New Avian B-Cell Pathway Discovered

Discovery of an alternative pathway for B-cell development in chickens.
Image Credit: ©Tomonori Nochi

Scientific Frontline: Extended "At a Glance" Summary: Alternative B-Cell Development Pathway in Birds

The Core Concept: Researchers have identified a secondary pathway for B-cell development in chickens where cells originate in the bone marrow and migrate directly to the cecal tonsils. This process completely bypasses the bursa of Fabricius, a specialized organ previously believed to be the exclusive site for avian B-cell maturation.

Key Distinction/Mechanism: Unlike the conventional pathway that relies on the bursa of Fabricius, this newly discovered bursa-independent mechanism allows B cells to transform directly into Immunoglobulin A (IgA)-producing plasma cells within the intestinal mucosa. These antibodies maintain intestinal homeostasis by coating beneficial bacteria and preventing harmful microorganisms from crossing the intestinal barrier.

Major Frameworks/Components: 

• Bursa-Dependent Pathway: The traditional, established model of avian B-cell maturation.
• Bursa-Independent Pathway: The newly discovered mechanism facilitating direct bone marrow-to-gut cellular migration.
• Intestinal Mucosal Immunity: The localized production of IgA to regulate gut microbiota and maintain immune homeostasis.
• Gut-Liver Axis Homeostasis: The cooperative immune function that prevents bacterial translocation, specifically the spread of harmful strains like Streptococcus alactolyticus, from the intestine to the liver.

Branch of Science: Avian Immunology, Veterinary Medicine, and Microbiology.

Future Application: This research provides the biological foundation for developing non-antibiotic, microbiota-driven strategies for avian disease prevention. By targeting these complementary pathways, scientists aim to combat global antimicrobial resistance in livestock and poultry production.

Why It Matters: Experimental inhibition of both B-cell pathways halts IgA production, leading to severe disruptions in gut microbiota, the proliferation of harmful bacteria, and subsequent liver inflammation. Understanding this dual-pathway cooperation is essential for ensuring healthy poultry development and preserving fundamental gut-liver homeostasis.

Cooperation between two B-cell pathways establishes an intestinal IgA barrier that safeguards gut-liver homeostasis.
Image Credit: ©Tomonori Nochi

Birds possess a specialized organ called the bursa of Fabricius, which mammals do not have. It has long been thought that B cells, a part of the immune system, develop exclusively in this organ. However, researchers from Tohoku University have discovered a previously unknown pathway for B-cell development in chickens, overturning this long-standing belief.

"For more than half a century, this unique organ was considered the sole site of B-cell development in birds," stress Ryota Hirakawa, an assistant professor, and Tomonori Nochi, a professor, at Tohoku University's Graduate School of Agricultural Studies. "We discovered that a distinct population of B cells originates in the bone marrow of chickens and migrates directly to the cecal tonsils, bypassing the bursa of Fabricius entirely."

Hirakawa and his colleagues revealed that after the B cells establish themselves in the cecal tonsils—tissues in a bird's gut that protect the intestinal tract against viral and bacterial pathogens—the B cells transform into IgA-producing plasma cells that populate the intestinal mucosa. The antibody IgA is responsible for maintaining intestinal immune homeostasis by coating beneficial bacteria and preventing harmful microorganisms from crossing the intestinal barrier.

To determine the biological significance of their landmark discovery, the team developed a model where both the conventional bursa-dependent pathway and the newly discovered bursa-independent pathway were inhibited. Inhibition caused IgA production to cease, which, in turn, led to major alterations in the gut microbiota. Harmful bacteria proliferated, including Streptococcus alactolyticus, which translocated from the intestine to the liver.

The bacterial invasion triggered liver inflammation and disrupted normal metabolic function, demonstrating that the two B-cell populations cooperate to establish an effective intestinal barrier and maintain gut-liver homeostasis.

Tomonori Nochi notes that the study comes at a time when antimicrobial resistance has become one of the greatest challenges facing livestock and poultry production worldwide. "Alternative, nonantibiotic strategies are needed for avian disease prevention. But to develop them, we need a more complete understanding of the immune system of birds. The findings in our study not only challenge our fundamental understanding of avian immunology but also elucidate how intestinal immunity develops and protects the host."

Looking ahead, the researchers seek to identify the molecular and environmental factors that stimulate both the conventional bursa-dependent and the newly discovered bursa-independent B-cell developmental pathways. Targeting these two complementary pathways could lead to ways to enhance intestinal IgA production and establish immune- and microbiota-based strategies that promote healthy poultry development.

Additional information: The study was conducted in collaboration with GENODAS Inc. and Kyoritsu Seiyaku Corporation, which hosts the Joint Research Laboratory of Animal Mucosal Immunology at Tohoku University.

Published in journal: Proceedings of the National Academy of Science

Title: Bursa of Fabricius–independent B cells establish an IgA-mediated intestinal barrier that safeguards gut–liver homeostasis

Authors: Ryota Hirakawa, Motoshi Hisamatsu, Sayoko Maekawa, Eiki Asai, Miyuko Ohta, Ayumi Matsuo, Kunihiro Okano, Toh Miyazaki, Motofusa Akiyama, Masaaki Toyomizu, Jahidul Islam, Mutsumi Furukawa, and Tomonori Nochi

Source/Credit: Tohoku University

Edited by: Scientific Frontline

Reference Number: imgy071626_01

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