The PML gene is not involved in the regulation of MHC class I expression in human cell lines

The promyelocytic leukemia gene, PML, is a growth and transformation suppressor. An additional role for PML as a regulator of major histocompatibility complex (MHC) class I antigen presentation has been proposed in a murine model, which would account for evasion from host immunity of tumors bearing malfunctioning PML, such as acute promyelocytic leukemia. Here we investigated a possible role of PML for the control MHC class I expression in human cells. PML function was perturbed in human cell lines either by PML/RAR alpha transfection or by PML- specific RNA interference. Impairment of wild-type PML function was proved by a microspeckled disassembly of nuclear bodies (NBs), where the protein is normally localized, or by their complete disappearance. However, no MHC class I down-regulation was observed in both instances. We next constructed a PML mutant, PML mut ex3, that is a human homolog of the murine PML mutant, truncated in exon 3, that was shown to down-regulate murine MHC class I. PML mut ex3 transfected in human cell lines exerted a dominant-negative effect since no PML molecules were detected in NBs but, instead, in perinuclear and cytoplasmic larger dot-like structures. Nevertheless, no down-regulation of MHC class I expression was evident. Moreover, neither transfection with PML mut ex3 nor PML-specific RNA interference affected the ability of gamma-interferon to up-regulate MHC class I expression. We conclude that, in human cell lines, PML is not involved directly in the regulation of MHC class I expression.