Knight et al. J Neurol Sci.
Several studies have demonstrated that stem cells are useful in the treatment of multiple sclerosis. The Cellmedicine clinic published previously in collaboration with the University of California San Diego that 3 patients treated with their own fat derived stem cells entered remission. Other studies are ongoing, including a study at the Cleveland Clinic in which bone marrow stem cells differentiated into mesenchymal stem cells are being administered into patients with multiple sclerosis. Unfortunately the mechanisms by which therapeutic effects occur are still largely unknown. One general school of thought believes that stem cells are capable of differentiating into damaged brain cells. The other school of thought believes that stem cells are capable of producing numerous growth factors, called trophic factors, that mediate therapeutic activity of the stem cells. Yet another school of thought propagates the notion that stem cells are merely immune modulatory cells. Before continuing, it is important to point out that stem cell therapy for multiple sclerosis involving autologous hematopoietic transplants is different than what we are discussing here. Autologous (your own) hematopoietic stem cell therapy is not based on regenerating new tissues, but to achieve the objective of extracting cells from a patients, purifying blood making (hematopoietic) stem cells, destroying the immune system of the recipient so as to wipe out the multiple sclerosis causing T cells, and subsequently readministering the patient’s own cells in order to regenerate the immune system. This approach, which was made popular by Dr. Richard Burt from Northwestern University.
In order to assess mechanisms of how stem cells work in multiple sclerosis it is necessary to induce the disease in animals. The most widely used animal model of multiple sclerosis is the experimental allergic encephalomyelitis model. This disease is induced in female mice that are genetically bred to have a predisposition to autoimmunity. These animals are immunized with myelin basic protein or myelin oligodendrocyte protein. Both of these proteins are components of the myelin sheath that protects the axons. In multiple sclerosis immune attack occurs against components of the myelin sheath. Therefore immunizing predisposed animals to components of the myelin sheath induces a disease similar to multiple sclerosis. The EAE model has been critical in development of some of the currently used treatments for multiple sclerosis such as copaxone and interferon.
Original studies have demonstrated that administration of bone marrow derived mesenchymal stem cells protects mice from development of EAE. This protection was associated with regeneration on oligodendrocytes as well as shifts in immune response. Unfortunately these studies did not decipher whether the protective effects of the stem cells were mediated by immune modulation, regeneration, or a combination of both. Other studies have shown that MSC derived from adipose tissue had a similar effect. One interesting point of these studies was that the stem cell source used was of human origin and the recipient mice were immune competent. One would imagine that administration of human cells into a mouse would result in rapid rejection. This did not appear to be the case since the human cells were found to persist and also to differentiated into human neural tissues in the mouse. One mechanism for this “immune privilege” of MSC is believed to be their low expression of immune stimulatory molecules such as HLA antigens, costimulatory molecules (CD80/86) and cytokines capable of stimulating inflammatory responses such as IL-12. Besides not being seen by the immune system, it appears that MSC are involved in actively suppressing the immune system. In one study MSC were demonstrated to naturally home into lymph nodes subsequent to intravenous administration and “reprogram” T cells so as to suppress delayed type hypersensitive reactions. In those experiments scientists found that the mechanism of MSC-mediated immune inhibition was via secretion of nitric oxide. Other molecules that MSC use to suppress the immune system include soluble HLA-G, Leukemia Inhibitor Factor (LIF), IL-10, interleukin-1 receptor antagonist, and TGF-beta. MSC also indirectly suppress the immune system by secreting VEGF which blocks dendritic cell maturation and thus prevents activation of mature T cells.
While a lot of work has been performed investigating how MSC suppress the immune system, relatively little is known regarding if other types of stem cells, or immature cells, inhibit the immune system. This is very relevant because there are companies such as Stem Cells Inc that are using fetally-derived progenitor cells therapeutically in a universal donor fashion. There was a paper from an Israeli group demonstrating that neural progenitors administered into the EAE model have a therapeutic effect that is mediated through immune modulation, however, relatively little work has been performed identifying the cell-to-cell interactions that are associated with such immune modulation.
Recently a paper by Knight et al. Cross-talk between CD4(+) T-cells and neural stem/progenitor cells. Knight et al. J Neurol Sci. 2011 Apr 12 attempted to investigate the interaction between immune cells and neural stem cells and vice versa. The investigators developed an in vitro system in which neural stem cells were incubated with CD 4 cells of the Th1 (stimulators of cell mediated immunity), Th2 (stimulators of antibody mediated immunity) and Th17 (stimulators of inflammatory responses) subsets. In order to elucidate the impact of the death receptor (Fas) and its ligand (FasL), the mouse strains lpr and gld, respectively, were used.
The investigators showed that Th1 type CD4 cells were capable of directly killing neural stem cells in vitro. Killing appeared to be independent of Fas activation on the stem cells since gld derived T cells or lpr derived neural stem cells still participated in killing. Interestingly, neural stem cells were capable of stimulating cell death in Th1 and Th17 cells but not in the Th2 cells. Killing was contact dependent and appeared to be mediated by FasL expressed on the neural stem cells. This is interesting because some other studies have demonstrated that FasL found on hematopoietic stem cells appears to kill activated T cells. In the context of hematopoietic stem cells this phenomena may be used to explain clinical findings that transplanting high numbers of CD34 cells results in a higher engraftment, mediated in part by killing of recipient origin T cells.
The finding that neural stem cells express FasL and selectively kill inflammatory cells (Th1 and Th17) while sparing anti-inflammatory cells (Th2) indicates that the stem cells themselves may be therapeutic by exerting an immune modulatory effect. One thing that the study did not do is to see if differentiated neural stem cells would mediate the same effect. In other words, it is essentially to know if the general state of cell immaturity is associated with inhibition of inflammatory responses, or whether this is an activity specific to neurons. As mentioned above, previous studies have demonstrated that mesenchymal stem cells (MSC) are capable of eliciting immune modulation through a similar means. Specifically, MSC have been demonstrated to stimulate selective generation of T regulatory cells. This cell type was not evaluated in the current study, however some activities of Th2 cells are shared with Treg cells in that both are capable of suppressing T cytotoxic cell activation. In the context of explaining biological activities of stem cell therapy studies such as this one stimulate the believe that stem cells do not necessarily mediate their effects by replacing damaged cells, but by acting on the immune system. Theoretically, one of the reasons why immature cells are immune modulatory in the anti-inflammatory sense may be because inflammation is associated with oxidative stress. Oxidative stress is associated with mutations. Conceptually, the body would want to preferentially protect the genome of immature cells given that the more immature the cells are, the more potential they have for stimulation of cancer. Mature cells have a limited self renewal ability, whereas immature cells, given they have a higher potential for replication are more likely to accumulate genomic damage and neoplastically transform.