Biphasic form of experimental autoimmune neuritis in dark Agouti rats and its oral therapy by antigen-specific tolerization.

A new and biphasic form of experimental autoimmune neuritis (EAN) is described in dark agouti rats (DA rats) and is inducible by a single immunization with bovine peripheral nerve myelin (BPM) in complete Freund's adjuvant (DA-EAN). Animals develop a mild episode of disease; after recovery, 66-100% of the rats suffer from a more severe bout of EAN with paraparesis 25-30 days after immunization. By histology, DA-EAN is an inflammatory and demyelinating polyradiculoneuropathy virtually without axonal damage. Demyelination affects mainly spinal roots. This is also reflected by markedly increased F-wave latencies in nerve conduction studies of sciatic nerves. In sciatic nerves, inflammation and demyelination are found only focally and may be the histopathologic basis for conduction failure in some fibers. Immunologic investigations revealed stronger proliferative responses of DA than of Lewis rat lymph node cells to BPM and various peptides derived from the P2 protein. Proliferative and Th1-cytokine responses were particularly pronounced in spleen during the late phase of DA-EAN as compared to the monophasic EAN of Lewis rats. The data suggest that persistent lymphocyte proliferation with secretion of interferon (IFN)-gamma may be relevant for the relapsing course of DA-EAN whereas epitope spreading may explain the increased severity of the second bout of disease. The extended Th1 response in DA rats did not go along with a lack of downregulatory mechanisms, because the second DA-EAN attack was self-limited and splenocytes from DA rats produced considerable amounts of interleukin (IL)-10 and transforming growth factor (TGF)-beta. To substantiate further a functional immunoregulation in DA rats, we modulated DA-EAN by antigen-specific oral tolerization, which is known to involve active suppressor mechanisms. Preventive feeding of BPM in combination with cholera toxin (CT) induced a long-lasting resistance to DA-EAN. Even therapeutic administration of BPM or BPM/CT after onset of signs of disease significantly mitigated the further course of disease and prevented development of paraparesis. Because DA-EAN is easily inducible and leads consistently to relapses in most rats, it can be used for studies of immune factors that determine a relapsing course of autoimmunity. Furthermore, DA-EAN may serve as a model for relapsing inflammatory demyelinating polyneuropathies such as chronic inflammatory demyelinating polyneuropathy (CIDP) and for treatment studies. Our findings on effective prevention and therapy of DA-EAN by oral application of myelin/CT corroborate this form of immunomodulation as a treatment strategy for cell-mediated processes in chronic inflammatory neuropathies.