A new paper focused on human and other chimeras just came out today in Cell reporting a number of findings, but most strikingly successful generation of human-pig chimeras in utero.
The paper, entitled, “Interspecies Chimerism with Mammalian Pluripotent Stem Cells” describes various chimeras including mouse-rat ones, although those have previously been reported. This work comes from a team led by Juan Carlos Izpisua Belmont including first author Jun Wu, and also with important contributions from Pablo Ross’s lab here at UC Davis
The big news is human-pig chimeras. The successful generation of chimeric embryos that were generated from mixtures of human stem cells (induced pluripotent stem cells or IPSC) and early pig embryos. The authors were able to get post implantation forms of these chimeric embryos and they then studied these. They tested the ability of different kinds of human pluripotent stem cells to contribute to the human-pig chimeras and found interestingly that an intermediate type of stem cell had the best ability.
In the rodents, the team made the chimeras using a neat CRISPR-based technology called interspecies blastocyst complementation to get one species cells to make a specific organ in the other species with the latter unable to make that organ due to an induced genetic change. While they did not use this technology with human chimeras, in theory if one were in the future to try to use chimeras to make human organs for transplantation, the complementation method or something like it would be needed. A related, very important chimera paper came out in Nature yesterday from pioneer Hiromitsu Nakauchi reporting functional islets in rat-mouse chimeras.
Residual pig cells in hypothetical human organ grown in chimera. Even with blastocyst complementation, let’s say with a human pancreas being the target organ to make in a human-pig chimera, the pancreas within the chimera to be used for transplantation would certainly contain some pig cells. These porcine cells likely would come from multiple sources including possibly a few pig pancreatic cells, mostly pig blood cells, and possibly others such as fibroblasts. These pig contributions would be a challenge in terms of successful organ transplantation into a human due to the threat of immune rejection.
CRISPR comes into play? Another potentially complementary technology that could come into play is one to reduce the immunogenicity of pigs using CRISPR to remove antigens. For instance, George Church’s team published a paper last year on using CRISPR for removing endogenous retroviral genes (coding for strong antigens in some cases) in pigs as a method to reduce immunoreactivity in pig-human chimeras. Of course, there are many antigenic proteins in pigs beyond those related to endogenous retroviruses.
Ideally, one could combine the chimera organ complementation and reduced antigenicity technologies to boost the odds of success.
Beyond technological challenges, thorny ethical issues are tightly interwoven into human chimera research. I wrote about these in a piece for Wired last year.
Even with complementation (where for example a pig chimera would ideally only have human cells contributing to one organ such as a kidney or pancreas) one of the ethical dilemmas is that the chimeras would have to be taken to term in order to get a usable human pancreas. It is unclear if taking a human-animal chimera to term could be ethically permissible. In today’s paper, the team isolated the human-pig chimeras for analysis very early in development.
Other ethical challenges include avoiding excessive (however one defines that) human cell contribution to chimeric brains and any human contribution to germ cells. Potential safeguards for the latter include never letting the animals be bred or always including a genetic change making them sterile or both.
Overall, this is exciting research in an ethically challenging arena. The real hope here long term for a new source of organs for transplants is extremely important given the massive need amongst patients, many of whom die on the waiting list. This development also makes starting to tackle the bioethical issues now rather than later a wise choice.