Recently, it was reported that RGI-2001, a CD1-binding synthetic derivative of alpha-galactosylceramide, activates and expands iNKT cells (166). cytokine receptors, such as IL-2 and CD25 (the subunit of the high affinity forms the IL-2 receptor) (36). Besides the basic biology, the blockade of one of these TCR-downstream signaling pathways, namely the NFAT calcium/calcineurin-dependent transduction pathway, was one of the first strategies explored to repress alloreactive T-cell activation after alloHCT in pioneered preclinical and clinical studies (37) and is still currently universally used as a standard approach for aGVHD prophylaxis (see below). Inhibition of the NF-(ICOS), OX40, and 4-1BB [nicely reviewed in (41, 42)] ( Figure 1 ). Their cognate ligands [namely B7 (-)-Epicatechin gallate ligands (CD86 or CD80), (B7RP-1), OX40L and 4-1BBL, respectively] are highly expressed at the surface of mature antigen presenting cells (APCs). Among all of the T-cell costimulatory receptors, the most extensively studied is CD28, which is constitutively expressed at the surface of naive T cells. Another B7 receptor, induced with T-cell activation, is (CTLA-4) that has similar structure to CD28 and acts as a competitor for CD80 and CD86 ligation, resulting in dowregulation of T-cell responses. Blockade of CD28/B7 interactions has been shown to attenuate alloreactive T-cell activation, induce tolerance to host alloantigens and to reduce aGVHD in studies and animal models of alloHCT (43C46). One of these approaches consists in using fusion proteins of the Fc region of human immunoglobulin with the extracellular domain of CTLA4 (CTLA4-Ig) (43, 45) and is tested for aGVHD prevention in clinical trials (see below). The third signal for sustained T-cell activation, acquisition of effector functions and survival is provided by cytokines [(mTOR) is another key signaling kinase in T cells that integrate an array of activating signals (including the three aforementioned signals of T-cell activation) and environmental cues to regulate cell survival, growth, proliferation, differentiation, and metabolism (56). Inhibition of mTOR Complex 1 (mTORC1) has demonstrated efficacy against aGVHD in preclinical models (56C58) and has been explored as GVHD prevention in clinical trials for several years (see below). Over the past decade, it has become increasingly clear that metabolic reprogramming of the T cell is required to enable the transition from a naive T cell to a proliferative and differentiated T cell that will drive immune effector functions and mediate aGVHD. Studies have reported that effector T cells use multiple metabolic pathways (glycolysis, oxidative phosphorylation, fatty acid oxidation, glutaminolysis) to keep the pace with high energy demands during aGVHD, (59, 60). Furthermore, the EMR2 metabolic demand of different T cell subsets is likely not identical. A key event in the initiation phase of aGVHD (-)-Epicatechin gallate is the interaction of CD4+ and CD8+ donor T cells with activated APCs (cross-presentation for the latter) that provide the three aforementioned signals. During the initiation phase of aGVHD, most of the APCs are host-derived hematopoietic APCs and host non-hematopoietic APCs (intestinal epithelial cells, keratinocytes, myofibroblasts…) (61, 62). By expressing pattern recognition receptors (PRR) such as Toll-like (TLR) and nucleotide oligomerization domain (NOD)-like receptors, innate immune cells and some epithelial cells are able to detect danger signals such as sterile DAMP (molecules, which are released from dying cells or disrupted extracellular (-)-Epicatechin gallate matrix) and PAMP (molecules, which can be released from invasive bacteria, fungi or viruses at the epithelial surfaces). After alloHCT, an increased number of DAMP and PAMP molecules can be released as a consequence of cytotoxic conditioning regimen or aGVHD [reviewed in (63)]. After alloHCT, several studies have demonstrated that host exposure.
