SOLUBLE-ANTIGEN THERAPY INDUCES APOPTOSIS OF AUTOREACTIVE T-CELLS PREFERENTIALLY IN THE TARGET ORGAN RATHER THAN IN THE PERIPHERAL LYMPHOIDORGANS

The administration of soluble myelin proteins is an effective way of d own-regulating the inflammation in the central nervous system (CNS) in experimental autoimmune encephalomyelitis (EAE), an animal model of m ultiple sclerosis. To shed more light on the mechanism of this antigen -specific therapy, we determined the effect of the intraperitoneal (i. p.) injection of soluble myelin basic protein (MBP) on T cell apoptosi s in the CNS and peripheral lymphoid organs of Lewis rats with EAE ind uced by inoculation with MBP and complete Freund's adjuvant. In partic ular we assessed the level of apoptosis of V beta 8.2(+) T cells, whic h constitute the predominant encephalitogenic MBP-reactive T cell popu lation in the Lewis rat. The daily i.p. injection of MBP for 3 days fr om the onset of neurological signs inhibited the further development o f neurological signs of EAE. Using two-color flow cytometry we found t hat a single i.p. injection of MBP increased the level of apoptosis of the V beta 8.2(+) T cell population in the CNS to 26.2 % compared to 7.4 % in saline-treated rats and 7.6 % in ovalbumin-treated rats. In c ontrast, treatment with MBP did not increase the level of apoptosis of the V beta 8.2(+) population in the popliteal lymph node draining the inoculation site (1.4 %) or in the spleen (1.6 %) above that occurrin g in saline-treated rats (1.6 % and 1.1 %, respectively). Limiting dil ution analysis revealed that the frequency of T cells reactive to the major encephalitogenic epitope, MBP72-89, was decreased in the CNS but not in the popliteal lymph node by this treatment. Three-color flow c ytometry in MBP-treated rats demonstrated that CNS V beta 8.2(+) T cel ls expressing Fas (CD95) and Fas ligand were highly vulnerable to apop tosis compared to V beta 8.2(+) T cells not expressing these proteins. We conclude that the i.p. injection of MBP increases the spontaneousl y occurring Fas-mediated activation-induced apoptosis of autoreactive T cells in the CNS in EAE and that this contributes to the therapeutic effect of the injection.