Find out whether an adult thymus can be made to work again, with an endpoint a clinician would act on

After a transplant or intensive therapy, cell counts recover and the range of things the immune system can recognise does not. That deficit is why revaccination programmes exist, why prophylaxis runs for months or years, and why a normal count can reassure falsely. Nothing in routine use shortens it in adults, and the agents tried, including thymic peptides, keratinocyte growth factor, growth hormone and sex steroid blockade, did not produce a treatment.

The honest reading is that this failed partly on biology and partly on design. Thymic output and repertoire diversity are research measures that no regulator has accepted as endpoints, and infection rates need trials larger than this population easily supports. So the field has neither a surrogate it is allowed to use nor a clinical endpoint it can afford.

The work splits in two. The regulatory half is to qualify an immune-recovery measure, by showing in existing transplant cohorts that it predicts infection and vaccine response well enough to be used as a trial endpoint. The biological half is then to test the candidates, old and new, against it. Doing the second without the first is what has already been tried.