Subproject A13

Lymphotoxin (LT) and tumor necrosis factor (TNF) contribute to both genesis and structural maintenance of mucosal lymphoid organs, such as Peyer's patches (PP), intestinal intraepithelial T lymphocytes (IELs) and nasal-associated lymphoid tissue (NALT). Using novel engineered mice we will dissect the contribution of TNF and LT produced by T cells, B cells and lymphoid tissue inducing cells (LTICs) in the development of PP, cryptopatches in the small intestine and IEL. We will also determine the role of TNF and LTα produced by these cell types in the development of experimental colitis, to define the hierarchy, complementation and redundancy in the etiopathogenesis of mucosal inflammation.

Lymphotoxin (LT) and TNF are known to contribute to both genesis and structural maintenance of mucosal lymphoid organs, such as Peyer's patches (PP), intestinal intraepithelial T lymphocytes (IELs), nasal-associated lymphoid tissue (NALT) and possibly to other tissues. TNF-TNFR1 signaling axis is a paradigm for the classical NFκB pathway activation, while LTα/LTβ-LTβR axis is a paradigm for the alternative NFκB pathways activation. However, the exact mechanisms of action of these cytokines are still incompletely defined and the target genes in the mucosal lymphoid tissues remain unknown. Considerable information has been obtained from the analysis of mice with targeted inactivating mutations of the genes for LT, TNF or the respective receptors. LTα-deficient and LT-receptor-deficient mice fail to develop any lymph nodes (LN) and PP. LTβ-deficient mice lack most of the lymph nodes, except mesenteric and cervical LN, and they completely lack PP. Neo cassette-free TNF-deficient mice develop all lymph nodes but lack PP. Lymphoid tissue inducer cells (LTICs) are associated with early development of lymph nodes and PP. Littman's lab has developed a system for specifically ablating genes in LTICs using ROR_Y(t)-Cre transgenic mice which will be utilized in our study. It is believed that activation of LN and PP mesenchyma creates the necessary platform for the subsequent development of mature lymphoid tissues. This process is known to involve LT, but the role of TNF is not yet defined. Using novel engineered mice we will dissect the contribution of TNF and LT produced by T cells, B cells and LTICs in the development of PP, cryptopatches in the small intestine and IEL. Using inducible gene ablation and transcriptome analysis we will attempt to define the group of genes responsible for the maintenance of the microstructure of mesenteric lymph nodes.

Finally, the roles of TNF and LT pathways on one hand, and of PP and mesenteric lymph nodes, on the other hand, in the pathology of experimental colitis have been previously investigated, but the results are controversial. In order to pinpoint the role of the LT/TNF system in mucosal lymphoid organogenesis and pathophysiology, mice in which the respective genes of interest can be inactivated conditionally, in distinct cell types or at distinct time points, are required. Using our novel panels of engineered mice we will be in a position to define the role of TNF and LTα produced by LTICs and lymphocytes in the development of experimental colitis, as we are planning to define the hierarchy, complementation and redundancy of TNF/LT signaling in the etiopathogenesis of mucosal inflammation.