The Spatiotemporal Development of Mesenteric Lymphatic Changes in the TNF Mouse Model of Terminal Ileitis.

Crohn's disease (CD) is a chronic inflammatory bowel disease which also encompasses significant alterations of the mesenteric lymphatic system. Whether these changes are a mere consequence of, or directly contribute to the inflammation is unknown. Here we characterized the spatial and temporal development of these events in the TNF mouse, which develops CD-like ileitis and significant mesenteric lymphatic alterations. At 8-, 12-, 20-, and 28 weeks of age, specific pathogen-free (SPF), germ-free (GF) TNF and WT mice were assessed for ileitis via myeloperoxidase activity (MPO) while mesenteric lymphatic alterations were assessed by confocal immunofluorescence imaging. Lymphatic alterations in the SPF TNF occurred in a stepwise manner between 8 and 28 weeks of age beginning with the development of mesenteric lymphadenopathy at 8 weeks despite no significant ileitis. By 12 weeks ileal MPO significantly elevates concomitantly with lymphangiectasia of the mesenteric collecting lymphatic vessels (CLV) and clustering of CD45 immune cells around them. At 20 weeks, significant lymphangiogenesis of the initials (ILV) and tertiary lymphoid organs aligned along lymphatic collectors (CA-TLOs) had developed. At 28 weeks, lymphangiectasia, lymphangiogenesis, and CA-TLOs increased. However, 28-week-old GF TNF, while displaying no ileitis, presented with mesenteric lymphadenopathy, lymphangiectasia, and lymphangiogenesis but no immune cell clustering nor CA-TLOs. The TNF mice develop terminal ileitis and lymphatic alterations in a stepwise manner beginning with MLN lymphadenopathy and ileal inflammation, followed by CLV dilation and lymphangiogenesis. These lymphatic alterations are exacerbated by the gut microbiome, with immune cell clustering and TLO formation being entirely dependent of its presence.