We found that 48/80, an MRGPRX2 agonist and commonly used mast cell activator, is toxic in HMC1.2 (human mast cells) and RBL-2H3 (rat basophils). Researchers should use caution and incorporate a viability test when performing assays in these cell lines.

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A spinoff of “The Americans” except it’s just the Deel spy at Rippling.

Have you used 48/80? How do you reliably measure mast cell activation? Let’s discuss! Comment on our paper or reply here to join the conversation. Please share this thread if you find this surprising or useful!.

Finally, others have reported 48/80 causing membrane permeabilization in bacteria (Pseudomonas) and potassium efflux in mammalian cells—consistent with our hypothesis that 48/80’s physicochemical properties are responsible for its toxicity.

Second, 48/80 is a short amphipathic polymer (usually 3–6 units long). Each monomer has a substituted phenyl ring + a secondary amine, which is positively charged at neutral pH. Other amphipathic molecules (like detergents + cell-penetrating peptides) permeabilize cell membranes.

First, we tested two other MRGPRX2 activators—PAMP12 and substance P—and found no β-hex release or toxicity. Thus, it seems that 48/80’s toxicity isn’t simply due to MRGPRX2 activation.

So, why is 48/80 toxic? Our focus has shifted to a qPCR-based assay to test our tick molecules, but here’s what we hypothesize based on our data and what's known about 48/80. .

We hoped there might be a non-toxic concentration of 48/80 that we could use to induce degranulation, so we did a dose-response assay. Unfortunately, these data confirmed that there is no non-toxic dose that we can use in either cell line.

We realized that β-hex in the supernatant didn’t necessarily signal degranulation. Could it actually indicate toxicity? We measured lactate dehydrogenase (LDH), a marker of cell damage, and found that 48/80 caused significant LDH release. So it’s toxicity, not activation!

Initially, 48/80 seemed promising—cells released β-hex! We planned to use this activator to screen our tick molecules, but our dose–response curves looked off and RNA yields (for a separate RNAseq experiment) from 48/80-treated cells were surprisingly low. Something was wrong.

We chose to model this process w popular mast cell lines, human HMC1.2 and rat RBL-2H3. They’re favored bc they’re easy to use and grow quickly. We tested a panel of activators—including 48/80—to identify which induce degranulation, detectable by measuring β-hex release.

Why mast cells? They're key skin sentinels against parasites. Ticks suppress mast cell activity so they can feed on blood for up to a week without detection. If we found a tick molecule that blocks mast cell degranulation, we’d have a promising anti-itch/inflammation candidate.

We want to leverage tick biology to develop new drugs for itch/inflammation. We computationally IDed tick saliva molecules that might help ticks evade host immune responses. To test them, we chose an assay measuring mast cell activation/degranulation.

How did we discover that the widely used mast cell activator 48/80 actually kills mast cells rather than selectively activating them? Here's the surprising story 🧵 (1/12).

Thread 🧵 to follow - stay tuned!.