Endothelial Cell Adhesion Molecules ECAMs in BECs versus LECs

Both LECs and BECs express several types of adhesion molecules including those in the Ig-superfamily and selectins family.

BECs basally express CD54/ICAM-1 and CD102/ ICAM-2 as well as CD31/PECAM. BECs also increase their expression of several adhesion molecules, such as VCAM-1, ICAM-1, and E- and P-selectin, after stimulation with several proinflammatory cytokines (TNF-a, IL-1b) or lipopolysaccharide in a tissue- and anatomy-specific manner. Unlike BECs, LEC may not densely express ICAM-1 and -2. LEC expression of ICAM-1 may also be anatomy specific; lymphatic and submandibular lymph node capillaries only weakly express ICAM-1, and ICAM-1 is among the BEC transcripts that are suppressed by the LEC fate determinant Prox-1. In cancer, ICAM-1 is however elevated in peritumoral lymphatics and involved lymph nodes.

PECAM-1 is constitutively expressed by most LECs, BECs, and HEV, but generally, PECAM-1 expression in LECs also appears lower than in other endothelial types.

VCAM-1 is not expressed basally, by either BEC or LEC but VCAM-1 (and ICAM-1, ICAM-3) are expressed by LEC during active inflammation (especially gut inflammation), in response to inflammatory cytokines. E-selectin is also mobilized in BEC during inflammation, and some tissues (e.g., tongue) even show constitutive LEC expression of E-selectin.

CD34 is a 90- to 120-kDa cell surface sialomucin expressed by BECs, but not by LECs. CD34 is expressed on early lymphoid hematopoietic stem and progenitors, small-vessel endothelial cells, embryonic fibroblasts, some fetal cells, and in adult nervous tissue. The function of CD34 is probably to control interactions of BEC with lymphocytes via L-selectin in high endothelial venules.

a9-Integrin also appears to participate in the development of lymphatic vessels, since a9-integrin knockout mice die from respiratory failure/chylothorax resulting from a failure in normal lymphatic development. a9-Integrin is densely expressed on several cell types including smooth muscle, epithelia, and neutrophils. a9-Integrin is also a ligand for VCAM-1, which may play a role in lymphocyte trafficking in forms of filariasis, an infestation of the lymphatic system. Although a9-integrin appears to play an important regulatory role in the maturation of the lymphatic system, it is not clear if it is expressed by LECs, or if it is involved in signaling that supports lymphatic maturation.

least within lymphatic buds. In the process of lymphatic budding, the lymphatic specific transcription factor, Prox-1, and Syk/SLP-76 must be activated for these cells to mature into lymphatics, which express VEGF-C/VEGF-R3 signaling, LYVE-1, and podoplanin.

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