Soluble nonclassical HLA generated by the metalloproteinase pathway

Soluble human leukocyte antigens (HLA-A, -B, and -C) proteins can be generated by a membrane-bound metalloproteinase (MPase). The MPase-mediated pathway produces soluble nonconformed HLA proteins susceptible to further degradation, and also HLA proteins with high affinity peptides stable at physiologic temperatures. Accessibility of classical HLA to the MPase cleavage inversely correlates with stability of heavy chain (HC) interactions with β₂-microglobulin (β₂M). Whether a MPase is involved in release of soluble non-classical HLA or CD1 proteins is unknown. We have investigated this question with transfectants expressing full-length HLA proteins. Native surface HLA-E and -G complexes, similar to HLA-A2, were unstable at low pH and dissociated giving rise to β₂m-free HC. Furthermore, HLA-E and -G proteins, similar to HLA-A2, were readily released from cell surface into supernatants as soluble 37-kilodalton β₂m-free HC. However, the stability of surface CD1d complexes was not affected by pH changes and no soluble CD1d was detected. Because β₂m-free CD1d HC were expressed on cells, the lack of cleaved soluble products cannot be explained by high stability of native complexes. Instead, absence of a CD1d-specific MPase in these cells or its impaired interactions with substrate HC may be responsible.