Self-Recognition Sensitizes Mouse and Human Regulatory T Cells to Low-Dose CD28 Superagonist Stimulation.

Daniela Langenhorst,Thomas Hünig,Charles D Surh,Bryan R Becklund,Tobias Gulde,Niklas Beyersdorf,Susanne Berr,Paula Tabares

doi:10.3389/fimmu.2017.01985

Abstract

In rodents, low doses of CD28-specific superagonistic monoclonal antibodies (CD28 superagonists, CD28SA) selectively activate regulatory T cells (Treg). This observation has recently been extended to humans, suggesting an option for the treatment of autoimmune and inflammatory diseases. However, a mechanistic explanation for this phenomenon is still lacking. Given that CD28SA amplify T cell receptor (TCR) signals, we tested the hypothesis that the weak tonic TCR signals received by conventional CD4+ T cells (Tconv) in the absence of cognate antigen require more CD28 signaling input for full activation than the stronger TCR signals received by self-reactive Treg. We report that in vitro, the response of mouse Treg and Tconv to CD28SA strongly depends on MHC class II expression by antigen-presenting cells. To separate the effect of tonic TCR signals from self-peptide recognition, we compared the response of wild-type Treg and Tconv to low and high CD28SA doses upon transfer into wild-type or H-2M knockout mice, which lack a self-peptide repertoire. We found that the superior response of Treg to low CD28SA doses was lost in the absence of self-peptide presentation. We also tested if potentially pathogenic autoreactive Tconv would benefit from self-recognition-induced sensitivity to CD28SA stimulation by transferring TCR transgenic OVA-specific Tconv into OVA-expressing mice and found that low-dose CD28SA application inhibited, rather than supported, their expansion, presumably due to the massive concomitant activation of Treg. Finally, we report that also in the in vitro response of human peripheral blood mononuclear cells to CD28SA, HLA II blockade interferes with the expansion of Treg by low-dose CD28SA stimulation. These results provide a rational basis for the further development of low-dose CD28SA therapy for the improvement of Treg activity.

Highlights

CD28 superagonists (CD28SA) are a unique class of CD28specific monoclonal antibodies able to activate T cells without overt stimulation of the T cell receptor (TCR) [1, 2]
regulatory T cells (Treg) expanded much more than Tconv, resulting in a sevenfold recovery over input at the highest CD28SA dose employed (30 μg/ml) as compared to a twofold recovery of conventional CD4+ T cells (Figure 1C)
These results were paralleled by an increase in the fraction of cells expressing the or MHC II ko APCs and 0 or 1.1 μg/ml of CD28SA. (B) Comparison of CFSE dilution in Treg and Tconv stimulated with WT APC or MHC II ko APC. (C,D)

Summary

Introduction

CD28 superagonists (CD28SA) are a unique class of CD28specific monoclonal antibodies (mAb) able to activate T cells without overt stimulation of the T cell receptor (TCR) [1, 2]. While CD28SA activate T cells both in vitro and in vivo without TCR ligation by mAb or MHC molecules presenting cognate peptide antigens, this activation strictly depends on “tonic” TCR signals [7, 8] generated by cellular interactions [9] during the process known as MHC scanning, in which the TCR briefly docks onto MHC peptide complexes in a MHC class and allele-nonspecific fashion and rapidly dissociates unless a cognate peptide is recognized [10] This strict dependence of the T cell response to CD28SA on preactivation through cell–cell contacts in the tissue results in the inability of human circulating T cells to respond to the human CD28SA TGN1412 ( called TAB08), which contributed to the failure to predict the cytokine release syndrome triggered by this antibody during a first-in-human (FIH) trial in 2006 [11, 12]. A method has been developed which resets human peripheral blood mononuclear cells (PBMC) to tissuelike status, allowing the in vitro analysis of the response to this potent T cell activating agent [9]

Methods

Results

Conclusion