ABSTRACT Reactive lymphangiogenesis requires B cell-expressed lymphotoxin (LT)α. LTα signals through TNFR1 or TNFR2 when it is presented to cells as a LTα3 homotrimer, or it signals through LTβR when it is part of the LTa1b2 heterotrimeric complex. Lymphangitis has been described as a feature of Crohn's disease, but the nature and role of lymphangitis in the disease has not been elucidated. In resected tissue from Crohn's disease patients, outflow lymphatic trunks (called collecting lymphatic vessels) running from the ileum through the mesentery and on to draining lymph nodes appeared obstructed by tertiary lymphoid organs (TLOs) that formed around them. The TLOs were rich in B cells. In a mouse model of ileitis driven by a mutation in the mRNA locus for the cytokine TNF (TNF∆ARE mice) that increases TNF abundance when the gene is environmentally triggered, TLOs developed along mesenteric collecting lymphatics draining the ileum, like patients. The mice display marked blockade of lymph flow due to obstruction by the TLOs, which raised lymph pressure so that flow did not move forward but pushed backwards through lymphatic collaterals toward the gut. Backflow characterizes compromised valves. Indeed, TLOs mainly formed where lymphatic valves lost their identity and quiescence. Instead, these valve sites supported lymphangiogenesis that partially encapsulated expanding TLOs. Before lymphangiogenesis got underway, B cells accumulated around lymphatic valves. These B cells may be the source of cytokines that act on the lymphatic vessels. Indeed, data so far suggest that the action of TNF on lymphatic endothelium may be sufficient in vitro and in vivo to reprogram valve- associated genes critical to the maintenance of normal lymph flow. That is, early blockade with anti-TNF neutralizing mAb restores lymph flow and prevents progression of TLOs. At late stages in disease, anti-TNF therapy becomes less effective as the TLOs have greatly expanded and enlarged, but nonetheless anti-TNF still partially restores mesenteric lymph flow, suggesting that TNF remains a key, and possibly required, signal in disease progression. However, it remains unclear if the TLOs per se drive worsened ileitis. What would happen if the TLOs did not form but other inflammatory pathways were left intact? In preliminary data for this application, we have identified a means to prevent TLO formation: TLOs do not form in the absence of B cells. Furthermore, TNF and/or LTα expressed by B cells is required for the formation of TLOs in the ileitis-affected mesentery of TNFARE mice. These data cause us to hypothesize that TNF receptor ligands TNF and/or LTα derived from B cells act through TNF receptors on lymphatic endothelium to drive tertiary lymphoid organ (TLO) formation by skewing signals that normally maintain lymphatic valve integrity. We predict, in turn, that TLOs govern the pathophysiology of the disease itself by trapping inflammatory cells and mediators rather than allowing ...