**4. Role of NFκB signaling during reprogramming to pluripotency**

Undifferentiated human iPSC have elevated NFκB activities, which play important roles in maintaining *OCT4* and *NANOG* expression in pluripotent hiPSC [111]. Innate immune TLR signaling was recently shown to enhance nuclear reprogramming probably through the induction of an open chromatin state, and global changes of epigenetic modifiers [39]. This normally increases cell plasticity in response to a pathogen, but may also enhance the induction of pluripotency, transdifferentiation and even malignant transformation [112-116].

The EBNA (Epstein-Barr virus nuclear antigen) is a virus-derived protein that is not only a critical component of episomal reprogramming vectors, where it mediates extra-chromosomal self-replication, but it is also known to activate several TLRs [117-119]. These include TLR3, which is known to augment reprogramming efficiencies through the activation of inflamma‐ tory pathways [39, 120]. TLR3 recognizes double-stranded RNA from retroviruses and signals through TRAF6 and NFκB [121-123]. The TLR3 agonist poly I:C was shown to have the same effect as retroviral particles in enhancing Yamanaka factor-induced iPSC production. TLR3 causes widespread changes in the expression of epigenetic modifiers and facilitates nuclear reprogramming by inducing an open chromatin state through down-regulation of histone deacetylases (HDACs) and H3K4 (histone H3 at lysine 4) trimethylations [38, 39, 124]. These epigenetic modifications mark transcriptionally active genes, whereas the H3K9me3 (Histone H3 at lysine 9) modification marks transcriptionally silenced genes [125, 126]. Histone deacetylation is generally associated with a closed chromatin state and HDAC inhibitors were shown to enhance nuclear reprogramming [127, 128]. Histone acetylation favors an open chromatin state, and is maintained by proteins containing histone acetyltransferase (HAT) domains, such as p300 and CBP [129, 130]. Interestingly, p300/CBP is able to interact with NFκB [131, 132]. RelB directly interacts with the methyltransferase G9a to mediate gene silencing of differentiation genes [133]. Epigenetic changes that allow an open chromatin state are crucial for giving the Yamanaka factors access to promoter regions necessary for the induction of pluripotency. Epigenetic chromatin modifications by TLRs are normally involved in the expression of host defense genes during infections [134-136]. This capability can be deployed to enable nuclear reprogramming as TLR3 was shown to change the methylation status of the *Oct4* and *Sox2* promoters. Interestingly, changes in these methylation marks were not observed with TLR3 activation alone but only in the presence of the reprogramming factors. Although TLR3 by itself promotes an open chromatin configuration, the reprogram‐ ming proteins are likely necessary to direct the epigenetic modifiers to the appropriate promoter sequences [137]. Lee *et al.* described the potential of inflammatory pathways to facilitate the induction of pluripotency as 'transflammation' [40, 138].
