Nitric Oxide and Chemokines Act in Concert to Mediate Immunosuppression by Mesenchymal Stem Cells
esenchymal stem cells (MSCs) are adult stem cells that exist in various tissues and are believed to supply new cells for the repair and remodelling of damaged or aging tissues. In laboratory settings, MSCs are most often isolated from bone marrow for study. These cells can develop into several types of tissues, including cartilage, bone, muscle and adipose tissue. Most importantly, MSCs have been shown to be able to turn off immune responses in various situations, including autoimmune disease and organ transplantation. However, the mechanisms by which MSCs suppress immune responses have not been clearly defined. An article we published in the journal Cell Stem Cell describes how specific inflammation factors, called cytokines, can activate MSCs and make them produce nitric oxide and chemokines, protein factors that specifically attract immune cells. The concerted action of the nitric oxide and chemokines accomplishes the mission of immunosuppression. Using a mouse model of graft-versus-host disease (GVHD) and MSCs from mice that lack the enzyme that makes nitric oxide or are non-responsive to certain inflammatory cytokines, our group showed that this mechanism orchestrates the suppression of immune reactions causing GVHD. Our findings allow a better understanding of the immunosuppression mediated by MSCs, and should lead to more efficient use of these adult stem cells in therapeutic context.
Stem cells have two distinct characteristics that distinguish them from other cell types. Firstly, they are unspecialized and can renew themselves for long periods without significant changes in their general properties. Secondly, under certain physiologic or experimental conditions, stem cells can be induced to differentiate into various specialized cell types. Thus, stem cells hold great promise for regenerative medicine in the treatment of many diseases. There are two major types of stem cells: embryonic stem cells and adult stem cells. Embryonic stem cells are the most primitive and pluripotent cell type, with the potential to differentiate into a wide variety of specialized cell types. On the other hand, adult stem cells are more differentiated than embryonic stem cells, but still can give rise to a number of specialized cell lineages. In some adult tissues, such as bone marrow, muscle, fat and brain, discrete populations of adult stem cells generate replacements for cells that are lost through normal aging, injury, or disease. Most studies of adult stem cells have used hematopoietic stem cells derived from bone marrow, peripheral blood or umbilical cord blood, although stem cells have also been obtained from many other tissues, even tumors. Only recently have mesenchymal stem cells (MSCs) attracted the attention of basic and clinical investigators. These adherent stromal MSCs are derived from many tissues, particularly bone marrow, and can be expanded in culture to generate differentiated progeny in response to a variety of in vitro protocols.
MSCs are low in frequency in the bone marrow; plating studies indicate that MSCs represent perhaps 0.001% to 0.01% of the nucleated cells, approximately 10-fold less abundant than hematopoietic stem cells (HSCs). The most important properties of MSCs are their readiness to grow in culture and their ability to suppress the immune system. Unlike embryonic stem cells, MSCs do not trigger immune rejection; instead, they actively mediate immune tolerance in recipient hosts. Thus, MSCs may be very useful in organ transplantation and the treatment of immune disorders. Due to their easy accessibility and handling, and their multi-lineage potential, studies of MSCs are now progressing rapidly. In fact, cultured MSCs are demonstrating great promise clinically, having already been infused into human subjects for safety studies and for early clinical testing to support bone marrow transplantation, and for the treatment of diseases like graft-versus-host disease (GVHD), osteogenesis imperfecta, and glycogen storage disease. It has also been reported that MSCs preferentially localize at tissue sites with damage or a wound, implying potential targeting of pathological sites.
Funded by stem cell grants from the New Jersey Commission on Science and Technology, scientists in our group, including graduate student Guangwen Ren, research and teaching specialist Dr. Gaungwu Xu and Dr. Liying Zhang, and others, have recently published an article in Cell Stem Cell (Volume 2:141–150, 2008), revealing the molecular mechanisms underlying the immunosuppressive effect of bone marrow-derived MSCs in mice. They used clonal bone marrow MSC lines, and combined a series of in vitro assays to demonstrate that T cell proliferation and cytokine production could be blocked by MSCs. This immunosuppressive effect of MSCs is not innate, but rather is stimulated by IFN-γ and at least one of three inflammatory cytokines (TNFα, IL-1α, or IL1-β). These combinations of cytokines induce MSCs to express inducible nitric oxide synthase (iNOS) and result in production of very high levels of NO, which suppress T cell responses. On the other hand, the same inflammatory cocktails also induce MSCs to secrete multiple chemokines. Our team suggests that this effect is likely responsible for the observed migration of activated T cells towards the treated MSCs. This cytokine-induced immunosuppression was absent in MSCs derived from iNOS-/- or IFN γR1-/- mice. Blockade of chemokine receptors also abolished the immunosuppression. Administration of wild-type MSCs, but not IFN γR1-/- or iNOS-/- MSCs, prevented graft-versus-host disease in mice, an effect that was reversed by anti-IFN γ or iNOS inhibitors. Wild-type MSCs also inhibited delayed-type hypersensitivity, while iNOS-/- MSCs aggravated it. Therefore, proinflammatory cytokines are required to induce immunosuppression by MSCs through the concerted action of chemokines and NO. These studies provide important insight into the pathways responsible for the therapeutic benefit observed in some cases of MSC transfusion. With a better handle on the mechanisms involved in MSC-mediated effects, therapies can be tailored to a given clinical situation such as cancer, heart disease, multiple sclerosis, type I diabetes, rheumatoid arthritis, systemic lupus, severe combined immunodeficiency syndrome, Parkinson’s disease, and spinal cord injury.
Yufang Shi, PhD, joined UMDNJ-Robert Wood Johnson Medical School faculty in 2001 as a University Professor. His major research interest is to understand the mechanisms through which the immune system is regulated and to apply this knowledge in the treatment of autoimmune disorders, cancer and infections.