Zag Bio Emerges with $80M Series A for Thymus-Targeted Autoimmune Therapies

Zag Bio, a Cambridge, Massachusetts-based startup, has emerged from stealth with an $80 million series A financing round to develop thymus-targeted medicines for autoimmune diseases. The round was led by T1D Fund and Polaris Partners, with participation from pharmaceutical giants AbbVie, Regeneron, and Sanofi.

Novel Approach to Regulatory T-Cell Therapies

Zag Bio is part of the growing field of regulatory T-cell (Treg) therapies, but sets itself apart with a unique thymus-targeting platform. The company aims to develop antibodies that deliver antigens to restore central immune tolerance in patients with various autoimmune diseases, including Type 1 diabetes.

"The name Zag Bio shows that we are intentionally moving in a new direction to treat autoimmunity by targeting the natural mechanisms in the thymus," said Jason Cole, the newly appointed CEO of Zag Bio. "When others in the autoimmune space are zigging, we are zagging."

The company's platform engages the body's natural mechanism for T-cell tolerance in the thymus, expanding antigen-specific Tregs while depleting antigen-specific T effector cells. These thymic Tregs then migrate to diseased tissues, providing protection. Zag Bio claims that thymic Tregs are epigenetically hardwired to be stable and durable, potentially resetting the immune system.

Industry Momentum and Nobel Recognition

The emergence of Zag Bio comes at a time of significant momentum in the Treg field. Earlier this month, three pioneering immunologists were awarded the 2025 Nobel Prize in Physiology or Medicine for their work on Tregs. Japanese scientist Shimon Sakaguchi, M.D., Ph.D., who discovered Tregs in 1995 as the "peacekeepers" of the immune system, was among the laureates.

While no Treg therapies are currently on the market, over 200 clinical trials are underway. Orca Bio is leading the pack, with an FDA decision expected in April for its blood disease candidate Orca-T.

Strategic Investments and Industry Partnerships

The involvement of AbbVie, Regeneron, and Sanofi in Zag Bio's financing round highlights the growing interest of major pharmaceutical companies in Treg therapies. This follows a similar pattern seen last year when Eli Lilly, Johnson & Johnson, and Pfizer invested in TRex Bio's $84 million series B round.

Two years ago, AstraZeneca partnered with Quell Therapeutics, committing $85 million to collaborate on Treg candidates, including one for Type 1 diabetes.

Alan Crane, a partner at Polaris and co-founder and chairman of Zag Bio, emphasized the company's potential: "Zag Bio is trailblazing an unprecedented therapeutic pathway for medicines to target the thymus, and this positions the company to be an innovator in autoimmune disease treatment."