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).

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-E(d)-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 alpha-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-A(d)-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.

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 x 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 x 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.

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.

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

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.

The role of self-peptides in the development of CD4+ CD25+ regulatory T cells.

The thymus produces a unique lineage of cells known as CD4+ CD25+ regulatory T cells, and the exact processes leading to their development continue to be defined. Highly specific interactions between developing thymocytes and cognate self-antigens expressed by radioresistant elements in the thymus have been shown to drive CD4+ CD25+ regulatory-T-cell development. The self-peptide(s) that mediate thymic selection of CD4+ CD25+ regulatory T cells can also promote their expansion in the periphery, and self-peptides might also play a role in the conversion of CD4+ CD25- T cells into CD25+ regulatory T cells.