Regulatory T cells in experimental allergic encephalomyelitis. I. Frequency and specificity analysis in normal and immune rats of a T cell subset that inhibits disease.

We have shown previously that administration of myelin basic protein (MBP)-reactive T cells to naive Lewis rats induces not only autoimmune encephalomyelitis (EAE) but also a near total resistance to subsequent disease. By isolating the effector cells that are responsible for the resistance, we demonstrated that disease protection paralleled with increased numbers of a CD8+ regulatory T cell (RTC) subset and that co-injection of this RTC subset with encephalitogenic T cells aborted the pathogenic activity of the latter cells. Here, we show that a radio-sensitive splenic population of RTC also exists in naive rats that can be recruited and activated to inhibit the onset of secondary episodes of adoptive EAE. In co-transfer experiments, this protective RTC subpopulation can be isolated to neutralize the pathogenic activity of stimulatory MBP-reactive T cells in vivo. We show that the frequency of RTC with specificity for MBP-reactive T cells in naive rats is two orders of magnitude higher than the frequency of MBP-specific precursors, the activity of RTC increases substantially with age and RTC frequencies increase as a consequence of immunization with MBP-reactive cells lines. In specificity studies, we show that RTC isolated from naive rats and RTC from animals primed with one MBP-reactive cell line show cross-reactive responses to a variety of different MBP-reactive T cell lines. However, following repeated stimulation with a given MBP line, these RTC display a more limited, clonotypic response to the selecting line and assume a uniform CD8 phenotype. Finally, functional studies with RTC indicate that proliferative and lytic specificities do not necessarily correlate and that activated rat RTC are especially lytic for a Fas-sensitive murine cell line.