L1 adhesion molecule on human lymphocytes and monocytes: expression and involvement in binding to αvβ3 integrin

The L1 adhesion molecule is a member of the immunoglobulin (Ig) superfamily initially identified in the nervous system which contains six Ig-like domains. Besides the known L1-L1 homotypic interaction, L1 was recently shown to bind to very late antigen (VLA)-5 in the mouse and αvβ3 in the human. The sixth Ig domain is critical for this function. We now demonstrate that human CD4⁺ peripheral blood T lymphocytes, monocytes and B lymphocytes, but not CD8⁺ T lymphocytes, express L1. When compared to the expression of CD31, another ligand for αvβ3 on T lymphocytes, only a small proportion of cells were CD31⁺L1⁺ double positive. L1 was also detected on the surface of human monocytic and lymphoid tumor lines and was shown to have a molecular mass of ∼220 kDa, similar to the molecule present on neuroblastoma cells. The function of the sixth Ig domain of human L1 as an integrin ligand was also investigated. Using an RGD-containing peptide derived from the sixth Ig domain as well as a fusion protein of the sixth Ig domain of L1 and the Fc portion of human IgG1 (6.L1-Fc), we demonstrated the binding of human MED-B1 (αvβ3^hi, α5β1^lo) tumor cells and this binding was blocked by αv-specific mAb. In contrast, human Nalm-6 cells (αvβ3^lo, α5β1^hi) did not bind to the 6.L1-Fc fusion protein. MED-B1 cells could also be stained with the 6.L1-Fc fusion protein. Our results suggest that human L1 binds predominantly to αvβ3 and that its presence on leukocytes could be important for adhesion and migration.