Lymph node formation and B cell homeostasis require IKK-a in distinct endothelial cell-derived compartments

Global inactivation of I?B kinase (IKK)-a results in defective lymph node (LN) formation and B cell maturation, and loss of IKK-a-dependent noncanonical NF-?B signaling in stromal organizer and hematopoietic cells is thought to underlie these distinct defects. We previously demonstrated that this pathway is also activated in vascular endothelial cells (ECs). To determine the physiologic function of EC-intrinsic IKK-a, we crossed IkkaF/F mice with Tie2-cre or Cdh5-cre mice to ablate IKK-a in ECs. Notably, the compound defects of global IKK-a inactivation were recapitulated in IkkaTie2 and IkkaCdh5 mice, as both lacked all LNs and mature follicular and marginal zone B cell numbers were markedly reduced. However, as Tie2-cre and Cdh5-cre are expressed in all ECs, including blood forming hemogenic ECs, IKK-a was also absent in hematopoietic cells (HC). To determine if loss of HC-intrinsic IKK-a affected LN development, we generated IkkaVav mice lacking IKK-a in only the hematopoietic compartment. While mature B cell numbers were significantly reduced in IkkaVav mice, LN formation was intact. As lymphatic vessels also arise during development from blood ECs, we generated IkkaLyve1 mice lacking IKK-a in lymphatic ECs (LECs) to determine if IKK-a in lymphatic vessels impacts LN development. Strikingly, while mature B cell numbers were normal, LNs were completely absent in IkkaLyve1 mice. Thus, our findings reveal that IKK-a in distinct EC-derived compartments is uniquely required to promote B cell homeostasis and LN development, and we establish that LEC-intrinsic IKK-a is absolutely essential for LN formation.

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