Estrogen Shields Females From Severe Hepatitis A, Mouse Study Finds

Hepatitis A virus (HAV) infects tens of millions of people each year, and although most patients recover fully, a subset develops severe, sometimes life-threatening liver disease. Epidemiological data have long hinted at an imbalance: men are hospitalized for severe hepatitis A more often than women. What has remained stubbornly unclear is why. Are men simply exposed more often, perhaps through occupational or behavioral differences, or does biology itself tilt the scales? A new study in PLOS Pathogens by Ichiro Misumi, Itoe Shiota, Tomoyuki Shiota, Jingyu Zhao, John M. Cullen, Stanley M. Lemon, and Jason K. Whitmire provides some of the strongest evidence yet that intrinsic biological sex differences shape the outcome of HAV infection, and that the female hormone estrogen plays a direct protective role against severe liver injury.

To dissect the question, the researchers turned to a mouse model. HAV is a fastidious virus with a narrow host range, and the team worked with mice lacking the gene for the type I interferon receptor, Ifnar1, which renders the animals permissive to HAV replication and allows the virus to reach the liver and cause measurable disease. By challenging male and female Ifnar1-deficient mice with HAV under controlled laboratory conditions, the investigators could hold exposure, diet, environment, and age constant, isolating sex as the variable. This design directly addresses the central confound of human epidemiology, where social and behavioral factors are difficult to disentangle from physiology.

The results were striking. Male mice replicated the virus to higher levels than females and shed more of it, indicating that the virus finds a more hospitable environment in the male body. More importantly for disease, the males developed accelerated and more severe liver injury. Serum alanine aminotransferase, or ALT, a standard clinical marker of hepatocyte damage, rose to higher levels in infected males than in females. Histological examination revealed greater immune cell infiltration into the livers of male mice, and molecular profiling showed elevated intrahepatic expression of chemokines, the signaling molecules that recruit inflammatory cells to tissue, as well as higher expression of interferon-stimulated genes, the cellular programs activated during antiviral responses.

That combination of findings paints a coherent picture of immunopathology. Hepatitis A is classically understood as a disease in which the immune response to the virus, rather than the virus itself, drives much of the liver damage. The male mice in this study mounted a more exuberant inflammatory response in the liver, with more chemokine production drawing more immune cells into the tissue, and the consequence was more hepatocyte injury. The higher viral loads in males suggest that the sex difference operates on both sides of the host-pathogen relationship, influencing not only how violently the immune system reacts but also how effectively the virus replicates and spreads within the host.

Crucially, the researchers tested whether age modified these effects by comparing young and older adult mice. The sex differences persisted across both age groups, indicating that the male disadvantage in HAV pathogenesis is not a feature of a particular developmental window but a stable characteristic of sex biology. This matters for interpreting human data, because hepatitis A incidence and severity vary with age, and any sex-based analysis must account for that. The mouse findings suggest that, at least in this model, age does not erase or substantially reshape the fundamental sex difference in infection outcome.

With the male-female difference firmly established, the team moved to the mechanistic question: what is it about being female that confers protection? The leading candidate was estrogen, the dominant female sex hormone, which is known to influence immune function and liver physiology in many contexts. The researchers performed ovariectomy, surgical removal of the ovaries, on female mice to eliminate endogenous estrogen production. When these ovariectomized mice were infected with HAV, they fared markedly worse than female mice that had undergone sham surgery. They carried higher viral loads in the liver, showed greater elevations in serum ALT, and displayed more severe liver pathology on histological examination. In other words, removing the ovaries made female mice look more like males in their response to the virus.

The converse experiment sealed the argument. When ovariectomized mice were given estradiol implants to restore estrogen signaling, their viral loads and ALT activity fell again, moving back toward the protected phenotype of intact females. This hormone-replacement result demonstrates that estrogen itself, rather than some other consequence of ovarian function, exerts a protective effect against HAV pathogenesis. The finding aligns with a broader literature on sex hormones and antiviral immunity, in which estrogen has been shown to modulate inflammatory signaling, enhance certain antiviral defenses, and generally temper the kind of tissue-damaging immunopathology that characterizes severe hepatitis.

The team then probed how estrogen delivers its protection at the cellular level. The canonical estrogen receptor, ESR1, is encoded by the Esr1 gene, and hepatocytes, the working cells of the liver, express this receptor and can respond directly to estrogen through it. One plausible mechanism was that estrogen acts on hepatocytes themselves, signaling through ESR1 to make them more resistant to injury or less permissive to viral replication. To test this, the researchers generated female Ifnar1-deficient mice lacking hepatocellular expression of the estrogen receptor. If direct ESR1 signaling in hepatocytes were the key to protection, these mice should have lost their female advantage. Instead, they were phenotypically similar to mice that still expressed Esr1 in their hepatocytes, indicating that direct ESR1 signaling in hepatocytes is not essential for protection against HAV pathogenesis.

This negative result is scientifically important because it redirects the search for mechanism. If estrogen is not protecting the liver by acting directly on hepatocytes, then its protective effect likely operates through other cell types or systemic pathways. Candidates include immune cells, which are richly responsive to estrogen and which drive the immunopathology of hepatitis; non-parenchymal liver cells such as Kupffer cells and liver sinusoidal endothelial cells, which orchestrate inflammatory signaling in the organ; or broader endocrine effects on metabolism and antiviral state. The study does not identify the responsible mechanism, but by ruling out the most obvious hepatocyte-autonomous explanation, it sharpens the questions for future work.

Taken together, the findings carry implications that extend beyond hepatitis A. They establish, in a controlled experimental system, that sex influences both the magnitude of viral replication and the severity of liver disease in HAV infection, and they identify female sex hormones as the protective factor. For clinicians and epidemiologists, the work supports the interpretation that the excess of severe hepatitis A among men reflects, at least in part, intrinsic biological vulnerability rather than behavior alone. For virologists, the study adds HAV to the growing list of infections whose course is shaped by sex-dependent biology, and it provides a tractable mouse model for dissecting the hormonal and immunological pathways involved. And for the many other liver diseases in which sex differences in severity have been observed but poorly explained, from viral hepatitis to drug-induced injury, the message is that sex hormones deserve a central place in the analysis. The road from mouse to medicine is long, but this study marks a clear step toward understanding why, when hepatitis A strikes, men and women do not face equal odds.

Subject of Research: Sex differences in hepatitis A virus pathogenesis and the protective role of estrogen in mice

Article Title: Sex differences in hepatitis A virus infection in mice

Article References: Misumi, I., Shiota, I., Shiota, T., Zhao, J., Cullen, J. M., Lemon, S. M., & Whitmire, J. K. (2026). Sex differences in hepatitis A virus infection in mice. PLOS Pathogens, 22(10), e1014660. https://doi.org/10.1371/journal.ppat.1014660

Image Credits: AI Generated

DOI: 10.1371/journal.ppat.1014660

Keywords: hepatitis A virus, sex differences, estrogen, liver injury, viral pathogenesis, interferon, immunopathology, ovariectomy, ESR1, hepatocytes, mouse model, PLOS Pathogens