Unopposed positive selection and autoreactivity in mice expressing class II MHC only on thymic cortex

Abstract

The normal development of T cells in the thymus requires both positive and negative selection. During positive selection, thymocytes mature only if their T-cell receptors react with some specificity to host major histocompatibility complex (MHC) and host peptides. During negative selection, thymocytes die if their T-cell receptors react with too high an affinity to the presenting cell, self MHC, and peptides to which they are exposed. These two processes are important for the development of the T-cell repertoire and the acquisition of self-tolerance, but their precise location and temporal relationship are not known. We have used the keratin 14 (K14) promoter to re-express a class II MHC antigen (I-Ab) in class II-negative mice. The transgenic I-A molecule is expressed only on thymic cortical epithelium; thymic medullary epithelium and bone-marrow-derived cells are I-A negative. CD4+ cells are positively selected in K14 mice, but clonal deletion does not ocur in K14 mice or in relB-negative mice, which lack a thymic medulla. The K14 CD4 cells are autoreactive, as they proliferate extensively to and specifically lyse I-Ab-positive target cells. These autoreactive cells make up 5% of the peripheral CD4 T cells, providing and estimate of the minimal frequency of positively selected cells that must subsequently undergo negative selection for self-tolerance to be preserved. Thus positive and negative selection occur in anatomically distinct sites.

Preferential Recognition of Self Antigens Despite Normal Thymic Deletion of CD4+CD25+ Regulatory T Cells

“…However, in contrast to earlier proposed explanations for the proposed autospecificity, we showed in this study that regulatory T cell precursors are as susceptible to induction of deletion as are their normal counterparts. Combined with the earlier reported increased generation of autospecific regulatory T cells in presence of agonist ligand (24), our data indicate that thymic generation of these cells depends on recognition of an MHC/peptide ligand exclusively at the surface of thymic elements incapable of induction of deletion, i.e., thymic cortical and/or medullary epithelial cells (4,6). Finally, our observation that significantly more regulatory T cells recognize self than non-self ligands would explain how these cells mainly inhibit autoimmunity, leaving useful non-self-specific immune responses to freely develop.…”

“…Increased (rather than decreased) differentiation of CD4 ϩ CD25 ϩ thymocytes has been observed in doubly transgenic mice expressing an influenza HA S1 peptide/I-E d -specific TCR as well as its agonist ligand (24), which would suggest their resistance to deletion. However, the low level of HA-transgene expression in the thymus has been reported to be limited to the cortical region (43), which is devoid of cells capable of inducing deletion in vivo (3,4,6). The transgenic HA expression patterns and/or levels in three other HA-transgenic mouse lines induced deletion rather than development of transgenic S1/I-E d TCR-expressing thymocytes (24).…”

“…Importantly, because only a part of self Ags is expressed by splenic APCs, the real frequency of autospecific regulatory T cells is probably even higher. Because thymic deletion by APCs normally applies to regulatory T cell precursors, the observed high frequency of autoreactivity reflects the level of autospecific precursors that either recognize their MHC/peptide ligand at the surface of cells incapable of apoptosis induction (e.g., thymic cortical (4,6) and probably also medullary epithelium (3)) or do not encounter it at all in the thymus (e.g., tissue-specific Ags). If a normal precursor recognizes its ligand at the surface of medullary epithelium, it will be rendered anergic and this unresponsive state can only inefficiently be reversed by IL-2 (3).…”

A novel role for CD28 in thymic selection: elimination of CD28/B7 interactions increases positive selection

“…Also consistent with the conclusion that the increase in CD8 SP cells in the thymus of CD28KO H-Y-specific TCR-Tg mice reflects increased positive selection, we find a distinct lack of selfreactivity in both CD28KO mice and B7DKO females, I-A b -specific or otherwise. This is in marked contrast to other systems where negative selection has been disrupted and detectable autoreactivity appears [23,24]. Both the pattern of CD5 expression and the lack of autoreactivity argue that enhanced CD8 SP differentiation is not due to disrupted negative selection but rather that the observed increase in the number of CD8 SP cells is due to increased positive selection.…”

Alternatively spliced transcripts of the thymus-specific protease PRSS16 are differentially expressed in human thymus

“…17 PRSS16 is a recently described serine protease that is also expressed exclusively in the thymic cortex, 1 exactly where the events leading to positive selection take place. 21 Moreover, the initiation of expression of Prss16 in the mouse has been mapped between days 11 and 15 of gestation, 3 a time point compatible with a role in T-cell development. We have identified four splice variants of the PRSS16 in addition to the full-length form, and analyzed their relative expression in human thymi.…”