Effective T-cell immune responses in the absence of the serine/threonine kinase RIP2

The potent and selective RIPK2 inhibitor BI 706039 improves intestinal inflammation in the TRUC mouse model of inflammatory bowel disease

Mouse and human data implicate the NOD1 and NOD2 sensors of the intestinal microbiome and the associated signal transduction via the receptor interacting protein kinase 2 (RIPK2) as a potential key signaling node for the development of inflammatory bowel disease (IBD) and an attractive target for pharmacological intervention. The TRUC mouse model of IBD was strongly indicated for evaluating RIPK2 antagonism for its effect on intestinal inflammation based on previous knockout studies with NOD1, NOD2, and RIPK2. We identified and profiled the BI 706039 molecule as a potent and specific functional inhibitor of both human and mouse RIPK2 and with favorable pharmacokinetic properties. We dosed BI 706039 in the spontaneous TRUC mouse model from age 28 to 56 days. Oral, daily administration of BI 706039 caused dose-responsive and significant improvement in colonic histopathological inflammation, colon weight, and terminal levels of protein-normalized fecal lipocalin (all P values <0.001). These observations correlated with dose responsively increasing systemic levels of the BI 706039 compound, splenic molecular target engagement of RIPK2, and modulation of inflammatory genes in the colon. This demonstrates that a relatively low oral dose of a potent and selective RIPK2 inhibitor can modulate signaling in the intestinal immune system and significantly improve disease associated intestinal inflammation.NEW & NOTEWORTHY The RIPK2 kinase at the apex of microbiome immunosensing is an attractive target for pharmacological intervention. A low oral dose of a RIPK2 inhibitor leads to significantly improved intestinal inflammation in the murine TRUC model of colitis. A selective and potent inhibitor of the RIPK2 kinase may represent a new class of therapeutics that target microbiome-driven signaling for the treatment of IBD.

T cell-intrinsic role for Nod2 in protection against Th17-mediated uveitis

Mutations in nucleotide-binding oligomerization domain-containing protein 2 (NOD2) cause Blau syndrome, an inflammatory disorder characterized by uveitis. The antimicrobial functions of Nod2 are well-established, yet the cellular mechanisms by which dysregulated Nod2 causes uveitis remain unknown. Here, we report a non-conventional, T cell-intrinsic function for Nod2 in suppression of Th17 immunity and experimental uveitis. Reconstitution of lymphopenic hosts with Nod2^-/- CD4⁺ T cells or retina-specific autoreactive CD4⁺ T cells lacking Nod2 reveals a T cell-autonomous, Rip2-independent mechanism for Nod2 in uveitis. In naive animals, Nod2 operates downstream of TCR ligation to suppress activation of memory CD4⁺ T cells that associate with an autoreactive-like profile involving IL-17 and Ccr7. Interestingly, CD4⁺ T cells from two Blau syndrome patients show elevated IL-17 and increased CCR7. Our data define Nod2 as a T cell-intrinsic rheostat of Th17 immunity, and open new avenues for T cell-based therapies for Nod2-associated disorders such as Blau syndrome.

RIP2 promotes FcγR-mediated reactive oxygen species production

Receptor-interacting protein 2 (RIP2) is a kinase that mediates signaling downstream of the bacterial peptidoglycan sensors NOD1 and NOD2. Genetic loss or pharmaceutical inhibition of RIP2 has been shown to be beneficial in multiple inflammatory disease models with the effects largely attributed to reducing proinflammatory signaling downstream of peptidoglycan recognition. However, given the widespread expression of this kinase and its reported interactions with numerous other proteins, it is possible that RIP2 may also function in roles outside of peptidoglycan sensing. In this work, we show that RIP2 undergoes tyrosine phosphorylation and activation in response to engagement of the Fc γ receptor (FcγR). Using bone marrow-derived macrophages from WT and RIP2-KO mice, we show that loss of RIP2 leads to deficient FcγR signaling and reactive oxygen species (ROS) production upon FcγR cross-linking without affecting cytokine secretion, phagocytosis, or nitrate/nitrite production. The FcγR-induced ROS response was still dependent on NOD2, as macrophages deficient in this receptor showed similar defects. Mechanistically, we found that different members of the Src family kinases (SFKs) can promote RIP2 tyrosine phosphorylation and activation. Altogether, our findings suggest that RIP2 is functionally important in pathways outside of bacterial peptidoglycan sensing and that involvement in such pathways may depend on the actions of SFKs. These findings will have important implications for future therapies designed to target this kinase.

T-Cell-Intrinsic Receptor Interacting Protein 2 Regulates Pathogenic T Helper 17 Cell Differentiation

Receptor interacting protein 2 (RIP2) plays a role in sensing intracellular pathogens, but its function in T cells is unclear. We show that RIP2 deficiency in CD4+ T cells resulted in chronic and severe interleukin-17A-mediated inflammation during Chlamydia pneumoniae lung infection, increased T helper 17 (Th17) cell formation in lungs of infected mice, accelerated atherosclerosis, and more severe experimental autoimmune encephalomyelitis. While RIP2 deficiency resulted in reduced conventional Th17 cell differentiation, it led to significantly enhanced differentiation of pathogenic (p)Th17 cells, which was dependent on RORα transcription factor and interleukin-1 but independent of nucleotide oligomerization domain (NOD) 1 and 2. Overexpression of RIP2 resulted in suppression of pTh17 cell differentiation, an effect mediated by its CARD domain, and phenocopied by a cell-permeable RIP2 CARD peptide. Our data suggest that RIP2 has a T cell-intrinsic role in determining the balance between homeostatic and pathogenic Th17 cell responses.

The Bacterial Peptidoglycan-Sensing Molecules NOD1 and NOD2 Promote CD8+ Thymocyte Selection

Nucleotide-binding and oligomerization domain (NOD)-like receptors NOD1 and NOD2 are cytosolic innate immune receptors that recognize microbial peptidoglycans. Although studies have addressed the role of NOD proteins in innate immune responses, little attention has been given to their impact on the developing adaptive immune system. We have assessed the roles of NOD1 and NOD2 deficiency on T cell development in mice. Our results demonstrate that NOD1 and NOD2 promote the positive selection/maturation of CD8 single-positive thymocytes in a thymocyte-intrinsic manner. TCR-mediated ERK phosphorylation is significantly reduced in the absence of NOD proteins, but receptor-interacting protein 2 is not involved in CD8 single-positive thymocyte selection or ERK signaling. Commensal bacteria-free animals have thymocyte maturation defects, and exogenous NOD ligands can enhance thymocyte maturation in culture. These results raise the intriguing possibility that abnormal lymphocyte responses observed in NOD-dependent inflammatory diseases are not driven solely by microbial signals in the gut, but may also involve intrinsic lymphocyte defects resulting from impaired CD8 T cell thymic development.

Nod2 activates NF-kB in CD4+ T cells but its expression is dispensable for T cell-induced colitis

Although the etiology of Crohn's disease (CD) remains elusive this disease is characterized by T cell activation that leads to chronic inflammation and mucosal damage. A potential role for maladaptation between the intestinal microbiota and the mucosal immune response is suggested by the fact that mutations in the pattern recognition receptor Nod2 are associated with higher risks for developing CD. Although Nod2 deletion in CD4⁺ T cells has been shown to impair the induction of colitis in the murine T cell transfer model, the analysis of T cell intrinsic Nod2 function in T cell differentiation and T cell-mediated immunity is inconsistent between several studies. In addition, the role of T cell intrinsic Nod2 in regulatory T cell (Treg) development and function during colitis remain to be analyzed. In this study, we show that Nod2 expression is higher in activated/memory CD4⁺ T cells and its expression was inducible after T cell receptor (TCR) ligation. Nod2 stimulation with muramyl dipeptide (MDP) led to a nuclear accumulation of c-Rel NF-kB subunit. Although functionally active in CD4⁺ T cells, the deletion of Nod2 did not impair the induction and the prevention of colitis in the T cell transfer model. Moreover, Nod2 deletion did not affect the development of Foxp3⁺ Treg cells in the spleen of recipient mice and Nod2 deficient CD4 T cells expressing the OVA specific transgenic TCR were able to differentiate in Foxp3⁺ Treg cells after OVA feeding. In vitro, CD25⁺ Nod2 deficient T cells suppressed T cell proliferation as well as wild type counter parts and T cell stimulation with MDP did not affect the proliferation and the cytokine secretion of T cells. In conclusion, our data indicate that Nod2 is functional in murine CD4⁺ T cells but its expression is dispensable for the T cell regulation of colitis.

Recognition of Extracellular Bacteria by NLRs and Its Role in the Development of Adaptive Immunity

Innate immune recognition of bacteria is the first requirement for mounting an effective immune response able to control infection. Over the previous decade, the general paradigm was that extracellular bacteria were only sensed by cell surface-expressed Toll-like receptors (TLRs), whereas cytoplasmic sensors, including members of the Nod-like receptor (NLR) family, were specific to pathogens capable of breaching the host cell membrane. It has become apparent, however, that intracellular innate immune molecules, such as the NLRs, play key roles in the sensing of not only intracellular, but also extracellular bacterial pathogens or their components. In this review, we will discuss the various mechanisms used by bacteria to activate NLR signaling in host cells. These mechanisms include bacterial secretion systems, pore-forming toxins, and outer membrane vesicles. We will then focus on the influence of NLR activation on the development of adaptive immune responses in different cell types.

Innate receptors for adaptive immunity

Pattern recognition receptors (PRRs) are commonly known as sensor proteins crucial for the early detection of microbial or host-derived stress signals by innate immune cells. Interestingly, some PRRs are also expressed and functional in cells of the adaptive immune system. These receptors provide lymphocytes with innate sensing abilities; for example, B cells express Toll-like receptors, which are important for the humoral response. Strikingly, certain other NOD-like receptors are not only highly expressed in adaptive immune cells, but also exert functions related specifically to adaptive immune system pathways, such as regulating antigen presentation. In this review, we will focus particularly on the current understanding of PRR functions intrinsic to B and T lymphocytes; a developing aspect of PRR biology.

NOD1 cooperates with TLR2 to enhance T cell receptor-mediated activation in CD8 T cells

Pattern recognition receptors (PRR), like Toll-like receptors (TLR) and NOD-like receptors (NLR), are involved in the detection of microbial infections and tissue damage by cells of the innate immune system. Recently, we and others have demonstrated that TLR2 can additionally function as a costimulatory receptor on CD8 T cells. Here, we establish that the intracytosolic receptor NOD1 is expressed and functional in CD8 T cells. We show that C12-iEDAP, a synthetic ligand for NOD1, has a direct impact on both murine and human CD8 T cells, increasing proliferation and effector functions of cells activated via their T cell receptor (TCR). This effect is dependent on the adaptor molecule RIP2 and is associated with an increased activation of the NF-κB, JNK and p38 signaling pathways. Furthermore, we demonstrate that NOD1 stimulation can cooperate with TLR2 engagement on CD8 T cells to enhance TCR-mediated activation. Altogether our results indicate that NOD1 might function as an alternative costimulatory receptor in CD8 T cells. Our study provides new insights into the function of NLR in T cells and extends to NOD1 the recent concept that PRR stimulation can directly control T cell functions.

Essential role of Rip2 in the modulation of innate and adaptive immunity triggered by Nod1 and Nod2 ligands

Muramyl peptides are the building blocks of bacterial peptidoglycan, and their biological functions in mammals have been extensively studied. In particular, muramyl peptides trigger inflammation, contribute to host defense against microbial infections, and modulate the adaptive immune response to antigens. These bacterial molecules are detected by nucleotide oligomerization domain 1 (Nod1) and Nod2, and recent evidence suggests that muramyl dipeptide also activates NLRP3 and NLRP1 inflammasomes. Here, we investigated the role of Rip2, the adaptor for Nod1- and Nod2-dependent signaling, in multiple aspects of the host response to muramyl peptides in vivo, such as inflammatory cytokine secretion, activation and recruitment of macrophages and neutrophils to the site of injection, systemic activation of myeloid, T and B cells in the spleen, adjuvanticity and capacity to polarize the adaptive response to ovalbumin. Our results demonstrate that Rip2 was crucial for all the biological functions studied. We also identified CD11c(int) CD11b(+) inflammatory dendritic cells as a major myeloid cell population responding to Nod stimulation in vivo. Together, our results highlight the importance of Rip2 for Nod-dependent induction of innate and adaptive immunity.

Signaling via the RIP2 adaptor protein in central nervous system-infiltrating dendritic cells promotes inflammation and autoimmunity

Peripheral peptidolgycan (PGN) is present within antigen-presenting cells in the central nervous system (CNS) of multiple sclerosis (MS) patients, possibly playing a role in neuroinflammation. Accordingly, PGN is linked with disease progression in the animal model of MS, experimental autoimmune encephalomyelitis (EAE), but the role of specific PGN-sensing proteins is unknown. Here we report that the progression of EAE was dependent on the intracellular PGN sensors NOD1 and NOD2 and their common downstream adaptor molecule, receptor interacting protein 2 (RIP2; also known as RIPK2 and RICK). We found that RIP2, but not toll-like receptor 2 (TLR2), played a critical role in the activation of CNS-infiltrating dendritic cells. Our results suggest that PGN in the CNS is involved in the pathogenesis of EAE through the activation of infiltrating dendritic cells via NOD1-, NOD2-, and RIP2-mediated pathways.

T cell-intrinsic role of Nod2 in promoting type 1 immunity to Toxoplasma gondii

Nod2 belongs to the (NOD)-like receptor (NLR) family of proteins, which function as intracellular pathogen sensors in innate immune cells. Nod2-deficiency results in an impaired immune response against bacterial pathogens. However, our understanding of how this protein promotes host defense against intracellular parasites is unknown. Here we found that Nod2^−/− mice showed reduced clearance of Toxoplasma gondii and decreased interferon-γ production. Reconstitution of T-cell deficient mice with Nod2^−/− T cells followed by T. gondii infection revealed a T cell-intrinsic defect. Nod2^−/− CD4⁺ T cells displayed poor helper T cell differentiation, which was associated with impaired IL-2 production and nuclear accumulation of c-Rel. These data revealed a T cell-intrinsic role of Nod2 signaling that is critical for host defense against T. gondii.