Programmed death 1 (PD-1) and its ligands, PD-L1 and PD-L2, deliver inhibitory signals that regulate the balance between T cell activation, tolerance, and immunopathology. Immune responses to foreign and self-antigens require specific and balanced responses to clear pathogens and tumors and yet maintain tolerance. Induction and maintenance of T cell tolerance requires PD-1, and its ligand PD-L1 on nonhematopoietic cells can limit effector T cell responses and protect tissues from immune-mediated tissue damage. The PD-1:PD-L pathway also has been usurped by microorganisms and tumors to attenuate antimicrobial or tumor immunity and facilitate chronic infection and tumor survival. The identification of B7-1 as an additional binding partner for PD-L1, together with the discovery of an inhibitory bidirectional interaction between PD-L1 and B7-1, reveals new ways the B7:CD28 family regulates T cell activation and tolerance. In this review, we discuss current understanding of the immunoregulatory functions of PD-1 and its ligands and their therapeutic potential.

PD-1 and its ligands in tolerance and immunity

▪ Abstract The discovery of new functions for the original B7 family members, together with the identification of additional B7 and CD28 family members, have revealed new ways in which the B7:CD28 family regulates T cell activation and tolerance. B7-1/B7-2:CD28 interactions not only promote initial T cell activation but also regulate self-tolerance by supporting CD4+CD25+ T regulatory cell homeostasis. CTLA-4 can exert its inhibitory effects in both B7-1/B7-2 dependent and independent fashions. B7-1 and B7-2 can signal bidirectionally by engaging CD28 and CTLA-4 on T cells and by delivering signals into B7-expressing cells. The five new B7 family members, ICOS ligand, PD-L1 (B7-H1), PD-L2 (B7-DC), B7-H3, and B7-H4 (B7x/B7-S1) are expressed on professional antigen-presenting cells as well as on cells within nonlymphoid organs, providing new means for regulating T cell activation and tolerance in peripheral tissues. The new CD28 families members, ICOS, PD-1, and BTLA, are inducibly expressed on T cells, and...

The b7 family revisited

Cellular processes can only be understood as the dynamic interplay of molecules. There is a need for techniques to monitor interactions of endogenous proteins directly in individual cells and tissues to reveal the cellular and molecular architecture and its responses to perturbations. Here we report our adaptation of the recently developed proximity ligation method to examine the subcellular localization of protein-protein interactions at single-molecule resolution. Proximity probes-oligonucleotides attached to antibodies against the two target proteins-guided the formation of circular DNA strands when bound in close proximity. The DNA circles in turn served as templates for localized rolling-circle amplification (RCA), allowing individual interacting pairs of protein molecules to be visualized and counted in human cell lines and clinical specimens. We used this method to show specific regulation of protein-protein interactions between endogenous Myc and Max oncogenic transcription factors in response to interferon-gamma (IFN-gamma) signaling and low-molecular-weight inhibitors.

Direct observation of individual endogenous protein complexes in situ by proximity ligation

Since the discovery of interferons (IFNs), considerable progress has been made in describing the nature of the cytokines themselves, the signalling components that direct the cell response and their antiviral activities. Gene targeting studies have distinguished four main effector pathways of the IFN-mediated antiviral response: the Mx GTPase pathway, the 2',5'-oligoadenylate-synthetase-directed ribonuclease L pathway, the protein kinase R pathway and the ISG15 ubiquitin-like pathway. As discussed in this Review, these effector pathways individually block viral transcription, degrade viral RNA, inhibit translation and modify protein function to control all steps of viral replication. Ongoing research continues to expose additional activities for these effector proteins and has revealed unanticipated functions of the antiviral response.

Interferon-inducible antiviral effectors

Immune-related adverse events with immune checkpoint blockade: a comprehensive review

Systemic lupus erythematosus (SLE, OMIM 152700) is a complex autoimmune disease that affects 0.05% of the Western population, predominantly women. A number of susceptibility loci for SLE have been suggested in different populations, but the nature of the susceptibility genes and mutations is yet to be identified. We previously reported a susceptibility locus (SLEB2) for Nordic multi-case families. Within this locus, the programmed cell death 1 gene (PDCD1, also called PD-1) was considered the strongest candidate for association with the disease. Here, we analyzed 2,510 individuals, including members of five independent sets of families as well as unrelated individuals affected with SLE, for single-nucleotide polymorphisms (SNPs) that we identified in PDCD1. We show that one intronic SNP in PDCD1 is associated with development of SLE in Europeans (found in 12% of affected individuals versus 5% of controls; P = 0.00001, r.r. (relative risk) = 2.6) and Mexicans (found in 7% of affected individuals versus 2% of controls; P = 0.0009, r.r. = 3.5). The associated allele of this SNP alters a binding site for the runt-related transcription factor 1 (RUNX1, also called AML1) located in an intronic enhancer, suggesting a mechanism through which it can contribute to the development of SLE in humans.

A regulatory polymorphism in PDCD1 is associated with susceptibility to systemic lupus erythematosus in humans

The Myc proto-oncoprotein family is considered to play an important role in the control of cell growth and differentiation. It appears that the interaction of Myc with its heterodimeric partner Max is essential for Myc function. Recently two other partners of Max, called Mad and Mxi1, have been identified. In an effort to gain insight into the network of these four proteins we have started to analyse the expression of the c-myc, max, mad and mxi1 genes at the mRNA level during hematopoietic cell growth and differentiation. In the human myeloid cell lines U-937, HL-60 and ML-1 c-myc expression was down-regulated as shown previously after induction of differentiation, whereas the expression of max was only slightly affected. In contrast to these two genes the expression of mad was induced upon differentiation in the three cell lines by TPA, retinoic acid, vitamin D3, dimethyl sulfoxide, and interferon-gamma and remained elevated for at least 3 days. A kinetic analysis showed that the induction of mad in U-937 in response to TPA was rapid (15 min) and at least in part transcriptional, reminiscent of immediate early genes. The expression of mxi1 was induced in U-937 by some inducers but not in HL-60 or ML-1. Its induction occurred slowly, peaking around 48 h. These analysis thus suggest that the expression of mad and c-myc is inversely regulated during induced hematopoietic differentiation.

Expression of mad, mxi1, max and c-myc during induced differentiation of hematopoietic cells: opposite regulation of mad and c-myc.

Monocytes and Macrophages

The proteasome pathway is an important target for anticancer drug development. Here, we identify the antialcoholism drug disulfiram and its analogue pyrrolidine dithiocarbamate (PDTC) as inhibitors of the 26S proteasome activity in a cell-based screening assay. As expected for proteasome inhibitors, these compounds also inhibited TNF-alpha-induced nuclear factor-KappaB (NF-KappaB) translocation and were cytotoxic. Disulfiram was more cytotoxic against chronic lymphocytic leukemia cells compared to peripheral blood mononuclear cells (PBMC) at clinically achievable concentrations. Proteasome and NF-KappaB inhibition were achieved with a potency in the same range as that of the clinically used proteasome inhibitor bortezomib. Disulfiram was also able to induce accumulation of p27(Kip1) and to prolong the half-life of c-Myc, both targets for proteasome-dependent degradation. It is concluded that the previously observed antitumoral and NF-KappaB inhibiting activity of disulfiram and PDTC could be attributed to their inhibition of the 26S proteasome.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/ijc.21534

Inhibition of proteasome activity, nuclear factor-KB translocation and cell survival by the antialcoholism drug disulfiram

1,25 alpha-Dihydroxicholecalciferol (VitD3) and retinoic acid (RA) are important regulators of the proliferation and differentiation of several cell types. This paper describes how the expression of the monocyte-macrophage Ag, CD14, and the low affinity Fc receptor for IgE, CD23, were inversely regulated during VitD3- and RA-induced monocytic differentiation of human U-937 monoblasts. PMA induced the expression of both CD14 and CD23 mRNA and protein. Exposure to VitD3 rapidly induced the de novo expression of CD14 mRNA and protein. The addition of cycloheximide completely blocked the VitD3 induction of CD14 mRNA expression, indicating that the induction was dependent on ongoing protein synthesis. While inducing CD14 expression, VitD3 concomitantly suppressed the basal, PMA-, and RA-inducible CD23 expression in a dose-dependent manner. In contrast, U-937 cells induced by RA strongly increased their expression of CD23 mRNA and protein, whereas they completely lacked detectable CD14 cell surface or mRNA expression. Furthermore, the VitD3- and the PMA-induced CD14 expression was inhibited as a temporal consequence of the RA-induced differentiation. The results suggest that there exists a functional antagonism between VitD3 and RA that may have important implications for the regulation of certain immune and inflammatory responses through their inverse effects on CD14 and CD23 gene expression.

Functional antagonism between vitamin D3 and retinoic acid in the regulation of CD14 and CD23 expression during monocytic differentiation of U-937 cells.

Fredrik Öberg

Papers

A regulatory polymorphism in PDCD1 is associated with susceptibility to systemic lupus erythematosus in humans

Expression of mad, mxi1, max and c-myc during induced differentiation of hematopoietic cells: opposite regulation of mad and c-myc.

Monocytes and Macrophages

Inhibition of proteasome activity, nuclear factor-KB translocation and cell survival by the antialcoholism drug disulfiram

Functional antagonism between vitamin D3 and retinoic acid in the regulation of CD14 and CD23 expression during monocytic differentiation of U-937 cells.