Vagus Nerve Controls Gut Inflammation II

Inflammatory Mast Cells Silenced

In a previous article, I outlined the role of the vagus nerve in responding to infection/damage signals by producing signals that inhibit inflammation. In a recent article (ref. below), the role of the vagus nerve in gut inflammation was examined using real-time biophotonic labeling. Basically that means that a video camera sensitive to infrared can be used to detect infrared dyes produced when NFkB is activated -- the camera is able to visualize regions of inflammation in living mice. Using this technique, researchers were able to demonstrate that cutting the vagus nerve produced heightened inflammation in gut treated with an irritant. The vagus nerve appears to stimulate regulatory T cells that lower the activity of inflammatory cells.

Inflammation/NFkB Activation Visualized in Live Mice

The studies were performed in a mouse line constructed to express an infrared fluorescent protein in cells in which the inflammation transcription factor, NFkB, is activated. Mice of this strain were prepared with and without the vagus nerve intact leading to the intestines. The mice were then exposed to sodium dextran sulfate (DSS) to simulate inflammatory bowel disease symptoms.

Cutting the Vagus Nerve Permits Inflammation

Mice with intact vagus nerves exhibited much less inflammation in their gut than those without vagus innervation. The cut vagus experiments demonstrated that the vagus nerve was responsible for suppressing inflammation. Further experiments were performed to determine if the inflammatory and anti-inflammatory reactions could be transferred to other mice by transferring cells from the treated mice.

Regulatory T Cells (CD4+, CD25+) Block Inflammation

Transfer experiments showed that inflammatory T cells (CD4+, CD25-) from cut vagus, DSS mice would cause bowel inflammation in other mice, but that did not happen with the same type of cells from mice with intact vagus nerves. Further tests showed that either cutting the vagus or adding inflammatory T cells from a mouse with a cut vagus, reduced the population of regulatory T cells (CD4+, CD25+) in control mice treated with DSS. So, without the vagus stimulation, the regulatory T cell population declined in the presence of inflammatory signals.

Absence of Regulatory T Cells Can Explain Many Inflammatory Diseases

In many inflammatory diseases, e.g. celiac, Crohn’s disease, rosacea, there appears to be a deficiency of regulatory T cells. In the absence regulatory T cells, signals from vagus nerves will no longer produce anti-inflammatory suppression. In fact the same nerve signals may become inflammatory. This would explain why rosaceans will become inflamed by hot or cold stimulation that would normally lead to anti-inflammatory stimulation of regulatory T cells. Similarly, capsaicin, castor oil and menthol, which normally produce an anti-inflammatory response, produce inflammation in rosaceans.

[Vagal stimulation exercise links:  here and here.]

reference:
O'Mahony C, van der Kleij HP, Bienenstock J, Shanahan F, O'Mahony L. 2009. Loss of vagal anti-inflammatory effect - in vivo visualization and adoptive transfer. Am J Physiol Regul Integr Comp Physiol. Aug 12. [Epub ahead of print]

Cure for Cancer, Autoimmunity, Allergies, etc.

The immune system is powerful enough to provide protection from disease. Unfortunately, to act decisively the cells of the immune system have to be able to discriminate between self and non-self. Poor discrimination can lead to autoimmunity, cancer or infection. New approaches promise the precise use of interleukins, to reset self-recognition, eliminate a wide range of diseases and liberalize organ transplantation.

IL-2 is the Cytokine Responsible for Suppression of Autoimmunity -- Tolerance

Self/non-self discrimination is dependent on cellular communication and much of that communication takes place via small proteins called interleukins. First and foremost among the interleukins is interleukin-2 (IL-2). IL-2 is made by cells of the immune system, lymphocytes. Mice that are either defective in producing IL-2 or the lymphocyte receptor for IL-2, IL2R alpha, also called CD25, rapidly develop autoimmune diseases, such as type I diabetes or inflammatory bowel disease. Thus IL-2 is necessary for both effective immunological defenses against pathogens and suppression of immune attacks on self tissues, i.e. autoimmunity.

IL-2 Balance Achieved with Complex of IL-2 and Anti-IL-2 Antibodies

Direct injection of IL-2 has some impact on cancers, but is very difficult to control. This should be expected, because local environments should determine if the IL-2 will stimulate aggressive immunological attacks or development of regulatory T cells, Tregs, that produce tolerance.

More subtle control is achieved by using antibodies that bind to particular regions of the IL-2. The resulting IL-2/anti-IL-2 complexes can be used to stimulate immunological reactions to an antigen, which is useful for vaccines, or can stimulate tolerance for use in organ transplantation.

Future applications may be in the cure of a wide variety of autoimmune diseases, e.g. type I diabetes, inflammatory bowel diseases, allergies, asthma; degenerative diseases, such as arthritis or athersclerosis, and cancers.

reference:
Webster KE, Walters S, Kohler RE, Mrkvan T, Boyman O, Surh CD, Grey ST, Sprent J. 2009. In vivo expansion of T reg cells with IL-2-mAb complexes: induction of resistance to EAE and long-term acceptance of islet allografts without immunosuppression. J Exp Med. Mar 30. [Epub ahead of print]

Arthritis Antibodies

Antibodies can be used to attack the signaling (TNF) molecule that mediates the autoimmune attack on arthritic joint tissues. These anti-TNF antibodies minimize inflammatory signaling, reduce joint inflammation and also reduce bone attrition.

Inflammation is an activated state of a tissue in which inflammatory cytokines, TNF, IL-1, IL-6 are secreted by T-cells and the tissue responds by expressing genes that cause characteristic vascular dilation and accumulation of migrating cells of the immune system. One particular type of blood cell, a macrophage, can also migrate to the site of inflammation and develop, in response to signals from the inflamed tissue and resident bone secreting cells, osteoblasts, into osteoclasts that degrade bone. Thus, inflammation of joints can result in bone destruction and increase in serum calcium.

TNF is particularly pivotal in the development of osteoclasts and bone destruction. Thus, drugs, such as thalidomide, that block TNF production, also block the symptoms of arthritis. Antibodies can also be developed that bind to TNF and some of these antibodies have been chemically and genetically modified to make them useful as drugs. Examples are Infliximab and Andalimumab. These are proteins that bind to and inactivate TNF. In a similar alternative strategy, a portion of the TNF receptor was engineer to serving as a neutralizing molecule to bind TNF in inflamed tissue. All of these TNF inactivators can reduce symptoms and provide effective therapy for arthritic joints.

The unanswered question in the use of TNF inactivators is, “What caused the inflammation of the joint in the first place?” Inactivation of TNF can provide a temporary return to approximately normal tissue function, but the symptoms are expected to return.

Thus, we come to the unifying question of what causes inflammatory disease mediated by the immune system and directed at normal tissue components. Two obvious candidates are diet and infectious agents.

Food ingredients can exacerbate or ameliorate the symptoms of inflammatory disease, and particular diets determine the risk of acquiring these diseases. Diet is a major factor in inflammation of any source. Bacterial or viral infections frequently precede inflammatory conditions.

The association of infection with inflammation remains controversial, but there is growing evidence that bacteria in particular reside in almost all inflamed tissues. Moreover, there is abundant anecdotal evidence of effective use of antibiotics in numerous inflammatory diseases, including arthritis, inflammatory bowel disease, atherosclerosis and cancers of various types.

I expect that elucidation of the link between chronic inflammation, diet and bacterial infection will provide increasingly effective and simple therapies for most diseases in the near future.

Mast Cell Heparin

Mast cells are sentinels in tissues. They respond to invading pathogens by releasing their stored histamine, enzymes and heparin. The heparin modifies the activity of enzymes and cytokines.

What are mast cells and why are they loaded with heparin (left)? Mast cells start in the bone marrow, like many other components of the immune system. They then move into the blood stream and offload in most of the tissues that typically encounter pathogens and parasites. Thus, the typical commercial source of the mast cell-produced heparin is pig intestines or cow lungs, i.e. since heparin is made and stored in mast cells and mast cells are abundant in lungs and intestines, those are the sources of crude heparin. Proteins bound to the crude heparin are removed as the heparin is cleaned up to be used as an anti-clotting drug.

Mast cells are sentinels near the surface of mucus membranes that line the airways of the lungs and the digestive tract. Diseases of the lungs and intestines, e.g. asthma and inflammatory bowel disease, that have an inflammatory and/or autoimmune component yield high levels of mast cells in the affected tissues. Pathogens or parasites coming in contact with mast cells trigger the sudden release of vesicles full of histamine, enzymes and heparin.

Heparin stored in vesicles in mast cells can also be readily visualized by staining the mast cells in microscope sections using the fluorescent dye berberine (left). Berberine binds quite specifically to heparin and is also used in herbal medicine as a treatment for many inflammatory diseases, such as arthritis. It would be very interesting to know whether berberine has any effect on asthma.

Mast cells display a variety of receptor proteins on their surfaces. Protein receptors work by binding target molecules, ligands, changing their shapes and transmitting a signal through the cytoplasm. A key aspect of the signal transmission is the requirement for the ligand binding to bring together receptors in pairs. The pairing of receptors during ligand binding is facilitated by the binding of heparin to both ligands and receptors. Two ligands, e.g. cytokine peptides, such as TNF, can bind to adjacent sites on a heparin molecule and this pair can then bind to two receptors brought together on the surface of a cell. The receptors bind to the ligand and to the heparin. Some ligands will bind to their receptors without heparin, but the presence of heparin greatly accelerates and intensifies the reactions.

Heparin is synthesized in the vesicles of mast cells and binds to enzymes, e.g. tryptase, also present in the vesicles. The tryptase enzyme proteins form tetramers with heparin wrapped around the edge (left, edge view showing one pair of tryptase proteins with heparin bound diagonally to blue heparin-binding domains; other pair of tryptase proteins is hidden).

Interestingly the active site for each tryptase in the tetramer faces a hole where the four proteins come together. Thus the tetramer can degrade small peptides, but large proteins cannot get access to the blocked active sites. Monomers change shape and are no longer active.

Activated mast cells release their vesicle contents with some enzymes active and their bound heparin is replaced by the heparan sulfate attached to adjacent cells. Other enzymes are initially inactive bound to heparin and are activated by dissociation of the heparin once they are released from the vesicles. In both cases some of the heparin is released from the mast cells into the surrounding tissue. The free heparin can bind to cytokines released from other cells and the combined pairs of cytokines bound to heparin can in turn bind to appropriate receptors on other cells. The abundance of heparan sulfate bound to other cells will determine whether additional heparin is required for receptor responses from particular cytokines. Cells with abundant heparan sulfates will sweep heparin binding ligands toward receptors aggregated in lipid rafts, as the heparan sulfate proteoglycans are internalized for recycling.

Mast cells can be activated by allergens, because of IgE receptors. IgEs are antibodies that trigger allergic responses. The IgEs produced by antibody producing B lymphocytes circulate in the blood serum and bind to mast cell receptor proteins. Allergen molecules bind to the IgE-receptor complexes, trigger the activation of the mast cells and release histamine. The histamine binds to receptors on other cells and produces the symptoms of allergy or asthma.
Heparin can be sprayed into the lungs of asthma sufferers and reduce symptoms. This suggests that the ratio of heparin to cytokines is important and that cytokine signaling required for asthma episodes of airway constriction can bind individually to different heparin molecules and minimize mast cell triggering and histamine release.

Asthma also responds to a general decrease in chronic systemic inflammation. Thus, an anti-inflammatory diet and lifestyle, can reduce episodes and potentially reverse symptoms. Omega-3 oils and glucosamine, for example are both effective.

Tryptase model: Sommerhoff CP, Bode W, Pereira PJ, Stubbs MT, Stürzebecher J, Piechottka GP, Matschiner G, Bergner A. 1999. The structure of the human betaII-tryptase tetramer: fo(u)r better or worse. Proc Natl Acad Sci U S A 96(20):10984-91.


Berberine staining of mast cell heparin: Feyerabend TB, Hausser H, Tietz A, Blum C, Hellman L, Straus AH, Takahashi HK, Morgan ES, Dvorak AM, Fehling HJ, Rodewald HR. 2005. Loss of histochemical identity in mast cells lacking carboxypeptidase A. Mol Cell Biol. 25:6199-210.