Demethylation of the FOXP3 gene in human melanoma cells precludes the use of this epigenetic mark for quantification of Tregs in unseparated melanoma samples

The human suppressive T cells that stably express transcription factor FOXP3, or regulatory T cells (Tregs), are thought to suppress antitumor immune responses. The most specific marker for human Tregs is the demethylation of CpG dinucleotides located in the first intron of FOXP3 ( FOXP3i1 ). FOXP3i1 is completely methylated in other hematopoietic cells, including nonsuppressive T cells that transiently express FOXP3 after activation. Previously, we and others reported estimations of the frequency of Tregs in the blood of melanoma patients using a FOXP3i1 methylation‐specific qPCR assay. Here, we attempted to quantify Tregs inside tumor samples using this assay. However, we found demethylated FOXP3i1 sequences in the melanoma cells themselves. This demethylation was not associated with substantial FOXP3 mRNA or protein expression, even though the demethylation extended to the promoter and terminal regions of the gene in some melanoma cells. Our results imply that analyzing Treg frequencies by quantification of demethylated FOXP3i1 will require that tumor‐infiltrating T cells be separated from melanoma cells.