Activation-dependent recognition by hematopoietic cells of the LDV sequence in the V region of fibronectin

Abstract

It has been shown that the α4β1 integrin is the lymphocyte receptor for the carboxy terminal cell-binding domain of fibronectin which comprises adhesion sites in Hep 2 and a high affinity site, CS-1, in the type III connecting segment or V (for variable) region. In the present studies, using a series of peptides derived from CS-1, we identify the tripeptide leu-asp-val (LDV), as the minimal peptide capable of supporting stable lymphocyte or melanoma cell adhesion. However, only cells which expressed an active form of the α4β1 complex were capable of attaching to and spreading on LDV peptide. On a molar basis, LDV minimal peptides were either not active or 10-20 times less active than intact CS-1 in promoting the adhesion of lymphocytes expressing the resting form of the receptor. In cells which express the high avidity form of the receptor, LDV and CS-1 were equally effective in promoting cell adhesion and spreading. The avidity of the α4β1 complex could be altered with mAbs to β1 which specifically activate β1 dependent function. The high avidity form of the α4β1 complex could be induced on U937 cells, T, and B lymphoblastoid cell lines, or PHA-stimulated T cell blasts. Resting PBL could not be induced to bind LDV peptide conjugates by activating antibodies to β1 implying that two signals are required for LDV recognition by T cells. In conclusion, these data show clearly that the minimal peptide for the α4β1 complex in CS-1 is the LDV sequence. Although numerous cell populations can interact with intact CS-1 only cells which express an active α4β1 complex can bind the LDV sequence. This implies that cell interaction with the carboxy terminal cell-binding domain of fibronectin can be regulated at several levels: (a) α4β1 expression; (b) activation of the α4β1 complex; and (c) alternate splicing of CS-1 into V+ isoforms of fibronectin.