The lysophospholipids sphingosine-1-phosphate (S1P) and lysophosphatidic acid (LPA) are pleiotropic signaling molecules with a broad range of physiological functions. Targeting the S1P1 receptor on lymphocytes with the immunomodulatory drug fingolimod has proven effective in the treatment of multiple sclerosis. An emerging body of experimental evidence points to additional direct effects on cells of the central and peripheral nervous system. Furthermore, fingolimod has been reported to reduce LPA synthesis via inhibition of the lysophospholipase autotaxin. Here we investigated whether modulation of particular signaling aspects of S1P as well as LPA by fingolimod might propagate peripheral nerve regeneration in vivo and independent of its anti-inflammatory potency. Sciatic nerve crush was performed in wildtype C57BL/6, in immunodeficient Rag1 −/− and Foxn1 −/− mice. Analyses were based on walking track analysis and electrophysiology, histology, and cAMP formation. Quantification of different LPA species was performed by liquid chromatography coupled to tandem mass spectrometry. Furthermore, functional consequences of autotaxin inhibition by the specific inhibitor PF-8380 and the impact of fingolimod on early cytokine release in the injured sciatic nerve were investigated. Clinical and electrophysiological measures indicated an improvement of nerve regeneration under fingolimod treatment that is partly independent of its anti-inflammatory properties. Fingolimod treatment correlated with a significant elevation of axonal cAMP, a crucial factor for axonal outgrowth. Additionally, fingolimod significantly reduced LPA levels in the injured nerve. PF-8380 treatment correlated with improved myelin thickness. Sciatic nerve cytokine levels were not found to be significantly altered by fingolimod treatment. Our findings provide in vivo evidence for direct effects of fingolimod on cells of the peripheral nervous system that may propagate nerve regeneration via a dual mode of action, differentially affecting axonal outgrowth and myelination by modulating relevant aspects of S1P and LPA signaling.

Autoimmune polyneuropathies are acquired inflammatory disorders of the peripheral nervous system (PNS) characterized by inflammation, demyelination, and axonal degeneration. Although the pathogenesis has not been fully elucidated, T cells recognizing self-antigens are believed to initiate inflammation in a subgroup of patients. However, the route and time of T cell entry into the PNS have not yet been described in detail. In this study, we analyzed both kinetics as well as localization of retrovirally transfected green fluorescent protein (GFP)-expressing neuritogenic T lymphocytes in experimental autoimmune neuritis (EAN). T lymphocytes obtained from rats following EAN induction by immunization with peripheral nerve protein peptide P255–78 were retrovirally engineered to express GFP. Non-specific T cells were negatively selected by in vitro restimulation, whereas GFP-expressing neuritogenic T cells (reactive to P255–78) were adoptively transferred into healthy rats (AT-EAN). Antigen-specific T cell tracking and localization was performed by flow cytometry and immunohistochemistry during the course of disease. After induction of autoimmune neuritis, P2-reactive T cells were detectable in the liver, spleen, lymph nodes, lung, peripheral blood, and the sciatic nerves with distinct kinetics. A significant number of GFP+ T cells appeared early in the lung with a peak at day four. In the peripheral nerves within the first days, GFP-negative T cells rapidly accumulated and exceeded the number of GFP-expressing cells, but did not enter the endoneurium. Very early after adoptive transfer, T cells are found in proximity to peripheral nerves and in the epineurium. However, only GFP-expressing neuritogenic T cells are able to enter the endoneurium from day five after transfer. Our findings suggest that neuritogenic T cells invade the PNS early in the course of disease. However, neuritogenic T cells cross the blood-nerve barrier with a certain delay without preference to dorsal roots. Further understanding of the pathophysiological role of autoagressive T cells may help to improve therapeutic strategies in immune-mediated neuropathies.

Neural Regen Res, Official publication of Publishing House of Neural Regeneration Research China Association of Rehabilitation Medicine,China