The CD28 superagonist (CD28SA) TGN1412 was administered to humans as an agent that can selectively activate and expand regulatory T cells but resulted in uncontrolled T cell activation accompanied by cytokine storm. The molecular mechanisms that underlie this uncontrolled T cell activation are unclear. Physiological activation of T cells leads to upregulation of not only activation molecules but also inhibitory receptors such as PD-1. We hypothesized that the uncontrolled activation of CD28SA-stimulated T cells is due to both the enhanced expression of activation molecules and the lack of or reduced inhibitory signals. In this study, we show that anti-CD3 antibody-stimulated human T cells undergo time-limited controlled DNA synthesis, proliferation and interleukin-2 secretion, accompanied by PD-1 expression. In contrast, CD28SA-activated T cells demonstrate uncontrolled activation parameters including enhanced expression of LFA-1 and CCR5 but fail to express PD-1 on the cell surface. We demonstrate the functional relevance of the lack of PD-1 mediated regulatory mechanism in CD28SA-stimulated T cells. Our findings provide a molecular explanation for the dysregulated activation of CD28SA-stimulated T cells and also highlight the potential for the use of differential expression of PD-1 as a biomarker of safety for T cell immunostimulatory biologics.
Keywords: APC, antigen presenting cell; CCR5, C-C chemokine receptor type 5; CD28 superagonist; CD28SA, CD28 superagonist; CK2, casein kinase 2; CTLA-4, cytotoxic T-Lymphocyte Antigen 4; IFNγ, interferon gamma; IL-2, interleukin 2; LAG-3, Lymphocyte-activation gene 3; LFA-1, lymphocyte function-associated antigen 1; MFI, mean fluorescence intensity; PBMC, peripheral blood mononuclear cells; PD-1; PD-1, programmed cell death protein 1; PD-L1, programmed cell death-ligand 1; PTEN, phosphatase and tensin homolog; S-phase, synthesis phase; T cells; TCR, T cell receptor; TEMs, effector memory T cells; TGN1412; TIM-3, T cell immunoglobulin mucin 3; immunostimulatory biologics.