We have previously discussed the importance of T cells in the initiation and maintenance of multiple sclerosis.  For example, it is known that CD4 T cells infiltrate the central nervous system of patients with multiple sclerosis and that through secretion of cytokines they induce microglial activation (associated with glutamate toxicity), as well as directly contribute to demyelination. 

An interesting component of the immune system that is only recently beginning to attract significant attention is the complement cascade.  This series of proteins is activated either by antibodies, or by proteins that secrete a “danger” signal to the body.  The activation of the complement cascade results in the formation of an “arrow-like” protein structure that inserts itself into the cellular membrane and results in the killing of the target cell.  This protein structure is called the “membrane attack complex”.

Usually immunologists think of complement as a means of the immune system clearing bacteria and other pathogens, however a recent study (Li et al. Augmenting DAF levels in vivo ameliorates experimental autoimmune encephalomyelitis. Mol Immunol 2009 Aug 4) demonstrated that complement may actually play a role in multiple sclerosis, or at least in the animal model of multiple sclerosis.

The investigators began by reporting data that in the experimental allergic encephalomyelities (EAE) model, when the mice are knocked out for the complement inhibitor decay accelerating factor (DAF), then the extent of autoimmune-mediated damage to the central nervous system is greatly amplified as compared to wild-type animals. 

If lack of the complement inhibitor exacerbates disease, it would seem logical that higher concentrations of the inhibitor may suppress disease.

The investigators generated DAF-transgenic mice, that is, mice that express high levels of the complement inhibitor DAF all throughout their bodies.  They observed that dendritic cells from DAF transgenic mice were poor stimulators of T cells.  This is an interesting observation because to my knowledge this is the first I see about complement affecting T cell proliferation.

The authors demonstrated that in contrast to wild-type (normal) mice, mice that were transgenic for DAF had reduction in inflammation and multiple sclerosis-like symptoms after induction of EAE with the administration of myelin oligodendrocyte glycoprotein antigen.

In conclusion this current study seems to suggest that augmenting levels of complement inhibitors may be a useful method of approaching multiple sclerosis.  Conversely, these data may stimulate research into small molecule inhibitors of complement activitors.  This reminds me…one of the well known complement inhibitors is cobra venom factor !  In fact, Tereny et al actually have a paper (Transient decomplementation of mice delays onset of experimental autoimmune encephalomyelitis and impairs MOG-specific T cell response and autoantibody production. Mol Immunol. 2009 Feb 6) demonstrating that cobra venom factor has some positive effects on the relapse remitting SJL model of multiple sclerosis.

We have discuss previously that numerous nervous system molecules have immune modulatory effects.  For example, the endocannabinoid anandamide is capable of converting microglial cells from secreting inflammatory compounds to producing antiinflammatory cytokines such as interleukin-10.  Microglial cells generally cause pathology in multiple sclerosis through production of glutamate, as well as release of inflammatory agents.  By inducing microglial cells to produce interleukin-10, mechanisms similar to those that mesenchymal stem cells use to control inflammation may be activated.  Neurologically-associated molecules also may play a role in homing of stem cells.  For example, the neurotransmitter Substance P has recently been shown to act as attractant of stem cells. 

Here we discuss an interesting new way of modulating T cells so as to prevent onset and progression of an animal model of multiple sclerosis.  The publication (Bittner et al. TASK1 modulates inflammation and neurodegeneration in autoimmune inflammation of the central nervous system.  Brain 2009 Jul 1) discusses the role of TASK1 in T cell activity in the experimental allergic encephalomyelitis model. 

What is TASK1?

TASK1 stands for “TWIK-related acid-sensitive potassium channel 1″, which is a the two-pore domain potassium channel family that is important for maintaining resting membrane potential and balancing neuronal excitability.  TASK1 activity is inhibited by low pH and is activated by certain anesthetics… and It is also known as OAT1; TASK; TBAK1; K2p3.1; and KCNK3, with the official name being KCNK3 potassium channel, subfamily K, member 3. 

In the publication it was demonstrated that mice made genetically deficient for the TASK1 gene were substantially resistant to induction of experimental allergic encephalomyelitis.  Interestingly, T cells from the TASK1 knockout mice had an inhibited proliferative and cytokine response in vitro, suggesting the resistance to EAE may be associated with alterations on the T cell side and not just on the neuronal side.  Conceptually, one may expect neurons from TASK deficient animals to be more resistant to damage due to possible role of TASK1 in induction of apoptosis.  Indeed, the authors did demonstrate using in vitro isolated neurons from TASK1 deficient and wild-type neurons that a protective effect was observed associated with TASK1 deficiency.

Anandamide, which we previously described as having potentially beneficial effects on the mouse model of multiple sclerosis (EAE) by virtue of its ability to alter microglial production of inflammatory agents was demonstrated to inhibit TASK1 activity.  In vitro administration of anandamide was shown to inhibit T cell production of inflammatory cytokines.

Perhaps most exciting from the pubication was that administration of anandamide was capable of inhibiting progression of EAE after disease onset was initiated. 

It will be interesting in the future to try to tease out the effects of anandamide between activities on the T cells and activities on the microglia, which do not necessarily need to be exclusive.  For example, one could envision a system where T regulatory cells may be selectively “reprogramming” microglia to reduce inflammatory activities.  Or conversely, T regulatory cells may be generated in the central nervous system as a result of microglial presentation of self-antigen in the presence of interleukin-10 generated by the anandamide-reprogrammed microglia.

In conclusion, the current work supports further investigation into the TASK1 channel as a possible target for drug development in multiple sclerosis.  Given that mesenchymal stem cells already have demonstrated therapeutic effects in multiple sclerosis but in mice and man, it will be interesting to see if co-administration of anandamide may enhance mesenchymal stem cell regenerative activity by modulating the local microenvironment.

When I started my training, if you wanted to know whether one gene goes up or down, it would take at least a week to figure out for every gene.  Now, in one afternoon a scientist in one shot take a look at level of expression of all known genes (about 30,000) of the human body!  This revolution in science has allowed for the discovery of new molecular pathways without having to know what one is looking for.  So in conditions such as multiple sclerosis, many scientists basically “go fishing” to try to find new genes whose expression correlates with disease.  People have even taken it further by being able to assess not only all genes, but also proteins made by the genes (called proteomics), and more recently, like my friend Gabriela Cezar at Stemina does, look for all small molecules (called metabolomics).

These very powerful techniques are beginning to bear their fruits.  A recent paper (Schulze-Topphoff et al.  Activation of kinin receptor B1 limits encephalitogenic T lymphocyte recruitment tothe central nervous system.  Nature Medicine.  June 28, 2009) identified that patients with multiple sclerosis, as well as in animals bearing a disease similar to multiple sclerosis (experimental allergic encephalomyelitis) have increased expression of the kinin receptor B1.  This receptor is activated by components of the kinin-kallikrein system, which are a group of proteins involved in pain, inflammation, and coagulation of blood.

The investigators found that giving mice developing experimental allergic encephalomyelitis (mouse model of multiple sclerosis) activators of the kinin receptor resulted in less disease, whereas administration of inhibitors of this receptor resulted in acceleration of disease onset.  This was demonstrated when the compounds were given before disease onset, but in other experiments even after disease onset. 

Manipulation of receptors using small molecules can be tricky business.  In other words, it may be that the small molecule receptor activator/inhibitors may have been working through other biological pathways to alter disease course.  Therefore, in order to know conclusively whether the kinin receptor B1 is responsible or not for alteration in disease process, the investigators used mice lacking the kinin receptor B1.  These mice suffered from accelerated disease, thus suggesting that the receptor is normally involved in controlling the disease. 

Expression of the receptor had to be on the T cells in order to mediate protection from disease.  It was demonstrated that activation of the kinin receptor B1 selectively suppressed the infiltration of Th17 cells into the central nervous system.  Most interestingly suppression of infiltration was limited to Th17, with Th1 cells still infiltrating.  One intersting question is whether there is selective expression of the kinin receptor B1 associated with various TCR clonotypes that are expanded in multiple sclerosis progression, as seen in this video describing a paper by Eli Sercarz.

These data suggest a brand new molecule that can be targetted in multiple sclerosis.  It also illustrates the power of using “discovery based” approaches.  If indeed selective inhibition of Th17 entry into the CNS can be achieved in humans, this may be useful as a synergistic agent with bone marrow or fat stem cell approaches to multiple sclerosis.

For stem cells to mediate their effects they must either be placed locally at the point of damage, or they must find their way there.  One of the questions that people ask is “how can intravenously administered stem cells home to where they need to go?”  To answer this, lets first think about the stem cell therapy that has been used for more than 4 decades: bone marrow transplantation.

In bone marrow transplantation the stem cells are injected intravenously into the recipient.  So how do they find their way to the bone marrow?  One of the may ways that this occurs is because the bone marrow expresses a protein called stromal derived factor (SDF)-1, which is also known as CXCL-12.  Specifically, bone marrow stromal cells are known to constitutively make this protein, which is what keeps the hematopoietic stem cells in the bone marrow.  So when donor hematopoietic stem cells are injected into a recipient, they selectively home to the bone marrow because of expression of SDF-1.  We know that SDF-1 is important for this process because if you block the interaction of SDF-1 with its receptor on the stem cell, called CXCR4, in a healthy person, then the healthy person’s bone marrow stem cells enter the blood.  The clinically used stem cell mobilizer mozobil works by interrupting this pathway.

We also know that SDF-1 is important for attracting stem cells because after heart attacks, this protein is produced by the injured heart muscle in large quantities, which attracts the patient’s own bone marrow cells to the area of injury.

A recently published study (McCandless et al. IL-1R Signaling within the Central Nervous System Regulates CXCL12 Expression at the Blood-Brain Barrier and Disease Severity during Experimental Autoimmune Encephalomyelitis. J Immunol. 2009 Jun 17) demonstrates that SDF-1 is expressed during the initial phases of disease progression in the mouse model of multiple sclerosis.

This study may provide one important clue as to how stem cells home into the central nervous system of patients with multiple sclerosis.  However this is a controversial area since, although mesenchymal stem cells prevent disease in animal models, some studies suggest that the stem cells do not need to actually home to the area of injury to inhibit multiple sclerosis, but instead may do this through immune modulation.

A recent paper from the University of Rochester (King et al. Circulating Ly-6C+ myeloid precursors migrate to the CNS and play a pathogenic role during autoimmune demyelinating disease. Blood 2009 Apr 2;113(14):3190-7) reports a novel mechanism by which the immune system may contribute to demyelination in the animal model of multiple sclerosis, experimental allergic encephalomyelitis (EAE).

The investigators observed that before relapses in the EAE model, a flux of myeloid progenitors enter the bloodstream, cross the blood brain barrier, and differentiate into macrophages and dendritic cells, which are believed to play an important role in immune mediated damage to the CNS.  Dendritic cells are known to be the most potent activators of T cells, whereas macrophages secrete various inflammatory mediators that contribute to demyelination.

It was found that the factor causing attraction of these myeloid progenitors was GM-CSF, since neutralization of this protein led to decreased myeloid recruitment and less disease severity.

This study suggests, at least in the animal model, that multiple sclerosis is not associated only with T cell activation, but that other cellular components are involved.  It is still unclear what causes the initial production of GM-CSF in the brain