ReIB in secondary lymphoid organ development: differential regulation by lymphotoxin and tumor necrosis factor signaling pathways

Abstract

Primary lymphoid organs are the major sites of lymphopoiesis where lymphocytes proliferate and mature into functional but naive cells. Secondary lymphoid organs are sites where these lymphocytes encounter antigens and elicit immune responses. RelB is a member of the Rel/NF-κB family of inducible dimeric transcription factors. RelB is abundantly expressed in secondary lymphoid organs, such as spleen, lymph nodes and Peyer's patches (PP). RelB-deficient mice have improper spleen structure and lack organizing centers for PPs, defects that can not be restored by the adoptive transfer of wild-type bone marrow cells. The work presented here revealed a significant reduction in expression of the homing chemokines B lymphocyte chemoattractant (BLC) and secondary lymphoid organ chemokine (SLC) in RelB-deficient spleen, suggesting a role for RelB in proper expression of chemokines by splenic stromal cells. Moreover, interleukin-7 (IL-7)-induced expression of lymphotoxin (LT) in intestinal cells - a crucial step in early PP development - was not impaired in RelB-deficient embryos, suggesting functional hematopoietic inducers and a defect in LTβ receptor (LTβR) expressing stromal responders. Activation of LTβR signaling in fibroblasts resulted in the specific induction of p52-RelB heterodimers, while tumor necrosis factor (TNF) induced classical p50-RelA NF-KB complexes. LTβR-induced RelB nuclear translocation and DNA binding of p52-RelB heterodimers required the degradation of the inhibitory p52 precursor, p100, which was dependent on the IKB kinase (IKK) complex subunit IKKa, but not on IKKβ or IKKy. In contrast to LTβR signaling, TNFR signaling increased p100 and RelB levels both in cytoplasm and nucleus and RelB was bound to p100 in both compartments. Despite the abundant presence of RelB in the nucleus, RelB DNA-binding was almost undetectable in TNF treated fibroblasts. Forced expression of p50 and p52 could not rescue the lack of DNA binding. In contrast, RelB DNA-binding significantly increased in cells lacking the C-terminus of p100, but not of p105, strongly suggesting that it is the specific inhibitory function of the C-terminal domain of p100, rather than the lack of the heterodimerization partner, which prevents RelB DNA-binding in TNF-stimulated fibroblasts. Thus, RelB and p52 in stromal cells could function in the proper development of the spleen by regulating the expression of chemokines such as BLC. Furthermore, generation of p52-RelB heterodimers by the LTβR pathway involving p100 degradation, appears to be a critical step in the formation of PP anlage.