Stimulation of human bone marrow stromal cell tyrosine kinases and IL-6 production by contact with B lymphocytes

The interaction of normal human B cell precursors with the bone marrow (BM) stromal cells that support their growth and maturation is mediated by adhesion of B cell CD49d/CD29 (α4β1 integrin or VLA-4) to stromal cell CD106 (vascular cell adhesion molecule-1). To investigate whether this interaction is involved in stromal cell signal transduction, we generated CD49d-deficient variants of the RAMOS early B Burkitt's lymphoma cell line. RAMOS was chosen due to its lack of expression of CD49e (α5 integrin) and CD44, which could potentially interact with stromal cells or their extracellular matrix. In contrast to the parental RAMOS cell line, CD49d- deficient RAMOS cell lines did not adhere to primary human BM stromal cells. Tyrosine phosphorylation of several stromal cell proteins was stimulated within 5 to 10 min of contact with either CD49d-expressing or CD49d-deficient RAMOS. These proteins included a 110-kDa substrate tentatively identified as pp125(FAK). In contrast, mAb cross-linking of stromal cell CD106 had no significant effect on protein tyrosine phosphorylation. IL-6 production by stromal cells was enhanced up to fourfold over 3 days when stromal cells were cultured in contact with RAMOS cells, regardless of CD49d expression. Stromal cell IL-6 production was not significantly enhanced by RAMOS cells cultured above the stromal cells in Transwell inserts. Thus, we have demonstrated through the use of paired CD49d-expressing and CD49d-deficient RAMOS cell lines that protein tyrosine kinase activity and IL-6 production by primary human BM stromal cells can be enhanced in a manner that is dependent on B cell contact but independent of CD49d-mediated adhesion. These results indicate the potential for B cells to interact with cells in their microenvironment and activate feedback mechanisms mediated by stromal cells, which may regulate lymphopoiesis.