Cryopreserved Thymus Tissue Transplants Successfully Rebuild Immune Systems

The thymus is the organ responsible for generating naïve T cells that distinguish self from nonself, providing protection against pathogens. In individuals born without a functional thymus — congenital athymia — untreated survival approaches zero percent. The current gold-standard therapy, cultured thymus tissue implantation (CTTI), requires that the donor thymus be used within 12 to 21 days of procurement, creating a logistical bottleneck that limits which patients can benefit. This study asked whether thymus tissue could be cryopreserved after culture and still reconstitute immunity when implanted.

Using an established rat model, the team implanted cryopreserved cultured thymus (cCTTI) under the kidney capsule of congenitally athymic nude rats and compared outcomes to fresh thymus and non-cryopreserved cultured thymus implants. At 10 months post-implantation, all six rats across all three conditions showed viable, robustly reconstituted thymus grafts with ongoing thymopoiesis. Critically, only recipients of cultured thymus (cryopreserved or not) were free of donor-derived T cells, while fresh non-cultured implant recipients showed transient donor T cell chimerism — reinforcing that the culture step eliminates graft-versus-host disease risk. cCTTI recipients showed slightly slower initial reconstitution but reached comparable naïve CD4 T cell levels by 10 months.

In a larger animal porcine model more anatomically and physiologically similar to humans, four thymectomized, T cell–depleted pigs received autologous cultured thymus either directly from culture or after cryopreservation. Biopsy at 4 months and full tissue examination at 7 months showed complete reconstitution with normal cortical and medullary thymus architecture in 100% of animals. Immature TdT-positive thymocytes undergoing T cell receptor rearrangement were present, confirming active thymopoiesis. Cytokeratin-14-positive thymic epithelial repopulating cells, Hassall bodies, and CCL21 chemokine expression were all preserved, demonstrating functional stromal integrity. CCL21 secretion in vitro post-thaw (11 ± 6 pg/mL) was approximately half that during fresh culture (23 ± 13 pg/mL), suggesting some reduction but meaningful retention of function.

Human thymus tissues processed identically — cultured for the standard duration and then cryopreserved — continued to meet all established histological and immunohistochemical quality criteria used to approve tissues for clinical implantation. This is a critical regulatory and translational milestone, as it demonstrates that cryopreserved human tissue would not fail existing release criteria.

The implications are substantial. Currently, CTTI must be coordinated tightly around both donor availability and recipient readiness within a three-week window. Cryopreservation would allow thymus tissue banks, pre-planned elective implantation schedules, and access for patients whose clinical condition makes the current narrow window impractical. Potential expanded indications include congenital athymia, post-organ transplant tolerance induction, and possibly other immunodeficiency conditions. The authors call for clinical trials to validate safety and efficacy in humans.