Thymocyte deletion can bias Treg formation toward low‐abundance self‐peptide

Abstract

Autoreactive CD4 1 T cells can undergo deletion and/or become CD25 1 Foxp3 1 Treg as they develop intrathymically, but how these alternative developmental fates are specified based on interactions with self-peptide(s) is not understood. We show here that thymocytes expressing an autoreactive TCR can be subjected to varying degrees of deletion that correlate with the amount of self-peptide. Strikingly, among thymocytes that evade deletion, similar proportions acquire Foxp3 expression. These findings provide evidence that Foxp3 1 Treg can develop among members of a cohort of autoreactive thymocytes that have evaded deletion by a self-peptide, and that deletion and Treg formation can act together to bias the Treg repertoire toward low-abundance self-peptide(s).Key words: Thymic selection . Tolerance . Transgenic models IntroductionDuring their development, thymocytes encounter an array of self-peptides that are expressed in different amounts and by distinct cell types. Self-peptides with which the TCR is strongly reactive can induce thymocyte deletion, which eliminates potentially autoagressive clones from the CD4 1 and CD8 1 T-cell repertoires [1]. Thymocytes can also undergo a program of differentiation to become CD4 1 CD25 1 Foxp3 1 Treg, which additionally participate in preventing autoimmunity [2,3]. The processes that instruct thymocytes to undergo deletion or to develop along a pathway to become Treg remain poorly understood.Evidence that interactions with self-peptides can promote Treg formation came from studies showing increased CD4 1 CD25 1 Treg development in TCR Tg mice that co-express their agonist peptide as a self-peptide [4][5][6]. The idea that signaling from self-peptides can promote Treg formation is supported by studies showing that TCR signaling of progenitor CD4 1 CD8 À (CD4SP) thymocytes can promote Foxp3 1 expression [7].However, as noted above, signaling from self-peptides can also promote thymocyte deletion, and the relationship between thymocyte deletion and Treg formation remains to be defined. One possibility is that differentiation along the Treg pathway is induced by cues other than recognition of self-peptides [8,9]. For example, non-peptide-mediated cues might induce differentiation along a Treg pathway and cause a subset of thymocytes to be more resistant to deletion than are the remaining thymocytes, which are fated to develop into conventional CD4 1 Foxp3 À T cells unless they are signaled by self-peptides to undergo deletion [8]. An alternative possibility is that the fate of an autoreactive thymocyte is dictated by the mode of self-peptide presentation, e.g. expression at high or low densities, or by particular thymic cell types, although recent studies argued against a role for specialized thymic cell types in promoting Treg formation [10,11].We have examined these questions by determining the extent of thymocyte deletion and/or Foxp3 1 Treg formation in Tg mice expressing an MHC class II-restricted TCR, and co-expressing the cognate peptide for this TCR under the control of ...

Distinct Modes of Antigen Presentation Promote the Formation, Differentiation, and Activity of Foxp3+ Regulatory T Cells In Vivo

“…There was a significant decrease in the number of 6.5 + CD4SP thymocytes in TS1xHA28 mice relative to TS1 mice, but the number of 6.5 + CD4SPFoxp3 + thymocytes was significantly increased, consistent with previous studies showing that the presence of the self-HA can induce both 6.5 + CD4SP thymocyte deletion and 6.5 + CD4SPFoxp3 + thymocyte formation in TS1xHA28 mice (9, 12) (Fig. 1C).…”

“…In HA28 mice, the SV40 early region promoter/enhancer drives expression of a truncated polypeptide corresponding to the NH 2 -terminal 273 amino acids of the PR8 HA polypeptide, and studies using bone marrow chimeras have shown that this polypeptide is expressed in radioresistant cell types (9, 12, 29, 33, 34). By contrast, the I-E a MHCII promoter directs transgene expression in HACII mice, and flow cytometric studies have demonstrated cell surface expression of the HA selectively by MHC Class II+ cells (35).…”

“…Although the exact signals that can specify an autoreactive thymocyte to undergo deletion versus development into a Foxp3 + Treg have not yet been defined, there is evidence that relatively high doses of a cognate peptide will induce substantial deletion of autoreactive thymocytes, while lower doses can lead to less thymocyte deletion, and in these circumstances significant formation of CD4SPFoxp3 + cells with specificity for the cognate self-Ag can occur (12, 13). Thymically-derived Tregs (tTregs) appear to constitute the majority of the Treg population (14, 15), but in certain circumstances CD4 + Foxp3 − cells that are present in the periphery can differentiate into Foxp3 + Tregs (termed peripherally-derived Tregs (pTregs)) upon recognition of cognate Ag (16).…”

Models of Self-Peptide Sampling by Developing T Cells Identify Candidate Mechanisms of Thymic Selection

“…A very similar trend was observed by Cozzo Picca et al [17] using TCR transgenic cells specific for an epitope of influenza virus in the presence of different levels of expression of this agonist. Atibalentja et al [18] also observed this trend following intravenous injection of varying concentrations of hen egg-white lysozyme (HEL), which was rapidly processed and presented in the thymus, resulting in the negative selection of specific TCR transgenic T and an increase in TCR transgenic T at low, but loss at higher, HEL concentrations.…”

Inventories of naive and tolerant mouse CD4 T cell repertoires reveal a hierarchy of deleted and diverted T cell receptors

“…TCR/model antigen transgenic systems showed that T reg cell differentiation, similar to clonal deletion, can be instructed by TCR agonists (2, 3), and this was later confirmed for polyclonal T cells and natural self-antigens (4). In some models, low antigen levels favored T reg cell generation, whereas high antigen doses favored deletion, suggesting that T reg cell differentiation occurs within an avidity window between positive selection and deletion (5–8). Other observations, however, are difficult to reconcile with a reductionist, purely signal strength-based model.…”