Cristina Cozzo Picca

Role of TCR specificity in CD4+CD25+ regulatory T‐cell selection

Summary:  CD4+CD25+ regulatory T cells play a crucial role in preventing autoimmune disease and can also modulate immune responses in settings such as transplantation and infection. We have developed a transgenic mouse system in which the role that T‐cell receptor (TCR) specificity for self‐peptides plays in the formation of CD4+CD25+ regulatory T cells can be examined. We have shown that interactions with a single self‐peptide can induce thymocytes bearing an autoreactive TCR to undergo selection to become CD4+CD25+ regulatory T cells and that thymocytes bearing TCRs with low affinity for the selecting peptide do not appear to undergo selection into this pathway. In addition, thymocytes with identical specificity for the selecting self‐peptide can undergo overt deletion versus abundant selection to become CD4+CD25+ regulatory T cells in response to variations in expression of the selecting peptide in different lineages of transgenic mice. Finally, we have shown that CD4+CD25+ T cells proliferate in response to their selecting self‐peptide in the periphery, but these cells do not proliferate in response to lymphopenia in the absence of the selecting self‐peptide. These studies are determining how the specificity of the TCR for self‐peptides directs the thymic selection and peripheral expansion of CD4+CD25+ regulatory T cells.

CD4+CD25+ Regulatory T Cells Inhibit the Maturation but Not the Initiation of an Autoantibody Response

To investigate the mechanism by which T regulatory (Treg) cells may control the early onset of autoimmunity, we have used an adoptive transfer model to track Treg, Th, and anti-chromatin B cell interactions in vivo. We show that anti-chromatin B cells secrete Abs by day 8 in vivo upon provision of undeviated, Th1- or Th2-type CD4+ T cell help, but this secretion is blocked by the coinjection of CD4+CD25+ Treg cells. Although Treg cells do not interfere with the initial follicular entry or activation of Th or B cells at day 3, ICOS levels on Th cells are decreased. Furthermore, Treg cells must be administered during the initial phases of the Ab response to exert full suppression of autoantibody production. These studies indicate that CD25+ Treg cells act to inhibit the maturation, rather than the initiation, of autoantibody responses.

CD4+CD25+ regulatory T cell repertoire formation shaped by differential presentation of peptides from a self-antigen.

We have used TCR transgenic mice directed to different MHC class II-restricted determinants from the influenza virus hemagglutinin (HA) to analyze how specificity for self-peptides can shape CD4+CD25+ regulatory T (Treg) cell formation. We show that substantial increases in the number of CD4+CD25+ Treg cells can occur when an autoreactive TCR directed to a major I-Ed-restricted determinant from HA develops in mice expressing HA as a self-Ag, and that the efficiency of this process is largely unaffected by the ability to coexpress additional TCR α-chains. This increased formation of CD4+CD25+ Treg cells in the presence of the self-peptide argues against models that postulate selective survival rather than induced formation as mechanisms of CD4+CD25+ Treg cell formation. In contrast, T cells bearing a TCR directed to a major I-Ad-restricted determinant from HA underwent little or no selection to become CD4+CD25+ Treg cells in mice expressing HA as a self-Ag, correlating with inefficient processing and presentation of the peptide from the neo-self-HA polypeptide. These findings show that interactions with a self-peptide can induce thymocytes to differentiate along a pathway to become CD4+CD25+ Treg cells, and that peptide editing by DM molecules may help bias the CD4+CD25+ Treg cell repertoire away from self-peptides that associate weakly with MHC class II molecules.

CD4+CD25+Foxp3+ regulatory T cell formation requires more specific recognition of a self-peptide than thymocyte deletion

CD4+CD25+Foxp3+ regulatory T (Treg) cells are generated during thymocyte development and play a crucial role in preventing the immune system from attacking the bodys cells and tissues. However, how the formation of these cells is directed by T-cell receptor (TCR) recognition of self-peptide:major histocompatibility complex (MHC) ligands remains poorly understood. We show that an agonist self-peptide with which a TCR is strongly reactive can induce a combination of thymocyte deletion and CD4+CD25+Foxp3+ Treg cell formation in vivo. A weakly cross-reactive partial agonist self-peptide could similarly induce thymocyte deletion, but failed to induce Treg cell formation. These studies indicate that CD4+CD25+Foxp3+ Treg cell formation can require highly stringent recognition of an agonist self-peptide by developing thymocytes. They also refine the “avidity” model of thymocyte selection by demonstrating that the quality of the signal mediated by agonist self-peptides, rather than the overall intensity of TCR signaling, can be a critical factor in directing autoreactive thymocytes to undergo CD4+CD25+Foxp3+ Treg cell formation and/or deletion during their development.

Activation of CD4+ CD25+ regulatory T cell suppressor function by analogs of the selecting peptide.

CD4+CD25+Foxp3+ regulatory T (Treg) cells can undergo both thymic selection and peripheral expansion in response to self peptides that are agonists for their T cell receptors (TCR). However, the specificity by which these TCR must recognize peptide:MHC complexes to activate Treg cell function is not known. We show that CD4+CD25+Foxp3+ Treg cells can mediate suppression in response to peptides that are only weakly cross‐reactive with the self peptide that induced their formation in vivo. Moreover, suppression could be efficiently activated by peptide analogs that were inefficient at inducing CD69 up‐regulation, and that also induced little or no proliferation of naïve CD4+CD25–Foxp3– T cells expressing the same TCR. These findings provide evidence that self peptide‐specific CD4+CD25+Foxp3+ Treg cells can exert regulatory function in response to self‐ and/or pathogen‐derived peptides with which they are only weakly cross‐reactive.

CD4+ T Cells Recognizing a Single Self-Peptide Expressed by APCs Induce Spontaneous Autoimmune Arthritis

We have examined processes leading to the spontaneous development of autoimmune inflammatory arthritis in transgenic mice containing CD4+ T cells targeted to a nominal Ag (hemagglutinin (HA)) and coexpressing HA driven by a MHC class II promoter. Despite being subjected to multiple tolerance mechanisms, autoreactive CD4+ T cells accumulate in the periphery of these mice and promote systemic proinflammatory cytokine production. The majority of mice spontaneously develop inflammatory arthritis, which is accompanied by an enhanced regional immune response in lymph nodes draining major joints. Arthritis development is accompanied by systemic B cell activation; however, neither B cells nor Ab is required for arthritis development, since disease develops in a B cell-deficient background. Moreover, arthritis also develops in a recombinase activating gene-deficient background, indicating that the disease process is driven by CD4+ T cells recognizing the neo-self HA Ag. These findings show that autoreactive CD4+ T cells recognizing a single self-Ag, expressed by systemically distributed APCs, can induce arthritis via a mechanism that is independent of their ability to provide help for autoantibody production.

How specificity for self‐peptides shapes the development and function of regulatory T cells

The cataclysmic disease that develops in mice and humans lacking CD4+ T cells expressing the transcription factor Foxp3 has provided abundant evidence that Foxp3+CD4+ Tregs are required to suppress a latent autoreactivity of the immune system. There is also evidence for the existence of tissue‐specific Tregs that can act to suppress regional autoimmune responses, suggesting that Tregs exert their effects, in part, through responding to self‐peptides. However, how the immune system generates a repertoire of Tregs that is designed to recognize and direct regulatory function to self‐peptides is incompletely understood. This review describes studies aimed at determining how T cell recognition of self‐peptide(s) directs Treg formation in the thymus, including discussion of a modified “avidity” model of thymocyte development. Studies aimed at determining how TCR specificity contributes to the ability of Tregs to suppress autoimmune diseases are also discussed.

Thymocyte deletion can bias Treg formation toward low-abundance self-peptide.

Autoreactive CD4+ T cells can undergo deletion and/or become CD25+Foxp3+ 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+ 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).

Spontaneous Autoreactive Memory B Cell Formation Driven by a High Frequency of Autoreactive CD4+ T Cells

Although somatically mutated autoantibodies are characteristic of many autoimmune diseases, the processes that can lead to their development remain poorly understood. We have examined the formation of autoreactive memory B cells in PevHA mice, which express the influenza virus PR8 hemagglutinin (HA) as a transgenic membrane bound neo-self-Ag. Using a virus immunization strategy, we show that PR8 HA-specific memory B cell formation can occur in PevHA mice, even though a major subset of PR8 HA-specific B cells is negatively selected from the primary repertoire. Moreover, PR8 HA-specific memory B cells develop spontaneously in TS1 × PevHA mice, which coexpress a transgenic PR8 HA-specific TCR and contain a high frequency of HA-specific CD4+ T cells. Notably, autoreactive memory B cell formation occurred in TS1 × PevHA mice even though approximately half of the HA-specific CD4+ T cells were CD25+Foxp3+ cells that could significantly attenuate, but did not completely abolish HA-specific autoantibody production in an adoptive transfer setting. The findings provide evidence that a high frequency of autoreactive CD4+ T cells can be sufficient to promote autoreactive memory B cell formation in the absence of signals provided by overt immunization or infection and despite the presence of abundant autoantigen-specific CD4+CD25+Foxp3+ regulatory T cells.

Thymocyte deletion can bias Treg formation toward low-abundance self-peptide: Highlights

Autoreactive CD4+ T cells can undergo deletion and/or become CD25+Foxp3+ 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+ 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).