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T helper cells integrate signals from their microenvironment to acquire distinct specialization programs for efficient clearance of diverse pathogens or for immunotolerance. Ionic signals have recently been demonstrated to affect T cell polarization and function. Sodium chloride (NaCl) was proposed to accumulate in peripheral tissues upon dietary intake and to promote autoimmunity via the Th17 cell axis. Here we demonstrate that high NaCl conditions induced a stable, pathogen-specific, anti-inflammatory Th17 cell fate in human T cells in vitro. The p38/MAPK pathway, involving NFAT5 and SGK1, regulated FoxP3 and interleukin (IL)-17A-expression in high-NaCl conditions. The NaCl-induced acquisition of an anti-inflammatory Th17 cell fate was confirmed in vivo in an experimental autoimmune encephalomyelitis (EAE) mouse model, which demonstrated strongly reduced disease symptoms upon transfer of T cells polarized in high NaCl conditions. However, NaCl was coopted to promote murine and human Th17 cell pathogenicity, if T cell stimulation occurred in a pro-inflammatory and TGF-β-low cytokine microenvironment. Taken together, our findings reveal a context-dependent, dichotomous role for NaCl in shaping Th17 cell pathogenicity. NaCl might therefore prove beneficial for the treatment of chronic inflammatory diseases in combination with cytokine-blocking drugs.

BackgroundHigh serum interleukin (IL-6) levels may cause resistance to immunotherapy by modulation of myeloid cells in the tumor microenvironment. IL-6 signaling blockade is tested in cancer, but as this inflammatory cytokine has pleiotropic effects, this treatment is not always effective.MethodsIL-6 and IL-6R blockade was applied in an IL-6-mediated immunotherapy-resistant TC-1 tumor model (TC-1.IL-6) and immunotherapy-sensitive TC-1.control. Effects on therapeutic vaccination-induced tumor regression, recurrence and survival as well on T cells and myeloid cells in the tumor microenvironment were studied. The effects of IL-6 signaling in macrophages under therapy conditions were studied in Il6rafl/fl×LysMcre+ mice.ResultsOur therapeutic vaccination protocol elicits a strong tumor-specific CD8+ T-cell response, leading to enhanced intratumoral T-cell infiltration and recruitment of tumoricidal macrophages. Blockade of IL-6 signaling exacerbated tumor outgrowth, reflected by fewer complete regressions and more recurrences after therapeutic vaccination, especially in TC-1.IL-6 tumor-bearing mice. Early IL-6 signaling blockade partly inhibited the development of the vaccine-induced CD8+ T-cell response. However, the main mechanism was the malfunction of macrophages during therapy-induced tumor regression. Therapy efficacy was impaired in Il6rafl/fl×LysMcre+ but not cre-negative control mice, while no differences in the vaccine-induced CD8+ T-cell response were found between these mice. IL-6 signaling blockade resulted in decreased expression of suppressor of cytokine signaling 3, essential for effective M1-type function in macrophages, and increased expression of the phagocytic checkpoint molecule signal-regulatory protein alpha by macrophages.ConclusionIL-6 signaling is critical for macrophage function under circumstances of immunotherapy-induced tumor tissue destruction, in line with the acute inflammatory functions of IL-6 signaling described in infections.

IL-6 binds to the IL-6R α-chain (IL-6Rα) and signals via the signal transducer gp130. Recently, IL-6 was found to also bind to the cell surface glycoprotein CD5, which would then engage gp130 in the absence of IL-6Rα. However, the biological relevance of this alternative pathway is under debate. In this study, we developed a mouse model, in which murine IL-6 is overexpressed in a CD11c-Cre–dependent manner. Transgenic mice developed a lethal immune dysregulation syndrome with increased numbers of Ly-6G+ neutrophils and Ly-6Chi monocytes/macrophages. IL-6 overexpression promoted activation of CD4+ T cells while suppressing CD5+ B-1a cell development. However, additional ablation of IL-6Rα protected IL-6–overexpressing mice from IL-6–triggered inflammation and fully phenocopied IL-6Rα–deficient mice without IL-6 overexpression. Mechanistically, IL-6Rα deficiency completely prevented downstream activation of STAT3 in response to IL-6. Altogether, our data clarify that IL-6Rα is the only biologically relevant receptor for IL-6 in mice.

The cellular sources of interleukin 6 (IL-6) that are relevant for differentiation of the TH17 subset of helper T cells remain unclear. Here we used a novel strategy for the conditional deletion of distinct IL-6-producing cell types to show that dendritic cells (DCs) positive for the signaling regulator Sirpα were essential for the generation of pathogenic TH17 cells. Using their IL-6 receptor α-chain (IL-6Rα), Sirpα+ DCs trans-presented IL-6 to T cells during the process of cognate interaction. While ambient IL-6 was sufficient to suppress the induction of expression of the transcription factor Foxp3 in T cells, trans-presentation of IL-6 by DC-bound IL-6Rα (called 'IL-6 cluster signaling' here) was needed to prevent premature induction of interferon-γ (IFN-γ) expression in T cells and to generate pathogenic TH17 cells in vivo. Our findings should guide therapeutic approaches for the treatment of TH17-cell-mediated autoimmune diseases.

The cellular sources of interleukin-6 (IL-6) that are relevant for the differentiation of TH17 cells remain unclear. Here, we used a novel strategy of IL-6 conditional deletion of distinct IL-6-producing cell types to show that Sirpα+ dendritic cells (DC) were essential for the generation of pathogenic TH17 cells. During the process of cognate interaction, Sirpα+ DCs trans-presented IL-6 to T cells using their own IL-6Rα. While ambient IL-6 was sufficient to suppress the induction of the transcription factor Foxp3 in T cells, IL-6 trans-presentation by DC-bound IL-6Rα (here defined as IL-6 cluster signaling) was required to prevent premature induction of IFN-γ in T cells and to generate pathogenic TH17 cells in vivo. These findings will guide therapeutic approaches for TH17-mediated autoimmune diseases.

Mitochondria vary in morphology and function in different tissues; however, little is known about their molecular diversity among cell types. Here we engineered MitoTag mice, which express a Cre recombinase-dependent green fluorescent protein targeted to the outer mitochondrial membrane, and developed an isolation approach to profile tagged mitochondria from defined cell types. We determined the mitochondrial proteome of the three major cerebellar cell types (Purkinje cells, granule cells and astrocytes) and identified hundreds of mitochondrial proteins that are differentially regulated. Thus, we provide markers of cell-type-specific mitochondria for the healthy and diseased mouse and human central nervous systems, including in amyotrophic lateral sclerosis and Alzheimer's disease. Based on proteomic predictions, we demonstrate that astrocytic mitochondria metabolize long-chain fatty acids more efficiently than neuronal mitochondria. We also characterize cell-type differences in mitochondrial calcium buffering via the mitochondrial calcium uniporter (Mcu) and identify regulator of microtubule dynamics protein 3 (Rmdn3) as a determinant of endoplasmic reticulum-mitochondria proximity in Purkinje cells. Our approach enables exploring mitochondrial diversity in many in vivo contexts.

Mitochondria vary in morphology and function in different tissues, however little is known about their molecular diversity among cell types. To investigate mitochondrial diversity in vivo, we developed an efficient protocol to isolate cell type-specific mitochondria based on a new MitoTag mouse. We profiled the mitochondrial proteome of three major neural cell types in cerebellum and identified a substantial number of differential mitochondrial markers for these cell types in mice and humans. Based on predictions from these proteomes, we demonstrate that astrocytic mitochondria metabolize long-chain fatty acids more efficiently than neurons. Moreover, we identified Rmdn3 as a major determinant of ER-mitochondria proximity in Purkinje cells. Our novel approach enables exploring mitochondrial diversity on the functional and molecular level in many in vivo contexts.

IL-17–producing CD8 + T (Tc17) cells are detectible in multiple sclerosis (MS) lesions; however, their contribution to the disease is unknown. To identify functions of Tc17 cells, we induced EAE, a murine model of MS, in mice lacking IFN regulatory factor 4 (IRF4). IRF4-deficient mice failed to generate Tc17 and Th17 cells and were resistant to EAE. After adoptive transfer of WT CD8 + T cells and subsequent immunization for EAE induction in these mice, the CD8 + T cells developed a Tc17 phenotype in the periphery but could not infiltrate the CNS. Similarly, transfer of small numbers of WT CD4 + T cells alone did not evoke EAE, but when transferred together with CD8 + T cells, IL-17–producing CD4 + (Th17) T cells accumulated in the CNS and mice developed severe disease. Th17 accumulation and development of EAE required IL-17A production by CD8 + T cells, suggesting that Tc17 cells are required to promote CD4 + T cell–mediated induction of EAE. Accordingly, patients with early-stage MS harbored a greater number of Tc17 cells in the cerebrospinal fluid than in peripheral blood. Our results reveal that Tc17 cells contribute to the initiation of CNS autoimmunity in mice and humans by supporting Th17 cell pathogenicity.

Th1 lymphocytes preferentially infiltrate into the spinal cord during EAE via a VLA-4–mediated mechanism while Th17 lymphocyte infiltration is dependent on LFA-1 expression., The integrin α4β1 (VLA-4) is used by encephalitogenic T cells to enter the central nervous system (CNS). However, both Th1 and Th17 cells are capable of inducing experimental autoimmune encephalomyelitis (EAE), and the molecular cues mediating the infiltration of Th1 versus Th17 cells into the CNS have not yet been defined. We investigated how blocking of α4 integrins affected trafficking of Th1 and Th17 cells into the CNS during EAE. Although antibody-mediated inhibition of α4 integrins prevented EAE when MOG35-55-specific Th1 cells were adoptively transferred, Th17 cells entered the brain, but not the spinal cord parenchyma, irrespective of α4 blockade. Accordingly, T cell–conditional α4-deficient mice were not resistant to actively induced EAE but showed an ataxic syndrome with predominantly supraspinal infiltrates of IL-23R+CCR6+CD4+ T cells. The entry of α4-deficient Th17 cells into the CNS was abolished by blockade of LFA-1 (αLβ2 integrin). Thus, Th1 cells preferentially infiltrate the spinal cord via an α4 integrin–mediated mechanism, whereas the entry of Th17 cells into the brain parenchyma occurs in the absence of α4 integrins but is dependent on the expression of αLβ2. These observations have implications for the understanding of lesion localization, immunosurveillance, and drug design in multiple sclerosis.

Summary Mice that lack interleukin-23 (IL-23) are resistant to T cell-mediated autoimmunity. Although IL-23 is a maturation factor for T helper 17 (Th17) cells, a subset of γδ T cells expresses the IL-23 receptor (IL-23R) constitutively. Using IL-23R reporter mice, we showed that γδ T cells were the first cells to respond to IL-23 during experimental autoimmune encephalomyelitis (EAE). Although γδ T cells produced Th17 cell-associated cytokines in response to IL-23, their major function was to prevent the development of regulatory T (Treg) cell responses. IL-23-activated γδ T cells rendered αβ effector T cells refractory to the suppressive activity of Treg cells and also prevented the conversion of conventional T cells into Foxp3 + Treg cells in vivo. Thus, IL-23, which by itself has no direct effect on Treg cells, is able to disarm Treg cell responses and promote antigen-specific effector T cell responses via activating γδ T cells.

Activation of naive CD8(+) T cells with antigen in the absence of skewing cytokines triggers their differentiation into effector CTL, which induces death of target cells. We show that CD8(+) T cells activated in the presence of the cytokines IL-6 or IL-21 plus TGF-beta similar to CD4(+) T cells, develop into IL-17-producing (Tc17) cells. These cells display greatly suppressed cytotoxic function along with low levels of the CTL markers: T-box transcription factor Eomesodermin, granzyme B and IFN-gamma. Instead, these cells express hallmark molecules of Th17 program including retinoic acid receptor-related orphan receptor (ROR)gammat, RORalpha, IL-21 and IL-23R. The expression of the type 17 master regulator RORgammat is causally linked to Tc17 generation, because its overexpression stimulates production of IL-17 in the presence of IL-6 or IL-21. Both, upregulation of the type 17 program as well as suppression of CTL differentiation are STAT3 dependent. Furthermore, Tc17 cells producing IL-17 but not granzyme B are also detectable in EAE, a mouse model for multiple sclerosis. Our data point to the existence of mutually exclusive CTL and Tc17 developmental pathways in vitro and in vivo.

The quality control of proteins mediated by the plasticity of the proteasome system is regulated by the timely and flexible formation of this multisubunit proteolytic enzyme complex. Adaptable biogenesis of the 20S proteasome core complex is therefore of vital importance for adjusting to changing proteolytic requirements. However, the molecular mechanism and the cellular sites of mammalian proteasome formation are still unresolved. By using precursor complex-specific antibodies, we now show that the main steps in 20S core complex formation take place at the endoplasmic reticulum (ER). Thereby, the proteasome maturation protein (POMP)--an essential factor of mammalian proteasome biogenesis--interacts with ER membranes, binds to alpha1-7 rings, recruits beta-subunits stepwise and mediates the association of mammalian precursor complexes with the ER. Thus, POMP facilitates the main steps in 20S core complex formation at the ER to coordinate the assembly process and to provide cells with freshly formed proteasomes at their site of function.

Interferon-regulatory factor 4 (IRF4) is essential for the development of T helper type 2 cells. Here we show that IRF4 is also critical for the generation of interleukin 17-producing T helper cells (T(H)-17 cells), which are associated with experimental autoimmune encephalomyelitis. IRF4-deficient (Irf4(-/-)) mice did not develop experimental autoimmune encephalomyelitis, and T helper cells from such mice failed to differentiate into T(H)-17 cells. Transfer of wild-type T helper cells into Irf4(-/-) mice rendered the mice susceptible to experimental autoimmune encephalomyelitis. Irf4(-/-) T helper cells had less expression of RORgammat and more expression of Foxp3, transcription factors important for the differentiation of T(H)-17 and regulatory T cells, respectively. Altered regulation of both transcription factors contributed to the phenotype of Irf4(-/-) T helper cells. Our data position IRF4 at the center of T helper cell development, influencing not only T helper type 2 but also T(H)-17 differentiation.

Peptide generation by the proteasome is rate-limiting in MHC class I-restricted antigen presentation in response to IFN-γ. IFN-γ-inducedde novoformation of immunoproteasomes, therefore, essentially supports the rapid adjustment of the mammalian immune system. Here, we report that the molecular interplay between the proteasome maturation protein (POMP) and the proteasomal β5i subunit low molecular weight protein 7 (LMP7) has a key position in this immune adaptive program. IFN-γ-induced coincident biosynthesis of POMP and LMP7 and their direct interaction essentially accelerate immunoproteasome biogenesis compared with constitutive 20S proteasome assembly. The dynamics of this process is determined by rapid LMP7 activation and the immediate LMP7-dependent degradation of POMP. Silencing of POMP expression impairs recruitment of both β5 subunits into the proteasome complex, resulting in decreased proteasome activity, reduced MHC class I surface expression, and induction of apoptosis. Furthermore, our data reveal that immunoproteasomes exhibit a considerably shortened half-life, compared with constitutive proteasomes. In consequence, our studies demonstrate that the cytokine-induced rapid immune adaptation of the proteasome system is a tightly regulated and transient response allowing cells to return rapidly to a normal situation once immunoproteasome function is no longer required.

Nat. Immunol. 18, 74–85 (2017); published online 28 November 2016; corrected after print 23 January 2017 In the version of this article initially published, the label for third bar from the left in Figure 4f ('Anti-IL-6α') was incorrect; the correct label is 'Anti-IL-6Rα'. Also, in the legend for Figure 4f, the description of the treatment conditions for middle four bars ('medium alone, or LPS and anti-IL-6 (MR16-1 or polyclonal antibody), anti-IL-6R (mAb#8)') was incorrect; the correct description is 'LPS alone (Medium), or LPS and anti-IL-6Rα (MR16-1), anti-IL-6 (polyclonal antibody or mAb#8)'.

Optic neuritis is a common inflammatory manifestation of multiple sclerosis (MS). In experimental autoimmune encephalomyelitis (EAE), the optic nerve is affected as well. Here, we investigated whether autoimmune inflammation in the optic nerve is distinct from inflammation in other parts of the central nervous system (CNS). In our study, inflammatory infiltrates in the optic nerve and the brain were characterized by a high fraction of Ly6G(+) granulocytes whereas in the spinal cord, macrophage infiltrates were predominant. At the peak of disease, IL-17 mRNA abundance was highest in the optic nerve as compared with other parts of the CNS. The ratio of IL-17 vs IFN-γ producing CD4(+) T cells was higher in the optic nerve and brain than in the spinal cord and more effector CD4(+) T cells were committed to the Th17 transcriptional program in the optic nerve than in the spinal cord. IL-17 producing γδ T cells but rather not Ly6G(+) granulocytes themselves contributed to IL-17 production. Optical coherence tomography (OCT) studies on murine eyes revealed a decline in thickness of the retinal nerve fiber layer (RNFL) and the common layer of ganglion cells and inner plexiform layer (GCL+) after the recovery from motor symptoms indicating that autoimmune inflammation induced a significant atrophy of optic nerve fibers during EAE. Neutralization of IL-17 by treatment with anti-IL-17 antibodies reduced but did not abrogate motor symptoms of EAE. However, RNFL and GCL+ atrophy were completely prevented by blocking IL-17. Thus, the optic nerve compartment is particularly prone to support IL-17 mediated inflammatory responses during CNS autoimmunity and structural integrity of the retina can be preserved by neutralizing IL-17.

Central nervous system (CNS) autoimmunity is regulated by the balance of pro-inflammatory cytokines and IL-10. Here we identify the transcriptional regulator Blimp1 as crucial to induce IL-10 in inflammatory T helper cells. Pre-committed Th17 cells respond to IL-27 and IL-12 by upregulating Blimp1 and adopt a Tr-1-like phenotype characterized by IL-10 and IFN-γ production. Accordingly, Blimp1-deficient effector T cells fail to produce IL-10, and deficiency in Tr-1 cell function leads to uncontrolled Th17 cell-driven CNS pathology without the need to stabilize the Th17 phenotype with IL-23. IL-23 counteracts IL-27 and IL-12-mediated effects on Tr-1-development reinforcing the pro-inflammatory phenotype of Th17 cells. Thus, the balance of IL-23 vs IL-12/IL-27 signals into CD4(+) effector T cells determines whether tissue inflammation is perpetuated or resolves.

Plasmacytoid dendritic cells (PDCs) have been shown to present Ags and to contribute to peripheral immune tolerance and to Ag-specific adaptive immunity. However, modulation of adaptive immune responses by selective Ag targeting to PDCs with the aim of preventing autoimmunity has not been investigated. In the current study, we demonstrate that in vivo Ag delivery to murine PDCs via the specifically expressed surface molecule sialic acid binding Ig-like lectin H (Siglec-H) inhibits Th cell and Ab responses in the presence of strong immune stimulation in an Ag-specific manner. Correlating with sustained low-level MHC class II-restricted Ag presentation on PDCs, Siglec-H–mediated Ag delivery induced a hyporesponsive state in CD4+ T cells leading to reduced expansion and Th1/Th17 cell polarization without conversion to Foxp3+ regulatory T cells or deviation to Th2 or Tr1 cells. Siglec-H–mediated delivery of a T cell epitope derived from the autoantigen myelin oligodendrocyte glycoprotein to PDCs effectively delayed onset and reduced disease severity in myelin oligodendrocyte glycoprotein-induced experimental autoimmune encephalomyelitis by interfering with the priming phase without promoting the generation or expansion of myelin oligodendrocyte glycoprotein-specific Foxp3+ regulatory T cells. We conclude that Ag delivery to PDCs can be harnessed to inhibit Ag-specific immune responses and prevent Th cell-dependent autoimmunity.

PI3Ks control signal transduction triggered by growth factors and G-protein-coupled receptors and regulate an array of biological processes, including cellular proliferation, differentiation, survival and migration. Herein, we investigated the role of PI3Kγ in the pathogenesis of EAE. We show that, in the absence of PI3Kγ expression, clinical signs of EAE were delayed and mitigated. PI3Kγ-deficient myelin oligodendrocyte glycoprotein (MOG)(35-55) -specific CD4(+) T cells appeared later in the secondary lymphoid organs and in the CNS than their WT counterparts. Transfer of WT CD4(+) cells into PI3Kγ(-/-) mice prior to MOG(35-55) immunisation restored EAE severity to WT levels, supporting the relevance of PI3Kγ expression in Th cells for the pathogenesis of EAE; however, PI3Kγ was dispensable for Th1 and Th17 differentiation, thus excluding an altered expression of these pathogenetically relevant cytokines as the cause for ameliorated EAE in PI3Kγ(-/-) mice. These findings demonstrate that PI3Kγ contributes to the development of autoimmune CNS inflammation.

Activation of naive CD8(+) T cells with antigen in the absence of skewing cytokines triggers their differentiation into effector CTL, which induces death of target cells. We show that CD8(+) T cells activated in the presence of the cytokines IL-6 or IL-21 plus TGF-beta similar to CD4(+) T cells, develop into IL-17-producing (Tc17) cells. These cells display greatly suppressed cytotoxic function along with low levels of the CTL markers: T-box transcription factor Eomesodermin, granzyme B and IFN-gamma. Instead, these cells express hallmark molecules of Th17 program including retinoic acid receptor-related orphan receptor (ROR)gammat, RORalpha, IL-21 and IL-23R. The expression of the type 17 master regulator RORgammat is causally linked to Tc17 generation, because its overexpression stimulates production of IL-17 in the presence of IL-6 or IL-21. Both, upregulation of the type 17 program as well as suppression of CTL differentiation are STAT3 dependent. Furthermore, Tc17 cells producing IL-17 but not granzyme B are also detectable in EAE, a mouse model for multiple sclerosis. Our data point to the existence of mutually exclusive CTL and Tc17 developmental pathways in vitro and in vivo.

The immune system can recognize antigenic peptides derived from tumors by their presentation on MHC class I complexes to CTLs. Immunoproteasomes (i20S) can substantially enhance the MHC class I peptide repertoire, making down-regulation of i20S an important strategy of tumor cells in manipulating immune surveillance. Here, we report that human cancer cells express the nonfunctional immunosubunit-variant LMP7E1, in addition to, or instead of LMP7E2, in response to IFN-γ. This preferential expression of LMP7E1 and the consequent down-regulation of LMP7E2 results in i20S deficiency. The molecular explanation for this phenomenon is the incapacity of LMP7E1 to interact efficiently with the proteasome maturation protein, which regularly recruits LMP7E2 into nascent i20S precursor complexes. In contrast to previous reports, i20S formation in these cancer cells cannot be restored by IFN-γ treatment. However, expression of LMP7E2 in these cells restores the i20S-deficient phenotype. Thus, our data describe a novel mechanism that contributes to the process of oncogenesis. (Cancer Res 2006; 66(2):649-52)

The proteasome system is a central component of a cascade of proteolytic processing steps required to generate antigenic peptides presented at the cell surface to cytotoxic T lymphocytes by major histocompatibility complex (MHC) class I molecules. The nascent protein pool or DRiPs (defective ribosomal products) appear to represent an important source for MHC class I epitopes. Owing to the destructive activities of aminopeptidases in the cytosol, at most 1% of the peptides generated by the ubiquitin-proteasome system seems to be made available to the immune system. Interferon-gamma (IFN-gamma) helps to override these limitations by the formation of immunoproteasomes, the activator complex PA28, and the induction of several aminopeptidases. Both immunoproteasomes and PA28 use cleavage sites already used by constitutive proteasomes but with altered and in some cases dramatically enhanced frequency. Therefore, two proteolytic cascades appear to have evolved to provide MHC class I epitopes. The 'constitutive proteolytic cascade' is designed to efficiently degrade proteins to single amino acid residues, allowing only a small percentage of peptides to be presented at the cell surface. In contrast, the IFN-gamma-controlled proteolytic cascade generates larger amounts of appropriate antigenic peptides, assuring more peptides to overcome the proteolytic restrictions of the constitutive system, thereby enhancing MHC class I antigen presentation.

Transgenic expression of B- and T-cell receptors (BCRs and TCRs, respectively) has been a standard tool to study lymphocyte development and function in vivo. The generation of transgenic mice is time-consuming and, therefore, a faster method to study the biology of defined lymphocyte receptors in vivo would be highly welcome. Using 2A peptide-linked multicistronic retroviral vectors to transduce stem cells, TCRs can be expressed rapidly in mice of any background. We aimed at adopting this retrogenic technology to the in vivo expression of BCRs. Using a well characterised BCR specific for hen egg lysozyme (HEL), we achieved surface expression of the retrogenically encoded BCR in a Rag-deficient pro B-cell line in vitro. In vivo, retrogenic BCRs were detectable only intracellularly but not on the surface of B cells from wild type or Rag2-deficient mice. This data, together with the fact that no BCR retrogenic mouse model has been published in the 7 years since the method was originally published for TCRs, strongly suggests that achieving BCR-expression in vivo with retrogenic technology is highly challenging if not impossible.

Interleukin-6 (IL-6) is a pivotal cytokine with a diverse repertoire of physiological functions that include regulation of immune cell proliferation and differentiation. Dysregulation of IL-6 signalling is associated with inflammatory and lymphoproliferative disorders such as rheumatoid arthritis and Castleman disease, and several classes of therapeutics have been developed that target components of the IL-6 signalling pathway. So far, monoclonal antibodies against IL-6 or IL-6 receptor (IL-6R) and Janus kinases (JAK) inhibitors have been successfully developed for the treatment of autoimmune diseases such as rheumatoid arthritis. However, clinical trials of agents targeting IL-6 signalling have also raised questions about the diseases and patient populations for which such agents have an appropriate benefit-risk profile. Knowledge from clinical trials and advances in our understanding of the complexities of IL-6 signalling, including the potential to target an IL-6 trans-signalling pathway, are now indicating novel opportunities for therapeutic intervention. In this Review, we overview the roles of IL-6 in health and disease and analyse progress with several approaches of inhibiting IL-6-signalling, with the aim of illuminating when and how to apply IL-6 blockade.

INTRODUCTION: Natalizumab blocks α4-integrins and is a prototypic agent for a series of anti-inflammatory drugs that impair trafficking of immune cells into the CNS. However, modulation of the access of immune cells to the CNS is associated with impaired immune surveillance and detrimental viral infections of the CNS. Here, we explored the potency of cellular immune responses within the CNS to protect against viral encephalitis in mice with T cell conditional disruption of VLA-4 integrin (α4β1) expression. RESULTS: While VLA-4 expression in virus specific Th1 cells is non-redundant for their ability to access the CNS, α4-integrin deficient Th17 cells enter the CNS compartment and generate an inflammatory milieu upon intrathecal vaccinia virus (VV) infection. However, in contrast to Th1 cells that can adopt direct cytotoxic properties, Th17 cells fail to clear the virus due to insufficient Eomes induced perforin-1 expression. CONCLUSION: The quality of the intrathecal cellular antiviral response under conditions of impaired VLA-4 function jeopardizes host protection. Our functional in vivo data extend our mechanistic understanding of anti-viral immunity in the CNS and help to estimate the risk potential of upcoming therapeutic agents that target the trafficking of immune cells into distinct anatomical compartments.