Nasal administration of soluble antigens is an exciting means of specifically down-regulating pathogenic T-cell reactivities in autoimmune diseases. The mechanisms by which nasal administration of soluble antigens suppresses autoimmunity are poorly understood. To define further the principles of nasal tolerance induction, we studied the effects of nasal administration of myelin basic protein (MBP) on experimental autoimmune encephalomyelitis (EAE) in the Lewis rat. EAE is a CD4+ T-cell-mediated animal model for human multiple sclerosis. Nasal administration of guinea-pig (gp)-MBP at a dose as low as 30 gtg/rat can completely prevent gp-MBP-induced EAE, whereas nasal administration of bovine (b)-MBP is not effective even at a much higher dosage. Cellular immune responses, as reflected by T-cell proliferation and interferon-y (IFN-y)- ELISPOT, were suppressed in rats receiving the two different doses (30 and 600 Jig/rat) of gp-MBP, but not after administration of b-MBP. Rats tolerized with both doses of gp-MBP had also abrogated MBP-induced IFN-y mRNA expression in popliteal and inguinal lymph node mononuclear cells compared with rats receiving phosphate-buffered saline nasally. However, adoptive transfer revealed that only spleen mononuclear cells from rats pretreated with a low dose, but not from those pretreated with a high dose, of gp-MBP transferred protection to actively induced EAE. Low-dose (30 Jg/rat) gp-MBP-tolerized rats also had high numbers of interleukin-4 (IL-4) mRNA-expressing lymph node cells, while high-dose (600 Jg/rat) gp-MBP-tolerized rats had low numbers of IL-4 mRNA-expressing lymph node cells. Our data suggest an exquisite specificity of nasal tolerance. Dose-dependent mechanisms also relate to nasal tolerance induction and protection against EAE in the Lewis rat.