PD-L1 (B7-H1) regulation in zones of axonal degeneration.

Fibre tract injury evokes recruitment of antigen-presenting- and T cells, but does not cause autoimmune demyelination. This implies that immune tolerance to myelin is actively maintained or readily re-established. Using entorhinal cortex lesion (ECL) to induce axonal degeneration in the hippocampus of adult mice, we studied the induction of B7-H1 (PD-L1) in zones of axonal degeneration. This member of the B7-family has been shown to be expressed on parenchymal cells of various organs, where it strongly down-modulates the activity of T cells. Real-time reverse transcriptase (RT)-PCR revealed low mRNA levels in brain compared to lung and spleen under normal conditions. After ECL, a twofold increase could be observed. Immunocytochemistry revealed astrocytes as source of B7-H1, while immune positive microglia were not detected. Thus, axonal degeneration induces astrocytes to express B7-H1, a potent inhibitor of effector T cells.