Rapid turnover of CTLA4 is associated with a complex architecture of reversible ubiquitylation.

The immune checkpoint regulator CTLA4 is an unusually short-lived membrane protein. Here we show that its lysosomal degradation is dependent on ubiquitylation at Lysine residues 203 and 213. Inhibition of the v-ATPase partially restores CTLA4 levels following cycloheximide treatment, but also reveals a fraction that is secreted in exosomes. The endosomal deubiquitylase, USP8, interacts with CTLA4 and its loss enhances CTLA4 ubiquitylation in cancer cells, mouse CD4+ T cells and in cancer cell-derived exosomes. Depletion of the USP8 adapter protein, HD-PTP, but not ESCRT-0 recapitulates this cellular phenotype, but shows distinct properties vis-à-vis exosome incorporation. Re-expression of wild-type USP8, but neither a catalytically inactive, nor a localization-compromised ΔMIT domain mutant can rescue delayed degradation of CTLA4, or counteract its accumulation in clustered endosomes. UbiCRest analysis of CTLA4-associated ubiquitin chain linkages identifies a complex mixture of conventional Lys63- and more unusual Lys27- and Lys29-linked polyubiquitin chains that may underly the rapidity of protein turnover.

Ubiquitin-specific protease 8 (USP8/UBPy): a prototypic multidomain deubiquitinating enzyme with pleiotropic functions.

Protein modification by ubiquitin is one of the most versatile posttranslational regulations and counteracted by almost 100 deubiquitinating enzymes (DUBs). USP8 was originally identified as a growth regulated ubiquitin-specific protease and is like many other DUBs characterized by its multidomain architecture. Besides the catalytic domain, specific protein-protein interaction modules were characterized which contribute to USP8 substrate recruitment, regulation and targeting to distinct protein complexes. Studies in mice and humans impressively showed the physiological relevance and non-redundant function of USP8 within the context of the whole organism. USP8 knockout (KO) mice exhibit early embryonic lethality while induced deletion in adult animals rapidly causes lethal liver failure. Furthermore, T-cell specific ablation disturbs T-cell development and function resulting in fatal autoimmune inflammatory bowel disease. In human patients, somatic mutations in USP8 were identified as the underlying cause of adrenocorticotropic hormone (ACTH) releasing pituitary adenomas causing Cushing's disease (CD). Here we provide an overview of the versatile molecular, cellular and pathology associated function and regulation of USP8 which appears to depend on specific protein binding partners, substrates and the cellular context.

DC Respond to Cognate T Cell Interaction in the Antigen-Challenged Lymph Node.

Dendritic cells (DC) are unrivaled in their potential to prime naive T cells by presenting antigen and providing costimulation. DC are furthermore believed to decode antigen context by virtue of pattern recognition receptors and to polarize T cells through cytokine secretion toward distinct effector functions. Diverse polarized T helper (TH) cells have been explored in great detail. In contrast, studies of instructing DC have to date largely been restricted to in vitro settings or adoptively transferred DC. Here we report efforts to unravel the DC response to cognate T cell encounter in antigen-challenged lymph nodes (LN). Mice engrafted with antigen-specific T cells were immunized with nanoparticles (NP) entrapping adjuvants and absorbed with antigen to study the immediate DC response to T cell encounter using bulk and single cell RNA-seq profiling. NP induced robust antigen-specific TH1 cell responses with minimal bystander activation. Fluorescent-labeled NP allowed identification of antigen-carrying DC and focus on transcriptional changes in DC that encounter T cells. Our results support the existence of a bi-directional crosstalk between DC and T cells that promotes TH1 responses, including involvement of the ubiquitin-like molecule Isg15 that merits further study.

The ubiquitin-specific protease USP8 is critical for the development and homeostasis of T cells.

The modification of proteins by ubiquitin has a major role in cells of the immune system and is counteracted by various deubiquitinating enzymes (DUBs) with poorly defined functions. Here we identified the ubiquitin-specific protease USP8 as a regulatory component of the T cell antigen receptor (TCR) signalosome that interacted with the adaptor Gads and the regulatory molecule 14-3-3ß. Caspase-dependent processing of USP8 occurred after stimulation of the TCR. T cell-specific deletion of USP8 in mice revealed that USP8 was essential for thymocyte maturation and upregulation of the gene encoding the cytokine receptor IL-7Rα mediated by the transcription factor Foxo1. Mice with T cell-specific USP8 deficiency developed colitis that was promoted by disturbed T cell homeostasis, a predominance of CD8(+) γδ T cells in the intestine and impaired regulatory T cell function. Collectively, our data reveal an unexpected role for USP8 as an immunomodulatory DUB in T cells.

B cell antigen receptor-induced activation of an IRAK4-dependent signaling pathway revealed by a MALT1-IRAK4 double knockout mouse model.

BACKGROUND: The B cell antigen receptor (BCR) and pathogen recognition receptors, such as Toll-like receptor 4 (TLR4), act in concert to control adaptive B cell responses. However, little is known about the signaling pathways that integrate BCR activation with intrinsic TLR4 stimulation. Antigen receptors initialize activation of the inducible transcription factor nuclear factor-κB (NF-κB) via recruitment of the membrane-associated guanylate kinase caspase recruitment domain protein 11 (CARD11), the adapter molecule B cell CLL/lymphoma 10 (BCL10), and the "paracaspase" mucosa-associated lymphoid tissue lymphoma translocation gene 1 (MALT1) into lipid rafts. Upon BCR triggering, this activation strictly depends on BCL10, but not on MALT1, leading to the hypothesis that a MALT1-independent NF-κB activation pathway contributes to BCR-induced NF-κB activation downstream of BCL10. The identity of this pathway has remained elusive. RESULTS: Using genetic and biochemical approaches, we demonstrate that the IRAK4- and IRAK1-dependent TLR signaling branch is activated upon BCR triggering to induce partial NF-κB activation. BCR-induced MALT1-independent IκB degradation and B cell proliferation were inhibited in MALT1/IRAK4 double knockout B cells. Moreover, IRAK1 was recruited into lipid rafts upon BCR stimulation and activated following transient recruitment of IRAK4. CONCLUSION: We propose that the observed crosstalk between BCR and TLR signaling components may contribute to the discrimination of signals that emanate from single and dual receptor engagement to control adaptive B cell responses.

CARD6 is interferon inducible but not involved in nucleotide-binding oligomerization domain protein signaling leading to NF-kappaB activation.

We have previously reported the cloning and characterization of CARD6, a caspase recruitment domain (CARD)-containing protein that is structurally related to the interferon (IFN)-inducible GTPases. CARD6 associates with microtubules and with receptor-interacting protein 2 (RIP2). RIP2 mediates NF-kappaB activation induced by the intracellular nucleotide-binding oligomerization domain (NOD) receptors that sense bacterial peptidoglycan. Here we report that the expression of CARD6 and RIP2 in bone marrow-derived macrophages is rapidly induced by beta IFN and gamma IFN. This IFN-induced upregulation of CARD6 is suppressed by lipopolysaccharide (LPS), in contrast to LPS's enhancement of IFN-induced RIP2 upregulation. We generated CARD6-deficient (CARD6(-/-)) mice and carried out extensive analyses of signaling pathways mediating innate and adaptive immune responses, including the NOD pathways, but did not detect any abnormalities. Moreover, CARD6(-/-) mice were just as susceptible as wild-type mice to infection by Salmonella enterica serovar Typhimurium, Listeria monocytogenes, Candida albicans, lymphocytic choriomeningitis virus, or mouse adenovirus type 1. Thus, although structural and in vitro analyses strongly suggest an important role for CARD6 in immune defense, the physiological function of CARD6 remains obscure.

CARD tricks: controlling the interactions of CARD6 with RICK and microtubules.

In recent years, a number of proteins have been identified that contain a homotypic interaction motif called the caspase recruitment domain (CARD). Most proteins containing a CARD are involved in pathways regulating apoptosis or adaptive or innate immunity. Examples of prominent CARD proteins are caspase-9 and Apaf1, which are involved in the intrinsic death pathway; BCL10 and CARD11, which mediate antigen receptor-induced NF-kappaB activation; and receptor-interacting protein (RIP)-like interacting caspase-like apoptosis regulatory protein kinase (RICK) and the nucleotide-binding oligomerization domain (NOD) proteins, which induce NF-kappaB activation in response to intracellular bacterial peptidoglycan. The most recently discovered pathway involving CARD proteins senses virally-derived double-stranded (ds) RNA and initiates a host defense signaling program. CARD6 is a CARD-containing protein with a domain structure not shared by any other CARD protein. Although the CARD6 cDNA was deposited in GenBank five years ago, the physiological function of full-length CARD6 has yet to be reported. Here we review our initial characterization of CARD6 and discuss the functional implications of various conserved modules found in the CARD6 protein sequence. We conclude that CARD6 is structurally and potentially functionally related to the superfamily of interferon (IFN)-inducible GTPases, a growing family of host defense proteins that confer cell-autonomous immunity.

Caspase recruitment domain protein 6 is a microtubule-interacting protein that positively modulates NF-kappaB activation.

Proteins containing a caspase recruitment domain (CARD) play pivotal roles in signal transduction leading to apoptosis and NF-kappaB activation and inflammation. Here we identify and characterize human and mouse CARD protein 6 (CARD6), CARD-containing proteins of unique structure. CARD6 associates with microtubules and interacts with receptor-interacting protein (RIP)-like interacting caspase-like apoptosis regulatory protein kinase (RICK), a CARD-containing member of the RIP family of protein kinases. These kinases are involved in multiple NF-kappaB signaling pathways important for innate and adaptive immune responses. Surprisingly, the CARDs of CARD6 and RICK were not required for their interaction; instead, mutational analysis revealed that the CARD of CARD6 negatively controls the association of these molecules. CARD6 also binds to RIP1, a RIP kinase homologue that lacks a CARD but contains a C-terminal death domain. Coexpression of RICK targets CARD6 to aggresomes via a mechanism that requires the CARD of RICK. Importantly, CARD6 expression has a synergistic effect on NF-kappaB activation induced by several independent signal transduction pathways. In summary, our results indicate that CARD6 is a regulator of NF-kappaB activation that modulates the functions of RIP kinase family members.