Spleen-derived stromal cells. Adhesion molecules expression and lymphocyte adhesion to reticular cells.

The final steps of lymphocyte differentiation occur in secondary lymphoid organs where B and T lymphocytes interact with the lymphoid microenvironment. Although numerous studies describe the interactions of murine lymphocytes with dendritic, follicular and other antigen presenting cells, little is known on the interactions between lymphocytes and reticular cells, an important cellular component of spleen stroma. In this work we describe the culturing of complete murine spleen stromas and of two cell lines, Sp-1 and Sp-2, identified as of possible reticular origin, and describe the adhesive interactions between murine lymphocytes and human lymphoid cells with murine spleen stromal cells. FACS analysis indicates that the Sp-1 cell line shows a single cell type expressing VCAM-1 and CD44 constitutively. They do not express any of the markers described for follicular cells, interdigitating cells, macrophages or endothelial cells. Our data suggests that these cells represent a population of spleen reticular cells. The Sp-2 cell line shows two phenotypically different cell types that grow in association. FACS analysis demonstrates that both cell types express VCAM-1 and CD44 constitutively, but that they can be differentiated by the expression of CD11b and FcR. These data suggest that the Sp-2 cell line is composed of one type of stromal cell growing over an adherent layer of reticular cells. Furthermore, analysis of the non-B non-T cell fraction prepared from murine spleen shows that approximately 30% of these cells correspond to the CD44/VCAM-1 double positive cells. Murine B and T cells adhere to the complete stromas and to Sp-1 and Sp-2 cell lines. Activation of B cells with LPS had no effect on binding while binding of T cells to complete stromas increased up to threefold after Con-A treatment. Adhesion of human lymphoblastoid Daudi cells to complete spleen stromas is blocked by an anti-(murine) VCAM-1 antibody but not by an antibody to the (human) integrin alpha 4 subunit, while adhesion to the Sp-1 and Sp-2 stromas is blocked by antibodies against both molecules. Also, adhesion of Ramos cells to Sp-2 stromas is inhibited by antibodies to the integrin alpha 4 subunit and to murine VCAM-1. Antibodies to other adhesion receptors such as the integrin beta 2 subunit, ICAM-1 or CD44 have no effect on human cell binding to these stromas. Our results suggest that we have isolated a fraction of splenic reticular cells and that these cells can be cultured as a distinct cell line. The finding that these cells express CD44 and VCAM-1 constitutively and use some of these molecules for lymphocyte binding suggests that spleen reticular cells may be involved in the regulation of normal lymphocyte traffic through the spleen.