The development of approaches for Ag delivery to the appropriate subcellular compartments of APCs and the optimization of Ag persistence are both of central relevance for the induction of protective immunity or tolerance. The expression of the neonatal Fc receptor, FcRn, in APCs and its localization to the endosomal system suggest that it might serve as a target for Ag delivery using engineered Fc fragment-epitope fusions. The impact of FcRn binding characteristics of an Fc fragment on in vivo persistence allows this property to also be modulated. We have therefore generated recombinant Fc (mouse IgG1-derived) fusions containing the N-terminal epitope of myelin basic protein that is associated with experimental autoimmune encephalomyelitis in H-2u mice. The Fc fragments have distinct binding properties for FcRn that result in differences in intracellular trafficking and in vivo half-lives, allowing the impact of these characteristics on CD4+ T cell responses to be evaluated. To dissect the relative roles of FcRn and the “classical” FcγRs in Ag delivery, analogous aglycosylated Fc-MBP fusions have been generated. We show that engineered Fc fragments with increased affinities for FcRn at pH 6.0–7.4 are more effective in delivering Ag to FcRn-expressing APCs in vitro relative to their lower affinity counterparts. However, higher affinity of the FcRn-Fc interaction at near neutral pH results in decreased in vivo persistence. The trade-off between improved FcRn targeting efficiency and lower half-life becomes apparent during analyses of T cell proliferative responses in mice, particularly when Fc-MBP fusions with both FcRn and FcγR binding activity are used.

There is a paucity of knowledge concerning the immunologic sequelae that culminate in overt autoimmunity. In the present study, we have analyzed the factors that lead to disease in the model of autoimmunity, murine experimental autoimmune encephalomyelitis (EAE). EAE in H-2u mice involves autoreactive CD4+ T cells that are induced by immunization with the immunodominant N-terminal epitope of myelin basic protein. The affinity of this epitope for I-Au can be increased by substituting lysine at position 4 with tyrosine, and this can be used to increase the effective Ag dose. Paradoxically, high doses of Ag are poorly encephalitogenic. We have used quantitative analyses to study autoreactive CD4+ T cell responses following immunization of mice with Ag doses that are at the extremes of encephalitogenicity. A dose of autoantigen that is poorly encephalitogenic results in T cell hyperresponsiveness, triggering an anti-inflammatory feedback loop in which IFN-γ plays a pivotal role. Our studies define a regulatory mechanism that serves to limit overly robust T cell responses. This feedback regulation has broad relevance to understanding the factors that determine T cell responsiveness.

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Previous studies have established a link between adverse early life events and subsequent disease vulnerability. The present study assessed the long-term effects of neonatal maternal separation on the response to Theiler's murine encephalomyelitis virus infection, a model of multiple sclerosis. Balb/cJ mouse pups were separated from their dam for 180-min/day (180-min MS), 15-min/day (15-min MS), or left undisturbed from postnatal days 2–14. During adolescence, mice were infected with Theiler's virus and sacrificed at days 14, 21, or 35 post-infection. Prolonged 180-min MS increased viral load and delayed viral clearance in the spinal cords of males and females, whereas brief 15-min MS increased the rate of viral clearance in females. The 15-min and 180-min MS mice exhibited blunted corticosterone responses during infection, suggesting that reduced HPA sensitivity may have altered the immune response to infection. These findings demonstrate that early life events alter vulnerability to CNS infection later in life. Therefore, this model could be used to study gene-environment interactions that contribute to individual differences in susceptibility to infectious and autoimmune diseases of the CNS.

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