Leukocyte adhesion in the liver: distinct adhesion paradigm from other organs

It is well known that leukocyte recruitment is a multi-step cascade that requires an initial tethering to the endothelium of post-capillary venules followed by rolling along the vessel wall until appropriate activating molecules are encountered which cause firm adhesion and emigration out of the vasculature. Recruitment of leukocytes in the post-sinusoidal venules of the liver follows a similar paradigm. However, distinct from most other organs is the observation that many leukocytes can also be seen adhering in the sinusoids which are specialized hepatic capillaries. In this review, the lack of importance of rolling in sinusoids is discussed. The molecular mechanisms leading to adhesion in the liver sinusoids can occur via integrin-dependent as well as integrin-independent mechanisms. In addition to the "classical" beta(1)- and beta(2)-integrin adhesion, some of the "non-classical" (non-integrin dependent) pathways including CD44 and vascular adhesion protein-1, are discussed.