T Cell Receptor Revision Does Not Solely Target Recent Thymic Emigrants

Abstract

CD4+Vβ5+ T cells enter one of two tolerance pathways after recognizing a peripherally expressed superantigen encoded by an endogenous retrovirus. One pathway leads to deletion, while the other, termed TCR revision, results in cellular rescue upon expression of an alternate TCR that no longer recognizes the tolerogen. TCR revision requires the rearrangement of novel TCR β-chain genes and depends on recombinase-activating gene (RAG) expression in peripheral T cells. In line with recent findings that RAG+ splenic B cells are immature cells that have maintained RAG expression, it has been hypothesized that TCR revision is limited to recent thymic emigrants that have maintained RAG expression and TCR loci in a recombination-permissive configuration. Using mice in which the expression of green fluorescent protein is driven by the RAG2 promoter, we now show that in vitro stimulation can drive reporter expression in noncycling, mature, peripheral CD4+ T cells. In addition, thymectomized Vβ5 transgenic RAG reporter mice are used to demonstrate that TCR revision can target peripheral T cells up to 2 mo after thymectomy. Both sets of experiments strongly suggest that reinduction of RAG genes triggers TCR revision. Approximately 3% of CD4+Vβ5+ T cells in thymectomized Vβ5 transgenic reporter mice have undergone TCR revision within the previous 4–5 days. TCR revision can also occur in Vβ5+ T cells from nontransgenic mice, illustrating the relevance of this novel tolerance mechanism in unmanipulated animals.

Cutting Edge: TCR Revision Occurs in Germinal Centers

“…Furthermore, TCR revision can occur in V␤5 ϩ CD4 ϩ T cells from mice thymectomized up to 2 mo previously (11). These results indicate that TCR revision does not target recent thymic emigrants with an immature transitional phenotype, cells known to disappear within 2 wk after removal of the thymus (18).…”

“…Water-lysed PBL, splenocytes, lymph node (LN) cells, and bone marrow cells were stained as described previously (11). In experiments analyzing surface phenotype, splenocytes were depleted of B220 ϩ cells using B220 microbeads and an AutoMACS column according to the manufacturer's protocol (Miltenyi Biotec, Auburn, CA).…”

“…19) were bred to V␤5 TCR Tg B6 mice (11) and are Mtv-8 ϩ and carry a GFP transgene driven by the RAG2 promoter. Five-week-old mice were anesthetized with tribromoethanol (Sigma-Aldrich, St. Louis, MO) and thymectomized as described previously (20).…”

“…The risks inherent in one recently described mechanism, TCR revision, appear particularly large because it requires extrathymic Ag receptor gene rearrangement. Despite this implied risk, evidence from both mice and humans indicates that rearrangement of TCR genes by peripheral T cells can occur (3)(4)(5)(6)(7)(8)(9)(10)(11)(12). Although TCR revision can tolerize T cells to the initiating Ag (3,4,7,8), the danger of generating autoreactive cells through this mechanism is especially acute in autoimmune-prone mice (10,13).…”

“…After encounter with Mtv-8, most of these cells are rendered anergic and deleted, while a minority initiates TCR revision (15,16). Approximately 3% of V␤5 ϩ CD4 ϩ T cells undergo TCR revision every 4 -5 days in mice carrying Mtv-8 (11). One hypothesis to explain how Mtv-8-experienced T cells enter one pathway vs the other suggests that when V␤5 ϩ CD4 ϩ T cells encounter Mtv-8, some receive a weak or partial signal that induces the TCR revision pathway and some perceive a stronger Mtv-8 signal and are thereby deleted (16).…”