Immune pathways that regulate neutrophil activation and replenishment prevent persistent pneumococcal colonization

Technical Summary

Problem Statement
Streptococcus pneumoniae (Spn) frequently colonizes the nasopharynx, a condition that is often asymptomatic but serves as the critical first step in the pathogenesis of invasive pneumococcal disease. While colonization typically resolves within days to weeks in wild-type (WT) hosts, the specific immune mechanisms that prevent this colonization from becoming persistent (lasting months) are not fully elucidated. Understanding these regulatory pathways is essential for comprehending how the host prevents long-term mucosal infection by this leading opportunistic pathogen.

Methodology
The study utilized a murine model to investigate the duration of primary nasopharyngeal colonization by Spn. The researchers employed genetic knockout models to disrupt specific immune signaling pathways, specifically focusing on the Interleukin-17 Receptor A (IL-17RA) and Interleukin-1 Receptor (IL-1R) pathways. To characterize the molecular and cellular consequences of these disruptions, the authors performed RNA-sequencing (RNA-seq) analysis on nasal tissue. This was complemented by flow cytometry and immunological assays to assess neutrophil populations, surface marker expression (specifically CD11b), and the expression of endothelial adhesion molecules.

Key Results
The investigation revealed distinct yet cooperative roles for IL-17RA and IL-1R signaling in controlling Spn colonization:

1. IL-17RA Signaling and Neutrophil Replenishment:

   • In the absence of IL-17RA signaling, Spn colonization became persistent, lasting over six months, whereas it resolved within days to weeks in WT mice.
   • RNA-seq analysis indicated that IL-17RA signaling is not primarily responsible for driving the expression of neutrophil-specific chemokines. Instead, its critical function is the replenishment of neutrophils within the nasal tissue, which are otherwise depleted during infection.
   • Mechanistically, IL-17RA signaling promotes the expression of endothelial cell adhesion molecules. These molecules facilitate the trafficking of neutrophils from the circulation into the nasal tissue, ensuring a sustained presence of these immune cells.

2. IL-1R Signaling and Neutrophil Activation:

   • Mice lacking IL-1R signaling also exhibited persistent Spn colonization.
   • Unlike IL-17RA, IL-1R signaling is not required for the expression of IL-17A or the initial recruitment of neutrophils.
   • Instead, IL-1R signaling is essential for the activation of neutrophils already present in the nasal tissue. This activation is characterized by increased surface expression of CD11b, a receptor critical for the complement-opsonized phagocytosis of Spn.

3. Synergistic Effect:

   • The loss of either pathway individually resulted in persistent colonization.
   • When both pathways were removed, an additive effect was observed, further compromising the host's ability to clear the bacteria.

Significance and Claims
The paper concludes that preventing persistent mucosal infection by S. pneumoniae requires the sustained presence and functional activity of neutrophils. The study identifies that these two requirements are met through the cooperation of two distinct immune signaling pathways: IL-17RA signaling ensures the continuous replenishment of neutrophils into the tissue via endothelial adhesion, while IL-1R signaling enhances the functional capacity of these cells to clear the pathogen through phagocytosis. These findings provide a mechanistic insight into how the host immune system coordinates neutrophil dynamics to resolve nasopharyngeal colonization and prevent the transition to invasive disease.