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Immune checkpoints balance initial antigen-driven T cell stimulation by enhancing or dampening activation, allowing co-existence of efficient immune responses and maintenance of self-tolerance. In oncology, checkpoints currently targeted by inhibitors to amplify activity of T cell, NK cells or myeloid cells responses comprise CTLA-4 (cytolytic T-lymphocyte-associated antigen 4 or CD152), PD-1 (programmed cell death 1, or CD279), PD-L1 ( programmed cell death-ligand 1, or CD274), LAG-3 (Lymphocyte-activation gene 3, or CD223), TIM3 (T-cell immunoglobulin and mucin-domain containing-3), TIGIT (T cell immunoreceptor with Ig and ITIM domains ), VISTA (V-domain Ig suppressor of T cell activation), B7/H3 (or CD276), KIR (killer-cell immunoglobulin-like receptor), NKG2A, CD39, CD73, CSF1R (colony-stimulating factor 1 receptor), CD47 or CD172a. Other "checkpoints" are being pharmacologically triggered in order to directly amplify T cell co-stimulation. Among these molecules, CD28, CD137 (also called 4-1BB), OX40 [also called tumor necrosis factor receptor superfamily, member 4 (TNFRSF4)], GITR (Glucocorticoid-induced tumor necrosis factor receptor family-related protein) or CD40 are also tested in oncology, most often in combination with an inhibitory checkpoint inhibitor. In autoimmune and inflammatory diseases, checkpoint inhibitors or activators (LAG-3, CD28, CD40L) are also being tested. In this review, we focus on some modulators of immune checkpoints for which the mechanism of action has been particularly studied. As this description cannot be exhaustive, we have grouped in Table I all monoclonal antibodies (MAbs) or recombinant proteins in clinical use (to our knowledge), modulating the action of a control point of the immune system., (© 2019 médecine/sciences – Inserm.)

Polyclonal anti-lymphoyctes antibodies were first successfully used in the 1970 in organ transplantation, but ten years later, monoclonal antibodies (mAb) emerged as a new class of immunosuppressive agents in transplantation with the potential to target highly specifically immune cells responsible for acute rejection. Some have proved their efficacy, such as mAb recognizing CD3- and CD25-positive T cells and have been extensively studied in clinical trials. Others such as mAb against CD52 and CD20, are still under investigation; finally, the next challenge is, based on our improved understanding of the mechanisms of immune recognition and allograft rejection, to use these mAb either alone or in combination with standard immunosuppressive regimens to manipulate the allogenic response to reach antigen-specific tolerance desired in solid-organ transplantation.

Transplantation represents a major advance in modern medicine with a major impact on the interactions between individuals and society. The numbers of patients undergoing organ transplantation increased steadily over the years and around 250,000 individuals are living nowadays in Europe with a transplanted organ. On the other hand, the numbers of cadaveric (brain-dead) donors used for organ transplantation remains stable, at around 5,000 each year, and the numbers of transplantation from living donors only slowly increase in Europe. Therefore, a gap is growing between the numbers of patients in need of a transplant and the numbers of organs available for transplantation. About 45,000 patients are currently on renal transplant waiting lists in Europe and, depending on the countries considered, 15 to 30 % of candidates for liver or heart transplantation die before a life-saving transplant becomes available to them. There is therefore an urgent need to implement innovative research and to take full advantage of recent biotechnological advances to explore new avenues in xenotransplantation, and to simultaneously address the ethical, societal and public health issues related to organ replacement. Much progresses have been accomplished in the understanding of xenograft rejection processes that include hyperacute, acute vascular and cellular rejection mechanisms. Strategies to promote xenograft survival that are currently under evaluation include genetic engineering of donor pigs, adapted immunosuppressive treatments and tolerance induction. Also, the psychological acceptance has been evaluated.