Neonatal Exposure to Antigen Induces a Defective CD40 Ligand Expression that Undermines Both IL-12 Production by APC and IL-2 Receptor Up-Regulation on Splenic T Cells and Perpetuates IFN-γ-Dependent T Cell Anergy

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Abstract
T cell deletion and/or inactivation were considered the leading mechanisms for neonatal tolerance. However, recent investigations have indicated that immunity develops at the neonatal stage but evolves to guide later T cell responses to display defective and/or biased effector functions. Although neonatal-induced T cell modulation provides a useful approach to suppress autoimmunity, the mechanism underlying the biased function of the T cells remains unclear. In prior studies, we found that exposure of newborn mice to Ig-PLP1, a chimera expressing the encephalitogenic proteolipid protein (PLP) sequence 139–151, induced deviated Th2 lymph node cells producing IL-4 instead of IL-2 and anergic splenic T cells that failed to proliferate or produce IFN-γ yet secreted significant amounts of IL-2. However, if assisted with IFN-γ or IL-12, these anergic splenic T cells regained full responsiveness. The consequence of such biased/defective T cells responses was protection of the mice against experimental allergic encephalomyelitis. In this study, investigations were performed to delineate the mechanism underlying the novel form of IFN-γ-dependent splenic anergy. Our findings indicate that CD40 ligand expression on these splenic T cells is defective, leading to noneffective cooperation between T lymphocytes and APCs and a lack of IL-12 production. More striking, this cellular system revealed a requirement for IL-2R expression for CD40 ligand-initiated, IL-12-driven progression of T cells into IFN-γ production.