Spontaneous clustering of Thy-1 antigens on CD4+ CD8+ thymocytes lacking TCR engagement by MHC/peptide complexes.

While much is known concerning CD4+ CD8+ thymocytes positively or negatively selected through interaction of their TCR with self peptides bound to self-MHC molecules, little is known of the majority of CD4+ CD8+ thymocytes lacking this interaction. We developed a monoclonal antibody (mAb) 1D11, the ligand of which (1D11-L) is expressed on 60-70% of CD4+ CD8+ thymocytes but not on other subsets of thymocytes and peripheral T cells. 1D11-L expression on CD4+ CD8+ thymocytes reversely correlates with their TCR engagement, in vitro and in vivo. In addition, 1D11-L+ CD4+ CD8+ thymocytes were more susceptible than 1D11-L- CD4+ CD8- thymocytes to apoptosis. We also found that T lineage cells other than CD4+ CD8+ thymocytes and a Thy-1-expressing fibroblast cell line became positive for 1D11-L by cross-linking their Thy-1 antigens with anti-Thy-1 mAb but not with their Fab fragment, suggesting that 1D11 recognizes multimerized Thy-1 antigens. Confocal laser scanning microscopy revealed that Thy-1 antigens as well as 1D11-L are clustered on some CD4+ CD8+ thymocytes but not on the other subsets of thymocytes. Taken together, we suggest that clustering of Thy-1 antigens spontaneously and specifically occurs on CD4+ CD8+ thymocytes lacking TCR engagement by MHC/peptide complexes.