The blood-brain barrier induces differentiation of migrating monocytes into Th17-polarizing dendritic cells

Trafficking of antigen-presenting cells into the CNS is essential for lymphocyte reactivation within the CNS compartment. Although perivascular dendritic cells found in inflammatory lesions are reported to polarize naive CD4⁺ T lymphocytes into interleukin-17-secreting-cells, the origin of those antigen-presenting cells remains controversial. We demonstrate that a subset of CD14⁺ monocytes migrate across the inflamed human blood-brain barrier (BBB) and differentiate into CD83⁺CD209 ⁺ dendritic cells under the influence of BBB-secreted transforming growth factor-β and granulocyte-macrophage colony-stimulating factor. We also demonstrate that these dendritic cells secrete interleukin-12p70, transforming growth factor-β and interleukin-6 and promote the proliferation and expansion of distinct populations of interferon-γ- secreting Th1 and interleukin-17-secreting Th17 CD4⁺ T lymphocytes. We further confirmed the abundance of such dendritic cells in situ, closely associated with microvascular BBB-endothelial cells within acute multiple sclerosis lesions, as well as a significant number of CD4⁺ interleukin-17⁺ T lymphocytes in the perivascular infiltrate. Our data support the notion that functional perivascular myeloid CNS dendritic cells arise as a consequence of migration of CD14⁺ monocytes across the human BBB, through the concerted actions of BBB-secreted transforming growth factor-β and granulocyte-macrophage colony-stimulating factor.