The Emerging World of Microbiome in Autoimmune Disorders: Opportunities and Challenges.

Trillions of commensal bacteria colonizing humans (microbiome) have emerged as essential player(s) in human health. The alteration of the same has been linked with diseases including autoimmune disorders such as multiple sclerosis, rheumatoid arthritis, systemic lupus erythematosus, and ankylosing spondylitis. Gut bacteria are separated from the host through a physical barrier such as skin or gut epithelial lining. However, the perturbation in the healthy bacterial community (gut dysbiosis) can compromise gut barrier integrity, resulting in translocation of bacterial contents across the epithelial barrier (leaky gut). Bacterial contents such as lipopolysaccharide and bacterial antigens can induce a systemic inflammatory environment through activation and induction of immune cells. The biggest question in the field is whether inflammation causes gut dysbiosis or dysbiosis leads to disease induction or propagation, i.e., it is inside out or outside in or both. In this review, we first discuss the microbiome profiling studies in various autoimmune disorders, followed by a discussion of potential mechanisms through which microbiome is involved in the pathobiology of diseases. A better understanding of the role of the microbiome in health and disease will help us harness the power of commensal bacteria for the development of novel therapeutic agents to treat autoimmune disorders.

Cancer-associated immunodeficiency and dendritic cell abnormalities mediated by the prostaglandin EP2 receptor.

Prostaglandin E(2) (PGE(2)), a major COX metabolite, plays important roles in several facets of tumor biology. We characterized the contribution of the PGE(2) EP2 receptor to cancer-associated immune deficiency using EP2(-/-) mice. EP2(-/-) mice exhibited significantly attenuated tumor growth and longer survival times when challenged with MC26 or Lewis lung carcinoma cell lines as compared with their wild-type littermates. While no differences in T cell function were observed, PGE(2) suppressed differentiation of DCs from wild-type bone marrow progenitors, whereas EP2-null cells were refractory to this effect. Stimulation of cells in mixed lymphocyte reactions by wild-type DCs was suppressed by treatment with PGE(2), while EP2(-/-)-derived DCs were resistant to this effect. In vivo, DCs, CD4(+), and CD8(+) T cells were significantly more abundant in draining lymph nodes of tumor-bearing EP2(-/-) mice than in tumor-bearing wild-type mice, and a significant antitumor cytotoxic T lymphocyte response could be observed only in the EP2(-/-) animals. Our data demonstrate an important role for the EP2 receptor in PGE(2)-induced inhibition of DC differentiation and function and the diminished antitumor cellular immune responses in vivo.

Effects of the physico-chemical nature of two biomimetic crystals on the innate immune response.

The influence of the physico-chemical features of particulates made of calcium phosphate (hydroxyapatite, HAP) crystals, or monosodium urate monohydrate (MSUM) crystals, on the innate immune response was investigated in mice after intraperitoneal injections. The phenotype and activation status of harvested peritoneal cells from C57BL/6 mice was determined by flow cytometry analysis at 24, 48 and 72 h after particulate injections and compared to a known adjuvant, aluminum phosphate (ALP). A rigorous characterization of the chemistry, structure, morphology and particle size of the particulates was completed. Mid-sized (10 mum mean size) particulates of both crystal types recruited the most cells, as compared to fine (1 mum) or large (100 mum) particulates. Analysis of sub-populations of the peritoneal cells revealed that MSUM induced fewer PMNs and eosinophils than HAP or ALP. MSUM also had the greatest effect on the expression of CD11b, MHC-Class II and CD86 on peritoneal macrophages indicating MSUM provides a robust antigen presenting and co-stimulatory bridge between the innate and adaptive immune systems. This study indicates that manipulation of the physico-chemical features of particulates is a means of controlling the innate immune response and that knife-like morphologies are more stimulatory than spherical or plate-like shapes. Proper utilization of the physico-chemical features of particulates offers a new direction for the development of more effective vaccine adjuvants.

Effects of LPS-mediated bystander activation in the innate immune system.

LPS induces dendritic cell (DC) activation, but the precise in vivo mechanism is unclear since DCs express low levels of TLR4. Here, it is shown that DCs can be activated in response to LPS through a bystander mechanism. This result was obtained using chimeric mice reconstituted with LPS-responsive and nonresponsive bone marrow cells. Thus, after indirect in vivo conditioning by LPS, bystander-activated DCs (LPS nonresponsive) up-regulated CD86. This up-regulation occurred even when LPS-responsive cells were MyD88 deficient. Functional analysis demonstrated that in vivo LPS conditioning endowed both the LPS-responsive and bystander cells with the ability to produce IFN-gamma in response to TLR9 stimulation in vitro. IFN-gamma production was also shown to be important for enhanced T-bet gene expression but not important for up-regulation of CD86. To investigate aspects of the mechanism, we used intracellular cytokine staining and found that NKDCs were responsible for at least some of the IFN-gamma production. Thus, our in vivo results demonstrated that bacterial LPS can bridge activation of various cellular populations of the innate immune system through a bystander mechanism.

Successful use of Rituximab in a patient with recalcitrant multisystemic eosinophilic granulomatosis with polyangiitis.

Eosinophilic granulomatosis with polyangiitis (EGPA) is a rare eosinophil-rich disorder characterised by necrotising granulomatous inflammation affecting small to medium sized vessels. Extrapulmonary manifestations can be life-threatening when heart, central nervous system (CNS), gastrointestinal tract or kidneys are affected. We describe a case of a 56-year-old woman with a long-standing history of asthma, who presented with an acute sudden painless loss of vision after she had been recently diagnosed with EGPA and induced with pulse steroids and cyclophosphamide. The patient had a complicated hospital course with multisystemic involvement of active vasculitis, involving heart, kidneys, muscles, eyes and CNS. The patient's devastating condition responded remarkably to Rituximab. The role of Rituximab in EGPA is not yet proven. Few cases are reported in the literature about the role of Rituximab in EGPA, of which only one described retinal artery occlusion as a presentation of a recently treated patient with EGPA.

Lupus-Associated Pulmonary Arterial Hypertension: Variable Course and Importance of Prompt Recognition.

We describe a critically ill young woman with systemic lupus erythematosus (SLE) presenting with circulatory shock, multiorgan dysfunction, and elevated right-sided heart pressures. She was found to have recurrent acute severe pulmonary arterial hypertension (PAH) in the setting of an SLE flare. Our report highlights the variable course that SLE-associated PAH can take in the same patient and the implications of this for instituting the most effective treatment approach with each episode. This report also highlights the potential for SLE-associated PAH to present with life-threatening symptoms requiring critical care level interventions. We also describe evidence-based therapies, which can result in significant improvement in symptoms, function, and long-term outcomes.

IL-18 bridges innate and adaptive immunity through IFN-gamma and the CD134 pathway.

IL-18 induces inflammation resulting in either enhanced protection from pathogens or exacerbation of autoimmunity, and T cells are profoundly activated during these responses. How IL-18 influences T cell activation is unknown, but this study in mice shows that IL-18 boosted Ag-specific T cell clonal expansion of effector T cells and induced a subpopulation of IFN-gamma superproducing T cells. Commitment to IFN-gamma production through IL-18 was independent of NK cells and IL-12 but dependent on host-derived IFN-gamma. To determine how expansion of these effectors occurred, IL-18 was shown to induce OX40L on dendritic cells, whereas peptide stimulation induced CD134 (OX40) on specific T cells. CD134 blockade inhibited T cell effector expansion thereby reducing the number of IFN-gamma superproducers by 12-fold. Thus, independent of IL-12, IL-18 impacts T cell immunity throughout lymphoid and nonlymphoid tissue by bridging the innate and adaptive arms of the immune system through IFN-gamma and the CD134 costimulatory pathway.

CCR6-mediated dendritic cell activation of pathogen-specific T cells in Peyer's patches.

T cell activation by dendritic cells (DCs) is critical to the initiation of adaptive immune responses and protection against pathogens. Here, we demonstrate that a specialized DC subset in Peyer's patches (PPs) mediates the rapid activation of pathogen specific T cells. This DC subset is characterized by the expression of the chemokine receptor CCR6 and is found only in PPs. CCR6(+) DCs were recruited into the dome regions of PPs upon invasion of the follicle associated epithelium (FAE) by an enteric pathogen and were responsible for the rapid local activation of pathogen-specific T cells. CCR6-deficient DCs were unable to respond to bacterial invasion of PPs and failed to initiate T cell activation, resulting in reduced defense against oral infection. Thus, CCR6-dependent regulation of DCs is responsible for localized T cell dependent defense against entero-invasive pathogens.

Duration of alloantigen presentation and avidity of T cell antigen recognition correlate with immunodominance of CTL response to minor histocompatibility antigens.

CD8 T lymphocytes (CTL) responsive to immunodominant minor histocompatibility (minor H) Ags are thought to play a disproportionate role in allograft rejection in MHC-identical solid and bone marrow transplant settings. Although many studies have addressed the mechanisms underlying immunodominance in models of infectious diseases, cancer immunotherapy, and allograft immunity, key issues regarding the molecular basis of immunodominance remain poorly understood. In this study, we exploit the minor H Ag system to understand the relationship of the various biochemical parameters of Ag presentation and recognition to immunodominance. We show that the duration of individual minor H Ag presentation and the avidity of T cell Ag recognition influence the magnitude and, hence, the immunodominance of the CTL response to minor H Ags. These properties of CTL Ag presentation and recognition that contribute to immunodominance have implications not only for tissue transplantation, but also for autoimmunity and tumor vaccine design.

Cutting edge: the minor histocompatibility antigen H60 peptide interacts with both H-2Kb and NKG2D.

Minor histocompatibility Ags elicit cell-mediated immune responses and graft rejection in individuals receiving MHC-matched tissues. H60 represents a dominant Ag that elicits a strong CTL response in C57BL/6 mice immunized against BALB.B. An 8-aa peptide in the H60 protein is presented by H-2K(b) and this is recognized by the TCR as an alloantigen. The intact H60 glycoprotein is a ligand for the costimulatory NKG2D receptor that is expressed by activated CD8(+) T cells. Thus, H60 may provide both an allogeneic peptide and its own costimulation. We show that mutation of an H-2K(b)-binding anchor residue in the H60 peptide completely abrogates binding of H60 glycoprotein to NKG2D and a synthetic H60 peptide partially blocks the binding of NKG2D to its ligand. Ligands of the human NKG2D receptor are remarkably polymorphic, suggesting that these may also serve as minor histocompatibility Ags.

The H4b minor histocompatibility antigen is caused by a combination of genetically determined and posttranslational modifications.

Minor histocompatibility (H) Ag disparities result in graft-vs-host disease and chronic solid allograft rejection in MHC-identical donor-recipient combinations. Minor H Ags are self protein-derived peptides presented by MHC class I molecules. Most arise as a consequence of allelic variation in the bound peptide (p) that results in TCR recognizing the p/MHC as foreign. We used a combinational peptide screening approach to identify the immune dominant H2K(b)-restricted epitope defining the mouse H4(b) minor H Ag. H4(b) is a consequence of a P3 threonine to isoleucine change in the MHC-bound peptide derived from epithelial membrane protein-3. This allelic variation also leads to phosphorylation of the H4(b) but not the H4(a) epitope. Further, ex vivo CD8(+) T lymphocytes bind phosphorylated Ag tetramers with high efficiency. Although we document the above process in the minor H Ag system, posttranslational modifications made possible by subtle amino acid changes could also contribute to immunogenicity and immune dominance in tumor immunotherapeutic settings.