CD28 Facilitates the Generation of Foxp3− Cytokine Responsive Regulatory T Cell Precursors

Abstract

The T cell costimulatory molecule CD28 plays an important role in the thymic generation of Foxp3+ regulatory T cells (Tregs) essential for the maintenance of self-tolerance. In this study, we show that a cell-intrinsic signal from CD28 is involved in the generation of cytokine-responsive Foxp3− precursors using studies of mixed bone marrow chimeras as well as TCR-specific generation of Foxp3+ cells using intrathymic transfer of TCR-transgenic thymocytes expressing a natural Treg TCR. Contrary to a previous report, the analysis of CD28 mutant knockin mice revealed that this cell-intrinsic signal is only partially dependent on the Lck-binding PYAP motif. Surprisingly, even though the absence of CD28 resulted in a 6-fold decrease in thymic Tregs, the TCR repertoires of CD28-deficient and sufficient cells were largely overlapping. Thus, these data suggest that CD28 does not operate by markedly enlarging the repertoire of TCRs available for Treg development, but rather by improving the efficiency of Treg development of thymocytes expressing natural Treg TCRs.

Regulatory T-Cell Therapy in Transplantation: Moving to the Clinic

“…Treg development in the thymus is a multistep process controlled by cytokines and signaling through T-cell receptor and CD28, which culminates in FOXP3 expression and epigenetic modification of the FOXP3 locus (Burchill et al 2008;Lio and Hsieh 2008;Long et al 2009;Lio et al 2010;Ohkura et al 2012). The TSDR of the FOXP3 locus is demethylated in cells that have committed to the Treg lineage to ensure stable inheritance of FOXP3 expression in dividing cells and the lineage stability of Tregs (Floess et al 2007;Huehn et al 2009;Josefowicz and Rudensky 2009;Zheng et al 2010;Ohkura et al 2012).…”

Treg Cells, Life History, and Diversity

“…Treg selection requires TCR:MHC (major histocompatibility complex) molecular interactions, as evidenced by their reduction in MHC-deficient mice (Krajina et al 2004;Bienvenu et al 2005;Fontenot et al 2005b;Stephens et al 2007), but also requires costimulatory signals from CD28, which seems to amplify the probability that cells expressing a TCR committing them to Treg fate are actually selected (Salomon et al 2000;Tai et al 2005;Lio et al 2010). Treg differentiation follows a two-step process, through a FoxP3-negative CD25 hi intermediate that secondarily converts to if only because "self-reactivity" is a relative concept, highly influenced by the mode of self-antigen presentation and the state of the responding cell.…”

TCR affinity and specificity requirements for human regulatory T-cell function

“…28 Previous studies have indicated that CD28-mediated recruitment of Lck is required for Treg development; 42 however, a more recent study using a knock-in rather than a transgenic model could not completely replicate this finding. 43 In any case, our studies examined the ability of mature Tregs to function in the absence of CD4-recruited Lck, whereas most other studies have examined differentiation of naive T cells to Tregs. It is unclear whether the signaling requirements to mediate suppressive activity for the development and differentiation of Tregs are the same.…”