Katalin Karikó and her discoveries that made mRNA vaccines possible

Synthetic mRNA technology is awesome - you can put messenger RNA (mRNA) copies of a protein recipe into cells and get those cells to make the protein. This can either replace a missing or defective protein, or it can get cells to make something “foreign” that the immune system will learn to recognize and make antibodies, etc. against. The latter is the basis of Pfizer/BioNTech’s & Moderna’s coronavirus vaccines. Those vaccines have proven wildly successful, beyond many people’s expectations. But the technology behind them was almost abandoned. Thankfully Katalin Karikó believed in it and was willing to take a demotion in order to pursue it. Today she’s a senior VP of BioNTech and her findings are saving countless lives around the world. ⠀
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blog form (has way more text, figures, & links): bit.ly/katalink... ⠀
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Karikó’s key finding was that, if you made some modifications to the mRNA to make it more “human-like,” cells would use it without setting off those generic immune system alarm bells. RNA only has 4 genetically-encoded letters (nucleotides) - A, C, U, & G - but those letters can get modified, such as with the addition of methyl (-CH₃) groups, or the rearrangement of some of the atoms (isomerization). Bacteria don’t modify their RNA letters very much, but our cells do. Therefore, if our immune system sees un-modified RNA, it might get suspicious that bacteria’s around. But how, at the biochemical level, does an immune system “get suspicious”? And how does it relay that message? One way is through proteins that act as receptors for Pathogen Associated Molecular Patterns (PAMPs). PAMPs can be anything from bacterial toxins to double-stranded RNA (characteristic of some viruses) to, as is relevant here, unmodified RNA. Different Pattern Recognition Receptors (PRRs) bind to different types of PAMPs and trigger signaling cascades, the release of chemical messengers called cytokines, etc. to set the immune system in motion.⠀
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From early on Karikó saw the huge potential for mRNAs, with her interests primarily in protein replacement. But it was all just pie-in-the-sky theoretical unless scientists could figure out how to get the mRNA into cells without the body attacking it (and attacking the body’s own tissues in the process). ⠀
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In an aha-moment paper, Karikó outlines how she, in collaboration with a UPenn immunologist named Drew Weissman who I’ll tell you more about later, solved the mystery of the RNA’s immunogenicity. They took a type of immune cell called an MDDC (monocyte-derived dendritic cell, if you really want to know what that stands for) and they added various types of RNA, including RNAs from bacteria and mammalian cells. When they added total bacterial RNA (i.e. a combination of all the types of RNA in bacteria), the cells set off immune system alarm bells (instead of ringing loudly, they secreted high levels of immune system signaling molecules called cytokines). But if they added total mammalian RNA, this response was much lower. In jargon, we can say that the bacterial RNA was much more immunogenic. ⠀
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Now they needed to figure out what was different about the two (bacterial vs mammalian) and they found a key clue when they divided those two groups of “total RNA” into their different subtypes and tested those subtypes individually. For example, although the total RNA fraction from bacteria was highly immunogenic, bacterial tRNA was much less so (tRNA stands for transfer RNA and it’s a form of RNA that brings amino acids to ribosomes to be added to a growing protein chain during translation). And, although total RNA from mammalian cells wasn’t very immunogenic, mitochondrial RNA, which is more “bacteria-like” was. ⠀
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But, what does it mean at the biochemical level for RNA to be more “bacteria-like?” A key thing to know about bacterial RNA is that, apart from tRNAs, it contains very few post-transcriptional modifications (methylations, etc.). This is in stark contrast with mammalian RNAs which, apart from mitochondrial RNAs, are extensively modified post-transcriptionally. ⠀
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Lack of modification could explain why most bacterial RNAs, as well as mitochondrial RNAs, were immunogenic. And presence of modification could explain why most mammalian RNAs, as well as bacterial tRNAs, were much less immunogenic. It was a beautiful, sensible, hypothesis, but a hypothesis is just an educated guess. Could they prove it?⠀
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To do this, Karikó would need to show that RNA modification could prevent RNA from triggering immune system activation. They tested various U modifications and found that one of the “best” was a natural U modification called “pseudouridine” in which the ring of the base is “rotated” and attached differently to the sugar backbone (best explained visually).⠀She made a series of RNAs with the same sequence but different fractions modified and showed that, the more modification there was, the lower the immunogenicity.