Clustering of class I HLA molecules on the surfaces of activated and transformed human cells

We surveyed cells for the clusters of class I HLA molecules, HLA-I, which we have previously found on JY lymphoblasts. Two fluorescence techniques, fluorescence resonance energy transfer and electron exchange quenching, detected clustered HLA-I molecules on activated normal B and T cells, on cells of B and T lymphoblast lines, and on transformed fibroblasts. No HLA-I clusters were detectable in the surfaces of resting B or T cells or normal fibroblasts. HLA clustering correlates perfectly with the presence of the HC-10 epitope of beta 2-microglobulin (beta 2m)-free heavy chains at the cell surface although not with the amount of this epitope expressed. Clustering was reversed by exogenous beta 2m, but this did not change the amount of HC-10 bound. This suggests that a form of beta 2m-free heavy chain in equilibrium with both native HLA molecules and fully denatured HC-10-positive heavy chains is involved in HLA-I cluster formation.