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.
Therapeutic vaccination is a potentially promising strategy to enhance T cell immunity and viral control in chronically infected individuals. However, therapeutic vaccination approaches have fallen short of expectations, and effective boosting of antiviral T cell responses has not always been observed. One of the principal reasons for the limited success of therapeutic vaccination is that virus-specific T cells become functionally exhausted during chronic infections. We now provide a novel strategy for enhancing the efficacy of therapeutic vaccines. In this study, we show that blocking programmed death (PD)-1/PD-L1 inhibitory signals on exhausted CD8(+) T cells, in combination with therapeutic vaccination, synergistically enhances functional CD8(+) T cell responses and improves viral control in mice chronically infected with lymphocytic choriomeningitis virus. This combinatorial therapeutic vaccination was effective even in the absence of CD4(+) T cell help. Thus, our study defines a potent new approach to augment the efficacy of therapeutic vaccination by blocking negative signals. Such an approach may have broad applications in developing treatment strategies for chronic infections in general, and perhaps also for tumors.
The development of airway hyperreactivity (AHR), a cardinal feature of asthma, requires the presence of invariant NKT (iNKT) cells. In a mouse model of asthma, we demonstrated that the induction of AHR required ICOS costimulation of iNKT cells. ICOS was highly expressed on both naive and activated iNKT cells, and expression of ICOS was greater on the CD4(+) iNKT than on CD4(-) iNKT cells. Furthermore, the number of CD4(+) iNKT cells was significantly lower in spleens and livers of ICOS(-/-) and ICOSL(-/-) mice, and the remaining iNKT cells in ICOS(-/-) mice were dysfunctional and failed to reconstitute AHR when adoptively transferred into iNKT cell-deficient Jalpha18(-/-) mice. In addition, direct activation of iNKT cells with alpha-GalCer, which induced AHR in wild-type mice, failed to induce AHR in ICOS(-/-) mice. The failure of ICOS(-/-) iNKT cells to induce AHR was due in part to an inability of the ICOS(-/-) iNKT cells to produce IL-4 and IL-13 on activation. Moreover, survival of wild-type iNKT cells transferred into ICOSL(-/-) mice was greatly reduced due to the induction of apoptosis. These results indicate that ICOS costimulation plays a major role in induction of AHR by iNKT cells and is required for CD4(+) iNKT cell function, homeostasis, and survival in the periphery.
Suppression of T-cell responses by host-derived regulatory factors is a key event leading to viral persistence. Antibody blockade of either IL-10 or programmed death-ligand 1 (PD-L1) during viral persistence enhances T-cell function and reduces viral titers. Because blockade of these immunoregulatory networks represents a powerful approach to establish immune control during persistent infection, it is important to determine whether these immunoinhibitory factors act independently or jointly and if combined blockade of these factors further enhances T-cell immunity and viral clearance. Herein, we demonstrate that the IL-10 and PD-L1 immunosuppressive pathways are mechanistically distinct. As a result, simultaneous blockade of IL-10 and PD-L1 was significantly more effective in restoring antiviral T-cell responses than blockade of either alone, and led to substantially enhanced control of an established persistent viral infection. Thus, combinatorial blockade of multiple immune-regulatory molecules may ultimately restore the T-cell responses required to tip the balance from viral persistence to immune-mediated control or elimination of persistent infection.
Numerous studies have pointed to the role of programmed death-1 ligand 1 (PD-L1) in regulating tolerance, chronic infection, and tumor immunity. Recently, we have identified murine B7-1 as a new binding partner for murine PD-L1. Human and mouse B7-1 share only 46% identity, leading us to question whether human B7-1 and PD-L1 can participate in a similar interaction. Here we show that human B7-1 can interact with human PD-L1 with affinity greater than that of B7-1 with CD28, but less than that of B7-1 with CTLA-4 or of PD-L1 with PD-1. We characterize a series of anti-human PD-L1 monoclonal antibodies and identify antibodies that can block interactions of PD-L1 with B7-1, PD-1, or both. Since PD-L1 and CD28 on T cells may compete for B7-1 as a binding partner and CD8 T cells may express high or low levels of CD28, we examined when PD-L1 and CD28 are co-expressed on CD8 T cells. We compared the time-course and extent of PD-L1 induction on CD8 CD28high versus CD28low T cells following stimulation with anti-CD3. We show that PD-L1 is induced to a higher level on CD28high T cells than on CD28low T cells upon activation. These results suggest that PD-L1 may play an important and undervalued role on human T cells.
Marginal zone (MZ) B cells contain a large number of autoreactive clones and the expansion of this compartment has been associated with autoimmunity. MZ B cells also efficiently transport blood-borne antigen to the follicles where they activate T cells and differentiate into plasma cells. Using the B6.NZM2410.Sle1.Sle2.Sle3 (B6.TC) model of lupus, we show that the IgM+ CD1d(hi)/MZ B-cell compartment is expanded, and a large number of them reside inside the follicles. Contrary to the peripheral B-cell subset distribution and their activation status, the intrafollicular location of B6.TC IgM+ CD1d(hi)/MZ B cells depends on both bone marrow- and stromal-derived factors. Among the factors responsible for this intrafollicular location, we have identified an increased response to CXCL13 by B6.TC MZ B cells and a decreased expression of VCAM-1 on stromal cells in the B6.TC MZ. However, the reduced number of MZ macrophages observed in B6.TC MZs was independent of the IgM+ CD1d(hi)/B-cell location. B7-2 but not B7-1 deficiency restored IgM+ CD1d(hi)/MZ B-cell follicular exclusion in B6.TC mice, and it correlated with tolerance to dsDNA and a significant reduction of autoimmune pathology. These results suggest that follicular exclusion of IgM+ CD1d(hi)/MZ B cells is an important B-cell tolerance mechanism, and that B7-2 signaling is involved in breaching this tolerance checkpoint.
MRL/MpJ-Fas(lpr) (MRL-Fas(lpr)) mice develop a spontaneous T cell and macrophage-dependent autoimmune disease that shares features with human lupus. Interactions via the programmed death 1/programmed death ligand 1 (PD-1/PD-L1) pathway down-regulate immune responses and provide a negative regulatory checkpoint in mediating tolerance and autoimmune disease. Therefore, we tested the hypothesis that the PD-1/PD-L1 pathway suppresses lupus nephritis and the systemic illness in MRL-Fas(lpr) mice. For this purpose, we compared kidney and systemic illness (lymph nodes, spleen, skin, lung, glands) in PD-L1 null (-/-) and PD-L1 intact (wild type, WT) MRL-Fas(lpr) mice. Unexpectedly, PD-L1(-/-);MRL-Fas(lpr) mice died as a result of autoimmune myocarditis and pneumonitis before developing renal disease or the systemic illness. Dense infiltrates, consisting of macrophage and T cells (CD8(+) > CD4(+)), were prominent throughout the heart (atria and ventricles) and localized specifically around vessels in the lung. In addition, once disease was evident, we detected heart specific autoantibodies in PD-L1(-/-);MRL-Fas(lpr) mice. This unique phenotype is dependent on MRL-specific background genes as PD-L1(-/-);MRL(+/+) mice lacking the Fas(lpr) mutation developed autoimmune myocarditis and pneumonitis. Notably, the transfer of PD-L1(-/-);MRL(+/+) bone marrow cells induced myocarditis and pneumonitis in WT;MRL(+/+) mice, despite a dramatic up-regulation of PD-L1 expression on endothelial cells in the heart and lung of WT;MRL(+/+) mice. Taken together, we suggest that PD-L1 expression is central to autoimmune heart and lung disease in lupus-susceptible (MRL) mice.
Evidence from many human and rodent studies has established that T lymphocytes enhance inflammation in atherosclerotic plaques and contribute to lesion progression and remodeling. Recent work also indicates that regulatory T cells are important in limiting proatherogenic T-cell responses. Given the important role of T cells in atherosclerosis, there is a need to fully understand how proatherogenic T cells are activated and regulated. Antigen-dependent activation of naïve T cells, leading to clonal expansion and effector T-cell differentiation, and effector and memory T cells, is enhanced by signals provided by costimulatory molecules expressed by antigen presenting cells, which bind to receptors on the T cells. In addition, T-cell responses to antigen are negatively regulated by coinhibitory molecules expressed by antigen-presenting cells, which bind to receptors on T cells. Two major families of costimulatory molecules include the B7 and the tumor necrosis factor (TNF) families. These molecules bind to receptors on T cells belonging to the CD28 or TNF receptor families, respectively. The best-defined coinhibitors and their receptors belong to the B7 and CD28 families. Recent work has begun to define how these T-cell costimulatory and coinhibitory pathways influence atherosclerosis, largely in mouse models of the disease. Profound effects are attributable to molecules in both the B7/CD28 (B7-1/2, ICOS, and PDL-1/2) and the TNF/TNF receptor (CD40, OX40, and CD137) families. One emerging theme is that both pathogenic effector T-cell responses and regulatory T cells are influenced by overlapping sets of costimulators and coinhibitors. These complexities must be considered as immunotherapeutic approaches for atherosclerotic disease are developed.
Cytosine-phosphorothioate-guanine oligodeoxynucleotides (CpG ODNs) are synthetic ODNs with unmethylated DNA sequences that mimic viral and bacterial DNA and protect against infectious agents and tumor challenge. We show that CpG ODNs markedly accelerated graft-versus-host disease (GVHD) lethality by Toll-like receptor 9 (TLR9) ligation of host antigen-presenting cells (APCs), dependent upon host IFNgamma but independent of host IL-12, IL-6, or natural killer (NK) cells. Imaging studies showed significantly more green fluorescent protein-positive (GFP(+)) effector T cells in lymphoid and nonlymphoid organs. In engraftment studies, CpG ODNs promoted allogeneic donor bone marrow (BM) rejection independent of host IFNgamma, IL-12, or IL-6. During the course of these studies, we uncovered a previously unknown and critical role of donor BM APCs in modulating the rejection response. CpG ODNs promoted BM rejection by ligation of donor BM, but not host, TLR9. CpG ODNs did not impair engraftment of TLR9(-/-) BM unless wild-type myeloid (CD11b(+)) but not B-lineage (CD19(+)) BM cells were added to the donor inoculum. The importance of donor BM APCs in modulating the strength of the host antidonor rejection response was underscored by the finding that B7-1/B7-2(-/-) BM was less likely than wild-type BM to be rejected. Collectively, these data offer new insight into the mechanism of alloresponses regulating GVHD and BM rejection.