# IgM-mediated protection drives early B-cell activation and mucosal containment of Vibrio anguillarum in Atlantic cod (Gadus morhua)

## TL;DR

The study shows how Atlantic cod, lacking certain immune components, use IgM antibodies and B-cells to protect against Vibrio anguillarum infection.

## Contribution

The study identifies a novel IgM-mediated immune coordination mechanism in Atlantic cod that operates without CD4+ T-cell help.

## Key findings

- Vaccinated and immune-serum-treated cod contained Vibrio anguillarum in gills with minimal systemic spread.
- A distinct B-cell subset expanded during infection, showing increased IgM and MHC class I expression.
- Vaccinated fish showed the largest B-cell expansions and reduced inflammatory responses compared to naïve fish.

## Abstract

Atlantic cod lack functional MHC class II and CD4, raising fundamental questions about how vaccination generates protection in this species.

We combined single-cell transcriptomic profiling of splenic cells with qRT-PCR across complementary active vaccination and passive serum-transfer experiments to define cellular and transcriptional correlates of immunity to Vibrio anguillarum.

Bath-vaccinated fish and recipients of immune serum showed effective containment of infection, with bacterial signals largely restricted to gills and minimal detection in spleen or head kidney, whereas naïve fish frequently developed high systemic bacterial loads by day 3 post-infection. All groups exhibited increased splenic macrophage abundance following challenge, but only naïve fish showed strong and sustained inflammatory activation, consistent with their higher pathogen burden. Vaccinated fish and immune-serum recipients displayed only transient or weak myeloid responses despite similar early neutrophil activation. Across both experimental models, a transcriptionally distinct B-cell subset expanded at the peak of infection. This population showed increased immunoglobulin and MHC class I expression together with innate sensing features, consistent with an activated B-cell state. Although this B-cell subset increased in all groups, the largest expansions were observed in vaccinated fish and immune-serum recipients. Overall, these findings are consistent with antigen-specific IgM enhancing early B-cell activation and contributing to protection against V. anguillarum through coordinated humoral and innate-like B-cell responses.

These findings identify an antibody-driven mode of immune coordination that operates independently of classical CD4+ T-cell help and provide insight into how effective vaccination can be achieved in vertebrates with divergent adaptive immune architectures.