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

Functional impairment of antigen-specific T cells is a defining characteristic of many chronic infections, but the underlying mechanisms of T-cell dysfunction are not well understood. To address this question, we analysed genes expressed in functionally impaired virus-specific CD8 T cells present in mice chronically infected with lymphocytic choriomeningitis virus (LCMV), and compared these with the gene profile of functional memory CD8 T cells. Here we report that PD-1 (programmed death 1; also known as Pdcd1) was selectively upregulated by the exhausted T cells, and that in vivo administration of antibodies that blocked the interaction of this inhibitory receptor with its ligand, PD-L1 (also known as B7-H1), enhanced T-cell responses. Notably, we found that even in persistently infected mice that were lacking CD4 T-cell help, blockade of the PD-1/PD-L1 inhibitory pathway had a beneficial effect on the 'helpless' CD8 T cells, restoring their ability to undergo proliferation, secrete cytokines, kill infected cells and decrease viral load. Blockade of the CTLA-4 (cytotoxic T-lymphocyte-associated protein 4) inhibitory pathway had no effect on either T-cell function or viral control. These studies identify a specific mechanism of T-cell exhaustion and define a potentially effective immunological strategy for the treatment of chronic viral infections.

Restoring function in exhausted CD8 T cells during chronic viral infection.

Functional impairment of antigen-specific T cells is a defining characteristic of many chronic infections, but the underlying mechanisms of T-cell dysfunction are not well understood. To address this question, we analyzed genes expressed in functionally impaired virus-specific CD8 T cells present in mice chronically infected with lymphocytic choriomeningitis virus (LCMV), and compared these with the gene profile of functional memory CD8 T cells. Here we report that PD-1 (programmed death 1; also known as Pdcd1) was selectively upregulated by the exhausted T cells, and that in vivo administration of antibodies that blocked the interaction of this inhibitory receptor with its ligand, PD-L1 (also known as B7-H1), enhanced T-cell responses. Notably, we found that even in persistently infected mice that were lacking CD4 T-cell help, blockade of the PD-1/PD-L1 inhibitory pathway had a beneficial effect on the 9helpless9 CD8 T cells, restoring their ability to undergo proliferation, secrete cytokines, kill infected cells and decrease viral load. Blockade of the CTLA-4 (cytotoxic T-lymphocyte-associated protein 4) inhibitory pathway had no effect on either T-cell function or viral control. These studies identify a specific mechanism of T-cell exhaustion and define a potentially effective immunological strategy for the treatment of chronic viral infections. This abstract was published in Cancer Immunity, a Cancer Research Institute journal that ceased publication in 2013 and is now provided online in association with Cancer Immunology Research.

https://cancerimmunolres.aacrjournals.org/content/canimmarch/8/Suppl_1/3.full.pdf

Restoring function in exhausted CD8 T cells during chronic viral infection

▪ 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

Regulatory T cells (Tregs) and the PD-1: PD-ligand (PD-L) pathway are both critical to terminating immune responses. Elimination of either can result in the breakdown of tolerance and the development of autoimmunity. The PD-1: PD-L pathway can thwart self-reactive T cells and protect against autoimmunity in many ways. In this review, we highlight how PD-1 and its ligands defend against potentially pathogenic self-reactive effector T cells by simultaneously harnessing two mechanisms of peripheral tolerance: (i) the promotion of Treg development and function and (ii) the direct inhibition of potentially pathogenic self-reactive T cells that have escaped into the periphery. Treg cells induced by the PD-1 pathway may also assist in maintaining immune homeostasis, keeping the threshold for T-cell activation high enough to safeguard against autoimmunity. PD-L1 expression on non-hematopoietic cells as well as hematopoietic cells endows PD-L1 with the capacity to promote Treg development and enhance Treg function in lymphoid organs and tissues that are targets of autoimmune attack. At sites where transforming growth factor-beta is present (e.g. sites of immune privilege or inflammation), PD-L1 may promote the de novo generation of Tregs. When considering the consequences of uncontrolled immunity, it would be therapeutically advantageous to manipulate Treg development and sustain Treg function. Thus, this review also discusses how the PD-1 pathway regulates a number of autoimmune diseases and the therapeutic potential of PD-1: PD-L modulation.

The PD-1 pathway in tolerance and autoimmunity

A delivery catheter for a radially compressible tubular prosthesis comprises an elongate shaft slidably received within an elongate sheath. The prosthesis is carried over the distal end of the shaft where it is contained in a radially compressed configuration by the sheath. After introducing the catheter to a desired target location within a body lumen, the prosthesis may be released by proximally retracting the sheath. The prosthesis will remain anchored to the shaft during at least part of the release procedure, permitting the user to recapture the prosthesis by distally advancing the sheath.

Apparatus and methods for placement and repositioning of intraluminal prostheses

A method and device for the automated biopsy and collection of soft tissue having a piercing needle with a receiving port to trap tissue prior to cutting. A motor drive directs and positions the tissue receiving port at a lesion site in arbitrary positions about and along the longitudinal axis of the device. A cutter advances into the receiving chamber and severs tissue which has prolapsed into the receiving port. The severed tissue is then removed from the receiving port without removing the piercing needle receiving port from the lesion site, thus allowing for the accurate, rapid removal of an arbitrary number of core samples with only one insertion. A tissue sample cassette provides storage for the samples as well as a means for coding and decoding the location from which the samples were obtained. Together, these features allow for more accurate and complete sampling of large lesions, for the complete removal of small lesions or removal of other tissue for a variety of reasons.

Method and apparatus for automated biopsy and collection of soft tissue

Interactions between CD8+ T cells and endothelial cells are important in both protective and pathologic immune responses. Endothelial cells regulate the recruitment of CD8+ T cells into tissues, and the activation of CD8+ T cells by antigen presentation and costimulatory signals. PD-L1 and PD-L2 are recently described B7-family molecules which bind to PD-1 on activated lymphocytes and down-regulate T cell activation. We found that PD-L1 is expressed on interferon-gamma stimulated cultured human and mouse endothelial cells, while PD-L2 was found on stimulated human but not mouse endothelial cells. Expression was further up-regulated by TNF-alpha. Antibody blockade of endothelial cell PD-L1 and PD-L2 enhanced endothelial cell costimulation of PHA-activated human CD8+ T cells. Antibody blockade of mouse endothelial cell PD-L1 enhanced both IFN-gamma secretion and cytolytic activity of CD8+ T cells in response to endothelial cell antigen presentation. These results show that IFN-gamma activated endothelial cells can inhibit T cell activation via expression of the immunoinhibitory PD-L1 and PD-L2 molecules. Endothelial expression of PD-ligands would allow activation and extravasation of T cells without excessive vessel damage. Our findings highlight a potentially important pathway by which endothelial cells down-regulate CD8+ T cell-mediated immune responses.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/eji.200324270

Endothelial expression of PD-L1 and PD-L2 down-regulates CD8+ T cell activation and cytolysis.

A method of functionally replacing a previously implanted prosthetic heart valve. The method includes positioning a replacement prosthetic heart valve within an internal region defined by the previously implanted prosthetic heart valve. The replacement prosthetic heart valve is then physically docked to the previously implanted prosthetic heart valve. With this technique, the previously implanted prosthetic heart valve serves as a platform for securement of the replacement prosthetic heart valve to the patient's native tissue.

Replacement prosthetic heart valve, system and method of implant

A percutaneous discectomy system 20 having a bendable discectomy probe 22 and steerable cannula 24 allows for a wide range of positions relative to the discectomy tissue to be severed. The bendable discectomy probe 22 includes a bendable section 78, 86 which can allow the probe 22 to be bent more than 90° and still maintain the function required in order to sever and remove tissue. The steerable cannula 24 has a similar bendable section 102 which can appropriately position the end 50 of the discectomy probe 22 in order to address tissue to be removed.

Percutaneous discectomy system having a bendable discectomy probe and a steerable cannula

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