# Transient lung eosinophilia during breakthrough influenza infection in vaccinated mice is associated with protective and balanced Type 1/2 immune responses

## TL;DR

The study shows that during influenza breakthrough infection in vaccinated mice, lung eosinophils are part of a balanced immune response that helps clear the virus without causing severe damage.

## Contribution

The study reveals that eosinophil infiltration during breakthrough influenza infection is non-pathological and distinct from allergic or primary infections.

## Key findings

- Eosinophils in vaccinated mice during breakthrough infection peak at 7–10 days without causing severe inflammation or lung damage.
- Breakthrough infection is associated with Siglec-Fhi eosinophils and preserved alveolar macrophages, distinguishing it from allergic or primary infections.
- Extracellular traps from neutrophils and eosinophils are absent in breakthrough infection, suggesting a balanced immune response.

## Abstract

Eosinophils are versatile cells that participate in a multitude of homeostatic and inflammatory responses in the lung, ranging from allergic asthma to antiviral defense against respiratory viral infection. In the context of vaccination followed by viral infection, such as breakthrough infection, eosinophils have been linked to aberrant Th2 responses like vaccine-enhanced respiratory disease. Here, we demonstrate that the lung immune cell composition, cytokine and chemokine repertoire, histopathological profile, and systemic humoral response of breakthrough influenza infection in mice are distinct from those of primary influenza infection or allergic sensitization, canonical Type 1 and 2 immune responses, respectively. Longitudinal comparison of breakthrough infection with allergic sensitization and primary influenza infection demonstrated major differences in lung immunity between treatment groups in female BALB/c mice. Breakthrough infection mice exhibit lung eosinophil infiltration that peaks at 7–10 days post-challenge, enriched for the Siglec-Fhi subset, but in the absence of overt pro-inflammatory cytokine/chemokine signals, high viral titers, severe lung lesions, goblet cell hyperplasia, allergic levels of total IgE, or enhanced morbidity. Multiparameter fluorescence imaging corroborated findings from flow cytometry and also unveiled interactions between CD101+Siglec-F+ cells and CD3+ cells in the lung tissue space. Imaging also revealed a marked absence of eosinophil or neutrophil extracellular traps in the lungs of breakthrough infection mice, in contrast with allergic sensitization and primary influenza infection, respectively. Altogether, our findings provide a deeper understanding of the kinetics and cell-cell interplay during non-pathological, balanced Type 1/2 immune responses in vaccinated hosts during breakthrough infection.

Our findings reveal that eosinophilic recruitment during influenza breakthrough infection is non-pathological and represents a balanced immune response, distinct from highly inflammatory environments seen in primary influenza infection or allergic sensitization. We observed eosinophil enrichment in the lungs of vaccinated hosts following infection, which coincided with rapid viral clearance and minimal lung damage, challenging traditional associations of eosinophils with adverse outcomes in vaccinated hosts, like for respiratory syncytial virus. We identified a phenotypic shift toward Siglec-Fhi eosinophil subset in breakthrough infection mice. Additionally, we highlight preservation of alveolar macrophages and absence of neutrophil and eosinophil extracellular traps in vaccinated hosts, key features that distinguish breakthrough infection from both Type 1- and Type 2-skewed disease models. These insights have broad implications for exploring eosinophil plasticity and cell-cell interactions in the lung, which could inform the development of strategies to harness the benefits of balanced Type 1/2 immune responses across different vaccine/respiratory virus pairs.