Laminin isoforms 8 and 10 are primary components of the subendothelial basement membrane promoting interaction with neoplastic lymphocytes

To determine whether subendothelial laminins (LNs) could be implicated in t he extravasation of neoplastic lymphocytes, we have examined the distributi on of a number of LN isoforms in human vascular structures of adult individ uals and have assayed the ability of the isolated LN molecules to promote a dhesion of lymphoma and leukemic cells in vitro using a novel cell adhesion assay, CAFCA, Centrifugal Assay for Fluorescence-based Cell Adhesion (E. G iacomello et al., Biotechniques, 26: 758-762, 1999; P. Spessotto et al., Me thods Mol. Biol., 139: 321-343, 2000). The use of previously characterized LN chain-specific antibodies showed that the vast majority of the smaller v ascular compartments, known to correspond to sites of lymphocyte transmigra tion, expressed the subunits involved in the structuring of 9 of the 12 LN isoforms known to date. Eight LN isoforms (i.e., LN-1, -2, -4, -5, -8, -9, -10, and -11) and four naturally occurring LN complexes were isolated from various tissues and cultured cells by combined gel filtration, ion exchange , and immunoaffinity chromatographies, and the identity/composition of the isolated LNs/LN complexes was asserted by immunochemical means and amino-ac id sequencing. Notwithstanding the widespread colocalization of LN isoforms , a panel of neoplastic B- and T-cell lines and lymphocytes isolated from p atients affected by chronic lymphocytic B-cell leukemia attached preferenti ally and with high avidity to purified LN-8, purified LN-10, and LN-10-cont aining protein complexes, whereas lymphocytes derived from patients diagnos ed with acute Lymphocytic leukemia failed to bind to these LNs. All of the tested neoplastic lymphocytes failed to adhere to the isolated LN-1, LN-4, LN-9, and LN-11 and attached moderately well to purified LN-2 and LN-5. The interaction of transformed lymphocytes with LNs was cation-dependent and i nterchangeably mediated by the alpha (3)beta (1) and alpha (6)beta (1) inte grins. The degree of engagement of the two LN receptors was dependent upon their relative levels of cell surface expression, whereas, irrespective of the phenotype, lymphocytes deprived of either of these receptors were incap able of LN binding. The findings suggest that LN-8 and LN-10 may act in an independent or complementary fashion as primary components of the endotheli al basement membrane favoring the interaction of extravasating neoplastic l ymphocytes. Thus, our results would demonstrate that different LN isoforms may evoke diverse cellular responses in different cell types and that this divergence may be the basis for the redundancy of LN distribution in a numb er of vascular structures.