The basis of its stimulatory function is not well understood but is thought to occur through an as yet unidentified receptor.43 In contrast, many of the studies published to date have focused on its inhibitory function, which occurs by signaling through programmed death-1 (PD-1). B7-H1 shares this receptor with the related B7 family member B7-DC. B7-DC appears to have higher affinity for PD-1 than B7-H1,47 but its expression is much more limited than B7-H1, and is found predominantly on macrophages and DCs following cytokine

induction.48 Like B7-H1, CP-690550 cell line B7-DC exhibits dual inhibitory and stimulatory functions, but its restricted expression to APCs suggests that it primarily affects the priming stage of immune responses.49,50 PD-1 is expressed on activated T cells, B cells, and cells of the myeloid lineage and contains two cytoplasmic signaling domains consisting of an intracellular tyrosine inhibitory motif (ITIM) and an intracellular tyrosine switch motif (ITSM).51In vitro studies have suggested that the ITSM on PD-1 is critical for its inhibitory activity

and acts by recruiting SHP-1 and/or SHP-2 phosphatases which then interfere with CD28 signaling by preventing activation of phosphoinositide 3-kinase (PI3K) activation – a critical enzyme in CD28 signaling.52–54 The ultimate effect of PD-1 ligation on self-reactive T cells can be apoptosis or anergy. This regulatory pathway appears essential, as peripheral tolerance to some MHC class I-restricted self-antigens requires PD-1.55,56 In addition, genetic deletion of PD-1 results in severe autoimmunity click here because of the loss of peripheral tolerance of self-reactive T cells.57,58 Blocking PD-1 accelerated the onset and worsened the severity of both spontaneous and induced autoimmune disease.59,60 Similarly, accumulation of self-reactive T cells occurs when B7-H1 and B7-DC are depleted, resulting in increased susceptibility to induced autoimmune disease.46,61 T-cell exhaustion, a state of gradually acquired unresponsiveness to antigen,

can also occur when PD-1 is chronically ligated by B7-H1, although this phenomenon has only been implicated in the failure to clear infection, and it is not certain whether this occurs in tolerance to self-antigen.62 Finally, B7-H1 has also many recently been recognized to have a novel role in inducing differentiation of Tregs from naïve CD4+ T cells.63,64 There is also evidence that binding of PD-1 to B7-H1 or B7-DC can induce signaling through their intracellular domains, back into the APC; although the biological roles of this reverse signaling are less clear. Tumor cells receiving this signal become resistant to CTL-induced cytolysis, without the requirement for PD-1 signaling into the T cell.65 The signaling mechanism for this remains enigmatic but does require the approximately 30 amino acid, evolutionarily conserved cytoplasmic domain of B7-H1. Reverse signaling appears to occur through B7-DC as well.