Expression of β2m‐Free HLA Class I Heavy Chains in Neuroblastoma Cell Lines

Flow cytometry with the specific monoclonal antibody (MoAb) L31 was used to analyse the expression of HLA class I heavy chains not bound with β 2 ‐microglobulin (β 2 m) by neuroblastoma (NB) cell lines IMR‐32 and LA‐N‐1. The cells, which express barely detectable amounts of β 2 m‐free (L31‐positive molecules) and β 2 m‐complexed HLA class I antigens (W6.32‐ and BBM. I‐reactive molecules), expressed MHC class I molecules not bound to light chains upon differentiation with either retinoic acid or serum starvation. The expression was not accompanied by an increase of surface heterodimers. Conversely, recombinant interferon‐γ (rIFN‐γ) treatment led IMR‐32 and LA‐N‐1 cells to almost exclusively express β 2 m‐complexed HLA class I heavy chains. Surface β 2 m‐free MHC class I molecules displayed a molecular mass of ~45 kDa and did not bind exogenously added β 2 m. No changes in the synthesis of either HLA class I and β 2 m mRNAs or of L31 proteins were observed in differentiated NB cells, thus suggesting that the surface exposure of unusual HLA class I antigens is regulated post‐translationally. These findings indicate that, in addition to activated lymphocytes, the surface expression of β 2 m‐free class I heavy chains is a feature of other cell types, such as NB cells.