Date of Award
Doctor of Philosophy in Biology - (Ph.D.)
Federated Department of Biological Sciences
G. Miller Jonakait
Nicholas M. Ponzio
Christine Maria Rohowsky-Kochan
Microglia are the resident immune cells of the CNS. In the healthy CNS, they express negligible levels of MHC II molecules as well as co-stimulatory molecules CD40, CD80 and CD86 necessary for antigen presentation to and activation of T cells. Co-stimulatory molecule expression can be induced in isolated microglia in vitro by sequential treatment with granulocyte monocyte colony-stimulating factor (GM-CSF) followed by lipopolysaccharide (LPS). Upon such treatment, they become mature dendritic cells (DCs), capable of activating T cells. However, microglia are not isolated in life, but rather exist in an environment enriched by other cells, notably astrocytes.
Therefore, to determine the effect of astrocytes on the assumption by microglia of the DC phenotype, microglia were treated with GM-CSF and LPS either in the presence or absence of astrocytes and assayed for the DC phenotypic marker CD11c and the co-stimulatory CD40, CD80, CD86 and MHC II by flow cytometry. When compared to isolated microglia, a significantly lower percentage of microglia co-cultured with astrocytes expressed these markers. Astrocytes also prevent the expression of these molecules in bone marrow-derived DCs. Neither interleukin-10, transforming growth factor-beta, nor prostaglandin E2 are responsible for the inhibition. Rather, contact with the astrocytic environment is responsible for the suppressive qualities.
Microglia cultured in the presence of astrocytes are also functionally distinct from those cultured in isolation. Microglia in association with astrocytes promote T cell proliferation more robustly than do microglia in isolation. By contrast, a significantly higher percentage of CD4+ T cells stimulated with αCD3 in the presence of isolated microglia acquire an anti-inflammatory Foxp3+ T regulatory phenotype when compared to T cells cultured with microglia in the presence of astrocytes. This is not due to interactions between CD80/CD86 and CTLA4. The elevated presence of Foxp3+ T cells appears to be responsible for the lower level of T cell proliferation in the presence of isolated microglia. Finally, analysis of the supernatants from the T cell co-culture experiments reveal that astrocytic interaction(s) with microglia suppress T cell production of pro-inflammatory cytoki nes i nterferon-γ and interleukin-17. These data taken together suggest that astrocytes play a crucial role in modulating the microgl i al immune response in the CNS.
Padala, Nischal K., "Astrocytes modulate microglial phenotype and dendritic cell-like properties" (2012). Dissertations. 347.