This video discusses how the hormone leptin inhibits expansion of T regulatory cells. Theoretically blocking of leptin may be one method of repairing the immune defects in multiple sclerosis and in other types of autoimmune conditions.
Treatment of multiple sclerosis patients with their own fat derived stem cells has generated promising results which we recently published. There are several mechanisms by which this treatment may mediate therapeutic effects. One particular mechanism that we will discuss here is production of soluble factors by fat derived mesenchymal stem cells that may be protective to neurons.
In the paper (Wei et al. Adipose stromal cells-secreted neuroprotective media against neuronal apoptosis. Neurosciences Letters. 2009 Jun 21) conditioned media from fat derived mesenchymal stem cells was assessed for therapeutic activity in terms of ability to protect neurons from cell death. The idea being that the fat stem cells produce factors that can inhibit apoptosis of brain cells.
The investigators used a model of neuronal cell death by culturing cerebellar granule neurons in absence of serum and potassium in order to induce apoptosis. A dose-dependent inhibition of neuronal apoptosis was observed when adipose stem cell conditioned media was added to the cultures. Inhibition of caspase-3 activity was observed in the protected neurons (which would make sense since caspase-3 is involved in the vast majority of apoptotic signalling). Additionally, the adipose stem cell conditioned media was capable of stimulating the anti-apoptotic akt signalling pathway.
In order to investigate what specific molecules were in the adipose stem cell conditioned media that induced protective effects on the neurons, it was found that neutralization of insulin growth factor (IGF)-1 resulted in loss of anti-apoptotic activity.
This study brings up several interesting questions.
Firstly, would the supernatant from the adipose derived stem cells also have effects on oligodendrocytes? In multiple sclerosis there is eventual neuronal death, however, activation of oligodendrocytes is very important in terms of re-myelinating the injured tissue. We do know that bone marrow cells are capable of inducing remyelination under specific conditions, so the question would be if adipose stem cell supernatant may also mediate such an effect. This would be relatively easy to test in an in vitro system, or using the experimental allergic encephalomyelitis model. However the issue would be whether the supernatant would also mediate immune modulatory properties that may mask the effects of potential remyelinating activity.
The second question is whether the adipose stem cell supernatant would have effects on other models of neuronal apoptosis. In the context of multiple sclerosis, neuronal apoptosis mediated by excess glutamate seems to be an important factor. Alternatively, the adipose stem cell supernatant may increase astrocyte uptake of glutamate.
The third question would be whether various conditions can be applied to the adipose stem cells so as to enhance secretion of protective factors in the conditioned media. For example, there are publications demonstrating that mesenchymal stem cells cultured in the presence of low oxygen actually increase secretion of various therapeutic factors, including IGF-1. This may be because the mesenchymal stem cells “feel” the lack of oxygen and as a natural reparative mechanism start secreting factors that protect other cells. This may be possible given that mesenchymal stem cells have been demonstrated to inhibit neuronal cell death in vivo in models of stroke. Alternatively, another way to “stress” mesenchymal stem cells in vitro may be treatment with inflammatory cytokines such as TNF-alpha. At the recent FOCIS meeting in San Francisco, data was presented demonstrating that mesenchymal stem cells secrete higher levels of antiinflammatory factors after exposure to immunological stress, as in the form of a mixed lymphocyte reaction. Collection of supernatant of stem cells under various conditions of stress may be one interesting way to increase efficacy of the approach described in the current paper.
One of the reasons why fat stem cells seem to have therapeutic activity in animal models and early clinical trials is likely associated with their ability to modulate the immune system. Specifically, the mesenchymal stem cell component of the fat is very interesting since administration of both mouse and human mesenchymal stem cells into animal models of multiple sclerosis has resulted in beneficial effects in the disease process.
How to mesenchymal stem cells affect multiple sclerosis? There is some evidence that mesenchymal stem cells produce the enzyme indolamine 2,3 deoxygenase, which depletes local tryptophan and causes death of nearby T cells. The importance of this enzyme is seen in studies in which stem cell mediated inhibition of multiple sclerosis is reversed by addition of a chemical inhibitor of indolamine 2.3 deoxygenase. In addition to inhibition of activated T cells, indolamine 2,3 deoxygenase causes production of various small molecules that can directly induce T cell apoptosis. This enzyme is one of the mechanisms by which tumors escape immune attack and is also involved in the ability of the fetus (which has different genes than the mother) to grow up inside the mother without immunological rejection.
Mesenchymal stem cells express molecules such as HLA-G which are known to send inhibitory signals to T cells and prevent their activation. Additionally, HLA-G is known to bind to immunoglobulin-like transcripts (ILTs) on dendritic cells and induce immune suppressive activities. We previously discussed that subsets of T regulatory cells express HLA-G.
Of course, besides indolamine 2,3 deoxygenase and HLA-G, mesenchymal stem cells modulate the immune system by secretion of cytokines. Notable cytokines that have been implicated include TGF-beta, IL-10 and leukemia inhibitory factor (LIF). Interestingly, the cytokines that are immune modulatory actually start getting produced in higher quantities when the mesenchymal stem cell is under allogeneic immunological pressure, such as in a mixed lymphocyte reaction (Nasef et al. Leukemia inhibitory factor: Role in human mesenchymal stem cells mediated immunosuppression. Cell Immunol 2008 May-Jun;253(1-2):16-22). This would make sense since why would mesenchymal stem cells constitutively secrete immune suppressants? They would theoretically secrete them only when they are needed by the host, which is what seems to be the case.
Today we wanted to mention a new type of mesenchymal stem cell mediated immune modulatory mechanism: cleavage of the interleukin-2 receptor protein CD25. The clinically used antibody daclizumab binds to anti-CD25 and has had some promising effects in multiple sclerosis patients. In a recent paper (Ding et al. Mesenchymal Stem Cells prevent the rejection of fully allogenic islet grafts by the immunosuppressive activity of Matrix Metalloproteinase-2 and -9. Diabetes 2009 Jun 9) it was demonstrated that mesenchymal stem cells can cut and thereby inactivate CD25 on T cells via expression of MMPs 2 and 9.
The investigators took the study one step further and shown that while the mesenchymal stem cells could provide prolongation of allogeneic tissue survival, this was associated with their ability to reduce expression of CD25.
This paper is very interesting not only for the finding that mesenchymal stem cells can modulate this interesting area of T cell biology, but also because it suggests modulation of MMP activity by other means may be a useful method of controlling the immune system. For example, numerous MMP inhibitory compounds have been developed for treatment of cancer (cancer needs angiogenesis, angiogenesis needs MMPs) but not many, well to my knowledge none, have worked in Phase III. This means that there is a possiblity that MMP modulators that are feasible from a clinical trial perspective may already be in existance.
Another interesting question that this study begs is whether the MMPs involved in angiogenesis of cancer are also involved in cleaving CD25 off immune cells and therefore may be one of the mechanisms by which cancer reduces the immune response.