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SummaryWe studied major histocompatibility complex (MHC) class I expression in 12 tumor cell culture lines established from patients with metastatic renal cell carcinoma (RCC). In one of these cell culture lines, UOK 123, we found no surface expression of β 2 -microglobulin (β 2 m) and MHC class I by flow cytometry. Immunofluorescence staining using three different monoclonal antibodies to β 2 m revealed no detectable β 2 m in the endoplasmic reticulum (ER), Golgi apparatus, cytoplasm, or on the cell surface. There was no evidence of folded class I molecules inside or on the surface of the cells; however, the ER stained intensively for unfolded class I molecules. Transient expression of β 2 m by UOK 123 after infection with a recombinant vaccinia virus containing the gene for β 2 m resulted in normal expression of both β 2 m and class I (HLA-A, B, C) determinants assessed by flow cytometry analysis. No expression of class I or β 2 m was seen with the recombinant vaccinia vector carrying a control gene. The inability of class I molecules to reach the cell surface is due to the requirement of β 2 m for proper folding and presentation of the class I MHC complex. The failure to assemble and express MHC class I complex on the cell surface renders these cells incapable of antigen presentation to cyto-toxic T cells and provides a mechanism for escape from immune recognition by the tumor.

A monoclonal antibody that identities a membrane molecule unique in rat lung for type I1 alveolar epithelial cells was used to isolate these cells from enzymatically dispersed lung cells by fluorescence-activated cell sorting. Although multistep physical separation techniques have permitted the isolation of large quantities of these cells and flow cytometry has been used by others to isolate lamellar body-containing cells, the application of this antibody-directed sorting has distinct advantages. Because the marker molecule is expressed on immature type I1 cells prior to the development of lamellar bodies, the antibody will also permit their isolation and study.

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