The same survival protein that cancer cells hijack for their growth is now being exploited by Staphylococcus aureus, creating a devastating double-whammy effect that worsens infections.
Imagine a microscopic world where the very cells your body sends to fight infection suddenly switch sides, becoming destructive forces that aid the enemy. This isn't science fiction—it's the startling discovery scientists have made about Staphylococcus aureus, a common bacterium that can turn deadly.
Found on the skin of about one in three people, this microbe typically lives harmlessly, but when it enters the body through a cut or medical procedure, it can cause devastating infections that claim thousands of lives annually.
What makes S. aureus so dangerous isn't just its growing resistance to antibiotics, but its arsenal of sophisticated biological weapons called leukotoxins.
These toxins function like specialized saboteurs, specifically targeting and dismantling the body's immune defenses. For years, scientists knew these toxins could kill immune cells, but new research reveals an even more insidious strategy: at certain concentrations, these toxins don't just kill our defenders—they reprogram them to work against us.
Recent breakthrough research has uncovered how S. aureus leukotoxins drive the formation of a specific type of harmful immune cell that expands during infection and worsens outcomes.
This discovery, made possible by cutting-edge genetic tracing and single-cell analysis technologies, opens promising new avenues for treating dangerous bacterial infections at a time when antibiotic resistance is rising.
About 30% of people carry S. aureus in their noses without symptoms, but it can cause serious infections if it enters the bloodstream or deep tissues.
Staphylococcus aureus deploys a collection of bi-component pore-forming toxins—so named because they consist of two protein components that assemble into pores on host cell membranes. Think of them as microscopic drills that puncture cell surfaces, causing contents to leak out and ultimately leading to cell death through osmotic imbalance 1 8 .
Different leukotoxins target different immune cells:
These toxins are not redundant; each appears to have specialized functions in attacking different aspects of host defenses. While their cell-killing abilities are destructive enough, the latest research reveals they have another, more subtle function—rewiring surviving immune cells to work against the host.
Bi-component toxins assemble into pores on cell membranes, disrupting cellular integrity and function.
Within our immune system, CX3CR1+ monocytes serve as precursors to tissue macrophages—versatile immune cells that patrol our tissues, consuming pathogens and cellular debris. These monocytes respond to chemical signals called chemokines, migrating to sites of inflammation where they transform into macrophages that adopt different roles based on environmental cues 2 3 .
The receptor CX3CR1 recognizes a specific chemokine called fractalkine (CX3CL1), which exists both as a membrane-anchored adhesion molecule and a soluble attractant 9 . This receptor is expressed on various immune cells, including monocytes, T lymphocytes, natural killer cells, and dendritic cells 9 .
In normal infections, CX3CR1+ cells help coordinate immune responses. But when S. aureus unleashes its leukotoxins, these same cells can be transformed into harmful actors.
These immune cells normally defend against pathogens but can be reprogrammed by bacterial toxins to become harmful.
BIRC5, better known as survivin, is a protein that plays a dual role in cell division and apoptosis inhibition 3 . It's most famously studied in cancer biology, where tumor cells exploit its ability to block programmed cell death and promote unchecked growth 3 .
Under normal circumstances, most mature tissues contain little BIRC5—it's primarily active during fetal development and in certain stem cell populations. Its reappearance in cancer cells represents a hijacking of developmental pathways for pathological growth.
The surprising discovery is that S. aureus infection can trigger this same protein in myeloid cells, creating populations of long-lived, proliferating immune cells that may persist in tissues instead of differentiating properly 2 3 .
A protein normally involved in cell division and preventing cell death, hijacked by both cancer cells and pathogenic bacteria.
| Leukotoxin | Key Target Cells | Notable Characteristics |
|---|---|---|
| PVL | Human neutrophils, monocytes, macrophages | Highly associated with community-acquired MRSA; species-specific (active on human/rabbit but not mouse cells) |
| LukED | Neutrophils, monocytes, red blood cells (rabbit) | Unique in effectively targeting mouse phagocytes; uses chemokine receptors as entry points |
| γ-hemolysin | Red blood cells, neutrophils, macrophages | Present in 99% of clinical strains; promotes survival in blood |
| LukAB/GH | Neutrophils, monocytes, macrophages, dendritic cells | Most divergent leukotoxin; can be surface-associated on bacteria |
The groundbreaking research that linked these elements began with an observation in other inflammatory conditions. Scientists had previously identified a population of stem cell-like proliferating myeloid cells in atherosclerotic plaques that could serve as reservoirs for tissue macrophages 3 . These cells could persist in a self-renewing state in inflamed tissue rather than immediately differentiating into macrophages.
Researchers wondered whether similar populations might appear during infections. Using single-cell RNA sequencing—a technology that reveals which genes are active in individual cells—they profiled immune cells in the liver during chronic infections with either the parasite Schistosoma mansoni or Staphylococcus aureus 2 3 .
The findings were striking: both infections led to the expansion of a cluster of BIRC5+ myeloid cells in the liver. But the real breakthrough came when the team tested what happened during S. aureus infection when leukotoxins were absent 2 3 .
When mice were infected with a genetically modified strain of S. aureus that lacked all four leukotoxins (ΔTOX strain), these BIRC5+ myeloid cells failed to appear. This demonstrated that the toxins were directly driving the formation of these pathogenic cells 3 .
Even more compelling, when researchers genetically deleted BIRC5 specifically from CX3CR1-expressing cells, mice showed significantly improved survival during S. aureus infection 2 3 . This confirmed that BIRC5 in these cells was playing a harmful role rather than a protective one.
The discovery that bacterial toxins can reprogram immune cells to express cancer-associated proteins represents a major shift in understanding host-pathogen interactions.
Proliferating myeloid cells found in atherosclerotic plaques
BIRC5+ myeloid cells identified in infection models
Leukotoxin-deficient strains fail to induce BIRC5+ cells
BIRC5 deletion improves survival in infection models
To definitively establish that the harmful BIRC5+ cells originated from CX3CR1+ precursors, researchers employed sophisticated genetic fate-mapping techniques 3 . Here's how they did it:
This technique allows researchers to permanently label specific cell populations and track their descendants, revealing cellular lineages during infection.
Genetically engineered mouse models were essential for tracing the origin and fate of pathogenic immune cells during infection.
| Research Tool | Function in This Research |
|---|---|
| Cx3cr1CreERT2 mice | Enables tamoxifen-dependent genetic labeling of CX3CR1+ cells and their descendants |
| Rosa26stop-tdTomato reporter | Provides red fluorescent label when Cre recombination occurs |
| Single-cell RNA sequencing | Profiles gene expression of individual cells to identify distinct cellular states |
| BIRC5-floxed mice | Allows conditional deletion of BIRC5 gene in specific cell types |
| S. aureus ΔTOX strain | Leukotoxin-deficient mutant (ΔlukAB, hlg::tet, lukED::kan, pvl::epc, hla::erm) |
The experimental results revealed a compelling narrative:
These findings demonstrate that S. aureus leukotoxins actively drive the formation of pathogenic immune cells through induction of BIRC5, and that these cells worsen infection outcomes rather than helping to control the infection.
| Condition | BIRC5+ Cells | Survival |
|---|---|---|
| Wild-type S. aureus | Expanded | Poor |
| ΔTOX S. aureus | No expansion | Improved |
| BIRC5 deleted | N/A | Improved |
| Experimental Condition | BIRC5+ Myeloid Cells | Mouse Survival | Interpretation |
|---|---|---|---|
| Wild-type S. aureus | Significant expansion | Poor | Leukotoxins drive pathogenic cell formation |
| Leukotoxin-deficient (ΔTOX) S. aureus | No expansion | Improved | Pathogenic cells depend on toxin presence |
| BIRC5 deleted from CX3CR1+ cells | Not applicable | Improved | BIRC5 expression harmful to host |
This discovery that bacterial toxins can drive the formation of pathogenic immune cells through BIRC5 represents a paradigm shift in how we view host-pathogen interactions. Rather than simply killing immune cells, S. aureus appears to actively reprogram the immune system to its advantage.
The clinical implications are substantial. Since BIRC5 inhibitors are already being explored as cancer therapeutics, these compounds might be repurposed as adjunct therapies for serious S. aureus infections 3 . Such host-directed therapies could work alongside antibiotics to address both the infection and the harmful host response.
This research also highlights the value of targeting virulence factors rather than just trying to kill bacteria outright. By neutralizing the specific leukotoxins responsible for driving harmful immune responses, we might prevent the immune system from being hijacked. Researchers have already made progress in developing antibodies that can neutralize multiple leukotoxins simultaneously 8 .
The surprising discovery that different leukotoxins can sometimes inhibit each other adds another layer of complexity 6 . For instance, PVL can antagonize LukED activity on certain cell types, suggesting these toxins exist in a delicate balance that might be manipulated therapeutically.
As we enter an era of increasing antibiotic resistance, understanding these sophisticated interactions between pathogens and our immune system becomes increasingly crucial. The more we learn about how bacteria manipulate our defenses, the better equipped we'll be to develop clever countermeasures that protect us from within.
With rising antibiotic resistance, understanding how bacteria manipulate host immunity becomes increasingly important for developing new treatment strategies.
The discovery of BIRC5+ myeloid proliferating cells driven by S. aureus leukotoxins reveals just how sophisticated the evolutionary battle between pathogens and our immune system has become. This bacteria doesn't just blast through defenses—it cunningly reprogram them, turning our cellular defenders into destructive forces.
While much remains to be understood about exactly how these reprogrammed cells worsen infection, and whether similar mechanisms occur in other infectious diseases, one thing is clear: the era of viewing infections as simple battles between bacteria and antibiotics is over. The future lies in understanding the complex three-way interaction between pathogen, host, and treatment—and using that knowledge to develop therapies that protect both the patient and their immune system from manipulation.
As research continues, we move closer to a time when we can not only kill dangerous bacteria but prevent them from turning our own bodies against us—a victory that would save countless lives from these cunning microscopic adversaries.