🛡️ immunology

In-Depth Immune Profiling Reveals Cell-Type, Sex and Infection-Stage Dependent Differences for Tuberculosis Control

This study utilizes mass cytometry to profile the sex-specific and infection-stage-dependent immune landscape of mouse lungs, revealing Thy1.2 expression on NK cells as a novel sex-specific correlate of protection and confirming the critical role of T-cell activation in vaccine-induced tuberculosis control.

Original authors: Watt, J., Sokolowski, D. J., Liu, J.

Published 2026-10-07

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Watt, J., Sokolowski, D. J., Liu, J.

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 ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed.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

Tuberculosis is a relentless bacterial infection that has shaped human history for millennia, living quietly in the lungs of billions before flaring into a deadly disease. For decades, the world has relied on a single vaccine, developed nearly a century ago, to prevent the worst outcomes, yet its ability to protect adults has remained frustratingly inconsistent. Scientists have long known that the immune system is a vast, complex network of different cell types, each with a specific job, but they have struggled to see exactly how these cells work together to fight the infection or how a vaccine might change their behavior. A critical piece of this puzzle has been missing: a detailed map of the immune landscape inside the lungs after vaccination, showing how it changes over time and how it differs between males and females. Understanding these differences is vital because, in both humans and animals, males often suffer more severe disease than females, a disparity that current medical strategies have not fully addressed.

A team of researchers set out to create this detailed map by studying the lungs of mice infected with tuberculosis bacteria. They wanted to see how the immune system reacts when the body is challenged by the disease after receiving different types of protection. To do this, they used a powerful imaging technique that allows scientists to look at thousands of individual immune cells at once, measuring the specific proteins on their surfaces to identify what kind of cell they are and what state they are in. The researchers prepared three groups of mice: one group received a standard tuberculosis vaccine, another received a newer, experimental vaccine made from a weakened version of the bacteria, and a third group was infected with the bacteria, treated with antibiotics to clear the active infection, and then left to see if their natural immunity could protect them later. They also included a control group that received no protection at all. Crucially, they studied both male and female mice to see if their immune systems responded differently.

The study tracked these mice over a long period, checking their lungs and immune systems at three distinct stages: before they were exposed to the bacteria, a few weeks after exposure when the infection was just starting, and many weeks later when the infection had either been controlled or had taken hold. The researchers measured the number of bacteria in the lungs and spleen to see which mice were protected and which were not. They found that the experimental vaccine and the natural immunity from the antibiotic-treated group worked well in female mice, keeping bacterial numbers low even months after the challenge. However, the results for male mice were more complicated. While the experimental vaccine worked well for males, the natural immunity that protected the females failed completely in the males, who ended up with bacterial loads similar to the unvaccinated control group. This confirmed that the immune system's ability to remember and fight the infection is deeply influenced by sex, and that a strategy that works for one sex does not automatically work for the other.

Digging deeper into the cellular details, the researchers discovered specific markers that could predict how well a mouse was protected. They found that in female mice, a particular type of immune cell called a natural killer cell showed high levels of a specific protein, which was strongly linked to keeping the bacterial infection under control. This protein was not a reliable sign of protection in male mice, highlighting a fundamental difference in how the two sexes fight the disease. The study also revealed that the experimental vaccine caused a lasting change in the immune system that the standard vaccine did not. Even months after the initial vaccination, mice that received the experimental strain showed signs of a sustained immune response, with certain immune cells remaining active and ready to fight. This suggests that the experimental vaccine might be better at training the immune system for the long haul, a quality that is essential for preventing a disease that can lie dormant for years.

The researchers also observed that the immune system's reaction to the infection changed dramatically over time. In the early stages of infection, the immune response was chaotic and varied widely between vaccinated and unvaccinated mice. By the later stages, the immune systems of the protected mice had settled into a more organized state, while the unprotected mice showed signs of immune exhaustion, where the cells were worn out and less effective. The study showed that the standard vaccine, while useful, did not provide the same level of long-term protection in males as it did in females, and that the natural immunity gained from a previous infection was not enough to protect males from a second infection. These findings underscore the importance of looking at both sexes when developing new vaccines and treatments. The research does not offer a new vaccine for immediate use, but it provides a clear, high-resolution view of how the immune system works in the lungs, identifying specific cells and proteins that could be targeted to create better protection for everyone, regardless of sex.

Problem Statement
Tuberculosis (TB) remains the leading cause of death from a single infectious agent, yet current treatment regimens are intensive and only moderately effective, while the Bacille Calmette-Guérin (BCG) vaccine requires updating. A critical gap in knowledge exists regarding the systematic profiling of the lung immune environment following vaccination and subsequent Mycobacterium tuberculosis (Mtb) challenge. Furthermore, sex differences in TB susceptibility and vaccine efficacy are well-documented in both humans and mice, yet the specific immune correlates of protection that vary by sex and infection stage remain largely undefined. Understanding how different vaccination strategies (including BCG, live attenuated Mtb strains like $\Delta$ lsr2, and natural immunity) shape the immune landscape across sexes is essential for developing improved TB prevention strategies.

Methodology
The authors employed a high-dimensional, single-cell proteomics approach using Mass Cytometry by Time of Flight (CyTOF) to profile the lung immune environment of male and female C57BL/6 mice. The experimental design included four distinct groups:

$\Delta$ lsr2 vaccinated: Mice inoculated with a live attenuated Mtb strain lacking the lsr2 gene.
BCG vaccinated: Mice inoculated with BCG-Japan.
Mtb+antibiotics (Natural Immunity): Mice infected with wild-type Mtb H37Rv for 4 weeks, treated with rifampicin and isoniazid for 12 weeks, and then rested.
PBS Control: Sham-inoculated mice.

All groups were aerosol challenged with ~100 CFU of Mtb H37Rv 18 weeks after initial inoculation. The study captured three critical timepoints:

Baseline: 18 weeks post-vaccination/pre-challenge.
Early Infection: 3 weeks post-challenge.
Late Infection: 12 weeks post-challenge.

Lung and spleen tissues were harvested to measure bacterial burden (Colony Forming Units, CFU) and to generate single-cell suspensions for CyTOF analysis. The researchers utilized a panel of 28 metal-tagged antibodies to profile 10 major immune cell subsets. Data analysis involved quality control, cell-type identification via clustering (FlowSOM and ConsensusClusterPlus), and cell-type label transfer between timepoints using MetaNeighbor. The authors utilized Principal Component Analysis (PCA) on pseudobulked protein expression data to identify coordinated trends in immune responses, correlating principal component scores and loadings with bacterial burden, vaccination strategy, and sex.

Key Results

Sex-Dependent Protection: Female mice consistently displayed superior long-term protection against Mtb challenge compared to males across vaccine strategies. In the "Mtb+abx" (natural immunity) group, male mice exhibited bacterial burdens comparable to unvaccinated controls at the late infection stage, whereas females maintained protection similar to vaccinated groups.
Vaccine Strategy Differences: The $\Delta$ lsr2 vaccine induced sustained immune reprogramming up to 18 weeks post-vaccination, characterized by long-term changes in CD39 expression on dendritic cells and granulocytes. Unlike BCG, which showed sex-biased efficacy (better in females), $\Delta$ lsr2 provided comparable protection in both sexes.
Immune Correlates of Protection:
- T-Cell Activation: As expected, T-cell activation and infiltration correlated with protection, particularly in the early infection phase.
- NK Cell Thy1.2 Expression: A novel finding identified Thy1.2 expression on Natural Killer (NK) cells as a strong correlate of protection specifically in female mice during late-stage infection. This marker showed no correlation in males, highlighting a sex-specific immune strategy.
- Exhaustion Markers: During early infection, unvaccinated male mice showed signs of CD4 T-cell exhaustion (high PD1) and altered alveolar macrophage profiles. In late infection, high expression of PDL1 and CD39 on dendritic cells and macrophages correlated with high bacterial burden.
Dynamic Immune Reprogramming: The study demonstrated that the lung immune system undergoes significant reprogramming over time. While cell-type abundance was most dynamic by timepoint, specific protein expression patterns were driven by the interaction of sex and vaccination strategy.

Significance and Claims
The authors claim this study provides the first global, systematic profiling of the mouse lung immune environment in response to multiple Mtb vaccination strategies, explicitly integrating sex as a biological variable. By utilizing PCA rather than simple pairwise comparisons, the study identifies coordinated immune trends and effect sizes that might be obscured by traditional differential expression analysis.

The paper emphasizes three main contributions:

Validation of Sex Bias: It recapitulates and extends the understanding of sex bias in TB, confirming that male mice are more susceptible and that vaccine efficacy can be sex-dependent.
Novel Correlates: It identifies Thy1.2 expression on NK cells as a potential sex-specific correlate of protection in females, suggesting that future vaccine development must account for sex-specific immune mechanisms.
Long-term Immune Rewiring: It demonstrates that live attenuated vaccines like $\Delta$ lsr2 can induce sustained immune changes (e.g., CD39 upregulation) long after the initial vaccination, offering a potential mechanism for durable protection that differs from the transient immunity of BCG.

The authors conclude that their findings underscore the necessity of including both male and female subjects in TB research to define accurate correlates of protection and improve the development of next-generation vaccines and treatments. They position their work as a foundation for hypothesis generation rather than definitive proof, noting that their findings require further validation and that the study's exploratory nature limits the scope of individual claims.

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