Neonatally-induced tolerance of class I H-2 alloanti-gens can be abolished in adult, long-term-tolerant mice by infusions of immunocompetent cells from donors syngeneic with the recipient. By contrast, neonatally-induced tolerance of Ia alloantigens can not be abolished easily, indicating that the Ia-specific tolerant state is maintained by an active process that can be impressed upon mature alloreactive lymphoid cells in the tolerant environment. This finding is concordant with previous observations that tolerance of Ia alloantigens can readily be transferred adoptively by inoculating lymphoid cells from tolerant mice into syngeneic, naive recipients. It IS concluded that class II H-2 antigens induce a type of unresponsiveness that class I antigens fail to evoke, an unresponsiveness that IS actively maintained among mature immunocompetent cells. It is proposed that, in addition to a central process of clonal deletion/inactivation that both class I and class II H-2 antigens induce in neonatal mice, Ia alloantigens also evoke a secondary, fail-safe mechanism that operates to prevent alloreactivity from emerging when cells with alloreactive potential escape the central mechanism, or mutate to alloreactivity at a later stage of maturation.
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