OVEREXPRESSED LY-6A.2 MEDIATES CELL-CELL ADHESION BY BINDING A LIGAND EXPRESSED ON LYMPHOID-CELLS

The Ly-6 locus encodes several cell surface proteins whose functions are unknown, Although it is hypothesized that these proteins may be receptors, there is no direct evidence that they bind a ligand, Herein we present evidence that Ly-6A.2, a Ly-6 protein expressed on T lymphocytes, binds a ligand expressed on normal thymocytes and splenic B and T cells, We find that transgenic thymocytes that overexpress Ly-6A.2 spontaneously aggregate in culture, This homotypic adhesion requires the overexpression of Ly-6A.2 because it is not observed in cultures of nontransgenic thymocytes, The aggregation of Ly-6A.2 transgenic thymocytes is inhibited by phosphatidylinositol specific phospholipase C (which removes Ly-6A.2 and other glycosylphosphatidylinositol-anchored proteins from the membrane). Some anti-Ly-6A.2 monoclonal antibodies, including nonactivating ones and Fab' fragments, inhibit this aggregation, In contrast, other anti-Ly-6A.2 monoclonal antibodies increase the aggregation of transgenic but not nontransgenic thymocytes, To further examine whether Ly-6A.2 mediates adhesion (versus inducing another adhesion pathway) reaggregation assays were performed with paraformaldehyde-fixed Tg(+) thymocytes, Paraformaldehyde-fixed Tg(+) thymocytes reaggregate in culture and this aggregation is also blocked by phosphatidylinositol-specific phospholipase C and anti-Ly-6A.2 monoclonal antibodies, These results indicate that the homotypic adhesion of cultured Ly-6A.2 transgenic thymocytes is directly mediated by Ly-6A.2 and, more importantly, strongly suggests that Ly-6A.2 binds a ligand that is expressed on thymocytes. Tg(+) thymocytes also bind to nontransgenic thymocytes, B cells, and T cells, indicating that normal cells naturally express the Ly-6A.2 ligand.