KMT2D regulates activation, localization, and integrin expression by T-cells.

Individuals with Kabuki syndrome present with immunodeficiency; however, how pathogenic variants in the gene encoding the histone-modifying enzyme lysine methyltransferase 2D (KMT2D) lead to immune alterations remain poorly understood. Following up on our prior report of KMT2D-altered integrin expression in B-cells, we performed targeted analyses of KMT2D's influence on integrin expression in T-cells throughout development (thymocytes through peripheral T-cells) in murine cells with constitutive- and conditional-targeted Kmt2d deletion. Using high-throughput RNA-sequencing and flow cytometry, we reveal decreased expression (both at the transcriptional and translational levels) of a cluster of leukocyte-specific integrins, which perturb aspects of T-cell activation, maturation, adhesion/localization, and effector function. H3K4me3 ChIP-PCR suggests that these evolutionary similar integrins are under direct control of KMT2D. KMT2D loss also alters multiple downstream programming/signaling pathways, including integrin-based localization, which can influence T-cell populations. We further demonstrated that KMT2D deficiency is associated with the accumulation of murine CD8+ single-positive (SP) thymocytes and shifts in both human and murine peripheral T-cell populations, including the reduction of the CD4+ recent thymic emigrant (RTE) population. Together, these data show that the targeted loss of Kmt2d in the T-cell lineage recapitulates several distinct features of Kabuki syndrome-associated immune deficiency and implicates epigenetic mechanisms in the regulation of integrin signaling.

A Novel Homozygous RHOH Variant Associated with T Cell Dysfunction and Recurrent Opportunistic Infections.

RHOH, an atypical small GTPase predominantly expressed in hematopoietic cells, plays a vital role in immune function. A deficiency in RHOH has been linked to epidermodysplasia verruciformis, lung disease, Burkitt lymphoma and T cell defects. Here, we report a novel germline homozygous RHOH c.245G > A (p.Cys82Tyr) variant in a 21-year-old male suffering from recurrent, invasive, opportunistic infections affecting the lungs, eyes, and brain. His sister also succumbed to a lung infection during early adulthood. The patient exhibited a persistent decrease in CD4+ T, B, and NK cell counts, and hypoimmunoglobulinemia. The patient's T cell showed impaired activation upon in vitro TCR stimulation. In Jurkat T cells transduced with RHOHC82Y, a similar reduction in activation marker CD69 up-regulation was observed. Furthermore, the C82Y variant showed reduced RHOH protein expression and impaired interaction with the TCR signaling molecule ZAP70. Together, these data suggest that the newly identified autosomal-recessive RHOH variant is associated with T cell dysfunction and recurrent opportunistic infections, functioning as a hypomorph by disrupting ZAP70-mediated TCR signaling.

Role of Rad18 in B cell activation and lymphomagenesis.

Maintenance of genome integrity is instrumental in preventing cancer. In addition to DNA repair pathways that prevent damage to DNA, damage tolerance pathways allow for the survival of cells that encounter DNA damage during replication. The Rad6/18 pathway is instrumental in this process, mediating damage bypass by ubiquitination of proliferating cell nuclear antigen. Previous studies have shown different roles of Rad18 in vivo and in tumorigenesis. Here, we show that B cells induce Rad18 expression upon proliferation induction. We have therefore analysed the role of Rad18 in B cell activation as well as in B cell lymphomagenesis mediated by an Eµ-Myc transgene. We find no activation defects or survival differences between Rad18 WT mice and two different models of Rad18 deficient tumour mice. Also, tumour subtypes do not differ between the mouse models. Accordingly, functions of Rad18 in B cell activation and tumorigenesis may be compensated for by other pathways in B cells.

Unsynchronized butyrophilin molecules dictate cancer cell evasion of Vγ9Vδ2 T-cell killing.

Vγ9Vδ2 T cells are specialized effector cells that have gained prominence as immunotherapy agents due to their ability to target and kill cells with altered pyrophosphate metabolites. In our effort to understand how cancer cells evade the cell-killing activity of Vγ9Vδ2 T cells, we performed a comprehensive genome-scale CRISPR screening of cancer cells. We found that four molecules belonging to the butyrophilin (BTN) family, specifically BTN2A1, BTN3A1, BTN3A2, and BTN3A3, are critically important and play unique, nonoverlapping roles in facilitating the destruction of cancer cells by primary Vγ9Vδ2 T cells. The coordinated function of these BTN molecules was driven by synchronized gene expression, which was regulated by IFN-γ signaling and the RFX complex. Additionally, an enzyme called QPCTL was shown to play a key role in modifying the N-terminal glutamine of these BTN proteins and was found to be a crucial factor in Vγ9Vδ2 T cell killing of cancer cells. Through our research, we offer a detailed overview of the functional genomic mechanisms that underlie how cancer cells escape Vγ9Vδ2 T cells. Moreover, our findings shed light on the importance of the harmonized expression and function of gene family members in modulating T-cell activity.

The glycoimmune checkpoint receptor Siglec-7 interacts with T-cell ligands and regulates T-cell activation.

Siglec-7 (sialic acid-binding immunoglobulin-like lectin 7) is a glycan-binding immune receptor that is emerging as a significant target of interest for cancer immunotherapy. The physiological ligands that bind Siglec-7, however, remain incompletely defined. In this study, we characterized the expression of Siglec-7 ligands on peripheral immune cell subsets and assessed whether Siglec-7 functionally regulates interactions between immune cells. We found that disialyl core 1 O-glycans are the major immune ligands for Siglec-7 and that these ligands are particularly highly expressed on naïve T-cells. Densely glycosylated sialomucins are the primary carriers of these glycans, in particular a glycoform of the cell-surface marker CD43. Biosynthesis of Siglec-7-binding glycans is dynamically controlled on different immune cell subsets through a genetic circuit involving the glycosyltransferase GCNT1. Siglec-7 blockade was found to increase activation of both primary T-cells and antigen-presenting dendritic cells in vitro, indicating that Siglec-7 binds T-cell glycans to regulate intraimmune signaling. Finally, we present evidence that Siglec-7 directly activates signaling pathways in T-cells, suggesting a new biological function for this receptor. These studies conclusively demonstrate the existence of a novel Siglec-7-mediated signaling axis that physiologically regulates T-cell activity. Going forward, our findings have significant implications for the design and implementation of therapies targeting immunoregulatory Siglec receptors.

Enrichment of T-cell proliferation and memory gene signatures of CD79A/CD40 costimulatory domain potentiates CD19CAR-T cell functions.

CD19 chimeric antigen receptor (CAR) T-cells have demonstrated remarkable outcomes in B-cell malignancies. Recently, the novel CD19CAR-T cells incorporated with B-cell costimulatory molecules of CD79A/CD40 demonstrated superior antitumor activity in the B-cell lymphoma model compared with CD28 or 4-1BB. Here, we investigated the intrinsic transcriptional gene underlying the functional advantage of CD19.79A.40z CAR-T cells following CD19 antigen exposure using transcriptome analysis compared to CD28 or 4-1BB. Notably, CD19.79A.40z CAR-T cells up-regulated genes involved in T-cell activation, T-cell proliferation, and NF-κB signaling, whereas down-regulated genes associated with T-cell exhaustion and apoptosis. Interestingly, CD19.79A.40z CAR- and CD19.BBz CAR-T cells were enriched in almost similar pathways. Furthermore, gene set enrichment analysis demonstrated the enrichment of genes, which were previously identified to correlate with T-cell proliferation, interferon signaling pathway, and naïve and memory T-cell signatures, and down-regulated T-cell exhaustion genes in CD79A/CD40, compared with the T-cell costimulatory domain. The CD19.79A.40z CAR-T cells also up-regulated genes related to glycolysis and fatty acid metabolism, which are necessary to drive T-cell proliferation and differentiation compared with conventional CD19CAR-T cells. Our study provides a comprehensive insight into the understanding of gene signatures that potentiates the superior antitumor functions by CD19CAR-T cells incorporated with the CD79A/CD40 costimulatory domain.

The relationship of mRNA with protein expression in CD8+ T cells associates with gene class and gene characteristics.

T cells are key players in our defence against infections and malignancies. When T cells differentiate or become activated, they undergo substantial alterations in gene expression. Even though RNA expression levels are now well documented throughout different stages of T cells, it is not well understood how mRNA expression translates into the protein landscape. By combining paired RNA sequencing and mass spectrometry data of primary human CD8+ T cells, we report that mRNA expression is a poor proxy for the overall protein output, irrespective of the differentiation or activation status. Yet, gene class stratification revealed a function-specific correlation of mRNA with protein expression. This gene class-specific expression pattern associated with differences in gene characteristics such as sequence conservation and untranslated region (UTR) lengths. In addition, the presence of AU-rich elements in the 3'UTR associated with alterations in mRNA and protein abundance T cell activation dependent, gene class-specific manner. In conclusion, our study highlights the role of gene characteristics as a determinant for gene expression in T cells.

CRISPR screening identifies T cell-intrinsic regulators of CD3-bispecific antibody responses.

CD3-engaging bispecific antibodies (BsAbs) enable the formation of an immune synapse between T cells and tumor cells, resulting in robust target cell killing not dependent on a preexisting tumor specific T cell receptor. While recent studies have shed light on tumor cell-specific factors that modulate BsAb sensitivity, the T cell-intrinsic determinants of BsAb efficacy and response durability are poorly understood. To better clarify the genes that shape BsAb-induced T cell responses, we conducted targeted analyses and a large-scale unbiased in vitro CRISPR/Cas9-based screen to identify negative regulators of BsAb-induced T cell proliferation. These analyses revealed that CD8+ T cells are dependent on CD4+ T cell-derived signaling factors in order to achieve sustained killing in vitro. Moreover, the mammalian target of rapamycin (mTOR) pathway and several other candidate genes were identified as intrinsic regulators of BsAb-induced T cell proliferation and/or activation, highlighting promising approaches to enhancing the utility of these potent therapeutics.

Differences in H3K4me3 and chromatin accessibility contribute to altered T-cell receptor signaling in neonatal naïve CD4 T cells.

Neonatal CD4+ T cells have reduced or delayed T-cell receptor (TCR) signaling responses compared with adult cells, but the mechanisms underlying this are poorly understood. This study tested the hypothesis that human neonatal naïve CD4+ TCR signaling and activation deficits are related to differences in H3K4me3 patterning and chromatin accessibility. Following initiation of TCR signaling using anti-CD3/anti-CD28 beads, adult naïve CD4+ T cells demonstrated increased CD69, phospho-CD3ε and interleukin (IL)-2, tumor necrosis factor-α (TNF-α), interferon-γ and IL-17A compared with neonatal cells. By contrast, following TCR-independent activation using phorbol myristate acetate (PMA)/ionomycin, neonatal cells demonstrated increased expression of CD69, IL-2 and TNF-α and equivalent phospho-ERK compared with adult cells. H3K4me3 chromatin immunoprecipitation-sequencing (ChIP-seq) and assay for transposase-accessible chromatin with high-throughput sequencing (ATAC-seq) were performed on separate cohorts of naïve CD4+ T cells from term neonates and adults, and RNA-seq data from neonatal and adult naïve CD4+ T cells were obtained from the Blueprint Consortium. Adult cells demonstrated overall increased chromatin accessibility and a higher proportion of H3K4me3 sites associated with open chromatin and active gene transcription compared with neonatal cells. Adult cells demonstrated increased mRNA expression of the TCR-associated genes FYN, ITK, CD4, LCK and LAT, which was associated with increased H3K4me3 at the FYN and ITK gene loci and increased chromatin accessibility at the CD4, LCK and LAT loci. These findings indicate that neonatal TCR-dependent defects in activation are epigenetically regulated and provide a potentially targetable mechanism to enhance neonatal CD4+ T-cell responses.

Immunophenotypic and functional analysis of lymphocyte subsets in common variable immunodeficiency patients without monogenic defects.

Common variable immunodeficiency (CVID) is accompanied by various lymphocyte abnormalities believed to be mostly responsible for disease features in patients with no diagnosed monogenic defects. Here, we evaluated the association of B and T lymphocyte abnormalities with the incidence of CVID. Twenty-six genetically unsolved CVID patients were examined for B and T lymphocyte subsets by flow cytometry and CD4+ T-cell proliferation by carboxyfluorescein succinimidyl ester (CFSE) test. We detected a reduction in total, naive, memory B cells and plasmablasts, and also total, naive, central memory and regulatory CD4+ T cells, besides naive CD8+ T cells. There was an increase in CD21low and transitional B cells, effector memory (EM) and terminally differentiated effector memory (TEMRA ) CD4+ T-cell subsets as well as total, EM, TEMRA , activated and cytotoxic CD8+ T cells among non-monogenic CVID patients. CD4+ T-cell proliferation response was reduced regarding both division index and percent divided. In conclusion, regarding the similarity of lymphocyte abnormalities between patients without genetic defects and those with monogenic defects, genetic mutations are not responsible for these specific lymphocyte changes. However, the novel correlations observed between lymphocyte alterations among genetically unsolved CVID patients may serve as a guide to predict the potential of future CVID development for hypogammaglobulinemia children.

A genome-scale screen for synthetic drivers of T cell proliferation.

The engineering of autologous patient T cells for adoptive cell therapies has revolutionized the treatment of several types of cancer1. However, further improvements are needed to increase response and cure rates. CRISPR-based loss-of-function screens have been limited to negative regulators of T cell functions2-4 and raise safety concerns owing to the permanent modification of the genome. Here we identify positive regulators of T cell functions through overexpression of around 12,000 barcoded human open reading frames (ORFs). The top-ranked genes increased the proliferation and activation of primary human CD4+ and CD8+ T cells and their secretion of key cytokines such as interleukin-2 and interferon-γ. In addition, we developed the single-cell genomics method OverCITE-seq for high-throughput quantification of the transcriptome and surface antigens in ORF-engineered T cells. The top-ranked ORF-lymphotoxin-ß receptor (LTBR)-is typically expressed in myeloid cells but absent in lymphocytes. When overexpressed in T cells, LTBR induced profound transcriptional and epigenomic remodelling, leading to increased T cell effector functions and resistance to exhaustion in chronic stimulation settings through constitutive activation of the canonical NF-κB pathway. LTBR and other highly ranked genes improved the antigen-specific responses of chimeric antigen receptor T cells and γδ T cells, highlighting their potential for future cancer-agnostic therapies5. Our results provide several strategies for improving next-generation T cell therapies by the induction of synthetic cell programmes.

Human epigenetic and transcriptional T cell differentiation atlas for identifying functional T cell-specific enhancers.

The clinical benefit of T cell immunotherapies remains limited by incomplete understanding of T cell differentiation and dysfunction. We generated an epigenetic and transcriptional atlas of T cell differentiation from healthy humans that included exhausted CD8 T cells and applied this resource in three ways. First, we identified modules of gene expression and chromatin accessibility, revealing molecular coordination of differentiation after activation and between central memory and effector memory. Second, we applied this healthy molecular framework to three settings-a neoadjuvant anti-PD1 melanoma trial, a basal cell carcinoma scATAC-seq dataset, and autoimmune disease-associated SNPs-yielding insights into disease-specific biology. Third, we predicted genome-wide cis-regulatory elements and validated this approach for key effector genes using CRISPR interference, providing functional annotation and demonstrating the ability to identify targets for non-coding cellular engineering. These studies define epigenetic and transcriptional regulation of human T cells and illustrate the utility of interrogating disease in the context of a healthy T cell atlas.

Potential role of inducible GPR3 expression under stimulated T cell conditions.

G protein-coupled receptor 3 (GPR3) constitutively activates Gαs proteins without any ligands and is predominantly expressed in neurons. Since the expression and physiological role of GPR3 in immune cells is still unknown, we examined the possible role of GPR3 in T lymphocytes. The expression of GPR3 was upregulated 2 h after phorbol 12-myristate 13-acetate (PMA)/ionomycin stimulation and was sustained in Jurkat cells, a human T lymphocyte cell line. In addition, the expression of nuclear receptor 4 group A member 2 (NR4A2) was highly modulated by GPR3 expression. Additionally, GPR3 expression was linked with the transcriptional promoter activity of NR4A in Jurkat cells. In mouse CD4+ T cells, transient GPR3 expression was induced immediately after the antigen receptor stimulation. However, the expression of NR4A2 was not modulated in CD4+ T cells from GPR3-knockout mice after stimulation, and the population of Treg cells in thymocytes and splenocytes was not affected by GPR3 knockout. By contrast, spontaneous effector activation in both CD4+ T cells and CD8+ T cells was observed in GPR3-knockout mice. In summary, GPR3 is immediately induced by T cell stimulation and play an important role in the suppression of effector T cell activation.

An Artifact in Intracellular Cytokine Staining for Studying T Cell Responses and Its Alleviation.

Intracellular cytokine staining (ICS) is a widely employed ex vivo method for quantitative determination of the activation status of immune cells, most often applied to T cells. ICS test samples are commonly prepared from animal or human tissues as unpurified cell mixtures, and cell-specific cytokine signals are subsequently discriminated by gating strategies using flow cytometry. Here, we show that when ICS samples contain Ly6G+ neutrophils, neutrophils are ex vivo activated by an ICS reagent - phorbol myristate acetate (PMA) - which leads to hydrogen peroxide (H2O2) release and death of cytokine-expressing T cells. This artifact is likely to result in overinterpretation of the degree of T cell suppression, misleading immunological research related to cancer, infection, and inflammation. We accordingly devised easily implementable improvements to the ICS method and propose alternative methods for assessing or confirming cellular cytokine expression.

FCER1G positively relates to macrophage infiltration in clear cell renal cell carcinoma and contributes to unfavorable prognosis by regulating tumor immunity.

BACKGROUND: Tumor-associated macrophages (TAMs) are closely related to unfavorable prognosis of patients with clear cell renal cell carcinoma (ccRCC). However, the important molecules in the interaction between ccRCC and TAMs are unclear. METHODS: TCGA-KIRC gene expression data of tumor tissues and normal tissues adjacent to tumor were compared to identify differentially expressed genes in ccRCC. TAMs related genes were discovered by analyzing the correlation between these differentially expressed genes and common macrophage biomarkers. Gene set enrichment analysis was performed to predict functions of TAMs related gene. The findings were further validated using RNA sequencing data obtained from the CheckMate 025 study and immunohistochemical analysis of samples from 350 patients with ccRCC. Kaplan-Meier survival curve, Cox regression analysis and Harrell's concordance index analysis were used to determine the prognostic significance. RESULTS: In this study, we applied bioinformatic analysis to explore TAMs related differentially expressed genes in ccRCC and identified 5 genes strongly correlated with all selected macrophage biomarkers: STAC3, LGALS9, TREM2, FCER1G, and PILRA. Among them, FCER1G was abundantly expressed in tumor tissues and showed prognostic importance in patients with ccRCC who received treatment with Nivolumab; however, it did not exhibit prognostic value in those treated with Everolimus. We also discovered that high expression levels of FCER1G are related to T cell suppression. Moreover, combination of FCER1G and macrophage biomarker CD68 can improve the prognostic stratification of patients with ccRCC from TCGA-KIRC. Based on the immunohistochemical analysis of samples from patients with ccRCC, we further validated that FCER1G and CD68 are both highly expressed in tumor tissue and correlate with each other. Higher expression of CD68 or FCER1G in ccRCC tissue indicates shorter overall survival and progression-free survival; patients with high expression of both CD68 and FCER1G have the worst outcome. Combining CD68 and FCER1G facilitates the screening of patients with a worse prognosis from the same TNM stage group. CONCLUSIONS: High expression of FCER1G in ccRCC is closely related to TAMs infiltration and suppression of T cell activation and proliferation. Combining the expression levels of FCER1G and macrophage biomarker CD68 may be a promising postoperative prognostic index for patients with ccRCC.

Erythroid Differentiation Regulator 1 Strengthens TCR Signaling by Enhancing PLCγ1 Signal Transduction Pathway.

Erythroid differentiation regulator 1 (Erdr1) has previously been reported to control thymocyte selection via TCR signal regulation, but the effect of Erdr1 as a TCR signaling modulator was not studied in peripheral T cells. In this report, it was determined whether Erdr1 affected TCR signaling strength in CD4 T cells. Results revealed that Erdr1 significantly enhanced the anti-TCR antibody-mediated activation and proliferation of T cells while failing to activate T cells in the absence of TCR stimulation. In addition, Erdr1 amplified Ca2+ influx and the phosphorylation of PLCγ1 in CD4 T cells with the TCR stimuli. Furthermore, NFAT1 translocation into nuclei in CD4 T cells was also significantly promoted by Erdr1 in the presence of TCR stimulation. Taken together, our results indicate that Erdr1 positively modulates TCR signaling strength via enhancing the PLCγ1/Ca2+/NFAT1 signal transduction pathway.

A20 and ABIN-1 cooperate in balancing CBM complex-triggered NF-κB signaling in activated T cells.

T cell activation initiates protective adaptive immunity, but counterbalancing mechanisms are critical to prevent overshooting responses and to maintain immune homeostasis. The CARD11-BCL10-MALT1 (CBM) complex bridges T cell receptor engagement to NF-κB signaling and MALT1 protease activation. Here, we show that ABIN-1 is modulating the suppressive function of A20 in T cells. Using quantitative mass spectrometry, we identified ABIN-1 as an interactor of the CBM signalosome in activated T cells. A20 and ABIN-1 counteract inducible activation of human primary CD4 and Jurkat T cells. While A20 overexpression is able to silence CBM complex-triggered NF-κB and MALT1 protease activation independent of ABIN-1, the negative regulatory function of ABIN-1 depends on A20. The suppressive function of A20 in T cells relies on ubiquitin binding through the C-terminal zinc finger (ZnF)4/7 motifs, but does not involve the deubiquitinating activity of the OTU domain. Our mechanistic studies reveal that the A20/ABIN-1 module is recruited to the CBM complex via A20 ZnF4/7 and that proteasomal degradation of A20 and ABIN-1 releases the CBM complex from the negative impact of both regulators. Ubiquitin binding to A20 ZnF4/7 promotes destructive K48-polyubiquitination to itself and to ABIN-1. Further, after prolonged T cell stimulation, ABIN-1 antagonizes MALT1-catalyzed cleavage of re-synthesized A20 and thereby diminishes sustained CBM complex signaling. Taken together, interdependent post-translational mechanisms are tightly controlling expression and activity of the A20/ABIN-1 silencing module and the cooperative action of both negative regulators is critical to balance CBM complex signaling and T cell activation.

Sialic acid blockade in dendritic cells enhances CD8+ T cell responses by facilitating high-avidity interactions.

Sialic acids are negatively charged carbohydrates that cap the glycans of glycoproteins and glycolipids. Sialic acids are involved in various biological processes including cell-cell adhesion and immune recognition. In dendritic cells (DCs), the major antigen-presenting cells of the immune system, sialic acids emerge as important regulators of maturation and interaction with other lymphocytes including T cells. Many aspects of how sialic acids regulate DC functions are not well understood and tools and model systems to address these are limited. Here, we have established cultures of murine bone marrow-derived DCs (BMDCs) that lack sialic acid expression using a sialic acid-blocking mimetic Ac53FaxNeu5Ac. Ac53FaxNeu5Ac treatment potentiated BMDC activation via toll-like receptor (TLR) stimulation without affecting differentiation and viability. Sialic acid blockade further increased the capacity of BMDCs to induce antigen-specific CD8+ T cell proliferation. Transcriptome-wide gene expression analysis revealed that sialic acid mimetic treatment of BMDCs induces differential expression of genes involved in T cell activation, cell-adhesion, and cell-cell interactions. Subsequent cell clustering assays and single cell avidity measurements demonstrated that BMDCs with reduced sialylation form higher avidity interactions with CD8+ T cells. This increased avidity was detectable in the absence of antigens, but was especially pronounced in antigen-dependent interactions. Together, our data show that sialic acid blockade in BMDCs ameliorates maturation and enhances both cognate T cell receptor-MHC-dependent and independent T cell interactions that allow for more robust CD8+ T cell responses.

Control of Memory Phenotype T Lymphocyte Homeostasis: Role of Costimulation.

Foxp3+ T regulatory cells (Tregs), CD4+Foxp3- T cells, and CD8+ T cells are composed of naive phenotype (NP) and memory phenotype (MP) subsets. Ten to 20% of each MP T cell population are cycling (Ki-67+) in vivo. We investigated the contribution of costimulatory (CD28) and coinhibitory (CTLA-4, PD-1) receptors on MP T cell homeostatic proliferation in vivo in the mouse. Blockade of CD28-CD80/CD86 signaling completely abolished MP Tregs and profoundly inhibited MP CD4+Foxp3- T cell proliferation, but it did not affect MP CD8+ T cell proliferation. Marked enhancement of homeostatic proliferation of MP Tregs and MP CD4+Foxp3- T cells was seen after blocking CTLA4-CD80/CD86 interactions and PD-1-PD-L1/2 interactions, and greater enhancement was seen with blockade of both pathways. The CD28 pathway also played an important role in the expansion of Tregs and MP T cells after treatment of mice with agonistic Abs to members of the TNF receptor superfamily, which can act directly (anti-GITR, anti-OX40, anti-4-1BB) or indirectly (anti-CD40) on T cells. Induction of a cytokine storm by blocking the interaction of NK inhibitory receptors with MHC class I had no effect on Treg homeostasis, enhanced MP CD4+ proliferation, and expansion in a CD28-dependent manner, but it enhanced MP CD8+ T cell proliferation in a CD28-independent manner. Because MP T cells exert potent biologic effects primarily before the induction of adaptive immune responses, these findings have important implications for the use of biologic agents designed to suppress autoimmune disease or enhance T effector function in cancer that may have negative effects on MP T cells.

Fine-tuning of MEK signaling is pivotal for limiting B and T cell activation.

MEK1 and MEK2, the only known activators of ERK, are attractive therapeutic candidates for both cancer and autoimmune diseases. However, how MEK signaling finely regulates immune cell activation is only partially understood. To address this question, we specifically delete Mek1 in hematopoietic cells in the Mek2 null background. Characterization of an allelic series of Mek mutants reveals the presence of distinct degrees of spontaneous B cell activation, which are inversely proportional to the levels of MEK proteins and ERK activation. While Mek1 and Mek2 null mutants have a normal lifespan, 1Mek1 and 1Mek2 mutants retaining only one functional Mek1 or Mek2 allele in hematopoietic cell lineages die from glomerulonephritis and lymphoproliferative disorders, respectively. This establishes that the fine-tuning of the ERK/MAPK pathway is critical to regulate B and T cell activation and function and that each MEK isoform plays distinct roles during lymphocyte activation and disease development.