What Fly-Fishing Can Teach Us About mRNA Immunogenicity

By Anna Rose Welch, Editorial & Community Director, Advancing RNA

I can count the number of times I’ve gone fishing on one hand. In fact, the only thing I remember about my last fishing experience a decade ago was sitting on a dock at dusk being eaten alive by ravenous mosquitos. Whether I was actively fishing or just observing my then-partner slips my memory. Admittedly, it was only the power of young love that brought me — and kept me — on that mosquito-laden dock in the first place.

Fast-forward 10+ (fishing-free) years later, I had the good fortune to befriend a fly-fisherman at an industry conference. Perhaps we can blame it on the bourbon in my hand, or maybe it was the fact I’d just given a presentation that day on the future of the mRNA therapeutics space. But as he described what fly-fishing entails, I found myself likening what a fly-fisherman does to the mRNA industry’s efforts to understand and better control the immunogenicity of mRNA vaccines and therapeutics.

Thanks to the versatility of mRNA as a drug substance, the mRNA industry offers a variety of therapeutic directions in which to turn. Be it a vaccine for an infectious disease or an immunotherapy, a protein replacement therapy, or an in vivo gene editing therapy, mRNA can feel like a go-to lure in the tackle box. But such versatility also means that “controlling” immunogenicity will mean something completely different depending on the product’s overall therapeutic intent.

The mRNA immunogenicity conundrum is nicely explained in this (pre-COVID) article exploring the use of mRNA as a therapeutic. As the authors explain, “The intrinsic immunogenicity of mRNA can be seen as a double-edged sword. On the one hand, the (systemic) delivery of conventional and unpurified IVT mRNAs can activate the immune system and subsequently lead to the unwanted (systemic) production of pro-inflammatory cytokines and type 1 interferons. This intrinsic immune-stimulatory activity can directly interfere with the aimed therapeutic outcome…. On the other hand, in certain application fields such as vaccination approaches, the inflammatory cytokine production resulting from the recognition of mRNA might add to the effectiveness of the evoked immune response, making the mRNA to become its own adjuvant.”

It was in reading this description that I realized fly-fishing is an elegant metaphor for how we as an industry are working to understand two very important aspects of mRNA therapeutic design and delivery. Of course, the goal of fly-fishing is to snare a fish — an act we can liken to provoking an immune response using a vaccine. But before we can attract that fish, we must also understand which mechanisms and tools will help dispel a fish’s fight or flight response. Just as a fish must see the fly as an element of its natural habitat and as a normal food source, we as mRNA therapeutics makers are making modification, design, and delivery decisions to ensure our therapeutic is welcomed and permitted to do its necessary therapeutic work in the body. 

In this two-part article, I intend to explore how I see the art and science of fly-fishing nicely correlating to our growing understanding of mRNA immunogenicity. Here in part one, I’ll start the discussion by spelling out the decisions fly-fishermen must make to “trick” a fish, and how these decisions are akin to those of the mRNA industry as we strive to successfully navigate “the enigmatic beast” that is the human immune system.

Fly-Fishing “R&D:” The “MOA” Of The Fly

That fly-fishing is a form of R&D was spelled out long before this article was even on my radar. In fact, this recognition goes all the way back to the late 1700s.

As Mark Kurlansky described in his book, The Unreasonable Virtue of Fly-Fishing, “[Sir Humprey Davy, the father of modern chemistry and an avid salmon fly-fisherman] believed that fly-fishing also required a strong scientific background, an understanding of fish and the animals they eat, knowledge of weather patterns, and the life of the river and character.” Kurlansky continued to explain that even the writings of the ancient Romans suggest that fishing was less about catching food than it was an attempt to understand nature by imitating nature. (Only occasionally did “imitating nature” mean dropping bottles of wine into the water to intoxicate fish or dressing up as a female goat, because apparently fish really liked female goats.)

Just as we are constantly striving to understand the intricacies of how our cells make decisions and will react to our therapeutics, a fly-fisherman must have the same curiosity about what makes a fish tick. We regularly emphasize that no two human bodies are the same in their genetic makeups, tolerances, and intolerances. The same can be said for a fish and the environment in which they’re living. Whether it be a wooded American river or an English chalk river, a rarely fished alpine lake or a heavily fished catch-and-release stream, the environment is one of the biggest factors influencing a fish’s behavior and dietary desires. In turn, that environment dictates what a fly-fisherman must do — and the tools they must use — to attract a fish.

There are several pieces of tackle that, in addition to casting technique, play critical roles in a fly-fisherman’s success. These include the fly, the hook, the rod, the line, and the reel — all of which have evolved drastically since their first descriptions in literature circa 1500. However, for the sake of brevity here, I see the greatest parallels between fly-fishing and the mRNA industry’s efforts within the many elaborate discussions, debates, and advancements of the fly throughout history.

We can easily liken the fly to that of our mRNA drug product. We are literally delivering the fly onto/into the water in the hopes it will have the desired “effect” (i.e., attracting/catching a fish). Just as there are a dizzying number of factors drug companies seek to understand about their drug products, picking a fly is no mean feat. Fly-fishermen regularly weigh and often debate which physical attributes — or CQAs, if you will — of a fly will convince a fish that the fly is the “real meal.” After all, the goal of fly-fishing isn’t just to catch a fish’s attention, though this is of course what we hope happens; rather, we must capture the fish’s eye at the right time and in just the right way, without causing alarm and inciting a flight response.

There are a multitude of factors to weigh when selecting a fly.

First and foremost, there are two broad kinds of flies from which to choose: imitators and attractors. While both breeds of fly are artificial “insects,” each fly accomplishes its goals in different ways. As the names clearly indicate, the imitator aims to resemble a real insect, while an attractor takes some creative albeit still “bug-like” liberties to capture a fish’s attention.

Regardless of whether we go with an imitator or attractor fly, we must consider how a fish might perceive the fly below the water’s surface — in particular, the fly’s physical characteristics (e.g., color, shape, material, size/weight). History is full of opinionated fisherman arguing about the situations and environments in which it’s crucial to use large or small flies, light-colored or dark flies, plain or wildly ornate/colorful flies. We can blame the 1800s exotic feather vendors for only some of these proffered “facts” — one of my favorites being that salmon would only be enticed by Prussian blue or cinnamon brown flies. (In other words: Get thee to a trash can with those basic — *cough* synthetic *cough* — blue and brown flies.)

Equally important are the “physics” of a fly — for example, how it lands, moves on/in the water, and floats and/or sinks. To ensure your fly embodies the appropriate “CQAs” requires knowledge of a fish’s feeding habits (e.g., are they surface or subsurface feeders?); the pace/direction of the current; the fish’s feeding location within that body of water; the clarity of the water (i.e., for hook size); and at which stage in the life cycle the local insects are.

In fact, given that some insects, like the juvenile mayfly, temporarily live underwater, and some fish are surface-feeders, means we’re met with another important fly-related variable to consider: do we pick the wet or dry fly? Wet flies (subsurface/sunken flies) may have historically been the traditional fly for catching subsurface feeders, like trout. However, the desire to catch surface feeders or to lure subsurface feeders to the surface for our own entertainment has made dry fly-fishing all the rage. (As you may expect, there have been hundreds of years of debate on the merits of wet/sunk vs. dry fly-fishing — and vice versa. If you’re interested in reading some of the most dignified arguing you’ll ever encounter on this or any other topic in the universe, I’d refer you to Ian Whitlaw’s The History of Fly-fishing In Fifty Flies.)    

Now, some of these many choices can be simplified and/or decided by the geographic location in which you’re fishing. For example, there are flies specific to Scottish Rivers, The Catskills, Maine, and Oregon, etc. — many of which will not be interchangeable across regions. Just as an mRNA-LNP targeting a liver disease will not be effective in reaching/treating a heart condition, you would not have success fishing in Maine with Idaho’s black mite lure, nor would Maine’s Parmachenee Belle be as effective on the West Coast. 

Bridging The Gap Between Fly-Fishing & mRNA  

To be clear, not everything in fly-fishing is strictly scientific. Though some fishermen will go so far as to study the mineral and oxygen content of water and different species’ color preferences, many others rely on their personal experiences and “lore” from other fishermen. At the end of the day, regardless of how studied your fly choice and casting may be, there remains a shroud of mystery around a fish’s choice of one fly over another. However, what all the details behind fly design and selection nicely demonstrate is that there are a multitude of specific factors that can lead to your overall success or failure in fly-fishing.

We can argue the same for how and why our therapies behave the way they do in vivo. It goes without saying that the stakes are a bit higher for us in the mRNA industry than for that of a fly-fisherman; the FDA and our patients likely won’t “bite” if we chalk up our molecules’ performance (or lack thereof) to good/bad luck or a change in weather. Seeing as we’re still treading into relatively “unfished” waters with mRNA products, our nascent understanding of our products’ structures and functions can and most likely will be as murky as a storm cloud. Over time, as we get smarter, the ways we design and develop our processes and products will go a long way to impacting the overall safety, potency, and durability of our products.

However, there is one critical element for which we must account to ensure our mRNA therapies will be clinically meaningful. We must deeply understand the “environment” (or "micro-milieu") of the immune system to determine how its response impacts our product’s overall safety and efficacy profile. Just as a fly-fisherman synthetically imitates nature to gain a fish’s trust, the mRNA industry is creating and delivering synthetic mRNA in specific ways to keep our drug products from being recognized as a threat by our immune system. The more we understand about the immune system’s response pathways and how our products may impact these pathways, the more educated our decisions will become around our products’ structural and compositional modifications. In turn, we will be able to achieve more efficient and functional protein expression and more desirable PK and safety profiles for our mRNA therapeutics and vaccines.

Stay tuned for part 2, in which I’ll continue to explore the connections between fly-fishing and mRNA immunogenicity. Specifically, I’ll be taking a deeper dive into the mRNA vaccine- & therapeutic- specific considerations behind achieving the most ideal immunogenicity profile.

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Works Cited/Consulted

This paper took weeks of additional reading, research, and collaboration.

Special thanks go out to Ryan Murray, senior consultant at ValSource and fly-fisherman extraordinaire, for introducing me to the whacky — but beautiful — world of fly-fishing and ultimately planting the seed for this project.   

Other works referenced and consulted in this article include:

Kurlansky, M. (2021). The Unreasonable Virtue of Fly-Fishing. Bloomsbury Publishing.

Van Hoecke, L. & Roose, K. (2019). How mRNA Therapeutics Are Entering The Monoclonal Antibody Field. Journal of Translational Medicine, (17).  https://translational-medicine.biomedcentral.com/articles/10.1186/s12967-019-1804-8

Whitelaw, Ian. (2015). The History of Fly-Fishing In Fifty Flies. Abrams Image.