Immunological characterization of the rainbow trout bursa

Area of Science:

• Immunology
• Ichthyology
• Comparative Anatomy

Background:

• The bursa of Fabricius is crucial for avian B cell development.
• A bursa-analogous tissue was previously identified in Atlantic salmon but lacked primary lymphoid functions.
• Understanding fish immune systems is vital for aquaculture and conservation.

Purpose of the Study:

• To identify and characterize a bursa-like immune organ in rainbow trout (Oncorhynchus mykiss).
• To investigate the cellular composition and functional responses of this tissue.
• To compare its characteristics with the avian bursa and salmon bursa-analog.

Main Methods:

• Histological identification of the bursa-like structure in rainbow trout at various developmental stages.
• Flow cytometry to analyze B cell populations (IgM+, IgT+, IgD+).
• Transcriptional analysis of immune genes following immunization via immersion and anal routes.

Main Results:

• A bursa-like lymphoepithelial structure was identified caudal to the anus in rainbow trout, exhibiting age-dependent regression.
• The tissue contained abundant IgM+, CD3+, and MHC II-expressing cells, with notable IgT+ B cells and few IgD+ B cells.
• Anal immunization significantly altered the transcription of immune genes, including IgM, IgD, BCMA, and PRDM1-a2, unlike bath immunization.

Conclusions:

• Rainbow trout possess a bursa-like immune organ homologous to that in birds and other salmonids.
• This tissue plays a role in B cell populations, particularly IgT+ B cells, and responds to localized immune challenges.
• The findings suggest a connection between this bursa-like structure and gut-associated immune responses in teleosts.