Dr. Daley began by observing that cell transplants will always face an immune barrier, and the degree of antigen matching correlates with the vitality and durability of the graft. Although an infrastructure has arisen in the United States to support current transplantation paradigms, a new infrastructure that includes banking of cells and patient-specific cells may help to address the immunity issue. An off-the-shelf therapy could therefore make use of stem cell banks, although HLA polymorphisms may render this resource impractical. Therefore, patient-specific cells may be a solution. It has been shown that a single-antigen match is much easier for a cell bank to achieve than a multiple-antigen match (Taylor CJ, et.al. Lancet 2005;366:2086), and a single-antigen match is acceptable for transplanting solid organs. However, a few donors are homozygous at two HLA haplotypes, thereby matching three of six HLA antigens. As few as ten such donors could therefore theoretically establish bank that would be useful to much of the population.
Dr. Daley then discussed the use of parthenogenesis to create HLA-homozygous ES cells. This approach offers several advantages over nuclear transfer techniques. Using genetic selection, patient-specific cells or cell types could be selected to stock a cell bank. Tissues from major histocompatibility complex (MHC)-matched parthenogenetic ES (pES) cells are histocompatible (Revazova ES, et.al. Cloning Stem Cells 2007;9:432-449; Kim K, et.al. Science 2007;315:482-486). Parthenogenetic tissues, however, raise concern regarding imprint abnormalities and loss of heterozygosity that can arise from the double set of maternal imprints. However, these cells can function normally in the mouse and could possibly be used in transplantation. Nuclear transfer methods could also be used for combined gene/cell therapy, although human oocytes are in limited supply and the technique has yet to succeed in humans (Rideout WM 3rd, et.al. Cell 2002;109:17-27).
These issues suggest that other methods, such as the reprogramming of somatic cells to pluripotency, may be required for regenerative medicine purposes. In 2006, Yamanaka and coworkers (Cell 126:1-14) introduced a library of 24 factors into fibroblasts and identified four transcription factors that would produce ES-like cells. These cells were called induced pluripotent stem (iPS) cells. However, the method failed to generate a live mouse pup. By selecting certain factors, however, an embryo can theoretically be established, although this has not been achieved to date in practice. Researchers then asked if fetal or neonatal cell types, rather than adult cells, could be used for this procedure, given that the efficiency of the reprogramming process declines with age. When a microarray is used to profile iPS and hES cells, clustering is observed in the former, suggesting a similarity (but not identity) between the cell types. Additionally, iPS cells satisfy all of the criteria for pluripotency. To date, at least five different research groups have published independent derivations of these cells. The major near-term benefit of iPS cells is the facilitation of disease modeling.
However, clinical application of iPS cells is precluded by the use of oncogenic retroviruses needed for reprogramming. Dr. Daley speculated that within the next couple of years, however, reprogramming could possibly be effected through the transient expression of regulatory factors. Moreover, current iPS protocols use mouse ES cells, and these techniques must be extrapolated to human ES cells. Human ES cells may actually be more similar to mouse epiblast-derived cells than to mouse ES cells, although available assay types limit full characterization. Also, the derivation efficiency remains low and may decrease with patient age. Finally, many questions remain about the mechanisms that govern reprogramming--are they the same mechanisms that govern oocyte-based reprogramming? If this strategy can work, can cells be reprogrammed directly to tissue fates; e.g., can an exocrine pancreatic cell be redirected toward an endocrine cell? Also, the pathways common to reprogramming and cancer must be explored and exploited for clinical benefit.
In conclusion, Dr. Daley reiterated that stem cell banks derived from parthenotes and iPS cells from persons homozygous for HLA haplotypes are possible. However, patient-specific cells are autologous and offer the opportunity for direct programming. While a non-viral approach must be developed, he noted that time will tell whether somatic cell nuclear transfer will ultimately be the most pristine approach for this application.
One attendee observed that approximately 90 genes define stemness. Given that fact, have iterative combinations of these genes been explored as means to influence particular programming pathways? Dr. Daley replied that many gene sets can be combined to influence numerous reprogramming pathways. Over time, more pathways will be defined, and a common set of pathways will likely emerge. Genes will therefore fit into complementation groups.
Another attendee asked if this approach will be cost-effective. Dr. Daley noted that it is difficult to ascertain the exact cost, but that social justice issues speak to the need for widely-available applications. The International Society for Stem Cell Research Task Force has begun to establish guidelines for social justice that will facilitate making this approach to therapy widely-available and applicable.