Down-regulation of neuronal L1 cell adhesion molecule expression alleviates inflammatory neuronal injury

In multiple sclerosis (MS), the immune cell attack leads to axonal injury as a major cause for neurological disability. Here, we report a novel role of the cell adhesion molecule L1 in the crosstalk between the immune and nervous systems. L1 was found to be expressed by CNS axons of MS patients and human T cells. In MOG_35-55-induced murine experimental neuroinflammation, CD4⁺ T cells were associated with degenerating axons in the spinal cord, both expressing L1. However, neuronal L1 expression in the spinal cord was reduced, while levels of the transcriptional repressor REST (RE1-Silencing Transcription Factor) were up-regulated. In PLP_139-151-induced relapsing-remitting neuroinflammation, L1 expression was low at the peak stage of disease, reached almost normal levels in the remission stage, but decreased again during disease relapse indicating adaptive expression regulation of L1. In vitro, activated CD4⁺ T cells caused contact-dependent down-regulation of L1, up-regulation of its repressor REST and axonal injury in co-cultured neurons. T cell adhesion to neurons and axonal injury were prevented by an antibody blocking L1 suggesting that down-regulation of L1 ameliorates neuroinflammation. In support of this hypothesis, antibody-mediated blocking of L1 in C57BL/6 mice as well as neuron-specific depletion of L1 in synapsin^Cre × L1^fl/fl mice reduces disease severity and axonal pathology despite unchanged immune cell infiltration of the CNS. Our data suggest that down-regulation of neuronal L1 expression is an adaptive process of neuronal self-defense in response to pro-inflammatory T cells, thereby alleviating immune-mediated axonal injury.