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

Abstract

Global inactivation of IκB kinase (IKK)-α results in defective lymph node (LN) formation and B cell maturation, and loss of IKK-α-dependent noncanonical NF-κB 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-α, we crossed <i>Ikkα^F/F</i> mice with <i>Tie2-cre</i> or <i>Cdh5-cre</i> mice to ablate IKK-α in ECs. Notably, the compound defects of global IKK-α inactivation were recapitulated in <i>Ikkα^Tie2</i> and <i>Ikkα^Cdh5</i> mice, as both lacked all LNs and mature follicular and marginal zone B cell numbers were markedly reduced. However, as <i>Tie2-cre</i> and <i>Cdh5-cre</i> are expressed in all ECs, including blood forming hemogenic ECs, IKK-α was also absent in hematopoietic cells (HC). To determine if loss of HC-intrinsic IKK-α affected LN development, we generated <i>Ikkα^Vav</i> mice lacking IKK-α in only the hematopoietic compartment. While mature B cell numbers were significantly reduced in <i>Ikkα^Vav</i> mice, LN formation was intact. As lymphatic vessels also arise during development from blood ECs, we generated <i>Ikkα^Lyve1</i> mice lacking IKK-α in lymphatic ECs (LECs) to determine if IKK-α in lymphatic vessels impacts LN development. Strikingly, while mature B cell numbers were normal, LNs were completely absent in <i>Ikkα^Lyve1</i> mice. Thus, our findings reveal that IKK-α in distinct EC-derived compartments is uniquely required to promote B cell homeostasis and LN development, and we establish that LEC-intrinsic IKK-α is absolutely essential for LN formation.