Endolysosomal TLRs and Lupus

Lupus erythmatosusLupus erythematosus is a chronic autoimmune disease that, worldwide, is more common than leukaemia, multiple sclerosis, and muscular dystrophy. There are three recognised forms of lupus: discoid (cutaneous) lupus which affects the skin, systemic lupus (SLE) which attacks multiple organ systems, and drug-induced lupus which generally resolves once the drug is discontinued. The symptoms of lupus, which can flare and subside, vary from patient to patient but include severe fatigue, joint pain, muscle aches, anaemia, and general malaise. Approximately 10% of discoid lupus cases develop into systemic lupus which can result in the destruction of vital organs.

The causes of lupus are poorly understood, but researchers at the Scripps institute have now shown that specific Toll-like receptors (TLRs) play a key role in the development of this disease. One of the principal diagnostic indicators of lupus is a high level of self-recognising antinuclear antibodies. Antinuclear antibodies normally form part of the immune response and are produced when bacteria or viruses are degraded in the endolysosome. TLRs inside this compartment specifically detect viral RNA and viral and bacterial DNA and stimulate immune cells to produce antibodies against these molecules.

Writing in the Proceedings of the National Academy of Sciences, the Scripps team have shown that three TLRs in the endolysosome are necessary for the generation of antinuclear antibodies in mice. Previous studies had pointed to endolysosomal TLRs – TLR3, TLR7, TLR8 (in humans but not mice) and TLR9 – as important for the production of antinuclear antibodies but, in mouse models of lupus, knocking out only TLR 7 or TLR9 had not dramatically reduced symptoms. The Scripps team wanted to eliminate all three TLRs in mice and achieved this by knocking out a transmembrane endoplasmic reticulum protein, UNC-93B, which is necessary for correct functioning of the endolysosomal TLRs. Knocking out UNC-93B in strains of laboratory mice that spontaneously develop many of the same symptoms as human lupus sufferers produced animals with fewer antinuclear antibodies and fewer and less severe symptoms of lupus. Even lipid A stimulation of TLR4 – which is known to promote production of autoantibodies – did not produce symptoms of lupus in the knockout animals. Nucleic acid-sensing TLRs may thus provide a critical pathway in the development of systemic autoimmunity by reducing tolerance to nucleic acid-containing antigens.

Towards Safer Immunosuppressants

Cyclosporin A (CsA) and tacrolimus (FK-506) are immunosuppressive drugs used to prevent rejection after allogeneic organ transplants. The drugs bind to specific immunophilins and the resulting complexes (IS-IP complexes) then inhibit calcineurin (CN) and hence production of interleukin-2 (IL-2), a cytokine instrumental in discriminating between self and non-self. Although both CsA and tacrolimus provide benefit in terms of patient and graft survival following transplantation, their potential for treating chronic inflammatory diseases is restricted by adverse side effects.

New research into the mechanism of action of these drugs, led by researchers at the Centro Nacional de Investigaciones Cardiovasculares in Madrid and published in the journal Molecular Cell, however, may offer the promise of safer immunosuppressants.

Previous work had shown that the phosphatase CN regulates a transcription factor known as nuclear factor of activated T cells (NFAT). The physical interaction between CN and NFAT is an essential step in the activation of NFAT-dependent genes by calcium signals, and the new study was designed to investigate how NFAT and IS-IP complexes interact with CN. The research showed that both NFAT and IS-IP complexes compete for the same hydrophobic pocket at the interface of the two calcineurin subunits, and identified small peptides that bind to this site. Identification of compounds that interact specifically with this binding site could lead to the discovery of less toxic immunosuppressants since at least some of the side effects of CsA and tacrolimus are thought to be related to formation of the IS-IP complex and to be independent of CN.

1tco crystal structure

MyD88 Contributes to Host Defense Against B. pseudomallei

rice fieldMelioidosis is an infectious disease caused by the bacterium, Burkholderia pseudomallei which is found in soil and water. The disease is endemic in parts of south east Asia and northern Australia, and affects other species such as goats, sheep and horses as well as humans. The route of infection is believed to be either through a break in the skin, or through the inhalation of aerosolized B. pseudomallei. The most severe form of the disease is melioidosis septic shock, and mortality remains high despite antibiotic treatment.

A recent report in the journal PLoS elucidates the pathways which confer susceptibility to disease. The research focused on Toll-like receptors (TLRs), which have a central role in the recognition of pathogens and the initiation of the innate immune response. Specifically, the new study looked at the effect of two important adaptor proteins involved in TLR signalling and, using experiments in mice, found that MyD88 but not TRIF is important for host defense against B. pseudomallei.

The authors had previously shown that, although both TLR2 and TLR4 contribute to cellular responsiveness to B. pseudomallei in vitro, only TLR2 knockout mice were protected against B. pseudomallei induced mortality. Together, the data indicate that MyD88 deficiency results in a strongly impaired resistance to melioidosis despite an interruption of harmful TLR2 signalling.

Gut Flora Play Role in Immune Response

gutIt has been estimated that there are ten times as many bacterial cells as human cells in the body, with the vast majority of bacteria living in the intestine. Around 500 bacterial species are present in the normal human gut and generally provide a beneficial service, synthesizing vitamins such as folic acid, vitamin K and biotin, fermenting complex carbohydrates, and converting lactose to lactic acid. The presence of such bacterial colonies also inhibits the growth of potentially pathogenic bacteria. The microorganisms which populate the gut, termed the commensal microbiotica, are also actively involved in immune regulation and homeostasis and the composition of the microbiota has been suggested to influence susceptibility to inflammatory bowel diseases.

In recent years, IL-17-producing T-helper (Th17) cells have been recognised to be involved in a wide variety of inflammatory conditions and autoimmune diseases. A report in the journal Cell Host and Microbe shows that the small intestine provides an environment that uniquely favours differentiation of Th17 cells which are scarce elsewhere in the body. The composition of the commensal intestinal bacteria was found to have a crucial role in the differentiation of SI LP Th17 cells and in their balance with Treg cells, which also make up a large proportion of CD4+ T cells in the intestinal mucosa. Only a subset of vancomycin-sensitive bacteria were found to induce Th17 cell differentiation, suggesting that unique innate immune signaling pathways, distinct from the TLR-mediated signals that can be initiated by numerous microorganisms, are required for this process. In experiments in mice, the presence of Th17 cells in the mucosa correlated with the presence of members of the cytophaga-flavobacter-bacteroidetes (CFB) phylum, implicating these bacteria as Th17 cell inducers. This is the first report linking a defined set of gut flora to a specific immune response and could help in the development of novel treatments for inflammatory bowel disease and other diseases.