The PD-1–PD-L pathway in immunological tolerance Taku Okazaki 1,2 and Tasuku Honjo 2 1 21st Century Center of Excellence Program, Graduate School of Medicine, Kyoto University, Yoshida-Konoe, Sakyo-ku, Kyoto, 606-8501, Japan 2 Department of Immunology and Genomic Medicine, Graduate School of Medicine, Kyoto University, Yoshida-Konoe, Sakyo-ku, Kyoto, 606-8501, Japan Since the first observation of spontaneous autoimmune diseases in programmed cell death 1 (PD-1) knockout mice, PD-1 has been postulated to have essential roles in the regulation of autoimmunity but the precise mech- anism was largely unknown. Recent studies clearly demonstrated that PD-1 has dual roles in immunological tolerance: induction and maintenance of peripheral tolerance. PD-1 ligands (PD-Ls) on antigen-presenting cells have been shown to switch off autoreactive T cells and induce peripheral tolerance, whereas those on parenchymal cells prevent tissue destruction by sup- pressing effector T cells to maintain tolerance. In addition, PD-1 and other immuno-inhibitory receptors have been shown to collaborate in the regulation of tolerance. Here, we review recent studies on the role of PD-1 in immunological tolerance and discuss possible clinical applications of PD-1 manipulation. PD-1 and PD-1 ligands Programmed cell death 1 (PD-1, encoded by Pdcd1) was isolated, using subtractive hybridization, as a molecule whose expression was enhanced in a thymic T-cell line by apoptotic stimuli [1]. The extracellular region of PD-1 consists of a single IgV-like domain and its cytoplasmic region contains an immunoreceptor tyrosine-based inhibi- tory motif (ITIM) and an immunoreceptor tyrosine-based switch motif (ITSM). PD-1 belongs to the CD28 family and shares 23% amino acid sequence homology with cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4). The expression of CTLA-4 is restricted to T cells, whereas the expression of PD-1 can be induced not only in T cells but also in B cells and myeloid cells, suggesting PD-1 has broader roles in immune regulation [2,3]. Following antigen stimulation, PD-1 recruits the protein tyrosine phosphatase src homology 2 domain-containing tyrosine phosphatase 2 (SHP-2) to ITSM but not to ITIM and dephosphorylates effector molecules downstream of anti- gen receptor, for example, Syk and phosphoinositide 3-kinase (PI3K) [4–6]. The ligands for PD-1 (PD-Ls) are PD-L1 (also known as B7-H1) and PD-L2 (B7-DC), which were identified by database searches on the basis of their similarity with B7 molecules [7–10]. PD-L1 and PD-L2 are type I transmembrane proteins with IgV- and IgC-like domains in the extracellular region. PD-L1 is constitutively expressed in T cells, B cells, macrophages and dendritic cells (DCs), and further upregulated following activation of these cells [11,12]. The expression of PD-L1 is also detected on non-lymphoid cells, for example, endothelial cells in the heart, b cells in the pancreas, glial cells in inflamed brain and muscle cells [13–17]. By contrast, PD- L2 expression is regulated more tightly; it is observed on activated macrophages and DCs [11,12]. PD-L1 and PD- L2 are also expressed in tumor cells from various origins including ovary, esophagus, kidney and brain [18–24], an expression pattern which is in stark contrast to the lymphoid-specific expression of ligands for CD28 and CTLA-4 (B7.1 and B7.2). Therefore, the PD-1–PD-L pathway has been postulated to regulate the immune response in both lymphoid and non-lymphoid organs. The binding of PD-L1 and PD-L2 to PD-1 inhibits lymphocyte activation. However, several groups have reported positive effects of PD-L1 and PD-L2 on T cells [8,10,25,26]. Recently, Yamazaki et al. [27] reported that PD-L1 augmented T-cell proliferation by inhibiting the interferon (IFN) g-induced production of nitric oxide. Although it is not yet clear whether the positive effects of PD-L1 and PD-L2 can be explained solely by the inhibition of negative signaling or whether there is another stimulatory receptor for PD-Ls, we focus on the negative effects of PD-L1 and PD-L2 in this review. Spontaneous autoimmune diseases in PD-1-deficient mice The negative co-stimulatory function of PD-1 was initially suggested because PD-1 knockout (Pdcd1 K/K ) mice developed spontaneous autoimmune diseases [28,29]. C57BL/6-Pdcd1 K/K mice develop lupus-like glomerulone- phritis and arthritis with deposition of IgG3 and C3 in the glomeruli [29]. Interestingly, different target organs are affected by the autoimmune response when the knockout mice are backcrossed to different strains of mice, as summarized in Table 1. BALB/c-Pdcd1 K/K mice develop a fatal dilated cardiomyopathy with concomitant production of autoantibodies against cardiac troponin I [30–32]. The effect of PD-1 on immunological tolerance has been elucidated using transgenic mice carrying 2C, a T-cell receptor (TCR) that recognizes H-2L d -bearing cells [29]. In Corresponding author: Honjo, T. (honjo@mfour.med.kyoto-u.ac.jp). Available online 24 February 2006 Review TRENDS in Immunology Vol.27 No.4 April 2006 www.sciencedirect.com 1471-4906/$ - see front matter Q 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.it.2006.02.001