Generation of Human iPSCs by Protein Reprogramming and Stimulation of TLR3 Signaling

The discovery of induced pluripotent stem cells (iPSCs) allows for establishment of human embryonic stem-like cells from various adult human somatic cells (e.g., fibroblasts), without the need for destruction of human embryos. This provides an unprecedented opportunity where patient-specific iPSCs can be subsequently differentiated to many cell types, e.g., cardiac cells and neurons, so that we can use these iPSC-derived cells to study patient-specific disease mechanisms and conduct drug testing and screening. Critically, these cells have unlimited therapeutic potentials, and there are many ongoing clinical trials to investigate the regenerative potentials of these iPSC-derivatives in humans. However, the traditional iPSC reprogramming methods have problem of insertional mutagenesis because of use of the integrating viral vectors. While a number of advances have been made to mitigate this issue, including the use of chemicals, excisable and non-integrating vectors, and use of the modified mRNA, safety remains a concern. Both integrating and non-integrating methods also suffer from many other limitations including low efficiency, variability, and tumorigenicity. The non-integrating mRNA reprogramming is of high efficiency, but it is sensitive to reagents and need approaches to reduce the immunogenic reaction. An alternative non-integrating and safer way of generating iPSCs is via direct delivery of recombinant cell-penetrating reprogramming proteins into the cells to be reprogrammed, but reprogramming efficiency of the protein-based approach is extremely low compared to the conventional virus-based nuclear reprogramming. Herein, we describe detailed steps for efficient generation of human iPSCs by protein-based reprogramming in combination with stimulation of the Toll-like receptor 3 (TLR3) innate immune pathway.