Natalizumab

Integrins are transmembrane receptors and adhesion molecules that facilitate the chemotaxis of leukocytes to inflammation sites. Made up of multiple subunits, α4 integrins form heterodimers with β-subunits to form functional molecules. During inflammation, endothelial cells lining blood vessels are activated by cytokines. There is increased expression of cell adhesion molecules on the vascular endothelium, such as vascular cell adhesion molecule-1 (VCAM-1) and mucosal addressin cell adhesion molecule-1 (MAdCAM-1), expressed on vascular endothelial cells of the gastrointestinal tract. These cell adhesion molecules act as ligands or counter-receptors for α4 integrin receptors expressed primarily on lymphocytes, monocytes, and eosinophils. The interaction between cell adhesion molecules and α4 integrin facilitates the transmigration of leukocytes across the endothelium into inflamed parenchymal tissue, activation and proliferation of lymphocytes, and enhanced activity of local cytokines and chemokines. α4 integrin can also interact with extracellular matrix molecules such as fibronectin and osteopontin to further propagate inflammation. Natalizumab binds to the α4 subunit of α4β1 and α4β7 integrin receptors to block the α4-mediated adhesion of leukocytes to their counter-receptors. In vitro, natalizumab also blocks α4-mediated cell binding to osteopontin and an alternatively spliced domain of fibronectin, connecting segment-1 (CS-1). In vivo, natalizumab may further inhibit the interaction of α4-expressing leukocytes with their ligand(s) in the extracellular matrix and on parenchymal cells, thereby inhibiting further recruitment and inflammatory activity of activated immune cells.

The specific mechanism(s) by which natalizumab exerts its effects in multiple sclerosis and Crohn’s disease have not been fully defined. Lesions in multiple sclerosis (MS) are believed to occur when activated inflammatory cells, including T-lymphocytes, cross the blood-brain barrier (BBB). Leukocyte migration across the BBB involves the interaction between adhesion molecules on inflammatory cells and their counter-receptors expressed on endothelial cells lining blood vessels. Natalizumab blocks the molecular interaction of α4β1-integrin expressed by inflammatory cells with VCAM-1 on vascular endothelial cells and with CS-1 and/or osteopontin expressed by parenchymal cells in the brain; thereby, natalizumab reduces leukocyte migration into brain parenchyma and reduces plaque formation associated with MS.

The interaction of the α4β7 integrin with the endothelial receptor MAdCAM1 has been implicated as an important contributor to chronic inflammation in Crohn’s disease (CD). MAdCAM-1 is mainly expressed on gut endothelial cells and is critical in homing T lymphocytes to gut lymph tissue found in Peyer’s patches. Increased MAdCAM-1 expression is often observed at active inflammation sites in patients with CD, suggesting that MAdCAM-1 may be involved in the recruitment of leukocytes to the mucosa. The clinical effect of natalizumab in CD may, therefore, be secondary to the blockade of the molecular interaction of the α4ß7 integrin receptor with MAdCAM-1 expressed on the venular endothelium at inflammatory foci. VCAM-1 expression has been found to be upregulated on colonic endothelial cells in a mouse model of inflammatory bowel disease and appears to play a role in leukocyte recruitment to sites of inflammation; however, the role of VCAM-1 in CD is unclear.