Cell-cell adhesion regulates Merlin/NF2 interaction with the PAF complex.

The PAF complex (PAFC) coordinates transcription elongation and mRNA processing and its CDC73/parafibromin subunit functions as a tumour suppressor. The NF2/Merlin tumour suppressor functions both at the cell cortex and nucleus and is a key mediator of contact inhibition but the molecular mechanisms remain unclear. In this study we have used affinity proteomics to identify novel Merlin interacting proteins and show that Merlin forms a complex with multiple proteins involved in RNA processing including the PAFC and the CHD1 chromatin remodeller. Tumour-derived inactivating mutations in both Merlin and the CDC73 PAFC subunit mutually disrupt their interaction and growth suppression by Merlin requires CDC73. Merlin interacts with the PAFC in a cell density-dependent manner and we identify a role for FAT cadherins in regulating the Merlin-PAFC interaction. Our results suggest that in addition to its function within the Hippo pathway, Merlin is part of a tumour suppressor network regulated by cell-cell adhesion which coordinates post-initiation steps of the transcription cycle of genes mediating contact inhibition.

SARS-CoV-2-reactive T-cell receptors isolated from convalescent COVID-19 patients confer potent T-cell effector function.

Both B cells and T cells are involved in an effective immune response to SARS-CoV-2, the disease-causing virus of COVID-19. While B cells-with the indispensable help of CD4+ T cells-are essential to generate neutralizing antibodies, T cells on their own have been recognized as another major player in effective anti-SARS-CoV-2 immunity. In this report, we provide insights into the characteristics of individual HLA-A*02:01- and HLA-A*24:02-restricted SARS-CoV-2-reactive TCRs, isolated from convalescent COVID-19 patients. We observed that SARS-CoV-2-reactive T-cell populations were clearly detectable in convalescent samples and that TCRs isolated from these T cell clones were highly functional upon ectopic re-expression. The SARS-CoV-2-reactive TCRs described in this report mediated potent TCR signaling in reporter assays with low nanomolar EC50 values. We further demonstrate that these SARS-CoV-2-reactive TCRs conferred powerful T-cell effector function to primary CD8+ T cells as evident by a robust anti-SARS-CoV-2 IFN-γ response and in vitro cytotoxicity. We also provide an example of a long-lasting anti-SARS-CoV-2 memory response by reisolation of one of the retrieved TCRs 5 months after initial sampling. Taken together, these findings contribute to a better understanding of anti-SARS-CoV-2 T-cell immunity and may contribute to paving the way toward immunotherapeutics approaches targeting SARS-CoV-2.

Rbpj expression in regulatory T cells is critical for restraining TH2 responses.

The transcriptional regulator Rbpj is involved in T-helper (TH) subset polarization, but its function in Treg cells remains unclear. Here we show that Treg-specific Rbpj deletion leads to splenomegaly and lymphadenopathy despite increased numbers of Treg cells with a polyclonal TCR repertoire. A specific defect of Rbpj-deficient Treg cells in controlling TH2 polarization and B cell responses is observed, leading to the spontaneous formation of germinal centers and a TH2-associated immunoglobulin class switch. The observed phenotype is environment-dependent and can be induced by infection with parasitic nematodes. Rbpj-deficient Treg cells adopt open chromatin landscapes and gene expression profiles reminiscent of tissue-derived TH2-polarized Treg cells, with a prevailing signature of the transcription factor Gata-3. Taken together, our study suggests that Treg cells require Rbpj to specifically restrain TH2 responses, including their own excessive TH2-like differentiation potential.

Dissecting and modeling the emergent murine TEC compartment during ontogeny.

The origin of the thymic epithelium, i.e. the cortical (cTEC) and medullary (mTEC) epithelial cells, from bipotent stem cells through TEC progenitors and lineage-specific progeny still remains poorly understood. We sought to obtain an unbiased view of the incipient emergence of TEC subsets by following embryonic TEC development based on co-expression of EpCAM, CD80 and MHC class II (MHCII) on non-hematopoietic (CD45- ) thymic stromal cells in wild-type BL6 mice. Using a combination of ex vivo analysis, Re-aggregate Thymic Organ Culture (RTOC) reconstitution assays and mathematical modeling, we traced emergent lineage commitment in murine embryonic TECs. Both experimental and mathematical datasets supported the following developmental sequence: MHCII- CD80- → MHCIIlo CD80- → MHCIIhi CD80- → MHCIIhi CD80hi TECs, whereby MHCIIhi CD80- and MHCIIhi CD80hi TECs bear features of cTECs and mTECs respectively. These emergent MHCIIhi CD80- cTECs directly generate mature MHCIIhi CD80hi mTECs in vivo and in vitro, thus supporting the asynchronous model of TEC lineage commitment.

Deficiency of N-myristoylation reveals calcineurin activity as regulator of IFN-γ-producing γδ T cells.

γδ T cell subsets can be characterized, in part, by their secretion of select proinflammatory cytokines. The molecular mechanisms driving the diverse fates of γδ T cells have not been elucidated. We have previously shown that the attachment of myristic acid to the N-terminal glycine of proteins, termed N-myristoylation, is essential for αß T cell development and activation. Here, we explore the potential role of this lipid modification on the activation of γδ T cells. In the absence of N-myristoylation, the CD27+ γδ T cell subset was dominantly affected. The cells produced high levels of IFN-γ upon stimulation. In addition, they were more sensitive to inhibition of the CaN-Nfat pathway than were γδ T cells with myristoylated CaN. N-Myristoylation was found to modulate activity of phosphatase CaN, a regulator of Nfat. In summary, the CaN-Nfat pathway regulates development and function of IFN-γ-producing γδ T cells, and its balanced activity is strongly dependent on CaN N-myristoylation.

Immune cell recruitment in teratomas is impaired by increased Wnt secretion.

Wnt signaling plays a central role in tumor initiation and tumor progression. Mutations in Wnt pathway components, such as the tumor suppressor APC, lead to malignant transformation. While previous studies focused on Wnt-related changes in cancer cells, the impact of aberrant Wnt signaling on the tumor microenvironment is only beginning to emerge. In order to investigate the role of increased Wnt secretion on tumor growth and the microenvironment, we generated a novel germ cell tumor model by overexpressing the Wnt secretion factor Evi/Wls in mouse embryonic stem cells. Evi-overexpressing teratoma were characterized by enhanced tumor growth in supporting a tumor-promoting role of Wnt secretion. Interestingly, enhanced Evi expression correlated with impaired immune cell recruitment. Specifically, T- and B-cell infiltration was reduced in Evi-overexpressing teratomas, which was independent of teratoma size and differentiation. Our study suggests that Wnt secretion impairs immunosurveillance. Since immune cell infiltration has been shown to have prognostic value, the levels of secreted Wnt activity might impact the efficiency of cancer immunotherapy.

Thymic Epithelial Cells Are a Nonredundant Source of Wnt Ligands for Thymus Development.

Wnt signaling has been implicated in T cell development. However, it remained unclear which cell type is the major source of Wnt ligands and to what extent thymic epithelial cell (TEC) development is dependent on Wnt signaling. In this study, we analyzed the role of Wnt ligands provided by TECs for the development of T cells and TECs without manipulating the intracellular Wnt signaling machinery in either cell type. To this end, we used conditional knockout mice (FoxN1-Gpr177) in which TECs are unable to secrete Wnt ligands. Gpr177 (Evi/Wls) is a Wnt-specific cargo receptor that is required for the secretion of Wnt ligands. We found that TECs are the main source of Wnt ligands in the thymus, which serves a nonredundant role, and lack of TEC-provided Wnt ligands led to thymic hypotrophy, as well as a reduced peripheral T cell pool. Despite being reduced in numbers, T cells that developed in the absence of TEC-secreted Wnt ligands were functionally competent, and the subset composition of the peripheral T cell pool was not affected. Thus, our data suggest that T cell development is not directly dependent on TEC-provided Wnt ligands. Rather, TEC-secreted Wnt ligands are essential for normal thymus development and normal peripheral T cell frequencies but are dispensable for T cell function in the periphery.

Immunosuppression and Aberrant T Cell Development in the Absence of N-Myristoylation.

N-myristoylation refers to the attachment of myristic acid to the N-terminal glycine of proteins and substantially affects their intracellular targeting and functions. The thymus represents an organ with a prominent N-myristoylation activity. To elucidate the role of protein N-myristoylation for thymocyte development, we generated mice with a T cell lineage-specific deficiency in N-myristoyl transferase (Nmt)1 and 2. Depletion of Nmt activity in T cells led to a defective transmission of TCR signals, a developmental blockage of thymocytes at the transition from double-negative 3 to 4 stages, and a reduction of all the following stages. We could demonstrate that Lck and myristoylated alanine-rich C kinase substrate, two main myristoylated kinases in T cells, were mislocalized in the absence of Nmt activity. N-myristoylation was also indispensable for early and distal TCR signaling events such as CD3ζ, Zap70, and Erk activation and for release of cytokines such as IFN-γ and IL-2. As a consequence, the initiation and propagation of the TCR signaling cascade was severely impaired. Furthermore, we showed that the absence of myristoylation had an immunosuppressive effect on T cells in vivo after treatment with CpG and stimulation of the TCR with the staphylococcal enterotoxin B superantigen. Therefore, protein myristoylation is indispensable in T cell development and activation and its inhibition might offer a novel strategy to achieve immunosuppression.

Adult thymus contains FoxN1(-) epithelial stem cells that are bipotent for medullary and cortical thymic epithelial lineages.

Within the thymus, two major thymic epithelial cell (TEC) subsets-cortical and medullary TECs-provide unique structural and functional niches for T cell development and establishment of central tolerance. Both lineages are believed to originate from a common progenitor cell, yet the cellular and molecular identity of these bipotent TEC progenitors/stem cells remains ill defined. Here we identify rare stromal cells in the murine adult thymus, which under low-attachment conditions formed spheres (termed "thymospheres"). These thymosphere-forming cells (TSFCs) displayed the stemness features of being slow cycling, self-renewing, and bipotent. TSFCs could be significantly enriched based on their distinct surface antigen phenotype. The FoxN1 transcription factor was dispensable for TSFCs maintenance in situ and for commitment to the medullary and cortical TEC lineages. In summary, this study presents the characterization of the adult thymic epithelial stem cells and demonstrates the dispensability of FoxN1 function for their stemness.