Peripheral expression of self-MHC-II influences the reactivity and self-tolerance of mature CD4⁺ T cells: Evidence from a lymphopenic T cell model

While intrathymic MHC expression influences the specificity of developing thymocytes, we considered that peripheral MHC expression might influence the reactivity of postthymic T cells. We now report for CD4⁺ T cells that peripheral MHC-II expression does influence their reactivity and self-tolerance. Upon transfer into MHC-II-deficient lymphopenic hosts, mature CD4⁺ T cells were found to acquire an activated memory phenotype and to become: (1) autoreactive against syngeneic MHC-II⁺ skin grafts, (2) hyperreactive against third-party MHC-II⁺ skin grafts, and (3) functionally dysregulated, resulting in a lymphoproliferative disorder characterized by intraepithelial infiltrations. Peripheral MHC-II expression appeared to influence CD4⁺ T cell reactivity by two complementary mechanisms: maintenance of CD4⁺CD25⁺ regulatory T cells ("suppression") and direct dampening of CD4⁺ T cell reactivity ("tuning").