T cell receptor revision does not solely target recent thymic emigrants

T Cell Receptor Chain Centricity: The Phenomenon and Potential Applications in Cancer Immunotherapy

T cells are crucial players in adaptive anti-cancer immunity. The gene modification of T cells with tumor antigen-specific T cell receptors (TCRs) was a milestone in personalized cancer immunotherapy. TCR is a heterodimer (either α/β or γ/δ) able to recognize a peptide antigen in a complex with self-MHC molecules. Although traditional concepts assume that an α- and β-chain contribute equally to antigen recognition, mounting data reveal that certain receptors possess chain centricity, i.e., one hemi-chain TCR dominates antigen recognition and dictates its specificity. Chain-centric TCRs are currently poorly understood in terms of their origin and the functional T cell subsets that express them. In addition, the ratio of α- and β-chain-centric TCRs, as well as the exact proportion of chain-centric TCRs in the native repertoire, is generally still unknown today. In this review, we provide a retrospective analysis of studies that evidence chain-centric TCRs, propose patterns of their generation, and discuss the potential applications of such receptors in T cell gene modification for adoptive cancer immunotherapy.

Exposing T Cell Secrets Inside and Outside the Thymus

I've had serious misgivings about writing this article, because from living the experience day by day, it's hard to believe my accomplishments merit the attention. To skirt this roadblock, I forced myself to pretend I was in a conversation with my trainees, trying to distill the central driving forces of my career in science. The below chronicles my evolution from would-be astronaut/ballerina to budding developmental biologist to devoted T cell immunologist. It traces my work from a focus on intrathymic events that mold developing T cells into self-major histocompatibility complex (MHC)-restricted lymphocytes to extrathymic events that fine-tune the T cell receptor (TCR) repertoire and impose the finishing touches on T cell maturation. It is a story of a few personal attributes multiplied by generous mentors, good luck, hard work, perseverance, and knowing when to step down. Expected final online publication date for the Annual Review of Immunology, Volume 40 is April 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Of the multiple mechanisms leading to type 1 diabetes, T cell receptor revision may play a prominent role (is type 1 diabetes more than a single disease?)

A single determinant factor for autoimmunity does not exist; disease development probably involves contributions from genetics, the environment and immune dysfunction. Type 1 diabetes is no exception. Genomewide-associated studies (GWAS) analysis in T1D has proved disappointing in revealing contributors to disease prediction; the only reliable marker has been human leucocyte antigen (HLA). Specific HLAs include DR3/DR4/DQ2/DQ8, for example. Because HLA molecules present antigen to T cells, it is reasonable that certain HLA molecules have a higher affinity to present self-antigen. Recent studies have shown that additional polymorphisms in HLA that are restricted to autoimmune conditions are further contributory. A caveat is that not all individuals with the appropriate 'pro-autoimmune' HLA develop an autoimmune disease. Another crucial component is autoaggressive T cells. Finding a biomarker to discriminate autoaggressive T cells has been elusive. However, a subset of CD4 helper cells that express the CD40 receptor have been described as becoming pathogenic. An interesting function of CD40 on T cells is to induce the recombination-activating gene (RAG)1/RAG2 T cell receptor recombination machinery. This observation is contrary to immunology paradigms that changes in TCR molecules cannot take place outside the thymic microenvironment. Alteration in TCR, called TCR revision, not only occurs, but may help to account for the development of autoaggressive T cells. Another interesting facet is that type 1 diabetes (T1D) may be more than a single disease; that is, multiple cellular components contribute uniquely, but result ultimately in the same clinical outcome, T1D. This review considers the process of T cell maturation and how that could favor auto-aggressive T cell development in T1D. The potential contribution of TCR revision to autoimmunity is also considered.

Distinct IL-7 signaling in recent thymic emigrants versus mature naïve T cells controls T-cell homeostasis

IL-7 is essential for T-cell survival but its availability is limited in vivo. Consequently, all peripheral T cells, including recent thymic emigrants (RTEs) are constantly competing for IL-7 to survive. RTEs are required to replenish TCR diversity and rejuvenate the peripheral T-cell pool. However, it remains unknown how RTEs successfully compete with resident mature T cells for IL-7. Moreover, RTEs express low levels of IL-7 receptors, presumably rendering them even less competitive. Here, we show that, surprisingly, RTEs are more responsive to IL-7 than mature naïve T cells as demonstrated by markedly increased STAT5 phosphorylation upon IL-7 stimulation. Nonetheless, adoptive transfer of RTE cells into lymphopenic host mice resulted in slower IL-7-induced homeostatic proliferation and diminished expansion compared to naïve donor T cells. Mechanistically, we found that IL-7 signaling in RTEs preferentially upregulated expression of Bcl-2, which is anti-apoptotic but also anti-proliferative. In contrast, naïve T cells showed diminished Bcl-2 induction but greater proliferative response to IL-7. Collectively, these data indicate that IL-7 responsiveness in RTE is designed to maximize survival at the expense of reduced proliferation, consistent with RTE serving as a subpopulation of T cells rich in diversity but not in frequency.

Receptor revision in CD4 T cells is influenced by follicular helper T cell formation and germinal-center interactions

Peripheral CD4 T cells in Vβ5 transgenic (Tg) C57BL/6J mice undergo tolerance to an endogenous superantigen encoded by mouse mammary tumor virus 8 (Mtv-8) by either deletion or T-cell receptor (TCR) revision. Revision is a process by which surface expression of the Vβ5(+) TCR is down-regulated in response to Mtv-8 and recombination activating genes are expressed to drive rearrangement of the endogenous TCRβ locus, effecting cell rescue through the expression of a newly generated, non-self-reactive TCR. In an effort to identify the microenvironment in which revision takes place, we show here that the proportion of T follicular helper cells (Tfh) and production of high-affinity antibody during a primary response are increased in Vβ5 Tg mice in an Mtv-8-dependent manner. Revising T cells have a Tfh-like surface phenotype and transcription factor profile, with elevated expression of B-cell leukemia/lymphoma 6 (Bcl-6), CXC chemokine receptor 5, programmed death-1, and other Tfh-associated markers. Efficient revision requires Bcl-6 and is inhibited by B lymphocyte-induced maturation protein-1. Revision completes less efficiently in the absence of signaling lymphocytic activation molecule-associated protein although initiation proceeds normally. These data indicate that Tfh formation is required for the initiation of revision and germinal-center interactions for its completion. The germinal center is known to provide a confined space in which B-cell antigen receptors undergo selection. Our data extend the impact of this selective microenvironment into the arena of T cells, suggesting that this fluid structure also provides a regulatory environment in which TCR revision can safely take place.

CD40 interacts directly with RAG1 and RAG2 in autoaggressive T cells and Fas prevents CD40-induced RAG expression

CD4(+) T cells expressing CD40 (Th40 cells) constitute a pathogenic T-cell subset that is necessary and sufficient to transfer autoimmune disease. We have previously demonstrated that CD40 signals peripheral Th40 cells to induce RAG1 and RAG2 expression, proteins necessary for the expression of T-cell receptor (TCR), leading to TCR revision. The dependency of TCR expression in the thymus on RAG proteins has long been known. However, despite numerous publications, there is controversy as to whether TCR expression can be altered in the periphery, post-thymic selective pressures. Therefore, a better understanding of TCR expression in primary peripheral cells is needed. We now show that the CD40 protein itself interacts with RAG1 and RAG2 as well as with Ku70 and translocates to the nucleus in Th40 cells. This indicates that the CD40 molecule is closely involved in the mechanism of TCR expression in the periphery. In addition, Fas signals act as a silencing mechanism for CD40-induced RAGs and prevent CD40 translocation to the nucleus. It will be important to further understand the involvement of CD40 in peripheral TCR expression and how TCR revision impacts auto-antigen recognition in order to effectively target and tolerize autoaggressive T cells in autoimmune disease.

Analysis of naïve lung CD4 T cells provides evidence of functional lung to lymph node migration

The proportion of CD4 T cells with phenotypic and functional properties of naïve cells out of total CD4 T cells is similar in the lung parenchyma and lymph nodes. On treatment with a sphingosine-1-phosphate agonist, the frequency of these cells falls precipitously, but with a delay of ∼14 h compared with blood CD4 T cells; neither anti-CD62L nor pertussis toxin prevents entry of naïve CD4 T cells into the lung. Based on treatment with anti-CD62L and the use of CCR7(-/-) cells, lung naïve CD4 T cells appear to migrate to the mediastinal lymph nodes along a CD62L-independent, CCR7-dependent pathway. Cells that have entered the node in this manner are competent to respond to antigen. Thus, a portion (approximately one-half) of naïve CD4 T cells appears to enter the mediastinal lymph nodes through a blood-to-lung-to-lymph node route.

Modulation of TCRβ surface expression during TCR revision

TCR revision is a tolerance mechanism by which self-reactive TCRs expressed by mature CD4(+) peripheral T cells are replaced by receptors encoded by genes generated by post-thymic DNA rearrangement. The downmodulation of surface TCR expression initiates TCR revision, and serves as a likely trigger for the induction of the recombinase machinery. We show here in a Vβ5 transgenic mouse model system that downregulation of the self-reactive transgene-encoded TCR is not maintained by transgene loss or diminished transcription or translation. The downregulation of surface TCR expression likely occurs in two stages, only one of which requires tolerogen expression.

Clonally related CD8+ T cells responsible for rapid population of both diffuse nasal-associated lymphoid tissue and lung after respiratory virus infection

The immune system has evolved to use sophisticated mechanisms to recruit lymphocytes to sites of pathogen exposure. Trafficking pathways are precise. For example, lymphocytes that are primed by gut pathogens can, in some cases, be imprinted with CCR9 membrane receptors, which can influence migration to the small intestine. Currently, little is known about T cell trafficking to the upper respiratory tract or the relationship between effectors that migrate to the diffuse nasal-associated lymphoid tissue (d-NALT), the lower airways, and the lung. To determine whether a T cell primed by Ag from a respiratory pathogen is imprinted for exclusive trafficking to the upper or lower respiratory tract or whether descendents from that cell have the capacity to migrate to both sites, we inoculated mice by the intranasal route with Sendai virus and conducted single-cell-sequencing analyses of CD8(+) T lymphocytes responsive to a K(b)-restricted immunodominant peptide, FAPGNYPAL (Tet(+)). Cells from the d-NALT, lung airways (bronchoalveolar lavage), lung, and mediastinal lymph node were examined 10 d postinfection to determine TCR usage and clonal relationships. We discovered that 1) Tet(+) cells were heterogeneous but preferentially used TCR elements TRAV6, TRAV16, and TRBD1; 2) both N and C termini of Vα and Vβ TCR junctions frequently encompassed charged residues, perhaps facilitating TCR αβ pairing and interactions with a neutral target peptide; and 3) T cells in the d-NALT were often clonally related to cells in the lower respiratory tract.

Clonal analysis reveals uniformity in the molecular profile and lineage potential of CCR9(+) and CCR9(-) thymus-settling progenitors

The entry of T cell progenitors to the thymus marks the beginning of a multistage developmental process that culminates in the generation of self-MHC-restricted CD4(+) and CD8(+) T cells. Although multiple factors including the chemokine receptors CCR7 and CCR9 are now defined as important mediators of progenitor recruitment and colonization in both the fetal and adult thymi, the heterogeneity of thymus-colonizing cells that contribute to development of the T cell pool is complex and poorly understood. In this study, in conjunction with lineage potential assays, we perform phenotypic and genetic analyses on thymus-settling progenitors (TSP) isolated from the embryonic mouse thymus anlagen and surrounding perithymic mesenchyme, including simultaneous gene expression analysis of 14 hemopoietic regulators using single-cell multiplex RT-PCR. We show that, despite the known importance of CCL25-CCR9 mediated thymic recruitment of T cell progenitors, embryonic PIR(+)c-Kit(+) TSP can be subdivided into CCR9(+) and CCR9(-) subsets that differ in their requirements for a functional thymic microenvironment for thymus homing. Despite these differences, lineage potential studies of purified CCR9(+) and CCR9(-) TSP reveal a common bias toward T cell-committed progenitors, and clonal gene expression analysis reveals a genetic consensus that is evident between and within single CCR9(+) and CCR9(-) TSP. Collectively, our data suggest that although the earliest T cell progenitors may display heterogeneity with regard to their requirements for thymus colonization, they represent a developmentally homogeneous progenitor pool that ensures the efficient generation of the first cohorts of T cells during thymus development.

TCR transfer induces TCR-mediated tonic inhibition of RAG genes in human T cells

Induction of the TCR signaling pathway terminates the expression of RAG genes, and a link between this pathway and their transcriptional control is evident from the recent demonstration of their re-expression if the TCR is subsequently lost or down-regulated. Since unstimulated T cells display a steady-state level of "tonic" TCR signaling, i.e. in the absence of any antigenic stimulus, it was uncertain whether this control was exerted through ligand-dependent or ligand-independent TCR signaling. Here we demonstrate for the first time that exogenous TCR α and β chains transferred into the human immature RAG(+) T cell line Sup-T1 by lentiviral transduction inhibit RAG expression through tonic signaling, and that this inhibition could itself be reverted by pharmacological tonic pathway inhibitors. We also suggest that mature T cells already expressing an endogenous TCR on their surface maintain some levels of plasticity at the RAG locus when their basal TCR signaling is interfered with. Lastly, we show that the TCR constructs employed in TCR gene therapy do not possess the same basal signaling transduction capability, a feature that may have therapeutic implications.

Bcl-2-interacting mediator of cell death influences autoantigen-driven deletion and TCR revision

Peripheral CD4(+)Vβ5(+) T cells are tolerized to an endogenous mouse mammary tumor virus superantigen either by deletion or TCR revision. Through TCR revision, RAG reexpression mediates extrathymic TCRβ rearrangement and results in a population of postrevision CD4(+)Vβ5(-) T cells expressing revised TCRβ chains. We have hypothesized that cell death pathways regulate the selection of cells undergoing TCR revision to ensure the safety and utility of the postrevision population. In this study, we investigate the role of Bcl-2-interacting mediator of cell death (Bim)-mediated cell death in autoantigen-driven deletion and TCR revision. Bim deficiency and Bcl-2 overexpression in Vβ5 transgenic (Tg) mice both impair peripheral deletion. Vβ5 Tg Bim-deficient and Bcl-2 Tg mice exhibit an elevated frequency of CD4(+) T cells expressing both the transgene-encoded Vβ5 chain and a revised TCRβ chain. We now show that these dual-TCR-expressing cells are TCR revision intermediates and that the population of RAG-expressing, revising CD4(+) T cells is increased in Bim-deficient Vβ5 Tg mice. These findings support a role for Bim and Bcl-2 in regulating the balance of survival versus apoptosis in peripheral T cells undergoing RAG-dependent TCR rearrangements during TCR revision, thereby ensuring the utility of the postrevision repertoire.

TCR revision generates functional CD4+ T cells

CD4(+)Vβ5(+) peripheral T cells in C57BL/6 mice respond to encounter with a peripherally expressed endogenous superantigen by undergoing either deletion or TCR revision. In this latter process, cells lose surface Vβ5 expression and undergo RAG-dependent rearrangement of endogenous TCRβ genes, driving surface expression of novel TCRs. Although postrevision CD4(+)Vβ5(-)TCRβ(+) T cells accumulate with age in Vβ5 transgenic mice and bear a diverse TCR Vβ repertoire, it is unknown whether they respond to homeostatic and antigenic stimuli and thus may benefit the host. We demonstrate in this study that postrevision cells are functional. These cells have a high rate of steady-state homeostatic proliferation in situ, and they undergo extensive MHC class II-dependent lymphopenia-induced proliferation. Importantly, postrevision cells do not proliferate in response to the tolerizing superantigen, implicating TCR revision as a mechanism of tolerance induction and demonstrating that TCR-dependent activation of postrevision cells is not driven by the transgene-encoded receptor. Postrevision cells proliferate extensively to commensal bacterial Ags and can generate I-A(b)-restricted responses to Ag by producing IFN-γ following Listeria monocytogenes challenge. These data show that rescued postrevision T cells are responsive to homeostatic signals and recognize self- and foreign peptides in the context of self-MHC and are thus useful to the host.

Cutting Edge: Rag deletion in peripheral T cells blocks TCR revision

Mature CD4(+)Vbeta5(+) T cells that recognize a peripherally expressed endogenous superantigen are tolerized either by deletion or TCR revision. In Vbeta5 transgenic mice, this latter tolerance pathway results in the appearance of CD4(+)Vbeta5(-)TCRbeta(+) T cells, coinciding with Rag1, Rag2, and TdT expression and the accumulation of V(beta)-DJ(beta) recombination intermediates in peripheral CD4(+) T cells. Because postthymic RAG-dependent TCR rearrangement has remained controversial, we sought to definitively determine whether TCR revision is an extrathymic process that occurs in mature peripheral T cells. We show in this study that Rag deletion in post-positive selection T cells in Vbeta5 transgenic mice blocks TCR revision in vivo and that mature peripheral T cells sorted to remove cells bearing endogenous TCRbeta-chains can express newly generated TCRbeta molecules in adoptive hosts. These findings unambiguously demonstrate postthymic, RAG-dependent TCR rearrangement and define TCR revision as a tolerance pathway that targets mature peripheral CD4(+) T cells.

T-cell receptor revision: friend or foe?

T-cell receptor (TCR) revision is a process of tolerance induction by which peripheral T cells lose surface expression of an autoreactive TCR, reinduce expression of the recombinase machinery, rearrange genes encoding extrathymically generated TCRs for antigen, and express these new receptors on the cell surface. We discuss the evidence for this controversial tolerance mechanism below. Despite the apparent heresy of post-thymic gene rearrangement, we argue here that TCR revision follows the rules obeyed by maturing thymocytes undergoing gene recombination. Expression of the recombinase is carefully controlled both spatially and temporally, and may be initiated by loss of signals through surface TCRs. The resulting TCR repertoire is characterized by its diversity, self major histocompatibility complex restriction, self tolerance, and ability to mount productive immune responses specific for foreign antigens. Hence, TCR revision is a carefully regulated process of tolerance induction that can contribute to the protection of the individual against invading pathogens while preserving the integrity of self tissue.

Functional roles for T cell CD40 in infection and autoimmune disease: the role of CD40 in lymphocyte homeostasis

CD40 stimulation on monocytes/macrophages, dendritic cells, and B-lymphocytes has been the subject of much study. It is well recognized that activation of CD40 on antigen presenting cells by its ligand, CD154, expressed on T-lymphocytes, contributes to the pro-inflammatory response necessary for eradication of infection, yet pathological in autoimmunity. However, there is evidence that CD40 is also expressed on T-lymphocytes and can act as a costimulatory molecule. While the exact role of CD40 on CD8 T cells remains controversial, it does appear to contribute to the adaptive immune response against infection. CD40 on CD4 T cells, on the other hand, plays a functional role in the autoimmune disease process. Further dissection of the exact nature and role of CD40 in T cell activation could lead the way to more effective vaccines and novel therapeutics for autoimmune diseases.

Impact of post-thymic cellular longevity on the development of age-associated CD4+ T cell defects

Elderly people are at higher risk for infections due to declining cellular and humoral immune responses. Central to this dysfunction is the reduced responsiveness of the naive CD4(+) T cell compartment. Previous data from our laboratory suggest that although defects in the aged naive CD4(+) T cell response are apparent in recent thymic emigrant populations, additional defects develop during extended post-thymic longevity in the periphery. To further investigate the factors that lead to aging defects, we took advantage of the OT-II TCR-transgenic (Tg) mouse model. We show that because of an apparent superantigen-mediated loss of naive Vbeta5(+) Tg CD4(+) T cells from the periphery of aging OT-II mice, this compartment becomes enriched for cells of reduced post-thymic longevity, resulting in a frequency of recent thymic emigrants in aged mice that is similar to that of young mice. Purification and functional analysis of aged OT-II cells with reduced post-thymic longevity reveal that they have an age-associated decrease in expansion and IL-2 production in response to Ag in vitro. However, the in vivo expansion, IL-2 production, and cognate B cell helper ability of these cells are similar to those of cells from young mice. In contrast, T cells from aged HNT Tg mice demonstrate extended post-thymic longevity and exhibit severe defects in the same in vitro and in vivo models. These data support a correlation between the requirement for increased post-thymic longevity and the development of the most severe naive CD4(+) T cell-aging defects.

Cutting edge: TCR revision affects predominantly Foxp3 cells and skews them toward the Th17 lineage

CD4(+) T cells respond to peripheral endogenous superantigen stimulation by undergoing deletion or TCR revision. The latter involves RAG re-expression, TCR gene rearrangement, and expression of a novel TCR. TCR-revised T cells are functional and express a diverse TCR repertoire. Because TCR revision harbors the potential to create self-reactivity, it is important to explore whether T cells known to be self-reactive (regulatory T cells) or those involved in autoimmunity (Th17 cells) arise from TCR revision. Interestingly, we observed that Foxp3(+) cells are excluded from revising their TCR and that only a small fraction of postrevision cells expresses Foxp3. In contrast, Th17 cells are 20 times more frequent among revised than among C57BL/6 CD4(+) T cells, indicating that postrevision cells are biased toward the Th17 lineage. The link between Th17 differentiation and TCR revision might be highly relevant to the role of Th17 cells in promoting autoimmunity.

An in vitro model of T cell receptor revision in mature human CD8+ T cells

V(D)J recombination is a mechanism peculiar to the somatic rearrangement of antigen receptor genes. It requires both expression of the RAG-1 and RAG-2 recombinases and accessibility of the substrate to its recombinase and post-cleavage/DNA repair stage. TCR revision is a genetic correction mechanism that changes T cell specificity by re-activating V(D)J recombination in peripheral T cells. This process is now well described in both normal or pathological murine and human settings. Many of its features, such as the question of whether it occurs in truly mature T cells, remain to be elucidated. Its occurrence in human CD8+ T cells is also an open question. We have therefore established an in vitro model of TCR revision in mature human CD8+ T cells to determine whether down-regulation of the TCR/CD3 complex from the cell surface in the presence of IL7 as a factor favouring chromatin remodelling initiates a TCR revision pathway. Only mature CD8+ T cells carrying already-formed antigen receptors were used. CD8+ T cells treated with anti-CD3 and IL7 showed rearrangement intermediates and expressed new Vbeta-chains on their surface. Investigation of the molecular pathway thus induced disclosed up-regulation of the RAG-2 transcript, but absence of the 'canonical' RAG-1 mRNA. A surprising finding was the demonstration of alternative splice forms of this mRNA, already expressed in untreated CD8+ T cells, encoding for the full-length RAG-1 protein, which was increased three-fold in the treated cells. All the V(D)J requirements were thus fulfilled when mature human CD8+ T cells were stimulated with anti-CD3 and IL7. Induction of TCR revision in vitro in mature T cells is an easily controllable system that could be employed in further studies to elucidate the molecular pathways involved in secondary V(D)J rearrangements in peripheral cells.

A unique T cell subset described as CD4loCD40+ T cells (TCD40) in human type 1 diabetes

Human T1D pancreatic lymph nodes contain diabetes-autoantigen responsive T cells but identification of such T cells in the periphery has proven difficult. Here we describe a unique T cell subset defined by CD4(lo) and CD40 expression (T(CD40)) that is significantly expanded in peripheral blood of T1D but not control or T2D subjects. The HLA-DR3 and DR4 alleles are considered high risk factors for T1D and T(CD40) expansion occurs in T1D subjects carrying HLA DR3 or DR4 haplotypes but, T1D subjects who do not carry either DR3 or DR4 haplotypes still have an expanded percentage of T(CD40) cells. Non-autoimmune subjects, even DR3(+) and DR4(+), do not have elevated percentages of T(CD40) cells. The majority of T(CD40) cells in T1D carry a memory phenotype and a portion of those proliferates when exposed to diabetes-associated self-antigens. A greater number of memory T(CD40) cells express CXCR3 when compared to CD40(-) memory cells and that number is significantly expanded in T1D compared to control subjects. If only total CD4(+) T cells are compared no difference in CXCR3 is seen. Furthermore, T(CD40) cells produce a Th1, pro-inflammatory cytokine profile. In healthy controls, T(CD40) cells have equally Th1 and Th2 profiles.

Revision of the antigen receptor of T-lymphocytes

This review considers a crucially new mechanism of T-cell antigen-recognizing repertoire formation. It includes the revision of T-cell antigen receptor (TCR), which implies the secondary rearrangement of TCR genes in peripheral T-lymphocytes and surface expression of a new antigen receptor with altered specificity. Factors and mechanisms involved in the induction of this process have been analyzed. Certain attention is paid to a possible role of TCR revision in the formation of peripheral tolerance in the processes of "avidity maturation" of T-lymphocytes during immune response and also negative consequences related to appearance of potentially autoreactive clones in the periphery.

Re-shaping the T cell repertoire: TCR editing and TCR revision for good and for bad

Protection against the universe of pathogens requires a functional, diverse T cell repertoire. However, the price that is paid for an evolved, effective immune system includes the potential danger of generating autoaggressive T cells. Autoimmune diseases result from inherent breach of tolerance to self-antigens leading to disruption of the regulatory to autoaggressive T cell homeostatic balance. The immune system has evolved mechanisms to control those processes. For T cells, positive and negative selection in the thymus assures that only fully functional, non-self-reactive T cells will populate the periphery. Failure of this central tolerance would result in autoaggressive T cells escaping into the periphery. However, other means of escaping negative selection can occur in the periphery, i.e., TCR revision, or the altering of TCR expression after thymic egress. Here the potential benefits, i.e., expansion and re-shaping of the T cell repertoire as potentiated by TCR editing and revision are considered. Furthermore, the potential to develop autoaggressive TCR and thus enhance autoimmunity is considered.

Thymic output in aged mice

Using GFP to mark recent thymic emigrants (RTEs) in mice carrying a GFP transgene driven by the recombination-activating gene 2 promoter, we demonstrate that RTEs are readily detectable even in 2-year-old mice, despite the fact that the proportion of the peripheral T cell pool comprised of RTEs declines with age. Although the number of RTEs decreases after reaching a peak at 6 weeks of age, thymic output as a function of thymic size is surprisingly age-independent. The CD4:CD8 ratio of RTEs declines with age, partly because of a striking decrease in steady-state proliferation of CD4+ RTEs in older mice. RTEs in aged mice undergo phenotypic maturation in the lymphoid periphery with delayed kinetics compared with young mice. RTEs from aged mice secrete less IL-2, proliferate less well, and achieve only weak expression of early-activation markers compared with more mature naïve peripheral T cells from the same mice. The proportion of GFP- cells in the CD4+ and CD8+ thymic compartments increases with age, partly as a result of leakiness in the aged thymus, allowing reentry of naïve peripheral T cells.

Control of Genotypic Allelic Inclusion through TCR Surface Expression

To gain insight into the molecular causes and functional consequences of allelic inclusion of TCR alpha-chains, we develop a computational model for thymocyte selection in which the signal that determines cell fate depends on surface expression. Analysis of receptor pairs on selected dual TCR cells reveals that allelic inclusion permits both autoreactive TCR and receptors not in the single TCR cell repertoire to be selected. However, in comparison with earlier theoretical studies, relatively few dual TCR cells display receptors with high avidity for thymic ligands because their alpha-chains compete aggressively for the beta-chain, which hinders rescue from clonal deletion. This feature of the model makes clear that allelic inclusion does not in itself compromise central tolerance. A specific experiment based on modulation of TCR surface expression levels is proposed to test the model.

Human peripheral CD2−/lo T cells: an extrathymic population of early differentiated, developing T cells

We previously reported that a subset of human peripheral blood CD3+ T cells expresses low-to-null CD2 levels (CD2-/lo), produces type 2 cytokines and is inducible to differentiate to functionally mature IFN-gamma+ cells. Multiple-color immunofluorescence analysis indicated that this population, representing <0.1% of the T cells in fresh lymphocytes, contains subsets that are phenotypically immature, including CD4-CD8- and CD3+TCR- cells. Ex vivo, the CD2-/lo cells can proliferate (carboxyfluorescein diacetate succinimidyl ester analysis) independently from exogenous stimulation, respond to CD3-mediated stimulation with significantly greater proliferation than the autologous mature cells and their subsets are inducible to undergo in vitro a developmental sequence similar to that reported for the phenotypically similar thymic populations. This is especially evident for the CD4+CD8+ subset. CD2-/lo T-cell populations exhibit a TCR repertoire (Vbeta chain distribution) that is complete but different (complementarity determining region R3 analysis) from that of the autologous CD2+ T cells. These characteristics distinguish peripheral CD2-/lo T cells as possible early differentiated T cells that may undergo extrathymic maturation, and potentially contribute to maintain the peripheral naive T-cell pool. These findings define the existence of phenotypically immature T cells in the periphery. Also, given the high numbers of CD2-/lo T cells generated, upon ex vivo culture, from peripheral lymphocytes of all adult and neonatal individuals tested, they have relevance to clinical applications for immune reconstitution of T cells, as well as myeloid cells, via myeloid colony-stimulating factors and type 2 cytokines.

T cells from epicutaneously immunized mice are prone to T cell receptor revision

Epicutaneous immunization of T cell receptor (TCR) transgenic (Tg) mice whose CD4(+) T cells are specific for the Ac1-11 fragment of myelin basic protein (MBP) with Ac1-11 elicits T cells with dominant suppressor/regulatory activity that confers protection against Ac1-11-induced experimental autoimmune encephalomyelitis. We now report that such disease-resistant MBP TCR Tg mice also harbor a sizeable fraction of peripheral CD4(+) T cells lacking surface expression of the Tg TCR beta chain and expressing diverse, endogenously rearranged TCR beta chains. Ex vivo incubation at physiological temperature caused the loss of neo-beta-chain expression and reversion to the MBP alphabeta TCR(+) phenotype. The presence of recombination activating gene 1 and 2 proteins in CD4(+) T cells with revised TCRs was consistent with effective V(D)J recombination activity. The emergence of these cells did not depend on the thymic compartment. We conclude that in mice epicutaneously immunized with an autoantigen, peripheral specific T cells are susceptible to multiple mechanisms of tolerance.

Cutting edge: TCR revision occurs in germinal centers

Mouse CD4(+)Vbeta5(+) T cells recognize a peripherally expressed superantigen encoded by an endogenous retrovirus. Ag encounter tolerizes the mature CD4 T cell compartment, either by deletion of autoreactive cells or by TCR revision. This latter process is driven by TCRbeta rearrangement through RAG activity and results in the rescue of cells expressing novel TCRs that no longer recognize the tolerogen. Consistent with the notion that revising T cells represent a distinct peripheral T cell population, we now show that these lymphocyte blasts express a hybrid effector/memory phenotype and are not undergoing cell division. A population of revising T cells is CD40(+), expresses the germinal center (GC) marker CXCR5, and is Vbeta5(low)Thy-1(low). Histology reveals that, consistent with their surface Ag phenotype, T cells undergoing TCR revision are enriched in splenic GCs. These data demonstrate that TCR revision is a multistep tolerance pathway supported by the unique microenvironment provided by GCs.

Receptor revision in T cells: an open question?

During lymphocyte development, both B and T cells assemble antigen receptor variable region genes from germline gene segments, allowing the expression of unique receptors in each clonally derived lymphocyte. Previously, it was shown that in certain cases, progenitor and immature B cells are capable of editing their receptors to a new specificity on encounter with self-antigens. Although the existence of such a process in T cells remains controversial, recent studies suggest that mature T cells are able to similarly revise their receptors in the periphery.

Differential regulation of peripheral CD4+ T cell tolerance induced by deletion and TCR revision

In Vbeta5 transgenic mice, mature Vbeta5(+)CD4(+) T cells are tolerized upon recognition of a self Ag, encoded by a defective endogenous retrovirus, whose expression is confined to the lymphoid periphery. Cells are driven by the tolerogen to enter one of two tolerance pathways, deletion or TCR revision. CD4(+) T cells entering the former pathway are rendered anergic and then eliminated. In contrast, TCR revision drives gene rearrangement at the endogenous TCR beta locus and results in the appearance of Vbeta5(-), endogenous Vbeta(+), CD4(+) T cells that are both self-tolerant and functional. An analysis of the molecules that influence each of these pathways was conducted to understand better the nature of the interactions that control tolerance induction in the lymphoid periphery. These studies reveal that deletion is efficient in reconstituted radiation chimeras and is B cell, CD28, inducible costimulatory molecule, Fas, CD4, and CD8 independent. In contrast, TCR revision is radiosensitive, B cell, CD28, and inducible costimulatory molecule dependent, Fas and CD4 influenced, and CD8 independent. Our data demonstrate the differential regulation of these two divergent tolerance pathways, despite the fact that they are both driven by the same tolerogen and restricted to mature CD4(+) T cells.

Continued maturation of thymic emigrants in the periphery

Developing thymocytes are selected for recognition of molecules encoded by the major histocompatibility complex, purged of self-reactive cells and committed to either the CD4 or CD8 lineage. The 1% of thymocytes that complete these tasks emigrate and join the population of peripheral lymphocytes. Whether T cell maturation is complete at the time of thymic exit has been a subject of debate. Using mice transgenic for green fluorescent protein driven by the recombination activating gene 2 promoter to identify recent thymic emigrants, we now show that T cell differentiation continues post-thymically, with progressive maturation of both surface phenotype and immune function. In addition, the relative contribution of CD4 and CD8 recent thymic emigrants was modulated as they entered the peripheral T cell pool. Thus, T cell maturation and subset contribution are both finalized in the lymphoid periphery.

T-helper cell tolerance to ubiquitous nuclear antigens

Systemic autoimmune diseases are characterized by the development of antinuclear autoantibodies. In order to understand the immunologic events leading to the development of such antibodies, knowledge of mechanisms of immune tolerance to nuclear antigens is required. By utilizing adoptive T-cell transfer strategies with transgenic mouse models expressing nuclear neo-self antigens, T-cell tolerance to the lupus-related nuclear antigens human La and nRNP A has been demonstrated. These findings also indicate the existence in normal animals of autoreactive B cells continuously presenting nuclear antigen, suggesting that nuclear antigens are not sequestered from the immune system. Investigations of CD4+ T-cell tolerance to non-nuclear antigens have revealed a number of mechanisms that protect the host from autoreactivity, including autoreactive T-cell deletion, regulatory T-cell development and anergy induction. Recent studies using T-cell receptor and neo-self nuclear antigen transgenic mice are revealing the importance of such mechanisms in maintaining tolerance to nuclear antigens. Mechanisms of tolerogenic antigen presentation, identification of tolerogenic antigen source(s) and the pathways leading to loss of tolerance to nuclear antigens in systemic autoimmune disease states are currently being sought.