Smoltification of Atlantic Salmon (Salmo salar L.) Is Associated with Enhanced Traffic and Renewal of B Cell Repertoire

Abstract

The smoltification of farmed Atlantic salmon is commonly associated with mild immunosuppression. However, B cells may deviate from this trend, showing increased proliferation and migration during this period. This study assessed the effects of smoltification and adaptation to seawater in a controlled experiment. Analyses were conducted on the head kidney, spleen, gill, and both visceral and subcutaneous fat (VAT, SAT) across four time points: parr, early and complete smoltification, and twelve weeks post-seawater transfer. Gene expression analysis was performed to track the distribution and developmental changes in their B cells. Expression profiles of three types of immunoglobulins (ig), including membrane-bound and secreted forms of igm, as well as B cell-specific markers pax1 and cd79, showed strong correlations and contrasted with profiles of other immune cell markers. The highest levels of expression were observed in the lymphatic tissue, followed by the VAT. Enhanced expression in the gill and adipose tissues of smolts suggested an increase in B cell populations. Parallel sequencing of the variable region of the IgM heavy chain was used to track B cell traffic, assessed by the co-occurrence of the most abundant sequences (clonotypes) across different tissues. Smoltification markedly enhanced traffic between all tissues, which returned to initial levels after twelve weeks in the sea. The preferred migration between the head kidney, spleen, and VAT supports the role of abdominal fat as a reservoir of lymphocytes. These findings are discussed in the context of recent studies that suggested the functional significance of B cell traffic in Atlantic salmon. Specifically, the migration of B cells expressing secreted immunoglobulins to virus-infected hearts has been identified as a key factor in the disease recovery and survival of fish challenged with salmon alphavirus (SAV); this process is accelerated by vaccination. Additionally, the study of melanized foci in the skeletal muscles revealed an association between antigen-dependent differentiation and the migration of B cells, indicating a transfer from local to systemic immune responses. Updating the antibody repertoire in the lymphatic and peripheral tissues of smolts may assist in their adaptation to the marine environment and in encountering new pathogens. Emerging evidence highlights B cell migration as an important and previously unrecognized immune mechanism in salmonids.

Keywords: B cell traffic; IgM sequencing; atlantic salmon; gene expression.

Figure 1

Relationship between gene expression profiles in the entire dataset. (A): Correlation (Pearson r). The border line delimits B cell-specific genes. (B): Hierarchical clustering (Pearson r, Ward’s method).

Figure 2

Tissue expression of immune genes. Data (−ΔCt) were centered as described in Section 2.2. Migm–sigm presents differences between secreted and membrane isoforms of igm. Bars not sharing common letters are significantly different (ANOVA, Tukey test, p < 0.05).

Figure 3

Expression of B cell-specific genes across tissues and time points. Data (−ΔCt) were centered as described in Section 2.2. Bars that do not share common letters are significantly different (ANOVA, Tukey test, p < 0.05).

Figure 4

IgM sequencing. B cell migration assessed by co-occurrence of largest clonotypes. Tissues were compared in pairs to determine how many of the largest hundred clonotypes in the first tissue were present in the second tissue. (A): Individual data, six fish were analyzed at each of the four time points. The number in each square means the amount of clonotypes detected in both tissues. (B,C): Temporal changes, average numbers of clonotypes in each tissue shared with all other tissues at every time point (B) and number of shared clonotypes in all tissues across each time point (C). Bars not sharing common letters are significantly different (ANOVA, Tukey test, p < 0.05). Capital and lowercase letters denote time points and tissues, respectively. (D): Overlap between tissues, averaged across all time points. Bars not sharing common letters are significantly different (ANOVA, Tukey test, p < 0.05).