves.

Conclusion

We successfully identified four phosphorus metabolism-related genes associated with prognosis, providing potential targets for biomarkers and therapeutics. A nomogram based on these genes was developed. Nevertheless, this study is based on bioinformatics, and experimental validation remains essential.

Lck Function and Modulation: Immune Cytotoxic Response and Tumor Treatment More Than a Simple Event

Lck, a member of the Src kinase family, is a non-receptor tyrosine kinase involved in immune cell activation, antigen recognition, tumor growth, and cytotoxic response. The enzyme has usually been linked to T lymphocyte activation upon antigen recognition. Lck activation is central to CD4, CD8, and NK activation. However, recently, it has become clearer that activating the enzyme in CD8 cells can be independent of antigen presentation and enhance the cytotoxic response. The role of Lck in NK cytotoxic function has been controversial in a similar fashion as the role of the enzyme in CAR T cells. Inhibiting tyrosine kinases has been a highly successful approach to treating hematologic malignancies. The inhibitors may be useful in treating other tumor types, and they may be useful to prevent cell exhaustion. New, more selective inhibitors have been documented, and they have shown interesting activities not only in tumor growth but in the treatment of autoimmune diseases, asthma, and graft vs. host disease. Drug repurposing and bioinformatics can aid in solving several unsolved issues about the role of Lck in cancer. In summary, the role of Lck in immune response and tumor growth is not a simple event and requires more research.

The CD4 transmembrane GGXXG and juxtamembrane (C/F)CV+C motifs mediate pMHCII-specific signaling independently of CD4-LCK interactions

CD4+ T cell activation is driven by five-module receptor complexes. The T cell receptor (TCR) is the receptor module that binds composite surfaces of peptide antigens embedded within MHCII molecules (pMHCII). It associates with three signaling modules (CD3γε, CD3δε, and CD3ζζ) to form TCR-CD3 complexes. CD4 is the coreceptor module. It reciprocally associates with TCR-CD3-pMHCII assemblies on the outside of a CD4+ T cells and with the Src kinase, LCK, on the inside. Previously, we reported that the CD4 transmembrane GGXXG and cytoplasmic juxtamembrane (C/F)CV+C motifs found in eutherian (placental mammal) CD4 have constituent residues that evolved under purifying selection (Lee et al., 2022). Expressing mutants of these motifs together in T cell hybridomas increased CD4-LCK association but reduced CD3ζ, ZAP70, and PLCγ1 phosphorylation levels, as well as IL-2 production, in response to agonist pMHCII. Because these mutants preferentially localized CD4-LCK pairs to non-raft membrane fractions, one explanation for our results was that they impaired proximal signaling by sequestering LCK away from TCR-CD3. An alternative hypothesis is that the mutations directly impacted signaling because the motifs normally play an LCK-independent role in signaling. The goal of this study was to discriminate between these possibilities. Using T cell hybridomas, our results indicate that: intracellular CD4-LCK interactions are not necessary for pMHCII-specific signal initiation; the GGXXG and (C/F)CV+C motifs are key determinants of CD4-mediated pMHCII-specific signal amplification; the GGXXG and (C/F)CV+C motifs exert their functions independently of direct CD4-LCK association. These data provide a mechanistic explanation for why residues within these motifs are under purifying selection in jawed vertebrates. The results are also important to consider for biomimetic engineering of synthetic receptors.

Gaseous air pollutants and DNA methylation in a methylome-wide association study of an ethnically and environmentally diverse population of U.S. adults

Epigenetic mechanisms may underlie air pollution-health outcome associations. We estimated gaseous air pollutant-DNA methylation (DNAm) associations using twelve subpopulations within Women's Health Initiative (WHI) and Atherosclerosis Risk in Communities (ARIC) cohorts (n = 8397; mean age 61.3 years; 83% female; 46% African-American, 46% European-American, 8% Hispanic/Latino). We used geocoded participant address-specific mean ambient carbon monoxide (CO), nitrogen oxides (NO2; NOx), ozone (O3), and sulfur dioxide (SO2) concentrations estimated over the 2-, 7-, 28-, and 365-day periods before collection of blood samples used to generate Illumina 450 k array leukocyte DNAm measurements. We estimated methylome-wide, subpopulation- and race/ethnicity-stratified pollutant-DNAm associations in multi-level, linear mixed-effects models adjusted for sociodemographic, behavioral, meteorological, and technical covariates. We combined stratum-specific estimates in inverse variance-weighted meta-analyses and characterized significant associations (false discovery rate; FDR<0.05) at Cytosine-phosphate-Guanine (CpG) sites without among-strata heterogeneity (PCochran's Q > 0.05). We attempted replication in the Cooperative Health Research in Region of Augsburg (KORA) study and Normative Aging Study (NAS). We observed a -0.3 (95% CI: -0.4, -0.2) unit decrease in percent DNAm per interquartile range (IQR, 7.3 ppb) increase in 28-day mean NO2 concentration at cg01885635 (chromosome 3; regulatory region 290 bp upstream from ZNF621; FDR = 0.03). At intragenic sites cg21849932 (chromosome 20; LIME1; intron 3) and cg05353869 (chromosome 11; KLHL35; exon 2), we observed a -0.3 (95% CI: -0.4, -0.2) unit decrease (FDR = 0.04) and a 1.2 (95% CI: 0.7, 1.7) unit increase (FDR = 0.04), respectively, in percent DNAm per IQR (17.6 ppb) increase in 7-day mean ozone concentration. Results were not fully replicated in KORA and NAS. We identified three CpG sites potentially susceptible to gaseous air pollution-induced DNAm changes near genes relevant for cardiovascular and lung disease. Further harmonized investigations with a range of gaseous pollutants and averaging durations are needed to determine the effect of gaseous air pollutants on DNA methylation and ultimately gene expression.

Enhancing and inhibitory motifs regulate CD4 activity

CD4+ T cells use T cell receptor (TCR)-CD3 complexes, and CD4, to respond to peptide antigens within MHCII molecules (pMHCII). We report here that, through ~435 million years of evolution in jawed vertebrates, purifying selection has shaped motifs in the extracellular, transmembrane, and intracellular domains of eutherian CD4 that enhance pMHCII responses, and covary with residues in an intracellular motif that inhibits responses. Importantly, while CD4 interactions with the Src kinase, Lck, are viewed as key to pMHCII responses, our data indicate that CD4-Lck interactions derive their importance from the counterbalancing activity of the inhibitory motif, as well as motifs that direct CD4-Lck pairs to specific membrane compartments. These results have implications for the evolution and function of complex transmembrane receptors and for biomimetic engineering.

Transcriptome-wide association study in UK Biobank Europeans identifies associations with blood cell traits

Previous genome-wide association studies (GWAS) of hematological traits have identified over 10 000 distinct trait-specific risk loci. However, at these loci, the underlying causal mechanisms remain incompletely characterized. To elucidate novel biology and better understand causal mechanisms at known loci, we performed a transcriptome-wide association study (TWAS) of 29 hematological traits in 399 835 UK Biobank (UKB) participants of European ancestry using gene expression prediction models trained from whole blood RNA-seq data in 922 individuals. We discovered 557 gene-trait associations for hematological traits distinct from previously reported GWAS variants in European populations. Among the 557 associations, 301 were available for replication in a cohort of 141 286 participants of European ancestry from the Million Veteran Program. Of these 301 associations, 108 replicated at a strict Bonferroni adjusted threshold ($\alpha$= 0.05/301). Using our TWAS results, we systematically assigned 4261 out of 16 900 previously identified hematological trait GWAS variants to putative target genes. Compared to coloc, our TWAS results show reduced specificity and increased sensitivity in external datasets to assign variants to target genes.

Tyr192 Regulates Lymphocyte-Specific Tyrosine Kinase Activity in T Cells

TCR signaling critically depends on the tyrosine kinase Lck (lymphocyte-specific protein tyrosine kinase). Two phosphotyrosines, the activating pTyr³⁹⁴ and the inhibitory pTyr⁵⁰⁵, control Lck activity. Recently, pTyr¹⁹² in the Lck SH2 domain emerged as a third regulator. How pTyr¹⁹² may affect Lck function remains unclear. In this study, we explored the role of Lck Tyr¹⁹² using CRISPR/Cas9-targeted knock-in mutations in the human Jurkat T cell line. Our data reveal that both Lck pTyr³⁹⁴ and pTyr⁵⁰⁵ are controlled by Lck Tyr¹⁹² Lck with a nonphosphorylated SH2 domain (Lck Phe¹⁹²) displayed hyperactivity, possibly by promoting Lck Tyr³⁹⁴ transphosphorylation. Lck Glu¹⁹² mimicking stable Lck pTyr¹⁹² was inhibited by Tyr⁵⁰⁵ hyperphosphorylation. To overcome this effect, we further mutated Tyr⁵⁰⁵ The resulting Lck Glu¹⁹²/Phe⁵⁰⁵ displayed strongly increased amounts of pTyr³⁹⁴ both in resting and activated T cells. Our results suggest that a fundamental role of Lck pTyr¹⁹² may be to protect Lck pTyr³⁹⁴ and/or pTyr⁵⁰⁵ to maintain a pool of already active Lck in resting T cells. This provides an additional mechanism for fine-tuning of Lck as well as T cell activity.

Potentiating the Antitumor Activity of Cytotoxic T Cells via the Transmembrane Domain of IGSF4 That Increases TCR Avidity

A robust T-cell response is an important component of sustained antitumor immunity. In this respect, the avidity of TCR in the antigen-targeting of tumors is crucial for the quality of the T-cell response. This study reports that the transmembrane (TM) domain of immunoglobulin superfamily member 4 (IGSF4) binds to the TM of the CD3 ζ-chain through an interaction between His177 and Asp36, which results in IGSF4-CD3 ζ dimers. IGSF4 also forms homo-dimers through the GxxVA motif in the TM domain, thereby constituting large TCR clusters. Overexpression of IGSF4 lacking the extracellular (IG4ΔEXT) domain potentiates the OTI CD8+ T cells to release IFN-γ and TNF-α and to kill OVA+-B16F10 melanoma cells. In animal models, IG4ΔEXT significantly reduces B16F10 tumor metastasis as well as tumor growth. Collectively, the results indicate that the TM domain of IGSF4 can regulate TCR avidity, and they further demonstrate that TCR avidity regulation is critical for improving the antitumor activity of cytotoxic T cells.

The Transmembrane Adaptor Protein LIME Is Essential for Chemokine-Mediated Migration of Effector T Cells to Inflammatiory Sites

Lck-interacting transmembrane adaptor 1 (LIME) has been previously identified as a raft-associated transmembrane protein expressed predominantly in T and B lymphocytes. Although LIME is shown to transduce the immunoreceptor signaling and immunological synapse formation via its tyrosine phosphorylation by Lck, a Src-family kinase, the in vivo function of LIME has remained elusive in the previous studies. Here we report that LIME is preferentially expressed in effector T cells and mediates chemokine-mediated T cell migration. Interestingly, in LIME-/- mice, while T cell receptor stimulation-dependent proliferation, differentiation to effector T cells, cytotoxic T lymphocyte (CTL) function and regulatory T lymphocyte (Treg) function were normal, only T cell-mediated inflammatory response was significantly defective. The reduced inflammation was accompanied by the impaired infiltration of leukocytes and T cells to the inflammatory sites of LIME-/- mice. More specifically, the absence of LIME in effector T cells resulted in the reduced migration and defective morphological polarization in response to inflammatory chemokines such as CCL5 and CXCL10. Consistently, LIME-/- effector T cells were found to be defective in chemokine-mediated activation of Rac1 and Rap1, and dysregulated phosphorylation of Pyk2 and Cas. Taken together, the present findings show that LIME is a critical regulator of inflammatory chemokine-mediated signaling and the subsequent migration of effector T cells to inflammatory sites.

Protein Palmitoylation in Leukocyte Signaling and Function

Palmitoylation is a post-translational modification (PTM) based on thioester-linkage between palmitic acid and the cysteine residue of a protein. This covalent attachment of palmitate is reversibly and dynamically regulated by two opposing sets of enzymes: palmitoyl acyltransferases containing a zinc finger aspartate-histidine-histidine-cysteine motif (PAT-DHHCs) and thioesterases. The reversible nature of palmitoylation enables fine-tuned regulation of protein conformation, stability, and ability to interact with other proteins. More importantly, the proper function of many surface receptors and signaling proteins requires palmitoylation-meditated partitioning into lipid rafts. A growing number of leukocyte proteins have been reported to undergo palmitoylation, including cytokine/chemokine receptors, adhesion molecules, pattern recognition receptors, scavenger receptors, T cell co-receptors, transmembrane adaptor proteins, and signaling effectors including the Src family of protein kinases. This review provides the latest findings of palmitoylated proteins in leukocytes and focuses on the functional impact of palmitoylation in leukocyte function related to adhesion, transmigration, chemotaxis, phagocytosis, pathogen recognition, signaling activation, cytotoxicity, and cytokine production.

Transcript specific regulation of expression influences susceptibility to multiple sclerosis

Genome-wide association studies (GWAS) have identified over 100 loci containing single nucleotide variants (SNVs) that influence the risk of developing multiple sclerosis (MS). Most of these loci lie in non-coding regulatory regions of the genome that are active in immune cells and are therefore thought to modify risk by altering the expression of key immune genes. To explore this hypothesis we screened genes flanking MS-associated variants for evidence of allele specific expression (ASE) by quantifying the transcription of coding variants in linkage disequilibrium with MS-associated SNVs. In total, we were able to identify and successfully analyse 200 such coding variants (from 112 genes) in both CD4+ and CD8+ T cells from 106 MS patients and 105 controls. Fifty-six of these coding variants (from 43 genes) showed statistically significant evidence of ASE in one or both cell types. In the Lck interacting transmembrane adaptor 1 gene (LIME1), for example, we were able to show that in both cell types, the MS-associated variant rs2256814 increased the expression of some transcripts while simultaneously reducing the expression of other transcripts. In CD4+ cells from an additional independent set of 96 cases and 93 controls we were able to replicate the effect of this SNV on the balance of alternate LIME1 transcripts using qPCR (p = 5 × 10^-24). Our data thus indicate that some of the MS-associated SNVs identified by GWAS likely exert their effects on risk by distorting the balance of alternate transcripts rather than by changing the overall level of gene expression.

pTRAPs: Transmembrane adaptors in innate immune signaling

Transmembrane adaptor proteins (TRAPs) are protein scaffolds and signaling regulators with established roles in signal-induced activation of lymphocytes. A subset of the TRAP family, the palmitoylated TRAPs (pTRAPs), are increasingly emerging with additional roles in innate immune cells. Targeted to lipid rafts, tetraspannin-enriched microdomains, and protein microclusters in membranes, pTRAP scaffolds exert spatiotemporal regulation by recruiting signaling kinases, particularly Src and Syk family members, as well as Csk, and other effectors. In this way, pTRAPs modulate signaling and influence resulting cell responses, including the selective output of inflammatory cytokines and other mediators. Here, we review studies revealing that different pTRAPs work together, often with overlapping or redundant roles, for positive and negative regulation of key innate immune pathways, including Fc receptor and pattern recognition receptor signaling. Recent findings show that pTRAPs can bind directly to innate immune receptors, in addition to other transmembrane binding partners. Thus, pTRAPs are important, multifunctional scaffolds in pathways that are fundamental to diverse innate immune responses.

Epigenome-wide analysis in newborn blood spots from monozygotic twins discordant for cerebral palsy reveals consistent regional differences in DNA methylation

Background

Cerebral palsy (CP) is a clinical description for a group of motor disorders that are heterogeneous with respect to causes, symptoms and severity. A diagnosis of CP cannot usually be made at birth and in some cases may be delayed until 2–3 years of age. This limits opportunities for early intervention that could otherwise improve long-term outcomes. CP has been recorded in monozygotic twins discordant for the disorder, indicating a potential role of non-genetic factors such as intrauterine infection, hypoxia-ischaemia, haemorrhage and thrombosis. The aim of this exploratory study was to utilise the discordant monozygotic twin model to understand and measure epigenetic changes associated with the development of CP.

Methods

We performed a genome-wide analysis of DNA methylation using the Illumina Infinium Human Methylation 450 BeadChip array with DNA from newborn blood spots of 15 monozygotic twin pairs who later became discordant for CP. Quality control and data preprocessing were undertaken using the minfi R package. Differential methylation analysis was performed using the remove unwanted variation (RUVm) method, taking twin pairing into account in order to identify CP-specific differentially methylated probes (DMPs), and bumphunter was performed to identify differentially methylated regions (DMRs).

Results

We identified 33 top-ranked DMPs based on a nominal p value cut-off of p < 1 × 10⁻⁴ and two DMRs (p < 1 × 10⁻³) associated with CP. The top-ranked probes related to 25 genes including HNRNPL, RASSF5, CD3D and KALRN involved in immune signalling pathways, in addition to TBC1D24, FBXO9 and VIPR2 previously linked to epileptic encephalopathy. Gene ontology and pathway analysis of top-ranked DMP-associated genes revealed enrichment of inflammatory signalling pathways, regulation of cytokine secretion and regulation of leukocyte-mediated immunity. We also identified two top-ranked DMRs including one on chromosome 6 within the promoter region of LTA gene encoding tumour necrosis factor-beta (TNF-β), an important regulator of inflammation and brain development. The second was within the transcription start site of the LIME1 gene, which plays a key role in inflammatory pathways such as MAPK signalling. CP-specific differential DNA methylation within one of our two top DMRs was validated using an independent platform, MassArray EpiTyper.

Conclusions

Ours is the first epigenome-wide association study of CP in disease-discordant monozygotic twin pairs and suggests a potential role for immune dysfunction in this condition.

Electronic supplementary material

The online version of this article (10.1186/s13148-018-0457-4) contains supplementary material, which is available to authorized users.

P-glycoprotein attenuates DNA repair activity in multidrug-resistant cells by acting through the Cbp-Csk-Src cascade

Recent studies have demonstrated that P-glycoprotein (P-gp) expression impairs DNA interstrand cross-linking agent-induced DNA repair efficiency in multidrug-resistant (MDR) cells. To date, the detailed molecular mechanisms underlying how P-gp interferes with Src activation and subsequent DNA repair activity remain unclear. In this study, we determined that the C-terminal Src kinase-binding protein (Cbp) signaling pathway involved in the negative control of Src activation is enhanced in MDR cells. We also demonstrated that cells that ectopically express P-gp exhibit reduced activation of DNA damage response regulators, such as ATM, Chk2, Braca1 and Nbs1 and hence attenuated DNA double-strand break repair capacity and become more susceptible than vector control cells to DNA interstrand cross-linking (ICL) agents. Moreover, we demonstrated that P-gp can not only interact with Cbp and Src but also enhance the formation of inhibitory C-terminal Src kinase (Csk)-Cbp complexes that reduce phosphorylation of the Src activation residue Y416 and increase phosphorylation of the Src negative regulatory residue Y527. Notably, suppression of Cbp expression in MDR cells restores cisplatin-induced Src activation, improves DNA repair capacity, and increases resistance to ICL agents. Ectopic expression of Cbp attenuates cisplatin-induced Src activation and increases the susceptibility of cells to ICL agents. Together, the current results indicate that P-gp inhibits DNA repair activity by modulating Src activation via Cbp-Csk-Src cascade. These results suggest that DNA ICL agents are likely to have therapeutic potential against MDR cells with P-gp-overexpression.

Csk-binding protein controls red blood cell development via regulation of Lyn tyrosine kinase activity

Erythropoiesis is controlled principally through erythropoietin (Epo) receptor signaling, which involves Janus kinase 2 (JAK2) and Lyn tyrosine kinase, both of which are important for regulating red blood cell (RBC) development. Negative regulation of Lyn involves C-Src kinase (Csk)-mediated phosphorylation of its C-terminal tyrosine, which is facilitated by the transmembrane adaptor Csk-binding protein (Cbp). Although Cbp has significant functions in controlling Lyn levels and activity in erythroid cells in vitro, its importance to primary erythroid cell development and signaling has remained unclear. To address this, we assessed the consequence of loss of Cbp on the erythroid compartment in vivo and whether Epo-responsive cells isolated from Cbp-knockout mice exhibited altered signaling. Our data show that male Cbp^-/- mice display a modest but significant alteration to late erythroid development in bone marrow with evidence of increased erythrocytes in the spleen, whereas female Cbp^-/- mice exhibit a moderate elevation in early erythroid progenitors (not seen in male mice) that does not influence the later steps in RBC development. In isolated primary erythroid cells and cell lines generated from Cbp^-/- mice, survival signaling through Lyn/Akt/FoxO3 was elevated, resulting in sustained viability during differentiation. The high Akt activity disrupted GAB2/SHP-2 feedback inhibition of Lyn; however, the elevated Lyn activity also increased inhibitory signaling via SHP-1 to restrict the Erk1/2 pathway. Interestingly, whereas loss of Cbp led to mild changes to late RBC development in male mice, this was not apparent in female Cbp^-/- mice, possibly due to their elevated estrogen, which is known to facilitate early progenitor self-renewal.

C1q and HMGB1 reciprocally regulate human macrophage polarization

A healthy immune system results from a balance of stimulatory and inhibitory pathways that allow effective responses to acute insults, without descending into chronic inflammation. Failed homeostasis is characteristic of autoimmune diseases such as systemic lupus erythematosus. Although HMGB1 induces proinflammatory M1-like macrophage differentiation, we describe a mechanism by which C1q modulates this activity and collaborates with HMGB1 to induce the differentiation of monocytes to anti-inflammatory M2-like macrophages. These anti-inflammatory macrophages are unresponsive to dendritic cell induction factors, effectively removing them from participation in an adaptive immune response. This pathway is mediated through a complex with RAGE and LAIR-1 and depends on relative levels of C1q and HMGB1. Importantly, these data provide insight into a homeostatic mechanism in which C1q and HMGB1 can cooperate to terminate inflammation, and which may be impaired in C1q-deficient patients with autoimmune disease.

Early onset combined immunodeficiency and autoimmunity in patients with loss-of-function mutation in LAT

Keller et al. describe for the first time human LAT deficiency, which causes severe immune dysregulation with autoimmunity, lymphoproliferation, and progressive immunodeficiency.

The adapter protein linker for activation of T cells (LAT) is a critical signaling hub connecting T cell antigen receptor triggering to downstream T cell responses. In this study, we describe the first kindred with defective LAT signaling caused by a homozygous mutation in exon 5, leading to a premature stop codon deleting most of the cytoplasmic tail of LAT, including the critical tyrosine residues for signal propagation. The three patients presented from early childhood with combined immunodeficiency and severe autoimmune disease. Unlike in the mouse counterpart, reduced numbers of T cells were present in the patients. Despite the reported nonredundant role of LAT in Ca²⁺ mobilization, residual T cells were able to induce Ca²⁺ influx and nuclear factor (NF) κB signaling, whereas extracellular signal-regulated kinase (ERK) signaling was completely abolished. This is the first report of a LAT-related disease in humans, manifesting by a progressive combined immune deficiency with severe autoimmune disease.

Functional evidence for TCR-intrinsic specificity for MHCII

How T cells become restricted to binding antigenic peptides within class I or class II major histocompatibility complex molecules (pMHCI or pMHCII, respectively) via clonotypic T-cell receptors (TCRs) remains debated. During development, if TCR-pMHC interactions exceed an affinity threshold, a signal is generated that positively selects the thymocyte to become a mature CD4(+) or CD8(+) T cell that can recognize foreign peptides within MHCII or MHCI, respectively. But whether TCRs possess an intrinsic, subthreshold specificity for MHC that facilitates sampling of the peptides within MHC during positive selection or T-cell activation is undefined. Here we asked if increasing the frequency of lymphocyte-specific protein tyrosine kinase (Lck)-associated CD4 molecules in T-cell hybridomas would allow for the detection of subthreshold TCR-MHC interactions. The reactivity of 10 distinct TCRs was assessed in response to selecting and nonselecting MHCII bearing cognate, null, or "shaved" peptides with alanine substitutions at known TCR contact residues: Three of the TCRs were selected on MHCII and have defined peptide specificity, two were selected on MHCI and have a known pMHC specificity, and five were generated in vitro without defined selecting or cognate pMHC. Our central finding is that IL-2 was made when each TCR interacted with selecting or nonselecting MHCII presenting shaved peptides. These responses were abrogated by anti-CD4 antibodies and mutagenesis of CD4. They were also inhibited by anti-MHC antibodies that block TCR-MHCII interactions. We interpret these data as functional evidence for TCR-intrinsic specificity for MHCII.

Diverse Gene Expression in Human Regulatory T Cell Subsets Uncovers Connection between Regulatory T Cell Genes and Suppressive Function

Regulatory T (Treg) cells have a critical role in the control of immunity, and their diverse subpopulations may allow adaptation to different types of immune responses. In this study, we analyzed human Treg cell subpopulations in the peripheral blood by performing genome-wide expression profiling of 40 Treg cell subsets from healthy donors. We found that the human peripheral blood Treg cell population is comprised of five major genomic subgroups, represented by 16 tractable subsets with a particular cell surface phenotype. These subsets possess a range of suppressive function and cytokine secretion and can exert a genomic footprint on target effector T (Teff) cells. Correlation analysis of variability in gene expression in the subsets identified several cell surface molecules associated with Treg suppressive function, and pharmacological interrogation revealed a set of genes having causative effect. The five genomic subgroups of Treg cells imposed a preserved pattern of gene expression on Teff cells, with a varying degree of genes being suppressed or induced. Notably, there was a cluster of genes induced by Treg cells that bolstered an autoinhibitory effect in Teff cells, and this induction appears to be governed by a different set of genes than ones involved in counteracting Teff activation. Our work shows an example of exploiting the diversity within human Treg cell subpopulations to dissect Treg cell biology.

Membrane raft association is a determinant of plasma membrane localization

The lipid raft hypothesis proposes lateral domains driven by preferential interactions between sterols, sphingolipids, and specific proteins as a central mechanism for the regulation of membrane structure and function; however, experimental limitations in defining raft composition and properties have prevented unequivocal demonstration of their functional relevance. Here, we establish a quantitative, functional relationship between raft association and subcellular protein sorting. By systematic mutation of the transmembrane and juxtamembrane domains of a model transmembrane protein, linker for activation of T-cells (LAT), we generated a panel of variants possessing a range of raft affinities. These mutations revealed palmitoylation, transmembrane domain length, and transmembrane sequence to be critical determinants of membrane raft association. Moreover, plasma membrane (PM) localization was strictly dependent on raft partitioning across the entire panel of unrelated mutants, suggesting that raft association is necessary and sufficient for PM sorting of LAT. Abrogation of raft partitioning led to mistargeting to late endosomes/lysosomes because of a failure to recycle from early endosomes. These findings identify structural determinants of raft association and validate lipid-driven domain formation as a mechanism for endosomal protein sorting.

Palmitoylated transmembrane adaptor proteins in leukocyte signaling

Transmembrane adaptor proteins (TRAPs) are structurally related proteins that have no enzymatic function, but enable inducible recruitment of effector molecules to the plasma membrane, usually in a phosphorylation dependent manner. Numerous surface receptors employ TRAPs for either propagation or negative regulation of the signal transduction. Several TRAPs (LAT, NTAL, PAG, LIME, PRR7, SCIMP, LST1/A, and putatively GAPT) are known to be palmitoylated that could facilitate their localization in lipid rafts or tetraspanin enriched microdomains. This review summarizes expression patterns, binding partners, signaling pathways, and biological functions of particular palmitoylated TRAPs with an emphasis on the three most recently discovered members, PRR7, SCIMP, and LST1/A. Moreover, we discuss in silico methodology used for discovery of new family members, nature of their binding partners, and microdomain localization.

The role of membrane rafts in Lck transport, regulation and signalling in T-cells

Tyrosine phosphorylation is one of the key covalent modifications that occur in multicellular organisms. Since its discovery more than 30 years ago, tyrosine phosphorylation has come to be understood as a fundamentally important mechanism of signal transduction and regulation in all eukaryotic cells. The tyrosine kinase Lck (lymphocyte-specific protein tyrosine kinase) plays a crucial role in the T-cell response by transducing early activation signals triggered by TCR (T-cell receptor) engagement. These signals result in the phosphorylation of immunoreceptor tyrosine-based activation motifs present within the cytosolic tails of the TCR-associated CD3 subunits that, once phosphorylated, serve as scaffolds for the assembly of a large supramolecular signalling complex responsible for T-cell activation. The existence of membrane nano- or micro-domains or rafts as specialized platforms for protein transport and cell signalling has been proposed. The present review discusses the signals that target Lck to membrane rafts and the importance of these specialized membranes in the transport of Lck to the plasma membrane, the regulation of Lck activity and the phosphorylation of the TCR.

Differential polarization of C-terminal Src kinase between naive and antigen-experienced CD8+ T cells

In CD8(+) T cells, engagement of the TCR with agonist peptide:MHC molecules causes dynamic redistribution of surface molecules including the CD8 coreceptor to the immunological synapse. CD8 associates with the Src-family kinase (SFK) Lck, which, in turn, initiates the rapid tyrosine phosphorylation events that drive cellular activation. Compared with naive T cells, Ag-experienced CD8(+) T cells make shorter contacts with APC, are less dependent on costimulation, and are triggered by lower concentrations of Ag, yet the molecular basis of this more efficient response of memory T cells is not fully understood. In this article, we show differences between naive and Ag-experienced CD8(+) T cells in colocalization of the SFKs and their negative regulator, C-terminal Src kinase (Csk). In naive CD8(+) T cells, there was pronounced colocalization of SFKs and Csk at the site of TCR triggering, whereas in Ag-experienced cells, Csk displayed a bipolar distribution with a proportion of the molecules sequestered within a cytosolic area in the distal pole of the cell. The data show that there is differential redistribution of a key negative regulator away from the site of TCR engagement in Ag-experienced CD8(+) T cells, which might be associated with the more efficient responses of these cells on re-exposure to Ag.

How does the kinase Lck phosphorylate the T cell receptor? Spatial organization as a regulatory mechanism

T cell signaling begins with the ligation of the T cell antigen receptor (TCR) by a cognate peptide and the phosphorylation of the receptor’s immunoreceptor tyrosine-based activation motif domains by the kinase Lck. However, the canonical receptor model is insufficient to explain how the constitutively active kinase Lck can discriminate between non-ligated and ligated TCRs. Here, we discuss the factors that are thought to regulate the spatial distribution of the TCR and Lck, and therefore critically influence TCR signaling initiation.

LIME mediates immunological synapse formation through activation of VAV

Lck Interacting Membrane protein (LIME) was previously characterized as a transmembrane adaptor protein mediating TCR-dependent T cell activation. Here, we show that LIME associates with Vav in response to TCR stimulation and is required for Vav guanine nucleotide exchange factor (GEF) activity for Rac1. Consistent with this finding, actin polymerization at the immunological synapse (IS) was markedly enhanced by overexpression of LIME, but was reduced by expression of a LIME shRNA. Moreover, TCR-mediated cell adhesion to ICAM-1, laminin, or fibronectin was downregulated by expression of LIME shRNA. In addition, in the IS, LIME but not LAT was found to localize at the peripheral-supramolecular activation cluster (p-SMAC) where the integrins were previously shown to be localized. Together, these results establish LIME as a transmembrane adaptor protein linking TCR stimulation to IS formation and integrin activation through activation of Vav.

IGSF4 is a novel TCR ζ-chain-interacting protein that enhances TCR-mediated signaling

Immunoglobulin superfamily member 4 (IGSF4) is a known ligand of CRTAM, a receptor expressed in activated NKT and CD8(+) T cells, but its function in T cell immunity has not been elucidated. In this study, we show that IGSF4 directly interacts with the T cell receptor (TCR) ζ-chain and enhances TCR signaling by enhancing ζ-chain phosphorylation. Ectopic overexpression of IGSF4 enhances TCR-mediated T cell activation. In contrast, IGSF4 knockdown shows a dramatic decrease in markers associated with T cell activation compared with those in control small interfering RNA. The transmembrane domain is essential for TCR ζ-chain association and clustering to the immunological synapse, and the ectodomain is associated with T cell interaction with antigen-presenting cells (APCs). IGSF4-deficient mice have impaired TCR-mediated thymocyte selection and maturation. Furthermore, these mice reveal attenuated effector T cell functions accompanied by defective TCR signaling. Collectively, the results indicate that IGSF4 plays a central role in T cell functioning by dual independent mechanisms, control of TCR signaling and control of T cell-APC interaction.

SCIMP, a transmembrane adaptor protein involved in major histocompatibility complex class II signaling

Formation of the immunological synapse between an antigen-presenting cell (APC) and a T cell leads to signal generation in both cells involved. In T cells, the lipid raft-associated transmembrane adaptor protein LAT plays a central role. Its phosphorylation is a crucial step in signal propagation, including the calcium response and mitogen-activated protein kinase activation, and largely depends on its association with the SLP76 adaptor protein. Here we report the discovery of a new palmitoylated transmembrane adaptor protein, termed SCIMP. SCIMP is expressed in B cells and other professional APCs and is localized in the immunological synapse due to its association with tetraspanin-enriched microdomains. In B cells, it is constitutively associated with Lyn kinase and becomes tyrosine phosphorylated after major histocompatibility complex type II (MHC-II) stimulation. When phosphorylated, SCIMP binds to the SLP65 adaptor protein and also to the inhibitory kinase Csk. While the association with SLP65 initiates the downstream signaling cascades, Csk binding functions as a negative regulatory loop. The results suggest that SCIMP is involved in signal transduction after MHC-II stimulation and therefore serves as a regulator of antigen presentation and other APC functions.

A tale of two TRAPs: LAT and LAB in the regulation of lymphocyte development, activation, and autoimmunity

Transmembrane adaptor proteins (TRAPs) link antigen receptor engagement to downstream cellular processes. Although these proteins typically lack intrinsic enzymatic activity, they are phosphorylated on multiple tyrosine residues following lymphocyte activation, allowing them to function as scaffolds for the assembly of multi-molecular signaling complexes. Among the many TRAPs that have been discovered in recent years, the LAT (linker for activation of T cells) family of adaptor proteins plays an important role in the positive and negative regulation of lymphocyte maturation, activation, and differentiation. Of the two members in this family, LAT is an indispensable component controlling T cell and mast cell activation and function; LAB (linker for activation of B cells), also called NTAL, is necessary to fine-tune lymphocyte activation and may be a key regulator of innate immune responses. Here, we review recent advances on the function of LAT and LAB in the regulation of development and activation of immune cells.

Phosphoprotein associated with glycosphingolipid-enriched microdomains differentially modulates SRC kinase activity in brain maturation

Src family kinases (SFK) control multiple processes during brain development and function. We show here that the phosphoprotein associated with glycosphigolipid-enriched microdomains (PAG)/Csk binding protein (Cbp) modulates SFK activity in the brain. The timing and localization of PAG expression overlap with Fyn and Src, both of which we find associated to PAG. We demonstrate in newborn (P1) mice that PAG negatively regulates Src family kinases (SFK). P1 Pag1^-/- mouse brains show decreased recruitment of Csk into lipid rafts, reduced phosphorylation of the inhibitory tyrosines within SFKs, and an increase in SFK activity of >/ = 50%. While in brain of P1 mice, PAG and Csk are highly and ubiquitously expressed, little Csk is found in adult brain suggesting altered modes of SFK regulation. In adult brain Pag1-deficiency has no effect upon Csk-distribution or inhibitory tyrosine phosphorylation, but kinase activity is now reduced (−20–30%), pointing to the development of a compensatory mechanism that may involve PSD93. The distribution of the Csk-homologous kinase CHK is not altered. Importantly, since the activities of Fyn and Src are decreased in adult Pag1^-/- mice, thus presenting the reversed phenotype of P1, this provides the first in vivo evidence for a Csk-independent positive regulatory function for PAG in the brain.

Associations of high-grade glioma with glioma risk alleles and histories of allergy and smoking

Glioma risk has consistently been inversely associated with allergy history but not with smoking history despite putative biologic plausibility. Data from 855 high-grade glioma cases and 1,160 controls from 4 geographic regions of the United States during 1997-2008 were analyzed for interactions between allergy and smoking histories and inherited variants in 5 established glioma risk regions: 5p15.3 (TERT), 8q24.21 (CCDC26/MLZE), 9p21.3 (CDKN2B), 11q23.3 (PHLDB1/DDX6), and 20q13.3 (RTEL1). The inverse relation between allergy and glioma was stronger among those who did not (odds ratio(allergy-glioma) = 0.40, 95% confidence interval: 0.28, 0.58) versus those who did (odds ratio(allergy-glioma) = 0.76, 95% confidence interval: 0.59, 0.97; P(interaction) = 0.02) carry the 9p21.3 risk allele. However, the inverse association with allergy was stronger among those who carried (odds ratio(allergy-glioma) = 0.44, 95% confidence interval: 0.29, 0.68) versus those who did not carry (odds ratio(allergy-glioma) = 0.68, 95% confidence interval: 0.54, 0.86) the 20q13.3 glioma risk allele, but this interaction was not statistically significant (P = 0.14). No relation was observed between glioma risk and smoking (odds ratio = 0.92, 95% confidence interval: 0.77, 1.10; P = 0.37), and there were no interactions for glioma risk of smoking history with any of the risk alleles. The authors' observations are consistent with a recent report that the inherited glioma risk variants in chromosome regions 9p21.3 and 20q13.3 may modify the inverse association of allergy and glioma.

PRR7 is a transmembrane adaptor protein expressed in activated T cells involved in regulation of T cell receptor signaling and apoptosis

Transmembrane adaptor proteins (TRAPs) are important organizers and regulators of immunoreceptor-mediated signaling. A bioinformatic search revealed several potential novel TRAPs, including a highly conserved protein, proline rich 7 (PRR7), previously described as a component of the PSD-95/N-methyl-d-aspartate receptor protein complex in postsynaptic densities (PSD) of rat neurons. Our data demonstrate that PRR7 is weakly expressed in other tissues but is readily up-regulated in activated human peripheral blood lymphocytes. Transient overexpression of PRR7 in Jurkat T cell line led to gradual apoptotic death dependent on the WW domain binding motif surrounding Tyr-166 in the intracellular part of PRR7. To circumvent the pro-apoptotic effect of PRR7, we generated Jurkat clones with inducible expression of PRR7 (J-iPRR7). In these cells acute induction of PRR7 expression had a dual effect. It resulted in up-regulation of the transcription factor c-Jun and the activation marker CD69 as well as enhanced production of IL-2 after phorbol 12-myristate 13-acetate (PMA) and ionomycin treatment. On the other hand, expression of PRR7 inhibited general tyrosine phosphorylation and calcium influx after T cell receptor cross-linking by antibodies. Moreover, we found PRR7 constitutively tyrosine-phosphorylated and associated with Src. Collectively, these data indicate that PRR7 is a potential regulator of signaling and apoptosis in activated T cells.

Dynamic palmitoylation and the role of DHHC proteins in T cell activation and anergy

Although protein S-palmitoylation was first characterized >30 years ago, and is implicated in the function, trafficking, and localization of many proteins, little is known about the regulation and physiological implications of this posttranslational modification. Palmitoylation of various signaling proteins required for TCR-induced T cell activation is also necessary for their proper function. Linker for activation of T cells (LAT) is an essential scaffolding protein involved in T cell development and activation, and we found that its palmitoylation is selectively impaired in anergic T cells. The recent discovery of the DHHC family of palmitoyl acyl transferases and the establishment of sensitive and quantitative proteomics-based methods for global analysis of the palmitoyl proteome led to significant progress in studying the biology and underlying mechanisms of cellular protein palmitoylation. We are using these approaches to explore the palmitoyl proteome in T lymphocytes and, specifically, the mechanistic basis for the impaired palmitoylation of LAT in anergic T cells. This chapter reviews the history of protein palmitoylation and its role in T cell activation, the DHHC family and new methodologies for global analysis of the palmitoyl proteome, and summarizes our recent work in this area. The new methodologies will accelerate the pace of research and provide a greatly improved mechanistic and molecular understanding of the complex process of protein palmitoylation and its regulation, and the substrate specificity of the novel DHHC family. Reversible protein palmitoylation will likely prove to be an important posttranslational mechanism that regulates cellular responses, similar to protein phosphorylation and ubiquitination.

The effects of membrane compartmentalization of csk on TCR signaling

The TCR signal transduction is initiated by the activation of Src-family kinases (SFK) which phosphorylate Immunoreceptor tyrosine-based activation motifs (ITAM) present in the intracellular parts of the T-cell receptor (TCR) signaling subunits. Numerous data suggest that after stimulation TCR interacts with membrane rafts and thus it gains access to SFK and other important molecules involved in signal transduction. However, the precise mechanism of this process is unclear. One of the key questions is how SFK access TCR and what is the importance of non-raft and membrane raft-associated SFK for the initiation and maintenance of the TCR signaling. To answer this question we targeted a negative regulator of SFK, C-terminal Src kinase (Csk) to membrane rafts, recently described "heavy rafts" or non-raft membrane. Our data show that only Csk targeted into "classical" raft but not to "heavy raft" or non-raft membrane effectively inhibits TCR signaling, demonstrating the critical role of membrane raft-associated SFK in this process.

Regulation of lymphocyte development and activation by the LAT family of adapter proteins

Transmembrane adapter proteins (TRAPs) are critical components of signaling pathways in lymphocytes, linking antigen receptor engagement to downstream cellular processes. While these proteins lack intrinsic enzymatic activity, their phosphorylation following receptor ligation allows them to function as scaffolds for the assembly of multi-molecular signaling complexes. Many TRAPs have recently been discovered, and numerous studies demonstrate their roles in the positive and negative regulation of lymphocyte maturation, activation, and differentiation. One such example is the linker for activation of T cells (LAT) family of adapter proteins. While LAT has been shown to play an indispensable role in T-cell and mast cell function, the other family members, linker for activation of B cells (LAB) and linker for activation of X cells (LAX), are necessary to fine-tune immune responses. In addition to its well-established role in the positive regulation of lymphocyte activation, LAT exerts an inhibitory effect on T-cell receptor-mediated signaling. Furthermore, LAT, along with LAB and LAX, plays a crucial role in establishing and maintaining tolerance. Here, we review recent data concerning the regulation of lymphocyte development and activation by the LAT family of proteins.

The adaptor protein LAD/TSAd mediates laminin-dependent T cell migration via association with the 67 kDa laminin binding protein

The adaptor protein, LAD/TSAd, plays essential roles in T cell activation. To further understand the functions of this protein, we performed yeast two-hybrid screening using TSAd as bait and identified 67 kDa laminin binding protein (LBP) as the interacting partner. Subsequently, TSAd-LBP interaction was confirmed in D1.1 T cell line. Upon costimulation by T cell receptor (TCR) plus laminin crosslinking or TCR plus integrin alpha6 crosslinking, LBP was coimmunoprecipitated with TSAd. Moreover, TCR plus laminin costimulation-dependent T cell migration was enhanced in D1.1 T cells overexpressing TSAd but was disrupted in D1.1 cells overexpressing dominant negative form of TSAd or TSAd shRNA. These data show that, upon TCR plus integrin costimulation, TSAd associates with LBP and mediates T lymphocyte migration.

SOCS-6 negatively regulates T cell activation through targeting p56lck to proteasomal degradation

The T cell-specific tyrosine kinase, p56(lck), plays crucial roles in T cell receptor (TCR)-mediated T cell activation. Here, we report that SOCS-6 (suppressor of cytokine signaling-6) is a negative regulator of p56(lck). SOCS-6 was identified as a protein binding to the kinase domain of p56(lck) through yeast two-hybrid screening. SOCS-6 bound specifically to p56(lck) (F505), which mimics the active form of p56(lck), but not to wild type p56(lck). In Jurkat T cells, SOCS-6 binding to p56(lck) was detected 1-2 h after TCR stimulation. Confocal microscopy showed that upon APC-T cell conjugation, SOCS-6 was recruited to the immunological synapse and colocalized with the active form of p56(lck). SOCS-6 promoted p56(lck) ubiquitination and its subsequent targeting to the proteasome. Moreover, SOCS-6 overexpression led to repression of TCR-dependent interleukin-2 promoter activity. These results establish that SOCS-6 acts as a negative regulator of T cell activation by promoting ubiquitin-dependent proteolysis.

Transforming potential of Src family kinases is limited by the cholesterol-enriched membrane microdomain

The upregulation of Src family kinases (SFKs) has been implicated in cancer progression, but the molecular mechanisms regulating their transforming potentials remain unclear. Here we show that the transforming ability of all SFK members is suppressed by being distributed to the cholesterol-enriched membrane microdomain. All SFKs could induce cell transformation when overexpressed in C-terminal Src kinase (Csk)-deficient fibroblasts. However, their transforming abilities varied depending on their affinity for the microdomain. c-Src and Blk, with a weak affinity for the microdomain due to a single myristate modification at the N terminus, could efficiently induce cell transformation, whereas SFKs with both myristate and palmitate modifications were preferentially distributed to the microdomain and required higher doses of protein expression to induce transformation. In contrast, disruption of the microdomain by depleting cholesterol could induce a robust transformation in Csk-deficient fibroblasts in which only a limited amount of activated SFKs was expressed. Conversely, the addition of cholesterol or recruitment of activated SFKs to the microdomain via a transmembrane adaptor, Cbp/PAG1, efficiently suppressed SFK-induced cell transformation. These findings suggest that the membrane microdomain spatially limits the transforming potential of SFKs by sequestering them away from the transforming pathways.

Transmembrane adaptor proteins positively regulating the activation of lymphocytes

Engagement of the immunoreceptors initiates signaling cascades resulting in lymphocyte activation and differentiation to effector cells, which are essential for the elimination of pathogens from the body. For the transduction of these immunoreceptor-mediated signals, several linker proteins termed transmembrane adaptor proteins (TRAPs) were shown to be required. TRAPs serve as platforms for the assembly and membrane targeting of the specific signaling proteins. Among seven TRAPs identified so far, LAT and LIME were shown to act as a positive regulator in TCR-mediated signaling pathways. In this review, we will discuss the functions of LAT and LIME in modulating T cell development, activation and differentiation.

SHP-1 and SHP-2 in T cells: two phosphatases functioning at many levels

Tyrosine phosphorylation and dephosphorylation of proteins play a critical role for many T-cell functions. The opposing actions of protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPs) determine the level of tyrosine phosphorylation at any time. It is well accepted that PTKs are essential during T-cell signaling; however, the role and importance of PTPs are much less known and appreciated. Both transmembrane and cytoplasmic tyrosine phosphatases have been identified in T cells and shown to regulate T-cell responses. This review focuses on the roles of the two cytoplasmic PTPs, the Src-homology 2 domain (SH2)-containing SHP-1 and SHP-2, in T-cell signaling, development, differentiation, and function.

Signaling defects in anti-tumor T cells

The immune response to cancer has been long recognized, including both innate and adaptive responses, showing that the immune system can recognize protein products of genetic and epigenetic changes in transformed cells. The accumulation of antigen-specific T cells within the tumor, the draining lymph node, and the circulation, either in newly diagnosed patients or resultant from experimental immunotherapy, proves that tumors produce antigens and that priming occurs. Unfortunately, just as obviously, tumors grow, implying that anti-tumor immune responses are either not sufficiently vigorous to eliminate the cancer or that anti-tumor immunity is suppressed. Both possibilities are supported by current data. In experimental animal models of cancer and also in patients, systemic immunity is usually not dramatically suppressed, because tumor-bearing animals and patients develop T-cell-dependent immune responses to microbes and to either model antigens or experimental cancer vaccines. However, inhibition of specific anti-tumor immunity is common, and several possible explanations of tolerance to tumor antigens or tumor-induced immunesuppression have been proposed. Inhibition of effective anti-tumor immunity results from the tumor or the host response to tumor growth, inhibiting the activation, differentiation, or function of anti-tumor immune cells. As a consequence, anti-tumor T cells cannot respond productively to developmental, targeting, or activation cues. While able to enhance the number and phenotype of anti-tumor T cells, the modest success of immunotherapy has shown the necessity to attempt to reverse tolerance in anti-tumor T cells, and the vanguard of experimental therapy now focuses on vaccination in combination with blockade of immunosuppressive mechanisms. This review discusses several potential mechanisms by which anti-tumor T cells may be inhibited in function.

Lipid rafts and T-lymphocyte function: implications for autoimmunity

Experimental evidence indicates that the mammalian cell membrane is compartmentalized. A structural feature that supports membrane segmentation implicates assemblies of selected lipids broadly referred to as lipid rafts. In T-lymphocytes, lipid rafts are implicated in signalling from the T-cell antigen receptor (TCR) and in localization and function of proteins residing proximal to the receptor. This review summarizes the current literature that deals with lipid raft involvement in T-cell activation and places particular emphasis in recent studies investigating lipid rafts in autoimmunity. The potential of lipid rafts as targets for the development of a new class of immune-modulating compounds is discussed.