Beatriz M. Carreno

PD-L2 is a second ligand for PD-1 and inhibits T cell activation.

Programmed death 1 (PD-1)–deficient mice develop a variety of autoimmune-like diseases, which suggests that this immunoinhibitory receptor plays an important role in tolerance. We identify here PD-1 ligand 2 (PD-L2) as a second ligand for PD-1 and compare the function and expression of PD-L1 and PD-L2. Engagement of PD-1 by PD-L2 dramatically inhibits T cell receptor (TCR)-mediated proliferation and cytokine production by CD4+ T cells. At low antigen concentrations, PD-L2–PD-1 interactions inhibit strong B7-CD28 signals. In contrast, at high antigen concentrations, PD-L2–PD-1 interactions reduce cytokine production but do not inhibit T cell proliferation. PD-L–PD-1 interactions lead to cell cycle arrest in G0/G1 but do not increase cell death. In addition, ligation of PD-1 + TCR leads to rapid phosphorylation of SHP-2, as compared to TCR ligation alone. PD-L expression was up-regulated on antigen-presenting cells by interferon γ treatment and was also present on some normal tissues and tumor cell lines. Taken together, these studies show overlapping functions of PD-L1 and PD-L2 and indicate a key role for the PD-L–PD-1 pathway in regulating T cell responses.

PD-1:PD-L inhibitory pathway affects both CD4(+) and CD8(+) T cells and is overcome by IL-2.

Programmed death‐1 (PD‐1) is an immunoreceptor tyrosine‐based inhibitory motif (ITIM)‐containing receptor expressed upon T cell activation. PD‐1–/– animals develop autoimmune diseases, suggesting an inhibitory role for PD‐1 in immune responses. Members of the B7 family, PD‐L1 and PD‐L2, are ligands for PD‐1. This study examines the functional consequences of PD‐1:PD‐L engagementon murine CD4 and CD8 T cells and shows that these interactions result in inhibition of proliferation and cytokine production. T cells stimulated with anti‐CD3/PD‐L1.Fc‐coated beads display dramatically decreased proliferation and IL‐2 production, while CSFE analysis shows fewer cells cycling and a slower division rate. Costimulation with soluble anti‐CD28 mAb can overcome PD‐1‐mediated inhibition by augmenting IL‐2 production. However, PD‐1:PD‐L interactions inhibit IL‐2 production even in the presence of costimulation and, thus, after prolonged activation, the PD‐1:PD‐L inhibitory pathway dominates. Exogenous IL‐2 is able to overcome PD‐L1‐mediated inhibition at all times, indicating that cells maintain IL‐2 responsiveness. Experiments using TCR transgenic CD4+ or CD8+ T cells stimulated with antigen‐presenting cells expressing PD‐L1 show that both T cell subsets are susceptible to this inhibitory pathway. However, CD8+ T cells may be more sensitive to modulation by the PD‐1:PD‐L pathway because of their intrinsic inability to produce significant levels of IL‐2.

Cardiovascular toxicity and titin cross-reactivity of affinity-enhanced T cells in myeloma and melanoma

An obstacle to cancer immunotherapy has been that the affinity of T-cell receptors (TCRs) for antigens expressed in tumors is generally low. We initiated clinical testing of engineered T cells expressing an affinity-enhanced TCR against HLA-A*01-restricted MAGE-A3. Open-label protocols to test the TCRs for patients with myeloma and melanoma were initiated. The first two treated patients developed cardiogenic shock and died within a few days of T-cell infusion, events not predicted by preclinical studies of the high-affinity TCRs. Gross findings at autopsy revealed severe myocardial damage, and histopathological analysis revealed T-cell infiltration. No MAGE-A3 expression was detected in heart autopsy tissues. Robust proliferation of the engineered T cells in vivo was documented in both patients. A beating cardiomyocyte culture generated from induced pluripotent stem cells triggered T-cell killing, which was due to recognition of an unrelated peptide derived from the striated muscle-specific protein titin. These patients demonstrate that TCR-engineered T cells can have serious and not readily predictable off-target and organ-specific toxicities and highlight the need for improved methods to define the specificity of engineered TCRs.

Engagement of Glucocorticoid-Induced TNFR Family-Related Receptor on Effector T Cells by its Ligand Mediates Resistance to Suppression by CD4+CD25+ T Cells

Nonactivated CD4+CD25+ regulatory T cells constitutively express glucocorticoid-induced TNFR family-related receptor (GITR), a TNFR family member whose engagement was presumed to abrogate regulatory T cell-mediated suppression. Using GITR−/− mice, we report that GITR engagement on CD25−, not CD25+ T cells abrogates T cell-mediated suppression. Mouse APCs constitutively express GITR ligand (GITR-L), which is down-regulated following TLR signaling in vivo. Although GITR−/−CD25− T cells were capable of mounting proliferative responses, they were incapable of proliferation in the presence of physiological numbers of CD25+ T cells. Thus, GITR-L provides an important signal for CD25− T cells, rendering them resistant to CD25+-mediated regulation at the initiation of the immune response. The down-regulation of GITR-L by inflammatory stimuli may enhance the susceptibility of effector T cells to suppressor activity during the course of an infectious insult.

Identification of an Interleukin 17F/17A Heterodimer in Activated Human CD4+ T Cells

IL-17F and IL-17A are members of the IL-17 pro-inflammatory cytokine family. IL-17A has been implicated in the pathogenesis of autoimmune diseases. IL-17F is a disulfide-linked dimer that contains a cysteine-knot motif. We hypothesized that IL-17F and IL-17A could form a heterodimer due to their sequence homology and overlapping pattern of expression. We evaluated the structure of recombinant IL-17F and IL-17A proteins, as well as that of natural IL-17F and IL-17A derived from activated human CD4+ T cells, by enzyme-linked immunosorbent assay, immunoprecipitation followed by Western blotting, and mass spectrometry. We find that both IL-17F and IL-17A can form both homodimeric and heterodimeric proteins when expressed in a recombinant system, and that all forms of the recombinant proteins have in vitro functional activity. Furthermore, we find that in addition to the homodimers of IL-17F and IL-17A, activated human CD4+ T cells also produce the IL-17F/IL-17A heterodimer. These data suggest that the IL-17F/IL-17A heterodimer may contribute to the T cell-mediated immune responses.

Blockade of Programmed Death-1 Engagement Accelerates Graft-Versus-Host Disease Lethality by an IFN-γ-Dependent Mechanism

Acute graft-vs-host disease (GVHD) is influenced by pathways that can enhance or reduce lethality by providing positive or negative signals to donor T cells. To date, the only reported pathway to inhibit GVHD is the CTLA-4:B7 pathway. Because absence of the programmed death-1 (PD-1) pathway has been implicated in a predisposition to autoimmunity and hence a lack of negative signals, the effect of PD-1 pathway blockade on GVHD was explored using several distinct approaches. In each, GVHD lethality was markedly accelerated. Coblockade of CTLA-4 and PD-1 was additive in augmenting GVHD, indicating that these pathways are not fully redundant. Although neither perforin nor Fas ligand expression was required for GVHD enhancement, donor IFN-γ production was required for optimal GVHD acceleration in the absence of PD-1 ligation. These data indicate that PD-1 ligation down-regulates GVHD through modulation of IFN-γ production and suggest a novel therapeutic target for inhibiting GVHD lethality.

Program Death-1 Engagement Upon TCR Activation Has Distinct Effects on Costimulation and Cytokine-Driven Proliferation: Attenuation of ICOS, IL-4, and IL-21, But Not CD28, IL-7, and IL-15 Responses

The program death 1 (PD-1) receptor and its ligands, PD-1 ligand (PD-L)1 and PD-L2, define a novel regulatory pathway with potential inhibitory effects on T, B, and monocyte responses. In the present study, we show that human CD4+ T cells express PD-1, PD-L1, and PD-L2 upon activation, and Abs to the receptor can be agonists or antagonists of the pathway. Under optimal conditions of stimulation, ICOS but not CD28 costimulation can be prevented by PD-1 engagement. IL-2 levels induced by costimulation are critical in determining the outcome of the PD-1 engagement. Thus, low to marginal IL-2 levels produced upon ICOS costimulation account for the greater sensitivity of this pathway to PD-1-mediated inhibition. Interestingly, exogenous IL-2, IL-7, and IL-15 but not IL-4 and IL-21 can rescue PD-1 inhibition, suggesting that among these cytokines only those that activate STAT5 can rescue PD-1 inhibition. As STAT5 has been implicated in the maintenance of IL-2Rα expression, these results suggest that IL-7 and IL-15 restore proliferation under conditions of PD-1 engagement by enhancing high-affinity IL-2R expression and hence, IL-2 responsiveness.

CTLA-4 (CD152) Can Inhibit T cell Activation by Two Different Mechanisms Depending on Its Level of Cell Surface Expression

CTLA-4 (CD152) engagement results in down-regulation of T cell activation. Two mechanisms have been postulated to explain CTLA-4 inhibition of T cell activation: negative signaling and competitive antagonism of CD28:B7-mediated costimulation. We assessed the contributions of these two mechanisms using a panel of T cell lines expressing human CTLA-4 with mutations in the cytoplasmic region. Under conditions of B7-independent costimulation, inhibition of IL-2 production following CTLA-4 engagement required the CTLA-4 cytoplasmic region. In contrast, under B7-dependent costimulation, inhibition of IL-2 production by CTLA-4 engagement was directly proportional to CTLA-4 cell surface levels and did not require its cytoplasmic region. Thus, CTLA-4 down-regulates T cell activation by two different mechanisms—delivery of a negative signal or B7 sequestration—that are operational depending on the levels of CTLA-4 surface expression. These two mechanisms may have distinct functional outcomes: rapid inhibition of T cell activation or induction of T cell anergy.

The Human IL-17F/IL-17A Heterodimeric Cytokine Signals through the IL-17RA/IL-17RC Receptor Complex

IL-17A and IL-17F, produced by the Th17 CD4+ T cell lineage, have been linked to a variety of inflammatory and autoimmune conditions. We recently reported that activated human CD4+ T cells produce not only IL-17A and IL-17F homodimers but also an IL-17F/IL-17A heterodimeric cytokine. All three cytokines can induce chemokine secretion from bronchial epithelial cells, albeit with different potencies. In this study, we used small interfering RNA and Abs to IL-17RA and IL-17RC to demonstrate that heterodimeric IL-17F/IL-17A cytokine activity is dependent on the IL-17RA/IL-17RC receptor complex. Interestingly, surface plasmon resonance studies indicate that the three cytokines bind to IL-17RC with comparable affinities, whereas they bind to IL-17RA with different affinities. Thus, we evaluated the effect of the soluble receptors on cytokine activity and we find that soluble receptors exhibit preferential cytokine blockade. IL-17A activity is inhibited by IL-17RA, IL-17F is inhibited by IL-17RC, and a combination of soluble IL-17RA/IL-17RC receptors is required for inhibition of the IL-17F/IL-17A activity. Altogether, these results indicate that human IL-17F/IL-17A cytokine can bind and signal through the same receptor complex as human IL-17F and IL-17A. However, the distinct affinities of the receptor components for IL-17A, IL-17F, and IL-17F/IL-17A heterodimer can be exploited to differentially affect the activity of these cytokines.

Duplication of primate and rodent B7-H3 immunoglobulin V- and C-like domains: divergent history of functional redundancy and exon loss

B7-H3 is a novel protein structurally related to the B7 family of ligands by the presence of a single set of immunoglobulin-V-like and immunoglobulin-C-like (VC) domains. By multiplex PCR, the dominantly expressed form of human B7-H3 was found to be a splice variant containing tandemly duplicated VC domains (VCVC). In contrast, mouse B7-H3 cDNA contained only one single VC form due to an exon structure corresponding to V-(pseudoexon C)-(pseudoexon V)-C. Comparisons of human, monkey, mouse, and hamster genomic B7-H3 reveal that primates, but not rodents, exhibited a higher degree of intramolecular sequence similarity between VC duplications than between molecules. Both VC and VCVC forms of human B7-H3 inhibited CD4(+) T cell proliferation and downregulated cytokine production upon TCR activation. These results suggest independent, but convergent, paths of B7-H3 active domain duplication followed by divergent histories of exon degeneration in rodents and exon maintenance by humans.