α4-integrin

α4-integrin is a transmembrane glycoprotein subunit that forms heterodimers with β1 integrin (creating VLA-4, very late antigen-4) or β7 integrin (creating LPAM-1). VLA-4 binds vascular cell adhesion molecule-1 (VCAM-1) expressed on inflamed endothelium, allowing firm adhesion and subsequent transmigration of lymphocytes and monocytes across the blood-brain barrier and other vascular beds. The α4β7 heterodimer binds mucosal addressin cell adhesion molecule-1 (MAdCAM-1), which is expressed on gut endothelium, directing lymphocyte homing to gut-associated lymphoid tissue (GALT) and intestinal mucosa.

Clinically, α4-integrin is central to understanding two major monoclonal antibody therapies: 1. Natalizumab - a humanized monoclonal antibody against α4-integrin (blocking both VLA-4 and α4β7) used in relapsing-remitting multiple sclerosis and Crohn's disease. By blocking VLA-4/VCAM-1 interaction, it prevents lymphocyte entry into the CNS, reducing MS relapses. However, this creates immunosuppression risk of JC virus reactivation causing progressive multifocal leukoencephalopathy (PML), a serious black-box warning. 2. Vedolizumab - a more gut-selective antibody targeting only the α4β7 integrin, blocking α4β7-MAdCAM-1 interaction specifically in gut-associated lymphoid tissue. This provides efficacy in ulcerative colitis and Crohn's disease with a theoretically lower PML risk due to gut selectivity, though rare cases have been reported.

Understanding this pathway highlights the concept of tissue-specific lymphocyte homing via integrin-adhesion molecule pairs, which is high-yield for both immunology and pharmacology sections of Step 1, and for therapeutic mechanisms in Step 2 CK (neurology and GI).