Differential susceptibility to recombinant interferon-γ-induced HLA-DQ antigen modulation among clones from a human metastatic melanoma

Twenty-one clones from an early culture of a histocompatibility leukocyte antigen (HLA) class II negative human metastatic melanoma (Me 9229) were screened for susceptibility to phenotypic modulation induced by recombinant interferon-γ (rIFN-γ) by using SPV-L3, a monoclonal antibody to HLA-DQ antigens, in indirect immunofluorescence followed by fluorescence-activated cell sorter analysis. After treatment with 500 U/ml of rIFN-γ for 3 days one of the clones (9229/18) expressed high levels of DQ antigens, in terms of percentage of positive cells, whereas many other clones were much less suceptible or remained DQ negative. Scatchard analysis of the data of specific binding of ¹²⁵I-labeled rIFN-γ revealed that one clone susceptible (9229/18) and one clone resistant (9229/5) to HLA-DQ modulation expressed similar numbers of interferon-γ binding sites per cell; dose-response experiments showed that all clones could be induced to express HLA-DR and -DP antigens after exposure to rIFN-γ. However, the DQ-negative profile of clone 9229/5 was not modified even after incubation with up to 1 x 10⁴ U/ml of rIFN-γ or by extending the culture time in the presence of this lymphokine up to 120 hr. Furthermore, Northern blot analysis indicated a direct correlation between changes in the levels of HLA-DR and -DQ-specific mRNA after rIFN-γ treatment, and the lack or expression of HLA class II antigens at the cell surface of the two different clones. Karyotype studies did not reveal differences between clones 9229/5 and 9229/18 and Southern blot analysis indicated that both clones had similar EcoRI and HindIII restriction patterns for DR and DQ gene sequences. Finally, strong DQ-specific mRNA signal and antigen expression at the cell surface could be induced even on clone 9229/5 by treating the cells with supernatants from mixed lymphocyte cultures, recently shown to contain a class II-inducing factor different from interferon-γ. Taken together whese results indicate that DQ antigens can be modulated even in clones resistant to rIFN-γ induction and suggest that the differential susceptibility observed in response to this lymphokine could play a role in the genesis of the phenomenon of intratumor heterogeneity.