🛡️ immunology

Caspase-1 and -11 defend against environmental but not host-adapted pathogens in genetically complex mice

Using genetically robust B6 x 129 F1 hybrid mice, this study demonstrates that caspase-1 and -11 are essential for survival against environmental pathogens like *Burkholderia thailandensis* and *Chromobacterium violaceum* but provide little protection against host-adapted pathogens like *Listeria monocytogenes* and *Salmonella Typhimurium*, supporting the hypothesis that these caspases evolved primarily to defend against microbes lacking specific evasion strategies.

Original authors: Li, L., McGlaughon, B. D., Larson, H. N., Koller, B. H., Miao, E. A.

Published 2026-11-02

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CC BY 4.0

Original authors: Li, L., McGlaughon, B. D., Larson, H. N., Koller, B. H., Miao, E. A.

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer

Inside every cell of our bodies, there is a sophisticated alarm system designed to detect when a bacterial invader has broken in. When this system senses trouble, it triggers a specific type of cell death called pyroptosis. This process is not a quiet shutdown; it is a violent, self-destructive explosion that bursts the cell open. By destroying itself, the infected cell releases its contents to sound a general alarm, calling in other immune forces to hunt down the bacteria. Two specific proteins, known as caspase-1 and caspase-11, act as the executioners that carry out this order. For decades, scientists have studied these proteins by infecting laboratory mice with bacteria that are adapted to live inside animals, such as Salmonella or Listeria. In these familiar scenarios, the mice lacking these proteins often die, but the protection offered by the proteins has seemed modest, merely delaying death by a few days rather than preventing it entirely. This led to a lingering question: if these proteins are so important, why do they seem so ineffective against the bacteria that have evolved to live inside us?

A team of researchers at Duke University and the University of North Carolina decided to test a different idea. They proposed that caspase-1 and caspase-11 did not evolve primarily to fight the bacteria that have mastered the art of hiding inside animal hosts. Instead, they suggested these proteins evolved to defend against the ubiquitous bacteria found in soil and water—microbes that have never faced the pressure of evolving to evade an animal immune system. To prove this, the scientists had to overcome a major hurdle in previous research. The standard laboratory mice used for these studies carry hidden genetic flaws. One common strain lacks a gene called Nramp1, which is crucial for fighting certain bacteria, while another strain lacks a working version of the caspase-11 gene entirely. These flaws made it impossible to tell if the immune system was failing because of the missing caspase proteins or because of these other genetic defects. To solve this, the researchers bred a new type of mouse by crossing these two strains. The resulting offspring possessed a complete, functional set of immune genes, including both Nramp1 and caspase-11, providing a clean genetic background to test their theory.

The team then put these genetically robust mice to the test with four different types of bacteria. First, they used the host-adapted pathogens: Listeria monocytogenes and Salmonella Typhimurium. These are the bacteria that have spent millions of years learning how to survive inside animal cells. When the researchers infected the new hybrid mice with these bacteria, the results were stark. Whether the mice had functional caspase proteins or not, they fared almost exactly the same. The missing proteins did not save the mice from Listeria at any dose tested. With Salmonella, the proteins offered a tiny, fleeting advantage only at the very lowest dose of bacteria, but they could not prevent death when the bacterial load was higher. This confirmed that these specific immune proteins are largely ineffective against bacteria that have evolved strategies to hide from them.

Next, the researchers turned to the environmental pathogens: Burkholderia thailandensis and Chromobacterium violaceum. These bacteria live in the soil and water and are not adapted to live inside humans or mice. When the scientists infected the mice with these environmental invaders, the outcome changed dramatically. The mice with functional caspase-1 and caspase-11 were incredibly resistant. They could survive doses of Burkholderia that were six million times larger than the dose that killed the mice lacking these proteins. For Chromobacterium, the difference was equally massive, with the functional mice surviving doses up to one hundred thousand times larger than those that killed the deficient mice. In the mice that lacked the caspase proteins, even a tiny number of these environmental bacteria, sometimes as few as one hundred cells, was enough to cause death. The presence of the caspase proteins acted as an almost impenetrable wall, raising the lethal dose by millions of times.

The study reveals a clear distinction in how our immune system handles different kinds of threats. The caspase proteins are not the primary defense against the sophisticated, host-adapted bacteria that have learned to play hide-and-seek with our immune system. Instead, their true evolutionary power lies in defending against the raw, unevolved bacteria of the natural world. These environmental microbes carry weapons, such as molecular injection systems, that accidentally trigger our alarm sensors the moment they enter a cell. Because these soil and water bacteria have never had to evolve to hide from us, they cannot escape the caspase-triggered explosion. The research suggests that the immune system maintains these proteins not to fight the battles of the Red Queen, where hosts and pathogens constantly evolve against each other, but to hold a permanent, unbreakable barrier against the vast majority of environmental microbes that have never learned to fight back.

Technical Summary: Caspase-1 and -11 Defend Against Environmental but Not Host-Adapted Pathogens in Genetically Complex Mice

Problem Statement
Caspase-1 and caspase-11 are inflammatory caspases that execute pyroptosis, a lytic form of programmed cell death intended to eliminate intracellular pathogens. While these enzymes are known to confer survival benefits in various infection models, the magnitude of this protection is often modest, particularly against host-adapted pathogens like Salmonella enterica serovar Typhimurium and Listeria monocytogenes. The authors propose a hypothesis: caspase-1 and -11 evolved primarily to defend against ubiquitous environmental microbes that have not faced selective pressure to evade them ("Red Pawn" pathogens), whereas host-adapted pathogens ("Red Queen" pathogens) have evolved specific evasion strategies to subvert these defenses.

Testing this hypothesis has been complicated by the genetic background of standard laboratory mouse strains. C57BL/6 (B6) mice carry a loss-of-function allele of Nramp1 (a critical gene for controlling Salmonella and Listeria), and 129 mice possess a natural deletion rendering them deficient in functional caspase-11. These inherent defects confound the assessment of caspase-1 and -11 efficacy, as susceptibility in these strains may result from dual gene deficiencies rather than the specific loss of the inflammatory caspases alone.

Methodology
To resolve these genetic confounders, the authors generated B6 × 129 F1 hybrid mice. These hybrids are gene-sufficient for both Nramp1 and Casp11, providing a genetically complex and robust background to assess immune function. The study utilized four distinct mouse backgrounds:

B6: Nramp1 deficient, Casp11 sufficient.
129: Nramp1 sufficient, Casp11 deficient.
F1 Hybrid: Nramp1 and Casp11 sufficient (gene-sufficient).
F1 Casp1–/–Casp11–/–: Nramp1 sufficient, but deficient in both caspase-1 and caspase-11.

The authors infected these mice with four model pathogens representing two categories:

Host-adapted pathogens: Listeria monocytogenes and Salmonella Typhimurium.
Environmental pathogens: Burkholderia thailandensis and Chromobacterium violaceum.

Infection was administered via intravenous (IV) or intraperitoneal (IP) routes at multiple titrated doses. The primary metric for protection was the fold change in lethal dose ( $\Delta$ LD100) between wild-type (WT) and caspase-deficient mice. Survival curves were monitored, and bacterial burdens in the liver and spleen were quantified at specific days post-infection (DPI). The study also investigated the impact of microbiota by comparing B6 mice from different commercial vendors.

Key Results

Host-Adapted Pathogens (L. monocytogenes and S. Typhimurium):
- Listeria monocytogenes: In B6, 129, and F1 mice, the absence of caspase-1 and -11 resulted in a $\Delta$ LD100 of 1. Survival rates and bacterial burdens were largely indistinguishable between WT and deficient mice across all tested inocula. The study noted that bacterial burdens in B6 mice varied significantly depending on the vendor, highlighting the influence of microbiota on infection outcomes, but the lack of caspase-1/11 protection remained consistent.
- Salmonella Typhimurium: In B6 mice, extreme susceptibility due to the Nramp1 mutation prevented the determination of a $\Delta$ LD100. In 129 and F1 mice, caspase-1/11 deficiency did not significantly alter survival at high inocula ( $5 \times 10^4$ and $5 \times 10^5$ CFU). At the lowest inoculum ( $5 \times 10^3$ CFU) in F1 mice, WT mice survived significantly longer, and 30% survived, whereas all deficient mice succumbed. This yielded a $\Delta$ LD100 of $>1$ in F1 mice but indicated that caspase-1/11 primarily delays infection kinetics rather than preventing lethality in host-adapted pathogens.
Environmental Pathogens (B. thailandensis and C. violaceum):
- Burkholderia thailandensis: Caspase-1 and -11 provided potent protection. In B6 and F1 mice, WT strains survived doses up to $6 \times 10^7$ and $2 \times 10^7$ CFU, respectively, while deficient mice succumbed to doses as low as $10^2$ CFU. This represents a $\Delta$ LD100 of at least $6 \times 10^5$ (600,000-fold) in B6 mice and $2 \times 10^5$ in F1 mice. In 129 mice, the WT strain was susceptible due to the endogenous Casp11 mutation, confirming that the susceptibility was driven by the lack of caspase-11.
- Chromobacterium violaceum: Similar to B. thailandensis, WT mice on B6 and F1 backgrounds were highly resistant (surviving up to $10^7$ CFU), while deficient mice died at low doses ( $10^2$ CFU), resulting in a $\Delta$ LD100 of $>1,000$ . In 129 mice, even WT (caspase-1 sufficient) mice were susceptible, suggesting additional genetic loci in the 129 background contribute to lethality, which were rescued in the F1 hybrid.

Significance and Claims
The paper claims that the evolutionary role of caspase-1 and -11 is best understood through the lens of "Red Pawn barriers." The data supports the view that these enzymes achieve a near-permanent victory against environmental microbes that have not evolved evasion mechanisms. In contrast, host-adapted pathogens have successfully evolved strategies (e.g., repression of flagellin, vacuolar sequestration) to evade detection, rendering caspase-1 and -11 largely ineffective at preventing death, though they may delay infection kinetics.

The study emphasizes that the failure of caspase-1 and -11 against host-adapted pathogens is not an artifact of specific mouse strain genetics (such as the Nramp1 mutation in B6 or the Casp11 deletion in 129) but persists even in genetically robust F1 hybrids. The authors argue that the massive difference in $\Delta$ LD100 (orders of magnitude for environmental pathogens vs. minimal for host-adapted ones) indicates that the selective pressure maintaining caspase-1 and -11 is defense against environmental microbes, rather than an ongoing "Red Queen's race" with vertebrate-adapted pathogens. The paper concludes that these enzymes act as a barrier that prevents environmental pathogens from entering the evolutionary race with the host, effectively demoting them from "Red Queens" to "Red Pawns."

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