Early sex-specific organ transcriptional divergence without physiological differences in a murine model of fecal-induced peritonitis

In a murine model of fecal-induced peritonitis, male and female mice exhibit identical physiological responses to early sepsis, yet transcriptomic analysis reveals that females display stronger immune activation and inflammatory responses in the kidney and lung compared to males.

The Gist

Imagine your body is a bustling, high-tech city. When a dangerous invader (like a severe infection) attacks, the city's emergency services go into overdrive. This state of chaos is called sepsis. It's a life-threatening condition where the body's own defense system gets confused and starts damaging its own organs, like the liver, kidneys, and lungs.

For a long time, scientists studying this "city under attack" in mice have mostly used male mice, assuming they represent everyone. But this study asked a simple, crucial question: What if the city's emergency response looks different depending on whether the mayor is male or female?

Here is the story of what they found, explained simply:

The Experiment: A Controlled Fire Drill

The researchers set up a "fire drill" for mice. They injected a small amount of dirty fecal slurry (a mix of bacteria) into the belly of some mice to simulate a severe infection. Other mice got a harmless sugar water injection (the control group).

They watched the mice for 8 hours. They checked their temperature, how active they were, and their blood work.

• The Result: Both the male and female mice got sick in exactly the same way. They both got cold, sluggish, and their blood showed signs of infection.
• The Surprise: At this 8-hour mark, if you looked at the mice from the outside, you couldn't tell the difference between the males and females. They were both equally "sick" in a physical sense.

The Secret Code: Reading the City's "Text Messages"

Since the outside looked the same, the scientists decided to look inside the cells. They used a technology called RNA sequencing, which is like reading the city's internal text messages to see what the cells are actually thinking and planning.

They looked at the "text messages" (gene activity) in the liver, kidneys, and lungs.

What they found was shocking:
Even though the male and female mice looked the same on the outside, their internal "text messages" were completely different.

• The Male Response: The male mice's organs sounded the alarm, but it was a standard, moderate alert.
• The Female Response: The female mice's organs sounded a massive, blaring siren. Their immune systems were revving up much harder and faster than the males. In the kidneys and lungs, the female cells were shouting, "We are under attack! Mobilize everything!" much louder than the males.

The Analogy: Two Fire Departments

Imagine two fire departments responding to the same small fire.

• Department A (Males): Sends two fire trucks. They do the job, but they are calm and measured.
• Department B (Females): Sends a full fleet of trucks, helicopters, and extra crews. They are going into "all-out war" mode.

From the street, both departments are putting out the fire. But if you looked at the dispatch center (the genes), you'd see Department B is working twice as hard.

Why Does This Matter?

This study is a big deal for three reasons:

1. The "Invisible" Difference: It proves that biology can be different before it shows up in a physical exam. Just because a male and female patient look the same doesn't mean their bodies are reacting the same way on a molecular level.
2. The Double-Edged Sword: The female mice had a stronger immune reaction. In the short term, this might seem good (they are fighting hard!), but in the long run, fighting too hard can sometimes cause more damage to the city (the organs). This might explain why, in some real-world human studies, women sometimes have higher death rates from sepsis in the hospital, even if they survive longer in the long run.
3. Fixing the Blueprint: For years, medical research has been built mostly on male mice. This study says, "Hey, we need to stop ignoring the female blueprint." If we want to cure sepsis in humans, we need to understand how male and female bodies react differently, right from the very first hour.

The Bottom Line

This paper tells us that sex matters, even when it doesn't seem to. In the early hours of a severe infection, male and female bodies might look identical, but their internal engines are running on completely different settings. To save lives, doctors and scientists need to start tuning their treatments to fit the specific "engine" of the patient, whether they are male or female.

Technical Summary

1. Problem Statement

Sepsis is a life-threatening organ dysfunction caused by a dysregulated host response to infection, affecting millions globally. Despite its clinical significance, there is no effective pharmacological therapy, partly due to the heterogeneity of sepsis pathophysiology and the lack of sex-stratified research.

• Gap in Knowledge: Preclinical sepsis studies are predominantly conducted in male animals, potentially overlooking biological sex differences that could explain discrepancies in clinical outcomes (e.g., incidence vs. mortality rates between men and women).
• Specific Challenge: It remains unclear whether biological sex influences the molecular response to sepsis at the earliest stages of the disease, before overt physiological differences (such as organ failure or mortality) become apparent.

2. Methodology

The study utilized a rigorous, standardized preclinical protocol developed by the National Preclinical Sepsis Platform (NPSP) to ensure reproducibility and translatability.

• Animal Model:
  • Subjects: 10–12-week-old C57BL/6 mice (both male and female).
  • Induction: Fecal-Induced Peritonitis (FIP) via intraperitoneal injection of rat fecal slurry (0.75 mg/g body weight) suspended in dextrose/glycerol. Control groups received the vehicle alone.
  • Timepoint: Assessments were performed at 8 hours post-injection to capture the acute phase of sepsis.
  • Groups: Male Control ($n=3$), Male Septic ($n=4$), Female Control ($n=3$), Female Septic ($n=5$).
• Physiological Monitoring:
  • Body temperature, body weight, and a Modified Murine Sepsis Score (MSS) were tracked at 0, 4, and 8 hours.
  • Blood biochemistry (glucose, sodium, pH, BUN) and peritoneal lavage fluid (PLF) bacterial counts were analyzed.
• Transcriptomic Analysis:
  • Tissues: Liver, Kidney, and Lung were harvested and snap-frozen.
  • Sequencing: RNA was extracted and sequenced using Illumina NovaSeq (PE100).
  • Bioinformatics:
    • Alignment to Mus musculus genome (mm39) using STAR.
    • Differential Expression Analysis (DEA) using DESeq2 (threshold: FC $\ge$ 1.5, FDR $\le$ 0.05).
    • Pathway Analysis: Gene Ontology (GO) enrichment and Gene Set Enrichment Analysis (GSEA) using the MSigDB Hallmark gene sets (50 pathways categorized into 6 groups: Immune, Stress, Proliferation, Metabolism, Signaling, Development).
  • Sex-Stratification: Analyses were performed on combined populations and then stratified by sex for Kidney and Lung (Liver was excluded from sex-stratified analysis due to low sample size/power).

3. Key Contributions

• Standardized Sex-Stratified Protocol: The study applies a harmonized, multi-organ, sex-inclusive protocol (NPSP) to sepsis research, addressing the historical bias toward male-only models.
• Decoupling Physiology from Transcription: The study demonstrates that significant molecular divergence between sexes can occur in the absence of detectable physiological differences at the acute stage (8 hours).
• Organ-Specific Insights: It provides a comparative transcriptomic map of the liver, kidney, and lung in early sepsis, highlighting that while inflammatory pathways are conserved across organs, the magnitude of the response is sex-dependent.

4. Key Results

A. Physiological Response (No Sex Differences)

• Sepsis Induction: Both male and female mice in the FIP group exhibited significant physiological signs of sepsis compared to controls, including:
  • Hypothermia (reduced body temperature).
  • Increased MSS scores.
  • Elevated bacterial counts in PLF.
  • Metabolic disturbances (reduced glucose, sodium, pH; elevated BUN).
• Sex Comparison: There were no statistically significant differences in any physiological parameters (temperature, MSS, blood chemistry) between male and female septic mice.

B. Transcriptomic Response (Combined Population)

• Global Changes: Sepsis induced extensive transcriptional changes in all three organs (Liver: ~6,100 DEGs; Kidney: ~4,900 DEGs; Lung: ~6,200 DEGs).
• Conserved Pathways: Principal Component Analysis (PCA) showed clear separation between septic and control groups. Gene Ontology (GO) analysis revealed that 13 of the top 20 pathways were shared across all three organs, dominated by immune response and inflammatory activation.
• GSEA Findings: The "Immune & Inflammation" and "Stress & Damage Response" hallmark pathways were significantly enriched in septic mice across all tissues.

C. Sex-Specific Divergence (Kidney and Lung)

Despite identical physiological outcomes, transcriptomic analysis revealed distinct sex-based differences:

• Unique Gene Sets:
  • Kidney: 1,892 shared DEGs, but 2,310 DEGs were exclusive to females vs. 516 exclusive to males.
  • Lung: 2,447 shared DEGs, but 2,407 DEGs were exclusive to females vs. 682 exclusive to males.
• Magnitude of Inflammation:
  • Heatmaps of inflammatory markers showed a more pronounced upregulation in female mice compared to males in both the kidney and lung.
  • GSEA Results: In septic females, there was a significant enrichment of "Immune & Inflammation" and "Stress & Damage Response" pathways compared to males. In contrast, male mice showed less robust enrichment in these specific pathways during the acute phase.
• Dose Control: Since female mice were slightly lighter and received a smaller absolute volume of fecal slurry (normalized by weight), the heightened inflammatory response in females is attributed to biological sex differences rather than a higher bacterial dose.

5. Significance and Conclusion

• Early Divergence: The study concludes that biological sex differences in the septic response manifest at the transcriptional level during the early acute phase (8 hours), well before they become clinically or physiologically evident.
• Mechanistic Insight: The heightened inflammatory activation in females suggests a more robust innate immune response. The authors hypothesize that while this may lead to early tissue damage, it could contribute to the "female survival advantage" often observed in long-term sepsis outcomes (lower long-term mortality despite potentially higher initial severity).
• Clinical Implication: These findings underscore the necessity of including both sexes in preclinical sepsis research. Relying solely on male models may miss critical molecular mechanisms that drive sex-specific outcomes, hindering the development of precision medicine therapies for sepsis.
• Future Directions: The study calls for longitudinal assessments to link these early transcriptional signatures to long-term survival and organ recovery, and suggests expanding models to include other variables like age and diet.