B Cell Derived IL-15 Enhances CD8 T Cell Cytotoxicity and is Increased in Multiple Sclerosis Patients

Multiple sclerosis (MS) is an autoimmune disease that causes damage to the tissue of central nervous system (CNS). Specifically, multiple sclerosis results from unregulated inflammation that damages a nerve’s myelin sheath – protective coating that protects and insulates the axon of a nerve cells. When the myelin is damaged, transmission of CNS signals is slow and inefficient [1]. Damaging inflammation can occur in the brain, optic nerve, and spine and result in a variety of debilitating symptoms including: muscle spasms, numbness, tremors, weakness, difficulty walking, and problems with coordination [2].

Previous research has confirmed that CD8 T cells significantly contribute to the pathogenesis, or development of multiple sclerosis. Activated CD8 T cells are believed to secrete pro-inflammatory cytokines, or intracellular chemical signals, which release lytic enzymes upon recognition of a target cell surface receptor. Lytic enzymes are proteins that facilitate the breakdown of cell components. CD8 T cells bind to target cells that are damaged or infected with a pathogen, and release a granule containing lytic enzymes. The binding of an activated CD8 target cell initiates granule mediated cell destruction. In the case of MS, autoreactive CD8 T cells bind to healthy nerve cells and release lytic enzymes, resulting in damage to the myelin sheath [3].

Schneider and colleagues argue that IL-15 significantly contributes to the development of multiple sclerosis. IL-15 is a cytokine that regulates the development, activation, and proliferation of CD8 T cells. Previous research has concluded that elevated levels of IL-15 have been detected in a number of inflammatory diseases including rheumatoid arthritis, celiac disease, psoriasis, and inflammatory bowl disease. Additionally, elevated levels of IL-15 have also been identified in the peripheral blood of MS patients. In the present study, researchers hope to explore the role that B cells play in producing IL-15 in the peripheral blood of MS patients. In addition, the authors hope to explore the effect of IL-15 on enhancing CD8 T cell activity.

Schneider and colleagues utilized a number of experiments to prove their hypothesis. First, flow cytometry, a method of identifying and examining microscopic particles, was used to investigate B cell and monocyte cell surface expression of IL-15, and the IL-15 cell surface receptor, IL-15R-a. IL-15 and IL-15R-a bind together to initiate a specific immune response. Data from 10 MS patients and 10 control subjects revealed that the IL-15/IL-15R-a expression of was significantly greater on the B cells of MS patients.

Flow cytometry was used a second time to explore the affect of B cell activation, via B cell receptor (BCR) and CD40L interactions, on IL-15 expression. CD40L is a protein found on the surface of T cells, which binds to a BCR to facilitate B cell activation. Researchers found that increased BCR/CD40L interactions significantly increased cell surface expression of IL-15 and Il-15R-a on B cells. Schneider and colleagues conclude that that CD40L is an important contributor to the production of IL-15 and to the damaging autoimmune response in MS.

Next, CD8 T cells and CD40L activated B cells were cultured together in order to observe the effect that IL-15 has on granzyme production. Granzyme is a lytic enzyme that decomposes proteins within a target cell. First, CD8 T cells were cultured in the presence of IL-15 blocking antibodies. Cells produced under these conditions displayed a significantly reduced expression of granzyme on the surface of replicating CD8 T cells. Second, CD8 T cells cultured in the presence of IL-15 resulted in elevated expression of granzyme. Subsequently, Schneider and colleagues then sought to determine if the CD8 T cells of MS patients are more susceptible to IL-15 exposure, when compared to healthy individuals. An analysis of fluorescent-labeled (CFSE) CD8 T cells cultured in the presence of IL-15 revealed that MS patients produced significantly greater numbers of CD8 T cells expressing granzyme.

Subsequently, researchers analyzed the implications of IL-15 on the capacity of CD8 T cells to cross the blood brain barrier (BBB). Immune cell infiltration of the BBB allows for damage to the myelin sheath cells, which ultimately results in MS symptoms. Migration assays were performed to examine the effect that IL-15 has on the migration of CD8 T cells through the BBB. Significantly more pretreated CD8 cells migrated through a monolayer of human brain endothelial cells (HBECs), when compared with untreated CD8 cells. Therefore, it can be concluded that IL-15 significantly enhances the progression of the disease and contributes to the onset of MS symptoms.

Lastly, a test was conducted to examine the effect of IL-15 on the cytotoxicity – capacity of T cells to kill target cells via granzyme release – of CD8 T cells. Two populations of CD8 T cells were cultured; one in the presence of IL-15, and one without IL-15. Cultured T cells were then incubated with target cells and analyzed by flow cytometry to assess the number of target cells destroyed by granzyme release. Results concluded that pre-exposed CD8 cells demonstrate a significantly increased ability to kill target cells [1].

Previous research has suggested that CD8 T cells significantly contribute to the autoimmune activity that results in the development of multiple sclerosis. Schneider and colleagues successfully build upon this research by exploring the origins and function of elevated IL-15 concentrations in the blood of MS patients. Ultimately, the present research clearly implicates B-cells in the elevated secretion of IL-15, a cytokine that increases the cytotoxicity and migration of CD8 T cells. The T cell help supplied by IL-15 significantly augments the development and progression of MS. A greater understanding of the role of IL-15 will enlighten future research and treatment of MS. Future research may explore therapeutic strategies that will restrict B-cell activity in the hopes of reducing IL-15 in the blood and limiting the progression of MS symptoms.

1. Schneider, R., Mohebiany, A.N., Ifergan, I., Beauseigle, D. 2011. B Cell Derived IL-15 Enhances CD8 T cell Cytotoxicity and is Increased in Multiple Sclerosis Patients. The Journal of Immunology. 187: 4119-4128.

Other References

2. What we know about MS. National Multiple Sclerosis Society http://www.nationalmssociety.org/about-multiple-sclerosis/what-we-know-about-ms/index.aspx.

3. Kap, Y., J. Bauer, N. van Driel, W. Bleeker, P. Parren, E. Kooi, J. Geurts, J. Laman, J. Craigen, E. Blezer, and B. ’t Hart. 2011. B-cell depletion attenuates white and gray matter pathology in marmoset experimental autoimmune encephalomyelitis. J Neropathol Exp Neurol. 70: 992-1005.

4. Bar-Or, A., L. Fawaz, B. Fan, P. Darlington, A. Rieger, C. Ghorayeb, P. Calabresi, E. Waubant, S. Hauser, J. Zhang, and C. Smith. 2010. Abnormal B-cell cytokine responses a trigger of T-cell—mediated disease in MS? Ann Neurol. 67:452-461.

5. Kap, Y., N. van Driel, E. Blezer, P. Parren, W. Bleeker, J. Laman, J. Craigen, and B. ‘t Hart. 2010. Late B cell depletion with a human anti-human CD20 IgG1kappa monoclonal antibody halts the development of experimental autoimmune encephalomyelitis in marmosets. J Immunol. 185: 3990—4003.