Nasal postbiotic therapy restores NALT architecture and enhances respiratory innate immunity in protein-malnourished mice

Nasal administration of *Lacticaseibacillus rhamnosus* CRL1505 or its peptidoglycan postbiotic effectively restores nasopharynx-associated lymphoid tissue architecture and enhances mucosal innate immunity, thereby protecting protein-malnourished mice against *Streptococcus pneumoniae* infection.

The Gist

The Big Picture: A Broken Fortress and a Smart Repair Crew

Imagine your body is a castle, and your nose is the main gate. Inside the gate, there is a special "training camp" called NALT (Nasopharynx-Associated Lymphoid Tissue). This camp is where the body's security guards (immune cells) are trained, fed, and organized to fight off invaders like the bacteria Streptococcus pneumoniae (which causes pneumonia).

The Problem:
When a person (or in this study, a mouse) doesn't get enough protein in their diet, it's like the castle runs out of food and money. The training camp (NALT) starts to crumble. The walls get thin, the guards starve and leave, and the camp becomes a ghost town. Because the camp is broken, the castle is wide open to invaders. Even if you start feeding the mouse a normal diet again, the camp doesn't automatically fix itself; the guards just don't know how to get back to work.

The Experiment:
The scientists wanted to see if they could act as a "repair crew" to fix this broken training camp. They tested two different tools:

1. The Live Worker: A specific good bacteria called Lacticaseibacillus rhamnosus (let's call him "Larry").
2. The Blueprint: A piece of Larry's cell wall (peptidoglycan), which acts like a set of instructions or a "wake-up call" without needing a living bacteria.

They gave these tools to the malnourished mice through their noses (like a nasal spray) while they were eating a healthy diet.

What Happened? (The Results)

1. The "Ghost Town" Came Back to Life
In the mice that just ate a healthy diet but got no spray, the training camp remained small and empty. But in the mice that got the "Larry" spray or the "Blueprint" spray, the camp magically rebuilt itself. The walls got thick again, and the number of immune cells exploded back to normal levels.

2. The Guards Got Their Jobs Back
The study looked at the specific types of guards:

• The Recruits (B and T cells): The sprays brought in new recruits and helped them grow up to be strong soldiers.
• The Scouts (Dendritic cells and Macrophages): These are the eyes and ears of the immune system. The sprays made sure there were enough scouts to spot the enemy early.
• The Heavy Hitters (Neutrophils): The sprays balanced the numbers so the mice had just the right amount of heavy firepower ready to go.

3. The "Wake-Up Call" Effect
Here is the coolest part: The mice got the spray before they were even attacked by the pneumonia bacteria. The spray didn't just fix the damage; it primed the immune system. It was like turning on the castle's alarm system and telling the guards, "Get ready, we might have trouble soon!" When the bacteria finally arrived, the malnourished mice treated with the spray fought them off almost as well as the perfectly healthy mice.

4. The Blueprint vs. The Live Worker
Both the live bacteria (Larry) and the blueprint (peptidoglycan) worked incredibly well.

• Larry (Live bacteria) was slightly better at bringing back certain types of T-cells and making the immune system very active.
• The Blueprint (Postbiotic) was almost just as good at rebuilding the camp and making sure the guards could recognize the enemy (a protein called MHCII).
• Why does this matter? If a patient is too sick to handle live bacteria (like someone with a very weak immune system), the "Blueprint" is a safe, non-living alternative that still does the heavy lifting.

The Takeaway

This study shows that when you are malnourished, your body's defense system doesn't just "wait" for you to eat better; it stays broken. You need a specific nudge to wake it up.

By spraying a specific good bacteria (or its parts) up the nose, you can rebuild the immune training camp in the nose. This turns a vulnerable, sick body back into a fortress that can fight off dangerous lung infections.

In short: If your body's immune system is starving and broken, a little bit of "good bacteria" up the nose can act like a construction crew, rebuilding the walls and training the guards so you can survive the next attack.

Technical Summary

1. Problem Statement

Protein malnutrition is a leading cause of secondary immunodeficiency, particularly in developing regions. It severely compromises mucosal immunity, specifically in the respiratory tract, increasing susceptibility to pathogens like Streptococcus pneumoniae.

• The Gap: While the impact of malnutrition on systemic immunity is well-documented, its specific effect on the structural and cellular organization of the Nasopharynx-Associated Lymphoid Tissue (NALT)—the primary inductive site for upper respiratory immunity in mice—remains undercharacterized.
• The Challenge: Current interventions often rely on nutritional repletion alone, which may restore body weight but fails to fully reverse mucosal immune atrophy and dysfunction. There is a need for non-invasive strategies to restore local respiratory defenses in immunocompromised hosts.

2. Methodology

The study utilized a murine model to evaluate the efficacy of nasal administration of Lacticaseibacillus rhamnosus CRL1505 (a live immunobiotic) and its peptidoglycan (a postbiotic) in restoring NALT function.

• Animal Model: Male Swiss albino mice were subjected to a 21-day protein-free diet (PFD) to induce severe malnutrition (reaching ~50% of control body weight).
• Experimental Groups: Following malnutrition, mice underwent a 7-day nutritional repletion phase with a Balanced Conventional Diet (BCD). They were then divided into five groups:
  1. WNC: Well-nourished controls (BCD only).
  2. MNC: Malnourished controls (PFD continued).
  3. BCD: Malnourished mice repleted with BCD only.
  4. BCD+Lr05: Malnourished mice repleted with BCD + nasal administration of live L. rhamnosus CRL1505 ($10^8$ CFU/mouse/day) for the final 2 days.
  5. BCD+PG05: Malnourished mice repleted with BCD + nasal administration of CRL1505-derived peptidoglycan (8 µg/ml) for the final 2 days.
• Challenge: Mice were intranasally challenged with S. pneumoniae serotype 6B.
• Assessments:
  • Histology: H&E staining of NALT to assess architecture.
  • Microbiology: Nasal lavage plating to quantify bacterial load and microbiota composition.
  • Flow Cytometry: Analysis of immune cell populations (B cells, T cells, macrophages, dendritic cells, neutrophils) in NALT, Cervical Lymph Nodes (CLN), and Axillary Lymph Nodes (ALN). Markers included CD3, CD4, CD8, B220, CD24, CD11b, CD11c, CD103, F4/80, Gr-1, and MHCII.
  • Cytokines: Quantification of TNF-α, IFN-γ, and IL-10 in nasal lavage fluid.

3. Key Contributions

• First Characterization of NALT in Malnutrition: The study provides the first detailed description of how protein deficiency causes NALT atrophy and cellular depletion, and demonstrates that these structural defects are reversible.
• Postbiotic Efficacy: It establishes that the peptidoglycan component of L. rhamnosus CRL1505 is sufficient to restore NALT architecture and immune function, offering a safe alternative to live bacteria for immunocompromised patients.
• Mucosal Priming: The research highlights that nasal immunomodulation can "prime" the mucosal immune system, enhancing resistance before pathogen exposure, rather than just reacting to infection.

4. Key Results

• Restoration of NALT Architecture: Protein malnutrition caused marked NALT atrophy and reduced cellularity. While nutritional repletion (BCD) alone only partially reversed these effects, both nasal treatments (Lr05 and PG05) fully restored NALT structure, cellularity, and the NALT-to-body-weight ratio.
• Immune Cell Reconstitution:
  • Lymphocytes: Both treatments significantly increased total B and T lymphocytes, including immature B cells (indicating stimulated local lymphopoiesis), and CD4+ T cells.
  • Myeloid Cells: Treatments restored populations of CD11b+CD11c- cells, CD103+ dendritic cells, and F4/80+ macrophages, which were depleted in malnourished mice.
  • Neutrophils: The treatments balanced the percentage of Gr-1+ cells and restored their absolute numbers, which had been suppressed by malnutrition.
• Enhanced Antigen Presentation: Nasal treatments increased MHCII expression (Mean Fluorescence Intensity), particularly in the PG05 group post-infection, indicating improved antigen-presenting cell activation.
• Pathogen Control:
  • Nasal bacterial loads (total and specific S. pneumoniae counts) were significantly reduced in treated groups compared to malnourished controls.
  • Treated groups showed a selective enrichment of beneficial microbiota (MRS agar counts).
• Systemic Impact: The immunomodulatory effects extended beyond the NALT. Nasal treatments fully normalized cell populations in draining Cervical Lymph Nodes (CLN) and distal Axillary Lymph Nodes (ALN), whereas dietary repletion alone failed to do so.
• Cytokine Profile: Interestingly, cytokine levels (TNF-α, IFN-γ, IL-10) in nasal fluids remained largely unchanged, suggesting that the protection mechanism is driven primarily by cellular reconstitution and priming rather than a massive cytokine surge.

5. Significance and Conclusion

• Clinical Relevance: The findings support the development of nasal immunobiotic formulations (both live and postbiotic) as non-invasive interventions to bolster respiratory defenses in malnourished or immunocompromised populations.
• Mechanism of Action: The study demonstrates that nutritional recovery alone is insufficient to restore mucosal immunity; specific immunomodulatory signals (via peptidoglycan) are required to reactivate the NALT and re-establish the mucosa-lymph node axis.
• Safety and Application: The efficacy of the peptidoglycan (postbiotic) is particularly significant, as it offers a safe therapeutic option for patients where live bacterial administration might be contraindicated (e.g., severe immunosuppression).
• Future Directions: The authors suggest further investigation into the molecular pathways of NALT reactivation and long-term efficacy in chronic infection models to support translational development.

In summary, this paper proves that targeted nasal delivery of L. rhamnosus CRL1505 or its peptidoglycan can reverse the structural and functional collapse of the upper respiratory immune system caused by protein malnutrition, providing robust protection against pneumococcal infection.