Serum β₂-microglobulin binds to a T-cell differentiation antigen and increases its expression

The human T-cell leukaemia and differentiation antigen HTA 1 is defined by the monoclonal antibody NA1/34 (ref. 1) and also recognized by the monoclonal antibody OKT6². Like class I products of the human major histocompatibility complex, it has a glycosylated heavy (α) chain of approximately 45-50,000 molecular weight (MW) in non-covalent association with β₂-microglobulin (β₂m) (MW 11,900). A particular feature of HTA 1 is the presence in significant amounts of an additional β₂m-like subunit, called β_t (refs 3,4). To facilitate biochemical studies we have prepared a high HTA 1 expressor variant (NH17) of the human thymoma line MOLT-4⁵. The N-terminal amino acid sequence of the β_t purified from this cell line was shown to be indistinguishable from that of bovine β₂m. Further, β_t was present when the cells were grown in medium containing fetal calf serum (FCS), but absent from cells grown with human serum (HuS). We show here that addition of human and bovine β₂m to MOLT-4 and NH17 cells grown in serum-free medium produces a significant elevation of HTA 1 antigen expression, providing evidence for a regulatory or stabilizing function for the exchange of extracellular β₂m with a cell-surface antigen.