Identify proteins at Foxp3 bound cis-regulatory elements in situ

We hypothesize that nuclear factors co-occupying the genetic elements with regulatory T (Treg) cell lineage–specifying factor Foxp3 play critical roles in transcriptional regulation of Treg immune suppression function, thus, offering a unique approach to investigate the factors and their mechanisms controlling Treg-mediated immune tolerance involved in self-tolerance and antitumor immunity. We seek to identify the proteins occupying Foxp3 targets in the resting state or after cells receiving stimulation. To this end, we projected the spatial information (PSI) of Foxp3, Histone H3, or Stat5 onto their adjacent proteins with peroxidase–catalyzed biotin-phenoxyl radicals and identify these biotinylated proteins with tandem mass tag (TMT)–based quantitative mass spectrometry (MS).