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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-A12%, 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-A100% and I-A12% 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-A12% mice, transfer of I-A12% CD25−CD4+ T cells into RAG-knockout hosts revealed increased autoaggression activity against the liver. Compared to I-A100% mice, infection of I-A12% 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-A12% mice previously infected with a relative low dose of L. monocytogenes. CD44hiCD4+ T cells from I-A100% and I-A12% 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-A12% CD44hiCD4+ 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-A12% 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.

Immunological memory provides faster and stronger immune responses to protect host from infectious pathogens and tumor cells. T cell memory is a major part of immunological memory, and plays an important role to eliminate tumor cells or cells infected by intracellular pathogen. Cytokines including IL-15, IL-7 and IL-4 have been reported to enhance the generation or maintenance of memory T cells. IL-15 enhances homeostatic proliferation and survival of memory CD8+ T cells. An ENU-affected mouse pedigree carried a Tyr301stop point mutation of Runx3, which resulted in a truncated form of Runx3. These mice started to show spontaneous diarrhea after weaning (spontaneous colitis, SC). SC mice also expressed CD4-derepression in the CD8+ T cells, and loss of memory phenotype CD8+ T cells. SC CD8+ T cells showed proliferation deficiency and decreased memory CD8+ T cell precursors after activation and also had poor response to IL-15-induced proliferation. However, SC host mice enhanced in vivo long-term survival of activated WT CD8+ cells through improvement of homeostatic proliferation. This improvement of homeostatic proliferation was due to elevated serum IL-15 levels. In summary, C-terminal truncated Runx3 in mice results in upregulation of IL-15 expression and provides enhanced homeostatic environment for memory CD8+ T cells; however, CD8+ T cells have intrinsic memory defect. Thus, the SC mouse model is a valuable tool for investigations on the effect of immune system upon spontaneous colitis.

Inflammatory bowel disease (IBD) is a common, chronic, progressive inflammatory intestinal disease in the world. The etiology of IBD is due to the imbalance of host immune responses to intestinal flora. Many human genes play the important role to induce improper immune responses, including IL-10, STAT3, IRGM, AGT16L1, NOD2 and RUNX3. It has been reported that homozygous Runx3-ko mice develop colitis spontaneously. However, homozygous Runx3-ko mice show very high mortality rate in the first 2 week of life, and are hard to study the role of Runx3 in the spontaneous colitis. A mouse pedigree (spontaneous colitis, SC) carried an ENU-induced Tyr301stop point mutation of Runx3, which resulted in a short form truncated form of Runx3. SC mice started to show spontaneous diarrhea after weaning. SC mice exhibited prominant enlargement of colon that revealed large amounts of inflammatory cells infiltration from mucosal to muscular layers. Immunofluorescent staining showed large amounts of CD4+ T cells involved in the inflammatory colon. Moreover, inflammatory colons of SC mice expressed higher mRNA levels of IL-17A, and serum IL-17A level were also upregulated in SC mice. In addition, SC CD4+ T cells expressed stronger IL-17A than WT CD4+ T cells after in vitro activation. Transfer of total SC CD4+ T cells into Rag1-ko host mice resulted in autoimmune colitis. In summary, C-terminal truncated Runx3 results in autoimmune colitis through elevated Th17 immune responses. Thus, the SC mouse model is a valuable tool for investigations on the effect of immune system upon spontaneous colitis.