The HEV sialomucins are also called peripheral node addressins
(PNAds), because they function as "address code" molecules for peripheral node HEVs.
They express high levels of CD44 and lack homing addressins
(L-selectin and CCR7) [114,116,117]; thus, they are not circulating T cells and can be found in nonlymphoid tissues.
L-selectin expressed on most leukocytes binds to ligands on endothelial cells and on other leukocytes; however, these molecules like the peripheral node addressins
and the CD34 family of transmembrane sialomucins, such as podocalyxin, are not a part of this review.
For example, the specialized endothelial cells that line the high endothelial venules in lymph nodes and Peyer's patches constitutively express so-called addressins
(mucosal vascular addressin
cell adhesion molecule 1 [MAdCAM-1]), which support the homing of naive lymphocytes, whereas endothelial cells elsewhere permit no leukocyte binding unless they are exposed to inflammatory mediators.
The activation, recruitment and retention of immune cells in the genital mucosa involve obligatory and intimate interactions between the pathogen and accessory molecules such as addressins
, chemokine receptors, integrins, adhesion molecules on the host cells.