Low-Level MHC Class II Expression Leads to Suboptimal Th Cell Response, Increased Autoaggression, and Heightened...

The development and activation of MHC class II (MHC-II)-restricted CD4⁺ T cells are distinct immunological processes that are strictly MHC-II-dependent. To address their relative dependence on MHC-II, we established a novel ENU-induced mutant mouse on the C57BL/6 background, named I-A^12%, with ∼8-fold reduced I-A expression on the surface of B cells, dendritic cells, cortical thymic epithelial cells, and medullary thymic epithelial cells. I-A^100% and I-A^12% mice are highly similar with respect to the numbers of double-positive thymocytes, CD4⁺CD8^- T cells, regulatory T cells, CD4⁺ T cell marker expression, lifespan, and Th/regulatory T cell function. Despite the demonstration of functional intrathymic negative selection in I-A^12% mice, transfer of I-A^12% CD25^-CD4⁺ T cells into RAG-knockout hosts revealed increased autoaggression activity against the liver. Compared to I-A^100% mice, infection of I-A^12% mice with graded doses of Listeria monotcytogenes or influenza virus revealed comparable and significantly reduced generation of Ag-specific CD4⁺ T cells at high and low infection doses, respectively. A significantly weakened Ag-specific recall cytokine production response was also found for I-A^12% mice previously infected with a relative low dose of L. monocytogenes CD44^hiCD4⁺ T cells from I-A^100% and I-A^12% mice previously infected with a relatively high L. monocytogenes dose displayed highly similar Ag-specific multicytokine production profiles. In contrast, polyclonal activation of endogenous memory-like I-A^12% CD44^hiCD4⁺ T cells revealed highly elevated production of multiple cytokines. Our results demonstrate that there exist distinct thresholds for different MHC-II-dependent immunological processes. The I-A^12% mutant mouse model we describe in the present study is a valuable tool for investigations on the quantitative cause-effect relationship in MHC-II-dependent normal and autoimmune responses.