The solute carrier SLC15A4 is required for optimal trafficking of nucleic acid-sensing TLRs and ligands to endolysosomes

A function-impairing mutation (feeble) or genomic deletion of SLC15A4 abolishes responses of nucleic acid–sensing endosomal toll-like receptors (TLRs) and significantly reduces disease in mouse models of lupus. Here, we demonstrate disease reduction in homozygous and even heterozygous Slc15a4 feeble mutant BXSB male mice with a Tlr7 gene duplication. In contrast to SLC15A4, a function-impairing mutation of SLC15A3 did not diminish type I interferon (IFN-I) production by TLR-activated plasmacytoid dendritic cells (pDCs), indicating divergence of function between these homologous SLC15 family members. Trafficking to endolysosomes and function of SLC15A4 were dependent on the Adaptor protein 3 (AP-3) complex. Importantly, SLC15A4 was required for trafficking and colocalization of nucleic acid–sensing TLRs and their ligands to endolysosomes and the formation of the LAMP2+VAMP3+ hybrid compartment in which IFN-I production is initiated. Collectively, these findings define mechanistic processes by which SLC15A4 controls endosomal TLR function and suggest that pharmacologic intervention to curtail the function of this transporter may be a means to treat lupus and other endosomal TLR-dependent diseases.

Silica exposure and chronic virus infection synergistically promote lupus-like systemic autoimmunity in mice with low genetic predisposition

Considerable evidence indicates that autoimmune disease expression depends on both genetic and environmental factors. Among potential environmental triggers, occupational airway exposure to crystalline silica and virus infections have been linked to lupus and other autoimmune diseases in both humans and mouse models. Here, we hypothesized that combined silica and virus exposures synergize and induce autoimmune manifestations more effectively than single exposure to either of these factors, particularly in individuals with low genetic predisposition. Accordingly, infection with the model murine pathogen lymphocytic choriomenigitis virus (LCMV) in early life, followed by airway exposure to crystalline silica in adult life, induced lupus-like autoantibodies to several nuclear self-antigens including chromatin, RNP and Sm, concurrent with kidney lesions, in non-autoimmune C57BL/6 (B6) mice. In contrast, given individually, LCMV or silica were largely ineffectual in this strain. These results support a multihit model of autoimmunity, where exposure to different environmental factors acting on distinct immunostimulatory pathways complements limited genetic predisposition and increases the risk of autoimmunity above a critical threshold.

Lupus acceleration by a MAVS-activating RNA virus requires endosomal TLR signaling and host genetic predisposition

Viruses have long been implicated in the pathogenesis of autoimmunity, yet their contribution remains circumstantial partly due to the lack of well-documented information on infections prior to autoimmune disease onset. Here, we used the lymphocytic choriomeningitis virus (LCMV) as a model to mechanistically dissect the impact of viral infection on lupus-like autoimmunity. Virus persistence strongly enhanced disease in mice with otherwise weak genetic predisposition but not in highly predisposed or non-autoimmune mice, indicating a synergistic interplay between genetic susceptibility and virus infection. Moreover, endosomal Toll-like receptors (TLRs) and plasmacytoid dendritic cells (pDCs) were both strictly required for disease acceleration, even though LCMV also induces strong TLR-independent type I interferon (IFN-I) production via RNA helicases and MAVS in conventional DCs. These results suggest that LCMV enhances systemic autoimmunity primarily by providing stimulatory nucleic acids for endosomal TLR engagement, whereas overstimulation of the MAVS-dependent cytosolic pathway in the absence of endosomal TLR signaling is insufficient for disease induction.

Murine Models for Viral Hemorrhagic Fever

Hemorrhagic fever (HF) viruses, such as Lassa, Ebola, and dengue viruses, represent major human health risks due to their highly contagious nature, the severity of the clinical manifestations induced, the lack of vaccines, and the very limited therapeutic options currently available. Appropriate animal models are obviously critical to study disease pathogenesis and develop efficient therapies. We recently reported that the clone 13 (Cl13) variant of the lymphocytic choriomeningitis virus (LCMV-Cl13), a prototype arenavirus closely related to Lassa virus, causes in some mouse strains endothelial damage, vascular leakage, platelet loss, and death, mimicking pathological aspects typically observed in Lassa and other HF syndromes. This model has the advantage that the mice used are fully immunocompetent, allowing studies on the contribution of the immune response to disease progression. Moreover, LCMV is very well characterized and exhibits limited pathogenicity in humans, allowing handling in convenient BSL-2 facilities. In this chapter we outline protocols for the induction and analysis of arenavirus-mediated pathogenesis in the NZB/LCMV model, including mouse infection, virus titer determination, platelet counting, phenotypic analysis of virus-specific T cells, and assessment of vascular permeability.

Induction of Systemic Autoimmunity by a Xenobiotic Requires Endosomal TLR Trafficking and Signaling from the Late Endosome and Endolysosome but Not Type I IFN

Type I IFN and nucleic acid-sensing TLRs are both strongly implicated in the pathogenesis of lupus, with most patients expressing IFN-induced genes in peripheral blood cells and with TLRs promoting type I IFNs and autoreactive B cells. About a third of systemic lupus erythematosus patients, however, lack the IFN signature, suggesting the possibility of type I IFN-independent mechanisms. In this study, we examined the role of type I IFN and TLR trafficking and signaling in xenobiotic systemic mercury-induced autoimmunity (HgIA). Strikingly, autoantibody production in HgIA was not dependent on the type I IFN receptor even in NZB mice that require type I IFN signaling for spontaneous disease, but was dependent on the endosomal TLR transporter UNC93B1 and the endosomal proton transporter, solute carrier family 15, member 4. HgIA also required the adaptor protein-3 complex, which transports TLRs from the early endosome to the late endolysosomal compartments. Examination of TLR signaling pathways implicated the canonical NF-κB pathway and the proinflammatory cytokine IL-6 in autoantibody production, but not IFN regulatory factor 7. These findings identify HgIA as a novel type I IFN-independent model of systemic autoimmunity and implicate TLR-mediated NF-κB proinflammatory signaling from the late endocytic pathway compartments in autoantibody generation.

An agonist antibody that blocks autoimmunity by inducing anti-inflammatory macrophages

We have devised a method of using intracellular combinatorial libraries to select antibodies that control cell fates. Many agonist antibodies have been selected with this method, and the process appears to be limited only by the availability of a phenotypic selection system. We demonstrate the utility of this approach to discover agonist antibodies that engage an unanticipated target and regulate macrophage polarization by selective induction of anti-inflammatory M2 macrophages. This antibody was used therapeutically to block autoimmunity in a classic mouse model of spontaneous systemic lupus erythematosus.

Anti-IFN-α/β receptor antibody treatment ameliorates disease in lupus-predisposed mice

The demonstration in humans and mice that nucleic acid-sensing TLRs and type I IFNs are essential disease mediators is a milestone in delineating the mechanisms of lupus pathogenesis. In this study, we show that Ifnb gene deletion does not modify disease progression in NZB mice, thereby strongly implicating IFN-α subtypes as the principal pathogenic effectors. We further document that long-term treatment of male BXSB mice with an anti-IFN-α/β receptor Ab of mouse origin reduced serologic, cellular, and histologic disease manifestations and extended survival, suggesting that disease acceleration by the Tlr7 gene duplication in this model is mediated by type I IFN signaling. The efficacy of this treatment in BXSB mice was clearly evident when applied early in the disease process, but only partial reductions in some disease characteristics were observed when treatment was initiated at later stages. A transient therapeutic effect was also noted in the MRL-Fas(lpr) model, although overall mortality was unaffected. The combined findings suggest that IFN-α/β receptor blockade, particularly when started at early disease stages, may be a useful treatment approach for human systemic lupus erythematosus and other autoimmune syndromes.

Transmethylation in immunity and autoimmunity

The activation of immune cells is mediated by a network of signaling proteins that can undergo post-translational modifications critical for their activity. Methylation of nucleic acids or proteins can have major effects on gene expression as well as protein repertoire diversity and function. Emerging data indicate that indeed many immunologic functions, particularly those of T cells, including thymic education, differentiation and effector function are highly dependent on methylation events. The critical role of methylation in immunocyte biology is further documented by evidence that autoimmune phenomena may be curtailed by methylation inhibitors. Additionally, epigenetic alterations imprinted by methylation can also exert effects on normal and abnormal immune responses. Further work in defining methylation effects in the immune system is likely to lead to a more detailed understanding of the immune system and may point to the development of novel therapeutic approaches.

Systemic autoimmunity and lymphoproliferation are associated with excess IL-7 and inhibited by IL-7Rα blockade

Lupus is characterized by disturbances in lymphocyte homeostasis, as demonstrated by the marked accumulation of activated/memory T cells. Here, we provide evidence that proliferation of the CD8⁺ precursors for the accumulating CD4^–CD8^– T cells in MRL-Fas^lpr lupus-predisposed mice is, in part, driven by commensal antigens. The ensuing lymphadenopathy is associated with increased production of IL-7 due to expansion of fibroblastic reticular cells, the primary source of this cytokine. The excess IL-7 is not, however, consumed by CD4^–CD8^– T cells due to permanent down-regulation of IL-7Rα (CD127), but instead supports proliferation of autoreactive T cells and progression of autoimmunity. Accordingly, IL-7R blockade reduced T cell activation and autoimmune manifestations even when applied at advanced disease stage. These findings indicate that an imbalance favoring production over consumption of IL-7 may contribute to systemic autoimmunity, and correction of this imbalance may be a novel therapeutic approach in lymphoproliferative and autoimmune syndromes.

Deletion of IgG-switched autoreactive B cells and defects in Fas(lpr) lupus mice

During a T cell-dependent Ab response, B cells undergo Ab class switching and V region hypermutation, with the latter process potentially rendering previously innocuous B cells autoreactive. Class switching and hypermutation are temporally and anatomically linked with both processes dependent on the enzyme, activation-induced deaminase, and occurring principally, but not exclusively, in germinal centers. To understand tolerance regulation at this stage, we generated a new transgenic mouse model expressing a membrane-tethered gamma2a-reactive superantigen (gamma2a-macroself Ag) and assessed the fate of emerging IgG2a-expressing B cells that have, following class switch, acquired self-reactivity of the Ag receptor to the macroself-Ag. In normal mice, self-reactive IgG2a-switched B cells were deleted, leading to the selective absence of IgG2a memory responses. These findings identify a novel negative selection mechanism for deleting mature B cells that acquire reactivity to self-Ag. This process was only partly dependent on the Bcl-2 pathway, but markedly inefficient in MRL-Fas(lpr) lupus mice, suggesting that defective apoptosis of isotype-switched autoreactive B cells is central to Fas mutation-associated systemic autoimmunity.

Commitment to the regulatory T cell lineage requires CARMA1 in the thymus but not in the periphery

Regulatory T (T_reg) cells expressing forkhead box P3 (Foxp3) arise during thymic selection among thymocytes with modestly self-reactive T cell receptors. In vitro studies suggest Foxp3 can also be induced among peripheral CD4⁺ T cells in a cytokine dependent manner. T_reg cells of thymic or peripheral origin may serve different functions in vivo, but both populations are phenotypically indistinguishable in wild-type mice. Here we show that mice with a Carma1 point mutation lack thymic CD4⁺Foxp3⁺ T_reg cells and demonstrate a cell-intrinsic requirement for CARMA1 in thymic Foxp3 induction. However, peripheral Carma1-deficient T_reg cells could be generated and expanded in vitro in response to the cytokines transforming growth factor beta (TGFβ) and interleukin-2 (IL-2). In vivo, a small peripheral T_reg pool existed that was enriched at mucosal sites and could expand systemically after infection with mouse cytomegalovirus (MCMV). Our data provide genetic evidence for two distinct mechanisms controlling regulatory T cell lineage commitment. Furthermore, we show that peripheral T_reg cells are a dynamic population that may expand to limit immunopathology or promote chronic infection.

In mammals, CD4⁺ T cells are essential for controlling infections, but have the potential to attack host tissues as well, resulting in autoimmune disease. A subset of CD4⁺ T cells, regulatory T cells (T_reg)—identified by the expression of the forkhead transcription factor Foxp3—serve to prevent immunopathology by dampening immune responses. These cells are unique among CD4⁺ T cell subsets, as only the T_reg lineage can develop in both the thymus and periphery. Using a genetic approach, we identified a mutation in the gene Carma1, a key component of T and B cell signaling, which in mice distinguishes T_reg cells derived from the periphery from thymic-derived regulatory T cells. The mutation caused an absence of thymic T_reg cells. However, a small population of T_reg cells was observed in the spleen, lymph nodes, and colon of Carma1-mutant mice that expanded after viral infection, suggesting that peripheral development of T_reg cells could still occur. Indeed, Carma1-mutant CD4⁺ T cells could be converted into the T_reg lineage in vitro. Our results demonstrate an organ-specific requirement for the CARMA1 signaling pathway that developing thymocytes need in order to become T_reg cells, but that naïve CD4⁺ T cells can bypass in the periphery. This dichotomy suggests that T_reg cells of thymic or peripheral origin may have different specificities or functions in vivo.

The organ-specific requirement for CARMA1-dependent signaling in the thymus suggests that regulatory T cells of thymic or peripheral origin may have different roles in vivo.

γδ T Cell Homeostasis Is Controlled by IL-7 and IL-15 Together with Subset-Specific Factors

Among T cell subsets, gamma delta T cells uniquely display an Ag receptor-based tissue distribution, but what defines their preferential homing and homeostasis is unknown. To address this question, we studied the resources that control gamma delta T cell homeostasis in secondary lymphoid organs. We found that gamma delta and alpha beta T cells are controlled by partially overlapping resources, because acute homeostatic proliferation of gamma delta T cells was inhibited by an intact alpha beta T cell compartment, and both populations were dependent on IL-7 and IL-15. Significantly, to undergo acute homeostatic proliferation, gamma delta T cells also required their own depletion. Thus, gamma delta T cell homeostasis is maintained by trophic cytokines commonly used by other types of lymphoid cells, as well as by additional, as yet unidentified, gamma delta-specific factors.

Tumor immunity via homeostatic T cell proliferation: mechanistic aspects and clinical perspectives

Efforts to develop effective anti-tumor immunotherapies are hampered by the difficulty of overcoming tolerance against tumor antigens, which in most instances are normal gene products that are over-expressed, preferentially expressed or re-expressed in cancer cells. Considering that lymphopenia-induced homeostatic T cell proliferation is mediated by self-peptide/MHC recognition and that the expanded cells acquire some effector functions, we hypothesized that this process could be used to break tolerance against tumor antigens. Studies by us and others in several mouse models demonstrated that availability of tumor antigens during homeostatic T cell proliferation indeed leads to effective anti-tumor autoimmunity with specificity and memory. This effect appears to be mediated by reduction in the activation threshold of low-affinity tumor-specific T cells, leading to their preferential engagement and expansion. In its simplicity, this approach is likely to have application in humans, since it relies on conventional lymphopenia-inducing cancer therapies, infusion of autologous lymphocytes and, optimally, tumor-specific vaccination.

Human adult tonsil xenotransplantation into SCID mice for studying human immune responses and B cell lymphomagenesis

OBJECTIVE

To generate a human-mouse xenochimeric model where human cells remain clustered in the animal to optimize their interactions and recovery.

MATERIALS AND METHODS

Severe combined immune deficient mice (SCID) were xenografted subcutaneously with human adult tonsil pieces (hu-ton-SCID mice). Such animals were: (a) compared with those receiving tonsil cells in suspension, and (b) immunized with de novo and recall antigens.

RESULTS

Human tonsil pieces survived a long period of time in SCID mice, while polyclonal human T- and B-lymphocytes persisted in close vicinity within the implantation area; however, little or no graft-versus-host disease was detectable. Not surprisingly, local development of lymphoproliferative disease was often observed in animals receiving lymphoid implants from donors previously infected by the Epstein-Barr virus. One month after surgery, higher serum levels of human IgG were found in SCID mice transplanted with tonsil pieces (2x10(7) cells/animal) than in animals injected with 5x10(7) tonsil cells in suspension (1.9 vs. 0.3 mg/mL, p < 0.002). Importantly, the production of human IgG in hu-ton-SCID mice remained polyclonal for at least 6 months and was linked to the presence of cells within the implants. Immunization of hu-ton-SCID mice with hepatitis B core, a de novo antigen, did not produce a significant IgG immune response; however immunization with tetanus toxoid (TT), a thymus-dependent recall antigen, yielded high (> 700-fold increase in anti-TT IgG levels) and long-lasting (> 6 months) secondary immune responses.

CONCLUSION

The hu-ton-SCID mouse xenochimeric model described in this report may improve our understanding of human lymphoid cell interactions, secondary immune responses, and lymphomagenesis.

Fas (CD95) participates in peripheral T cell deletion and associated apoptosis in vivo

Following exposure to some types of antigen (superantigens), responsive T cells expand and then decline in numbers, a phenomenon that has been called 'peripheral deletion'. This process may play a role in limiting autoimmune reactions and in the maintenance of immune homeostasis. Here we describe experiments on peripheral deletion in mice carrying the lpr/lpr defect, which has been shown to be due to defective production of the CD95/Fas molecule. Young lpr/lpr mice with no apparent immunologic abnormalities display a defect in bacterial superantigen-induced peripheral deletion. Apoptotic death of the expanded T cell population associated with such peripheral deletion. Apoptotic death of the expanded T cell population associated with such peripheral deletion in normal animals is dramatically reduced in the mutant mice. Further, the levels of Fas on responding cells in normal mice increases and decreases together with increases and decreases in cell numbers, suggesting that cells with the highest levels of Fas are preferentially deleted. These observations are consistent with the known ability of CD95 to transduce a signal leading to apoptosis, and they implicate this signal transduction pathway in peripheral deletion. In contrast, bacterial superantigen-induced deletion of thymocytes appears to be fully functional in these mice, and thus Fas/APO-1 does not appear to be required for this process. Further, antibody ligation of the TCR on activated T cells from normal or young lpr/lpr mice can induce apoptosis and therefore under some circumstances this phenomenon is not dependent upon CD95/Fas. Thus, to avoid autoreactivity and ensure immune homeostasis, several different apoptotic mechanisms exist in peripheral T lymphocytes, only some of which involve Fas.(ABSTRACT TRUNCATED AT 250 WORDS)