Thymocytes May Persist and Differentiate Without Any Input from Bone Marrow Progenitors

Brief Definitive Report Thymocytes may persist and differentiate without any input from bone marrow progenitors Laetitia Peaudecerf,1 Sara Lemos,1 Alessia Galgano,1 Gerald Krenn,1 Florence Vasseur,1 James P. Di Santo,2 Sophie Ezine,1 and Benedita Rocha1 1Institut National de la Santé et de la Recherche Médicale, Unit 1020, Faculty of Medicine Descartes Paris V, 75015 Paris, France 2Innate Immunity Unit, Pasteur Institute, 75724 Paris, France Thymus transplants can correct deficiencies of the thymus epithelium caused by the complete DiGeorge syndrome or FOXN1 mutations. However, thymus transplants were never used to correct T cell–intrinsic deficiencies because it is generally believed that thymocytes have short intrinsic lifespans. This notion is based on thymus transplantation experiments where it was shown that thymus-resident cells were rapidly replaced by progenitors originating in the bone marrow. In contrast, here we show that neonatal thymi transplanted into interleukin 7 receptor–deficient hosts harbor populations with extensive capacity to self-renew, and maintain continuous thymocyte generation and export. These thymus transplants reconstitute the full diversity of peripheral T cell repertoires one month after surgery, which is 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; transplants induced partial or total protection to infection. These results challenge the current dogma that thymocytes cannot self-renew, and indicate a potential use of neonatal thymus transplants to correct T cell–intrinsic deficiencies. Finally, as found with mature T cells, they show that thymocyte survival is determined by the competition between incoming progenitors and resident cells. CORRESPONDENCE T lymphocytes are fundamental for the control cannot correct T cell deficiencies once the thy- Benedita Rocha: of infection. In rare cases, T cell deficiencies mus atrophies in adults. [email protected] are congenital, caused by mutations preventing Thymus transplants could constitute an The Journal of Experimental Medicine Abbreviations used: DP, the expression of any gene required for T cell advantageous alternative or complementary CD4+CD8+ double-positive generation. However, in most cases they are therapy; these grafts could be a source of both thymocytes; GVH, graft- induced in the adult either by infections such as a functional thymus epithelia and functional versus-host; LM, Listeria mono- cytogenes; LSK, Lineage Sca-1+ AIDS, by aggressive anticancer therapies, or by T cells, and thus might correct T cell deficien- c-Kit+ BM precursors; TN, aging. In the current clinical practice, these sit- cies in both children and adults. They would Lineage CD4CD8CD3 uations have become frequent, rendering the not necessarily require the conditioning of the triple-negative thymocyte. reconstitution of the peripheral T cell pool an patient and should export mature T cells im- important clinical goal. In children who do not mediately, overcoming the long lag-time re- yet have competent thymus epithelia, T cell quired for thymus T cell generation after BM reconstitution may be achieved by the trans- transplantation. However, although thymus grafts plantation of a competent BM. However, because were successful in correcting deficiencies of the BM precursors must transit through the thy- thymus epithelium, as found in the complete mus to generate T cells, the peripheral T cell DiGeorge syndrome (Markert et al., 2007) or reconstitution is delayed by many months, during which time patients are very susceptible to infec- © 2012 Peaudecerf et al. This article is distributed under the terms of an Attribution–Noncommercial–Share Alike–No Mirror Sites license for the first six tions (Parkman and Weinberg, 1997; Holländer, months after the publication date (see http://www.rupress.org/terms). After six months 2008; Cavazzana-Calvo et al., 2009; Reimann it is available under a Creative Commons License (Attribution–Noncommercial– Share Alike 3.0 Unported license, as described at http://creativecommons.org/ et al., 2010). In addition, BM transplantation licenses/by-nc-sa/3.0/). The Rockefeller University Press $30.00 J. Exp. Med. 2012 Vol. 209 No. 8 1401-1408 1401 www.jem.org/cgi/doi/10.1084/jem.20120845 in FOXN1 mutations (Markert et al., 2011), they were never Mechanisms involved in autonomous thymocyte renewal used to correct intrinsic T cell deficiencies, as it is generally Several possibilities could explain the persistence of resident accepted that the thymus does not harbor precursors with self- thymocytes in the thymus transplants grafted into IL-7R– renewal capacities. Indeed, in thymus transplants, resident deficient hosts. We excluded contamination by circulating thymocytes generate a single wave of mature T cells because hematopoietic stem cells, thereby ensuring continuous graft precursors originating from the BM rapidly replace resident colonization. The host BM did not contain CD45.1+ precur- cells (Berzins et al., 1998). Moreover, this occurs even when sors derived from the graft; the only precursors present were the host cannot generate mature T cells. When WT thymi are CD45.2+ ; LSK, Lineage Sca-1+c-Kit+ BM precursors (LSKs) transplanted into SCID or Rag2-deficient hosts, the compe- from the host, as it is characteristic for Rag IL-7R or Ragc tent thymocyte populations from the graft are rapidly re- mice (Fig. 1 C). Thymocyte persistence was also not caused by placed by the incompetent precursors from the host BM any mechanism preventing the colonization of the graft by (Frey et al., 1992; Takeda et al., 1996), and mature T cell ex- the host BM: all grafts were colonized by host BM-derived port fails by 3 wk after surgery. Based on these data, the cur- progenitors, which progressed through differentiation as char- rent dogma postulates that all thymocyte subpopulations are acteristic of each set of host mice: WT BM generating all short-lived, with their maintenance being strictly dependent Lineage CD4CD8CD3 triple-negative thymocytes on the continuous input of BM-derived progenitors. (TN) precursor types, Rag2 BM-derived cells arresting their In contrast, we describe that the neonatal thymus harbors differentiation at the TN3, and IL-7R or Ragc BM at populations with self-renewal capacity that maintain the thymo- the TN2 differentiation stage (Fig. 1 D; Egerton et al., 1990; cyte populations independently of any input from the BM, Mombaerts et al., 1992; Di Santo et al., 1999). Finally, Ly5.1+ rapidly reconstitute the peripheral T cell pools, and the ca- thymocytes of graft origin could have stopped dividing, but pacity to clear infections in T cell–deficient mice. Moreover, this was not the case (Fig. 2): we concluded that the persis- these transplants function across histocompatibility barriers, tence of a conserved CD4+/CD8+ profile in the trans- recalling early studies on the susceptibility of neonatal cells to planted thymi indicated the presence of one or several become tolerant to alloantigens (Billingham et al., 1953). populations with self-renewal capacity. These results highlight a possible application of thymus trans- We first considered that CD4+CD8+ double-positive plants in correcting intrinsic T cell deficiencies. thymocytes(DP) cells might continuously renew in the ab- sence of any intake from more immature precursors, although RESULTS AND DISCUSSION their BrdU incorporation was similar to that of DP cells from The fate of thymus transplants in IL-7R–deficient hosts a normal thymus (Fig. 2 A). To address this possibility, we When searching for thymocyte populations able to generate investigated DP cells’ TCRA repertoires, as the continuous gut intraepithelial lymphocytes (Lambolez et al., 2006; division of the same cohort of DP cells should reduce diver- Peaudecerf et al., 2011) we found that thymus transplants be- sity and/or switch TCRA repertoires to the preferential usage haved differently when transplanted into Ragc hosts. Indeed, of 5 V and 3 J genes (Guo et al., 2002; Pasqual et al., as described previously (Berzins et al., 1998), when a single 2002; Krangel et al., 2004). However, the high-throughput neonatal CD45.1+ thymus lobe is grafted into WT CD45.2+ analysis of 107 TCRA chains expressed by normal DP thymo- hosts, the transplant is rapidly invaded by precursor cells de- cytes and 8 × 106 TCRA chains expressed by DP cells from rived from the host BM; 1 mo later, only rare mature T cells transplanted thymi showed no modifications of TCRA from the graft remain (Fig. 1 A). This substitution occurs repertoires (Fig. S1). Diversity was maintained because sam- even when the host cannot generate mature T cells (Frey ples had equivalent number of unique in-frame CDR3s et al., 1992; Takeda et al., 1996); when CD45.1+ thymi are (Fig. S1 A), and VA and JA usage were also comparable transplanted into CD45.2+ Rag2- or CD3-deficient mice (Fig. S1 B). Therefore, DP differentiation was not modified, (not depicted) thymocytes of graft origin are also substi- indicating that maintenance of DP cells in these grafts should tuted by incompetent CD45.2+ precursors from the host be ensured by more immature progenitors. BM (Fig. 1 A). In contrast, we were surprised to observe that Analysis of CD45.1+ TN cells revealed the persistence of this substitution did not occur when the hosts could not re- T cell progenitors of graft origin. Resident ETPs and TN2s + spond to IL-7; in CD45.2 Rag2 IL-7R or Rag2c hosts, declined rapidly (Fig. 2 B), likely substituted by the hosts’ the vast majority of thymocytes residing in the graft were TN precursors (Fig. 1 D). CD45.1+ TNs were progressively CD45.1+ cells of graft origin (Fig. 1 A). These resident cells enriched in CD44+CD25low TN1–TN2 transition cells, with persisted and maintained a normal CD4/CD8 profile up a CD44+ Sca-1+ c-kit+ IL-7Rlow, Fl3L+/ phenotype (Fig. 2, to 7 mo after transplant, the latest time point studied (Fig.

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