Biosynthesis and Functional Significance of Peripheral Node Addressin in Cancer-Associated TLO

Abstract

Peripheral node addressin (PNAd) marks high endothelial venules (HEV), which are crucial for the recruitment of lymphocytes into lymphoid organs in non-mucosal tissue sites. PNAd is a sulfated and fucosylated glycoprotein recognized by the prototypic monoclonal antibody, MECA-79. PNAd is the ligand for L-selectin, which is expressed on the surface of naive and central memory T cells, where it mediates leukocyte rolling on vascular endothelial surfaces. Although PNAd was first identified in the HEV of peripheral lymph nodes, recent work suggests a critical role for PNAd in the context of chronic inflammatory diseases, where it can be used as a marker for the formation of tertiary lymphoid organs (TLOs). TLO form in tissues impacted by sustained inflammation, such as the tumor microenvironment where they function as local sites of adaptive immune cell priming. This allows for specific B- and T-cell responses to be initiated or reactivated in inflamed tissues without dependency on secondary lymphoid organs. Recent studies of cancer in mice and humans have identified PNAd as a biomarker of improved disease prognosis. Blockade of PNAd or its ligand, L-selectin, can abrogate protective antitumor immunity in murine models. This review examines pathways regulating PNAd biosynthesis by the endothelial cells integral to HEV and the formation and maintenance of lymphoid structures throughout the body, particularly in the setting of cancer.

Keywords: L-selectin; high endothelial venule; peripheral node addressin; tertiary lymphoid organ; tumor.

Figure 1

PNAd biosynthesis. (A) (1) LTβR is expressed on blood vessel endothelial cells. Membrane-bound LTα1β2 or secreted LTα3 secreted from cDC can signal through this receptor. (2) LTβR-mediated signaling promotes a physical change in vascular endothelial cells from a flat to cuboidal morphology. This signaling cascade also leads to the expression of PNAd on the surface of vascular endothelial cells, promoting HEV status. (3) LTβR signaling further induces HEV secreted chemokines, including CCL19, CCL21, and CXCL13. (4) Chemokines form gradients and “decorate” the blood vessel wall, initiating the recruitment of CCR7⁺ T cells or CXCR5⁺ B lymphocytes from the peripheral blood circulation into chronically inflamed tissues. (5) L-selectin on the surface of T cells is able to bind PNAd on the surface of HEV. These cells are then able to adhere to the vessel wall and extravasate into the tissue. (B) PNAd is synthesized from a Core 1 O-glycan. The extended Core 1 O-glycan serves as the MECA-79 recognition motif. The fucosylated Core 2 O-glycans are able to be recognized by L-selectin. Sulfation of the extended Core 1 and Core 2 O-glycans at the sixth position is mediated by GlcNAc6ST-1 and -2; α3 fucosylation is added by FucT-IV and -VII.

Figure 2

Structure of classical and non-classical tertiary lymphoid organs. (A) Classical TLO contain a nucleated core of fDC and germinal center B cells (BGC), surrounded by an HEV-containing T cell zone. (B) Non-classical TLO do not contain fDC or BGC, but may contain sparse B cell, T cell, and DC infiltrates surrounding HEV.