The scientific article of October, 2023 [Updated]

- 🏭 The Immune System's Bleach Factory: A Delicate Balance -

😲 Did you know some immune cells of your body produce bleach to maintain its health?

👽 It might sound a bit like science fiction, but it's a scientific fact! Let's see it...

Illustrations inspired on my post-doctoral project at The Hospital for Sick Children, 2018-2022.

Read some articles from my postdoc I"
Read some articles from my postdoc II"

💪 Who are these cells?
- Neutrophils -

- Neutrophils are the body's first responders against infections. Their primary function is to detect and destroy pathogens, ensuring your body stays free from harmful invaders. They are the inflammatory crew.

1. Rapid Response: Upon detecting an invasion, neutrophils swiftly move to the site of infection, a process known as chemotaxis. They follow a gradient of molecules released by other immune cells or the invaders themselves, guiding them to the front lines.

2. Phagocytosis: Once they reach the battleground, neutrophils engulf and internalize pathogens, a process termed as phagocytosis. Within the neutrophil, these invaders are trapped in tiny compartments called phagosomes.

3. Killing Mechanisms: Neutrophils employ a series of powerful tools to neutralize these threats. One of their primary weapons is the production of reactive oxygen species (ROS), specially bleach.

4. Neutrophil Extracellular Traps (NETs): In certain situations, neutrophils can eject a web-like structure of DNA filled with antimicrobial agents, called NETs. These traps can capture and neutralize bacteria, preventing their spread.

5. Short Lifespan: Neutrophils are designed for rapid response and have a relatively short lifespan, ranging from a few hours to a couple of days. Once they've completed their mission, they undergo a process called apoptosis or programmed cell death.

💪 Who are these cells?

- Macrophages -

- On the other hand, macrophages act as the cleanup crew. They perform a vital process called efferocytosis – essentially, they "eat up" and recycle dead cells, keeping your tissues free from cellular debris. They are the anti-inflammatory team.

Originating from monocytes in the bloodstream, they differentiate and migrate to tissues, where they mature into macrophages.

1. Phagocytosis: Much like their neutrophil counterparts, macrophages excel at engulfing and digesting foreign invaders, debris, and dead cells. This process, termed phagocytosis, aids in keeping our body clean from potential threats.

2. Antigen Presentation: Beyond mere cleanup, macrophages play a crucial role in alerting the immune system. After ingesting pathogens, they display fragments, known as antigens, on their surface. This 'presentation' signals specific immune cells to launch a targeted attack.

3. Tissue Repair: Macrophages are not just warriors. In the aftermath of an infection or injury, they switch roles to support tissue repair and regeneration, releasing growth factors and clearing debris to aid healing.

4. Lifespan: Unlike the short-lived neutrophils, macrophages can persist for weeks to months, adapting and responding to changing tissue environments.

🦠But, how is that bleach produced?

Well, scientists in the 30’s put neutrophils with bacteria in a barometer and observed that the pressure dropped. The secret lies in a process called the respiratory burst, a process that consumes high amounts of oxygen, during which Reactive Oxygen Species (ROS) are produced. This burst is triggered when these cells encounter pathogens. The primary purpose of this oxidative burst is to destroy the invading microbes. Process description:
1. Activation: When immune cells, such as neutrophils, recognize and engulf pathogens, it triggers an intracellular signalling cascade either in phagosomes and cytoplasm.
2. Killing the Pathogen: These ROS are potent antimicrobial agents. Within the oxygen-dependent killing mechanism, the superoxide anion can be further converted into other ROS like hydrogen peroxide, which can combine with chloride ions to form hypochlorous acid (bleach).
3. Protection: However, this molecule is a double-edged sword; while it's toxic to invading pathogens, it can also harm host tissues if not controlled. To prevent this tissue damage, these cells also contain antioxidants and enzymes like catalase and superoxide dismutase, which rapidly break down ROS.

Read here the "barometer experiment"

🫧How do these cells produce that bleach?

They use the enzyme Nox2 NADPH oxidase. This protein complex consists of five sub-units. When activated, it positions itself on the phagosome membrane, where it converts oxygen molecules into superoxide anions, the primary form of ROS.

1. ROS Production: At the core of Nox2's function is its ability to catalyze the production of superoxide anion, the primary ROS.

2. Structure and Activation. The Nox2 complex consists of six crucial sub-units. At the heart of this assembly is the transmembrane core, flavocytochrome b558, formed by the union of the gp91 and p22 proteins. In addition to this core, four cytoplasmic sub-units – p67, p47, p40, and Rac2 proteins – bind to it, both activating and stabilizing the entire complex. Nox2 becomes operational when these components assemble on the phagosome membrane during an immune response.

3. Regulation is Key: The activity of Nox2 needs meticulous regulation. An overactive Nox2 can lead to excessive ROS, contributing to inflammation and tissue damage. If this turns chronic can promote auto-immune diseases and cancer. Conversely, a deficient Nox2, as we will see next, results in a compromised immune response.

A little bit of history

The Nox2 history it's an interesting line of facinating scientific descoveries! Let see some of them:

1. Agammaglobulinemia disease (1952). In the 50’s, Bruton et al reports the first case of patients that were unable to produce antibodies and suffered recurrent and chronic infections. They called it Agammaglobulinemia.

2. Hyper-gammaglobulinemia (1954). Looking for more cases of this rare condition (54’s), where patients suffered chronic infections, Janeway et al found the opposite: patients who were recurrently infected but displayed hyper-gammaglobulinemia.

3. Chronic granulomatous disease (1957): Even producing high levels of antibodies, these patients died during their childhood because infections. Bridges et al named this condition as fatal chronic granulomatous disease.

4. Phagocytosis and digestion (1960): In the 60’s, Holmes found neutrophils from CGD patients were able to eat bacteria, but unable to kill it. That means eating process is separated from digestion process. Huge milestone.

5. The root of evil (1960): In the same year, Baehner et al found neutrophils from CGD patients could not consume oxygen in a colorimetric test. They and Klebanoff point out the defect is on the leukocyte oxidase: Nox2 NADPH Oxidase.

6. Everything it's clear now: However was almost 20 years after (1980) that Segal et al identified the mutated gene that encodes the protein core of this enzyme: Cybb. In 2010 was discovered the last subunit of this enzyme.

🤕 What happen when Nox2 doesn't function correctly?

Individuals that are born with inactivating mutations in the Nox2 suffer from Chronic Granulomatous Disease (CGD), a severe immunodeficiency where the immune system struggles to combat infections that most of us can effortlessly ward off.
1. Chronic Granulomatous Disease (CGD) is a rare inherited immune disorder that compromises the body's ability to fend off certain bacteria and fungi, leading to recurrent infections. Moreover, CGD patients don't effectively clean dead cells, leading to persistent inflammation and granulomas.
2. The Root Issue: At the heart of CGD are inactivatin mutations on any of the Nox2 NADPH oxidase's subunits. This defect disrupts the bleach-dependent killing of ingested pathogens and the efferocytosis of dead neutrophils from our tissues.
3. Infection Susceptibility: Individuals with CGD are especially vulnerable to particular bacteria and fungi, causing recurrent bouts of pneumonia, skin infections, and more severe complications.
4. Granuloma Formation: One defining feature of CGD is the formation of granulomas—clumps of immune cells formed in an attempt to isolate pathogens they can't eradicate. These granulomas can obstruct vital body pathways, causing additional health problems.
5. Diagnosis and Treatment: Early diagnosis is essential. While traditionally diagnosed. Early diagnosis is essential. While traditionally diagnosed through a test measuring the neutrophil's oxidative burst, genetic testing is also available. Stem cell transplantation offers a potential cure, but antibiotic and antifungal medications remain the mainstay of management.

⚖ It's all about the disruption of a delicate balance:

🚑 As we can see In the first row representing healthy patients, infections are primarily countered by neutrophils producing ROS (bleach), which which sacrifice themselves to eliminate pathogens. While macrophages also contribute to destroying these pathogens, their primary role is to clear the mess, the dead neutrophils, a process depending of ROS as well. This cleanup prevents tissue from accumulating cellular debris and toxins produced during inflammation. Once this recycling is accomplished, the tissue begins its healing process, restoring its original structure.

In contrast, as we can see in the second row, (A) while CGD's neutrophils rush to fight infections, their lack of Nox2 means they can't manage the pathogens. (B) As these neutrophils die off, without clear effectively pathogens, CGD's macrophages face challenges in clearing them from tissues, (C) resulting in sustained inflammation.

🤓 Take-home message.

The bleach producer Nox2 embodies the "Janus god" of your body: a pro-inflammatory role controling infections, and an anti-inflammatory role, cleaning dead cells, highlighting both its protective benefits and potential pitfalls, and emphasizing the need to understand its complexity.

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The Hospital for Sick Children, Universidad Nacional de Córdoba, University of Harvard, Universidad Austral de Chile, UCT-H Oñativia, SIIC, Hospital Dr. Oñativia, University of Toronto. :)