The original Yamanaka iPSC reprogramming workflow used integrating retroviruses to deliver OCT4, SOX2, KLF4, and c-MYC into somatic cells, with mouse embryonic fibroblast (MEF) feeder layers supporting the resulting iPSC colonies. Both elements — integrating viruses and feeder layers — are now optional. The published iPSC reprogramming literature has steadily moved toward non-integrating delivery (episomal plasmids, mRNA, Sendai virus) and feeder-free, xeno-free, chemically defined culture systems. This survey covers the published feeder-free iPSC reprogramming literature with focus on episomal and lentiviral methods, defined matrix substrates, and the practical workflow for current iPSC generation.
- Non-integrating episomal plasmids carrying OCT4, SOX2, KLF4, NANOG, LIN28, non-transforming L-MYC, and dominant-negative p53 reprogram normal human fibroblasts to iPSCs under defined xeno-free and feeder-free conditions, with characterized pluripotency and karyotype stability (PMID 33517122).
- The published episomal protocol uses nucleofection delivery into fibroblasts or PBMCs, reprogramming in chemically defined media, and approximately 30 days from delivery to established iPSC colonies (PMID 33226617).
- iPSC production is more efficient and consistent in chemically defined media than in undefined or serum-supplemented systems. Lentivirus-mediated reprogramming remains a published research workhorse where integration is acceptable (PMID 33226615).
- Vitronectin is a published defined matrix substrate for iPSC expansion that supports feeder-free culture without animal-derived components (PMID 33226617).
Why feeder-free reprogramming became the published standard
MEF feeder layers were the original support for iPSC derivation because feeders provide the cytokine and matrix environment that pluripotent cells require to maintain self-renewal. The published reasons for moving away from feeders are practical: feeder layers add a separate cell preparation step, introduce mouse-derived components into a human cell line intended for human use, and create a heterogeneous culture in which the iPSC product is mixed with the feeder population.
The published feeder-free alternatives provide the missing signals through chemically defined basal media (commercial formulations such as E8, mTeSR, StemFlex) and through defined matrix substrates (vitronectin, laminin-521, recombinant fragments). The combination removes the feeder cell population while maintaining the pluripotent niche that iPSCs need to self-renew.
Non-integrating episomal reprogramming
The published episomal plasmid system uses nucleofection to deliver multiple plasmids encoding the reprogramming transcription factors into somatic cells. Inzunza et al. (PMID 33517122) describe the KISCOi001-A iPSC line generated from normal human foreskin fibroblasts with non-integrating episomal plasmid vectors encoding OCT4, SOX2, KLF4, NANOG, LIN28, non-transforming L-MYC, and dominant-negative p53. The resulting iPSCs are transgene-free, with pluripotency confirmed by stem cell marker expression and trilineage differentiation, and identity and karyotype stability confirmed by genomic assays.
The episomal system delivers transient transgene expression, and the plasmids are diluted out as the cells divide. Established iPSC colonies become transgene-free over subsequent passages, typically verified by PCR for plasmid backbone before downstream use, which avoids the long-term genomic integration concern that retroviral and lentiviral methods carry. Febbraro et al. (PMID 33226617) provide the detailed protocol for generating human iPSCs from fibroblasts and peripheral blood mononuclear cells using non-integrating episomal plasmids delivered by nucleofection, with reprogramming in chemically defined media. The full process takes approximately 30 days from delivery to established iPSC colonies.
Reprogramming efficiency from PBMCs is typically lower than from fibroblasts on a per-cell basis, but PBMC sourcing is less invasive and supports patient-specific iPSC generation from blood draws. Donor-specific factors (age, health status, cell quality) also affect efficiency. Published protocols recommend titrating plasmid dose and reprogramming-medium timing for each new donor source.
Lentiviral reprogramming in chemically defined media
For published research applications where genomic integration is acceptable, lentivirus-mediated reprogramming remains a high-efficiency option. Shao et al. (PMID 33226615) describe a protocol for human fibroblast iPSC generation using GFP-marked lentiviral vectors in chemically defined medium, allowing visualization and tracking of transduced cells through the reprogramming process.
The chemically defined medium produces more efficient and consistent iPSC generation than undefined or serum-supplemented systems, even when the delivery method itself is integrating. The combination of lentiviral delivery for high transduction efficiency with chemically defined medium for consistent reprogramming environment is the published research pattern for understanding pluripotency mechanisms and developing new reprogramming approaches.
Vitronectin and other defined matrix substrates
Feeder-free iPSC culture requires a matrix substrate that supports adhesion and self-renewal in the absence of feeder-cell-secreted components. 
Vitronectin, a glycoprotein found in serum and extracellular matrix, is a published defined-matrix substrate for human iPSC culture that supports feeder-free expansion (PMID 33226617).
Other published defined-matrix options include laminin-521 (LN521), Matrigel (animal-derived, not xeno-free), and recombinant matrix protein fragments. The choice of substrate affects iPSC morphology, colony shape, and scaling characteristics, and published protocols typically validate the substrate for the specific iPSC line and downstream differentiation protocol.
Reprogramming in chemically defined media: the protocol consensus
The published protocol consensus for feeder-free iPSC generation centers on three components:
- Non-integrating delivery of OCT4/SOX2/KLF4 (with optional NANOG, LIN28, and small-molecule supports) by nucleofection of episomal plasmids, mRNA transfection, or Sendai virus.
- Reprogramming in chemically defined media that support both the starting somatic cells and the emerging iPSC colonies, with media transitions during the reprogramming window.
- Culture on defined matrix substrates (vitronectin, laminin-521) that support feeder-free expansion and downstream differentiation.
The Febbraro et al. (PMID 33226617) protocol covers this end-to-end, from somatic cell preparation through nucleofection, reprogramming, colony picking, and long-term expansion on vitronectin. The Inzunza et al. (PMID 33517122) protocol applies the same components to derive a fully characterized iPSC line with documented pluripotency and karyotype stability.
Frequently asked questions
What is the difference between episomal and lentiviral iPSC reprogramming?
Episomal reprogramming uses non-integrating plasmids delivered by nucleofection that are diluted out as cells divide, leaving transgene-free iPSCs. Lentiviral reprogramming integrates the transgenes into the host genome, providing higher transduction efficiency but leaving permanent transgene insertions. Episomal is the published preference for clinical-grade iPSC lines; lentiviral remains useful for research where integration is acceptable (PMID 33517122, PMID 33226615).
How long does iPSC reprogramming take in chemically defined media?
The Febbraro et al. protocol reports approximately 30 days from somatic cell delivery to established iPSC colonies using episomal plasmid nucleofection in chemically defined media (PMID 33226617). Lentiviral reprogramming can be slightly faster, with colonies emerging in 2–3 weeks. Established colonies require additional passages to confirm pluripotency markers and karyotype stability.
What transcription factors are used in modern episomal reprogramming?
The Inzunza et al. KISCOi001-A protocol used OCT4, SOX2, KLF4, NANOG, LIN28, non-transforming L-MYC, and dominant-negative p53 (PMID 33517122). The non-transforming L-MYC variant and dominant-negative p53 reduce oncogenic risk relative to the original c-MYC-based Yamanaka factors while maintaining reprogramming efficiency.
What chemically defined media are used for iPSC reprogramming?
Commercial chemically defined media used in published feeder-free iPSC workflows include E8/Essential 8, mTeSR1/mTeSR Plus, and StemFlex. Each is a defined formulation that supports both the late stages of reprogramming and ongoing iPSC maintenance. Selection depends on the specific protocol, downstream differentiation pathway, and supplier qualification status.
What substrate replaces MEF feeders for iPSC culture?
Vitronectin is a published defined-matrix substrate for feeder-free iPSC culture that supports adhesion and self-renewal (PMID 33226617). Laminin-521 is another published defined option. Matrigel is widely used but is animal-derived and not xeno-free, which limits its use for clinical-grade workflows.
Does PBMC reprogramming work as well as fibroblast reprogramming?
PBMC reprogramming works under the same episomal nucleofection protocol as fibroblast reprogramming but typically with lower per-cell efficiency. PBMCs are less invasive to obtain than skin biopsies and support patient-specific iPSC generation from a blood draw. Published protocols (PMID 33226617) cover both starting cell sources with adjusted plasmid dose and timing.
Sources
- Inzunza J, Arias-Fuenzalida J, Segura-Aguilar J, Nalvarte I, Varshney M. Generation of nonviral integration-free human iPS cell line KISCOi001-A from normal human fibroblasts, under defined xeno-free and feeder-free conditions. Stem Cell Research. 2021 Mar. PMID 33517122
- Febbraro F, Chen M, Denham M. Generation of Human iPSCs by Episomal Reprogramming of Skin Fibroblasts and Peripheral Blood Mononuclear Cells. Methods in Molecular Biology. 2021. PMID 33226617
- Shao Z, Cevallos RR, Hu K. Reprogramming Human Fibroblasts to Induced Pluripotent Stem Cells Using the GFP-Marked Lentiviral Vectors in the Chemically Defined Medium. Methods in Molecular Biology. 2021. PMID 33226615
