Thymocytes self-renewal: a major hope or a major threat?

ABSTRACT

Thymus transplants were never used to correct T-cell intrinsic deficiencies, as it is generally believed that thymocytes have short intrinsic lifespans. This notion is based on multiple thymus transplantation experiments, where it was shown that thymus-resident cells were rapidly replaced by progenitors migrating from the bone marrow (BM). This substitution occurs even when bone marrow precursors are unable to generate T cells, as in Rag1/2(-) or severe combined immunodeficiency (SCID)-deficient mice. In contrast, two groups reported that neonatal thymi transplanted into mice that cannot respond to IL-7 harbor populations with extensive capacity to self-renew, which maintain continuous thymocyte generation for several months after surgery. The consequences of this self-renewal capacity differed in these two laboratories. We found that these thymus transplants rapidly reconstitute the full diversity of peripheral T-cell repertoires 1 month after surgery, the earliest time point studied. Moreover, transplantation experiments performed across major histocompatibility barriers show that allogeneic-transplanted thymi are not rejected, and allogeneic cells do not induce graft-versus-host disease, both syngeneic and allogeneic transplants inducing rapid protection from infection. These results indicate a potential use of neonatal thymus transplants to correct T-cell intrinsic deficiencies. The other group observed that continuous thymocyte renewal from BM precursors was fundamental to prevent tumor development. In the absence of this input, thymocytes from the transplanted thymus generated tumors with all the characteristics of T-cell acute lymphoblastic leukemia (T-ALL). Moreover, they suggested that the absence of BM competition was responsible for the T-ALLs developing in X-linked severe combined immunodeficiency (SCID)-X1 patients, deficient in the expression of IL2-Rγc . These patients were treated with autologous CD34(+) cells transfected with virus vectors expressing γc in the absence of myeloablation. We here review the potential therapeutic impact of thymus transplantation and compare the results of these two laboratories aiming to find an answer to the 'Dr Jekill versus Mr. Hyde' status of thymus transplantation experiments.