This document addresses thymus tissue transplantation (also known as culture thymus tissue [CTT] transplantation) using allogeneic processed thymus tissue (allogeneic processed thymus tissue-agdc [RETHYMIC^®], Enzyvant Therapeutics, Inc. Cambridge, MA) a regenerative therapy used for immune reconstitution in children with congenital athymia.

Note: This document does not address therapeutic uses of stem cells for hematopoietic indications (FDA-approved products derived from stem cells that are approved for limited use in individuals with disorders involving the hematopoietic system).

Note: For additional information on related topics, please see the applicable document:

• TRANS.00035 Therapeutic use of Stem Cells, Blood and Bone Marrow Products

Position Statement

Medically Necessary:

A single administration of allogeneic processed thymus tissue is considered medically necessary for immune reconstitution in the pediatric population* with congenital athymia when all of the following criteria are met:

1. Congenital athymia is confirmed via flow cytometry as less than 50 naïve T cells/mm³ in the peripheral blood or less than 5% of total T cells being naïve in phenotype; and
2. Documentation that infection control measures, including immunoprophylaxis, can reasonably be maintained until the development of thymic function is established; and
3. Absence of comorbidities, in the opinion of the treating clinician, that are reasonably likely to result in severe complications, including death from administration of allogeneic processed thymus tissue (for example, pre-existing renal impairment, or cytomegalovirus or Epstein-Barr virus infection).

*Note: Pediatric population (generally age below 18 years of age)

Investigational and Not Medically Necessary:

Use of allogeneic processed thymus tissue is considered investigational and not medically necessary for administration of all other uses, including but not limited to immune reconstitution in individuals with severe combined immunodeficiency.