We previously discussed a paper demonstrating augmentation of T regulatory cell (Treg) activity in patients with relapse-remitting multiple sclerosis after initiation of interferon beta therapy. The possibility that one can increase activity of these cells whose physiological function is to prevent autoimmunity is very intriguing. Therefore, we thought it may be worthwhile to see what else has been published on Treg cells in the area of multiple sclerosis patients, so we will talk about a publication (Viglietta et al. Loss of functional suppression by CD4+CD25+ regulatory T cells in patients with multiple sclerosis. J Exp Med 2004 Apr 5;199(7):971-9) from Dr. David Hafler’s group at Harvard investigating differences between healthy volunteers and patients with multiple sclerosis in terms of Treg activity.
Before we begin discussing the paper, let us ask ourselves, how would one measure Treg activity? If you think about it, it is actually very difficult if done in the most pure fashion. What we mean is that theoretically, the cells that are protecting the body from immunological attack would have a specific receptor, a specific T cell receptor (TCR) that recognizes the myelin basic protein and that stops the “conventional” T cells, or “T effector cells” which also are recognizing the myelin basic protein from attacking the myelin sheath. In other words, quantification of the suppressive activity of all the Tregs in the body may or may not be the best way to assess whether the Tregs are working or not. The most important Tregs are the ones that inhibit the attack on the myelin, the other Tregs, that prevent attack against, say, collagen II (antigen in rheumatoid arthritis), or GAD65 (antigen in type I diabetes) are not important for the situation of multiple sclerosis.
Unfortunately, we dont know all of the antigens that the T cells are attacking in multiple sclerosis, and it is difficult to measure only the Treg cells that are specific for antigens that we do know. When one is trying to quantify effector cells, there is something called “tetramer technology” in which peptides are bound to labelled proteins that resemble the MHC complex, and flowcytometry can be used for assessment. I wonder why we dont really see this being done with quantification of Treg cells. My guess is that they are found in much smaller numbers than the effector cells and thats why its difficult. Just to give you an idea, Treg cells comprise approximately 5% of the CD4 population, with the other 95% being conventional T cells, or T effector cells.
So in the publication, assessment of Treg function was performed by adding increasing numbers of Treg cells (defined as CD4+ CD25+) to a fixed number of T effector cells (CD4+ CD25+) and providing an activation signal (anti-CD3 monoclonal antibody) that nonspecifically activates the T cell receptor of both the Treg and the T effector cells. Activation of the cells can easily be measured by the rate at which the cells divide, as well as cytokines that they make.
So one would expect that if only T effector cells were mixed with the anti-CD3 antibody, there would be proliferation, and with the increasing number of Treg cells added to the mix, there would be a suppression of the proliferation. As seen in the figure below, with increasing number of Treg cells there is an increase in suppression. Most interestingly the addition of Tregs from MS patients did not seem to suppress the anti-CD3 stimulated proliferation as well as the Tregs derived from healthy volunteers. The data is representative of a total of 21 healthy controls and 15 patients with multiple sclerosis.
These data seem to suggest that Treg cell function is compromised in patients with multiple sclerosis. The question is, what could be compromising it? There are many things that inhibit function of Treg, or example, ligation of the protein GITR-ligand has been demonstrated to abolish Treg activity. Interleukin-6 in some situations has also been demonstrated to inhibit Treg activity. Additionally, inflammatory stimuli such as activation of various toll like receptors has also been associated with suppression of activity. However, none of these factors really come to mind when one thinks of multiple sclerosis patients.
One other question that is posed by these data is whether multiple sclerosis patients would be predisposed to other autoimmune diseases? Clinically multiple sclerosis seems to present as a distinct entity. So if the immune attack is only against nervous system tissue, specifically the myelin sheath, why would ALL the Tregs seem to have deficient function?
Another question is whether the lack of Treg activity is a cause of disease or whether it is a symptom. For example, it may be that the intial immune reaction against the myelin sheath may stimulate systemic changes and inflammation that could in turn somehow modulate Treg activity. In fact, systemic inflammatory mediators such as serum amyloid A protein is actually increased in patients with multiple sclerosis (Boylan et al. Interferon-beta1a administration results in a transient increase of serum amyloid A protein and C-reactive protein: comparison with other markers of inflammation. Immunol Lett 2001 Jan 15;75(3):191-7).
The ability of mesenchymal stem cells to reduce systemic inflammation is best demonstrated in clinical studies of patients with steroid refractory GVHD which seem to respond after administration of non-matched bone marrow derived MSC (www.osiris.com). It therefore makes sense to see some of the animal and early human data demonstrating activity of mesenchymal stem cells in multiple sclerosis.