{"entity":{"id":"rejuv-agenda-thymus-does-not-regrow","kind":"bottleneck","name":"Nothing in routine use rebuilds an adult's thymus","aka":[],"tldr":"After a transplant or intensive chemotherapy, the part of the immune system that makes new kinds of T cell recovers slowly in adults and sometimes not at all. The deficit is well described, well measured and currently not correctable.","summary":"The transplant facet of this round put the gap in a sentence: there is no randomised evidence that any intervention accelerates thymic recovery in adults, and the agents trialled for it, including thymic peptides, keratinocyte growth factor, growth hormone and sex steroid blockade, have not produced a treatment in routine use.\n\nThe consequence is specific rather than general. Cell counts come back; the repertoire does not. Normal numbers of T cells can coexist with a narrowed range of things those T cells can recognise, which is why a normal count can reassure falsely, and why repertoire measurement remains a research tool rather than a clinical test. For a person this shows up as susceptibility to infections years after counts normalised, as vaccine responses that are weaker than the schedule assumes, and as the reason revaccination exists as a programme at all.\n\nIt is a moderate rather than a critical bottleneck because the problem is largely managed around rather than solved: prophylaxis by phase, immunoglobulin replacement where antibodies are low, and revaccination schedules do most of the work, and the honest framing is that these substitute for an immune system rather than rebuild one. The commonest failure in practice is not biological at all, it is that nobody owns the revaccination schedule between the transplant centre and primary care.\n\nWhat would change it is a trial, and the reason there has not been one is partly that the readout is contested. Thymic output can be estimated from T-cell receptor excision circles, repertoire diversity can be sequenced, and neither has been accepted as an endpoint that a regulator would license a drug against. An intervention that moved a repertoire measure without changing infection rates would not be worth having, and an intervention that reduced infections would be adopted whatever it did to the thymus, which is the design problem.","asOf":"2026-10-02","links":[{"label":"Cordonnier et al., Vaccination of haemopoietic stem cell transplant recipients: ECIL 7 guidelines (Lancet Infectious Diseases 2019)","url":"https://doi.org/10.1016/S1473-3099(18)30600-5"}],"tags":["rejuvenation","survivorship","open-problem","immune"],"related":["rejuv-tx-immune-reconstitution-timeline","rejuv-tx-revaccination","rejuv-tx-b-cell-aplasia-and-immunoglobulin","rejuv-tx-infection-by-phase","rejuv-frontier-immune-reconstitution","idea-rejuv-thymic-regeneration-in-adults","rejuvenation-roadmap"],"cancers":[],"sections":["rejuvenation","supportive-care"],"technologies":[],"targets":[],"drugs":[],"companies":[],"institutions":[],"pathways":[],"terms":[],"trials":[],"people":[],"bottlenecks":[],"keyPapers":[],"journals":[],"dependsOn":[],"notes":[],"stage":"biology","severity":"moderate","metrics":[{"label":"Interventions in routine use that accelerate thymic recovery in adults","value":"None","source":"OnCo transplant facet, rejuv-tx-immune-reconstitution-timeline"},{"label":"Agents trialled without producing a routine treatment","value":"Thymic peptides, keratinocyte growth factor, growth hormone, sex steroid blockade","source":"OnCo transplant facet, rejuv-tx-immune-reconstitution-timeline"}],"causes":["The thymus involutes with age, so adults start from a much smaller organ than the children in whom recovery is fastest.","No agreed clinical endpoint exists: repertoire diversity and thymic output are research measures, infection rates need very large trials.","The population is small and heterogeneous in conditioning regimen, graft source and graft-versus-host disease.","Agents with a plausible mechanism are old or off-patent, so nobody is funding a licensing programme.","Chronic graft-versus-host disease and its treatment themselves suppress recovery, confounding any trial run in the population that needs it most."],"currentEfforts":["Phase-based infection prophylaxis and revaccination schedules from ECIL and the CDC manage the consequence rather than the cause.","Immunoglobulin replacement covers the antibody deficit after B-cell-directed cell therapy, on evidence the transplant facet grades moderate because the strongest study is a 37-patient retrospective comparison.","Repertoire sequencing is being used in cohorts to characterise what recovers and what does not."],"successLooksLike":"An intervention that shortens the period of susceptibility after transplant or intensive therapy on an endpoint a clinician would act on, with a measure of immune recovery that regulators accept."},"route":"/bottlenecks/rejuv-agenda-thymus-does-not-regrow/","neighbours":{"technology":[{"id":"rejuv-tx-b-cell-aplasia-and-immunoglobulin","kind":"technology","name":"B-cell aplasia and low antibodies after CAR-T and bispecifics, and immunoglobulin replacement","route":"/technologies/rejuv-tx-b-cell-aplasia-and-immunoglobulin/"},{"id":"rejuv-tx-infection-by-phase","kind":"technology","name":"Infection risk after transplant and cell therapy, phase by phase, and the prophylaxis that follows it","route":"/technologies/rejuv-tx-infection-by-phase/"},{"id":"rejuv-tx-immune-reconstitution-timeline","kind":"technology","name":"Rebuilding an immune system: the timeline, lineage by lineage","route":"/technologies/rejuv-tx-immune-reconstitution-timeline/"},{"id":"rejuv-frontier-immune-reconstitution","kind":"technology","name":"Rebuilding the immune system after treatment","route":"/technologies/rejuv-frontier-immune-reconstitution/"},{"id":"rejuv-tx-revaccination","kind":"technology","name":"Revaccination after transplant: the schedules, and where the UK and the US differ","route":"/technologies/rejuv-tx-revaccination/"}],"idea":[{"id":"idea-rejuv-thymic-regeneration-in-adults","kind":"idea","name":"Find out whether an adult thymus can be made to work again, with an endpoint a clinician would act on","route":"/ideas/idea-rejuv-thymic-regeneration-in-adults/"}],"roadmap":[{"id":"rejuvenation-roadmap","kind":"roadmap","name":"Recovery and rejuvenation roadmap: cure is not enough → survivorship gets a name → the cohorts that measured the cost → exercise proven as treatment → biological ageing measured and sold → repair, if anyone funds it","route":"/roadmaps/rejuvenation-roadmap/"}],"section":[{"id":"rejuvenation","kind":"section","name":"Recovery & Rejuvenation","route":"/fronts/rejuvenation/"},{"id":"supportive-care","kind":"section","name":"Supportive Care & Survivorship","route":"/fronts/supportive-care/"}]}}