Interspecies Chimerism

Let's talk about everyone’s new favorite topic: interspecies chimerism. You start with a rat embryo, with the PDX1 gene knocked out; normally that rat embryo can’t grow a pancreas. But you inject mouse iPSCs into the rat embryo very early in development. It turns out that this rat will be totally healthy, just living with a mouse pancreas. Then you can take that mouse pancreas and transplant it into a mouse, and it works. Crazy. The trillion dollar question here, just to state it explicitly, is whether you could grow a human organ in a pig that could be transplanted into a human — unlocking a 1000X increase in organ supply and transforming how we do medicine.

Now, in just a moment we will get into where the challenges are, but let’s pause for a minute to talk about how impactful this would be. In the USA we do ~2000 heart transplants a year, but ~700,000 people die every year of heart disease. These heart transplants are ~$1M a pop and involve a massive logistical team to pull off, since there just aren’t that many organ donors dying in usable ways and the shelf-life for organs is short. The unmet need for kidney & liver transplants is arguably a bit less extreme, but only marginally. If our supply chain for organs looked a bit more like our supply chain for wagyu beef, this would ease tremendous amounts of suffering. Whoever eventually figures out how to do this will have invented one of the most impactful therapeutics ever, right up in the pantheon alongside penicillin.

Sadly, this turns out to be a hard problem. Mice and rats are a special case — they are closely related; humans & pigs have a much broader evolutionary distance to cover. Different species have different growth rates and use different signaling hormones etc to direct development. Embryos with large contributions from interspecies iPSCs either die or dramatically reduce in their percentage of foreign species tissue; there are clear drop-off points in embryonic development which is sometimes referred to as the “xenobarrier”. One other possible confounder is that mice iPSCs are higher quality than human iPSCs (although that may be improving). Even doing the PDX1 experiment in reverse (rat iPSCs into a mouse embryo) shows worse results, which is plausibly because we are a little bit worse at making rat iPSCs. For some hard numbers: the mouse pancreas grown in rat is >90% mouse, a rat pancreas grown in mice is ~70% rat, and in most pig chimerism experiments, anything >10% is considered noteworthy.

What if we ditched the pigs and went for monkeys instead? Growing human organs in non-human primates has been proposed as one way to simplify this mismatch problem. But the ethical & logistical challenges are serious — if baboons were used to grow organs, the scale of organ production necessary to meet demand would mean that 99%+ of all baboons would be born for the ultimate purpose of extracting their organs. And at the end of the day, it isn't totally obvious that all of this would be worth it — it is still possible that engineering high-contribution percentage human organs in baboons could fail.

Understanding these signaling & development problems is hard, of course, but I would assert that it does seem more tractable with modern tools like spatial transcriptomics and genome editing technologies — screen to identify genes that turn on in normal development but aren't turning on during interspecies chimerism (perturbseq with a bunch of cas9 knockouts?) and then work your way through a list of edits. But this sort of work is expensive, and right as those tools were getting started, funding in this area got dramatically cut down. In 2015 a paper showed that the injected iPSCs can drift into the brain, which freaked many people out (no one wants to put human consciousness in a lab-experiment pig) — the NIH "temporarily" halted funding to chimerism, and many of the labs doing serious chimerism research had to pivot to collaborating with international labs (many in China) where grant funding still enables this work. It seems plausible that there are workarounds to this (a CNS-specific promoter to trigger apoptosis?) — but there is no sign that the NIH is interested in wading back into this.

So where does that leave us? There are lot of avenues still left to explore to improve chimeric contribution rates. So-called "conditional blastocyst complementation" has been able to increase contribution rates in specific cell types by selectively knocking out the host's ability to generate those tissues. One particularly interesting subarea is vascular replacement — if the vascular system was replaced, it is possible that many of the immune rejection issues encountered in xenotransplantation would be significantly reduced. Approaches that slow down host development or speed up human iPSC growth rates have also shown improved complementation, although problems persist and clearly growth rate alone is not the full story explaining limited compatibility. As mentioned above, recent transcriptomics datasets may provide a solid baseline for understanding what pathways are disregulated during organogenesis in interspecies chimeras — which may indicate some new directions to pursue for opening up these directions. Further afield, there are a few startups developing next-gen immunosuppression that might make pig organs directly-usable. 3D printing of organs seems to be doing fairly well, although the fine cellular-scale structure is generally absent on these models which instead rely on vascularization as an end-step once an approximate structure has been made.

If you've read this far, email me at hello@fuisz.xyz! If there are people interested in pursuing this further, I think there's something worth doing here.