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

The Ly-6 locus encodes several cell surface proteins whose functions a re unknown, Although it is hypothesized that these proteins may be rec eptors, 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 lymphocy tes, binds a ligand expressed on normal thymocytes and splenic B and T cells, We find that transgenic thymocytes that overexpress Ly-6A.2 sp ontaneously aggregate in culture, This homotypic adhesion requires the overexpression of Ly-6A.2 because it is not observed in cultures of n ontransgenic thymocytes, The aggregation of Ly-6A.2 transgenic thymocy tes is inhibited by phosphatidylinositol specific phospholipase C (whi ch removes Ly-6A.2 and other glycosylphosphatidylinositol-anchored pro teins from the membrane). Some anti-Ly-6A.2 monoclonal antibodies, inc luding nonactivating ones and Fab' fragments, inhibit this aggregation , In contrast, other anti-Ly-6A.2 monoclonal antibodies increase the a ggregation of transgenic but not nontransgenic thymocytes, To further examine whether Ly-6A.2 mediates adhesion (versus inducing another adh esion pathway) reaggregation assays were performed with paraformaldehy de-fixed Tg(+) thymocytes, Paraformaldehyde-fixed Tg(+) thymocytes rea ggregate in culture and this aggregation is also blocked by phosphatid ylinositol-specific phospholipase C and anti-Ly-6A.2 monoclonal antibo dies, These results indicate that the homotypic adhesion of cultured L y-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 ex pressed on thymocytes. Tg(+) thymocytes also bind to nontransgenic thy mocytes, B cells, and T cells, indicating that normal cells naturally express the Ly-6A.2 ligand.