Allorecognition by T Lymphocytes and Allograft Rejection

Recognition of donor antigens by recipient T cells in secondary lymphoid organs initiates the adaptive inflammatory immune response leading to the rejection of allogeneic transplants. Allospecific T cells become activated through interaction of their T cell receptors with intact allogeneic MHC molecules on donor cells (direct pathway) and/or donor peptides presented by self-MHC molecules on recipient antigen presenting cells (APCs) (indirect pathway). In addition, recent studies show that alloreactive T cells can be also stimulated through recognition of allogeneic MHC molecules displayed on recipient APCs (MHC cross-dressing) after their transfer via cell-cell contact or through microvesicles (semi-direct pathway). The specific allorecognition pathway used by T cells is dictated by intrinsic and extrinsic factors to the allograft and can influence the nature and magnitude of the alloresponse and rejection process. Consequently, various organs and tissues such as skin, cornea and solid organ transplants are recognized differently by pro-inflammatory T cells through these distinct pathways, which may explain why these grafts are rejected in a different fashion. On the other hand, the mechanisms by which anti-inflammatory regulatory T cells (Tregs) recognize alloantigen and promote transplantation tolerance are still unclear. It is likely that thymic Tregs are activated through indirect allorecognition while peripheral Tregs recognize alloantigens in a direct fashion. As we gain insights into the mechanisms underlying allorecognition by pro-inflammatory and regulatory T cells, novel strategies are being designed to prevent allograft rejection in the absence of ongoing immunosuppressive drug treatment in patients

Tolerance induction after organ transplantation, “delayed tolerance,” via the mixed chimerism approach: Planting flowers in a battle field

We have previously reported that peri-transplant conditioning leads to successful induction of renal allograft tolerance via the mixed chimerism approach in nonhuman primates (NHP) and humans. However, this strategy requires treatments beginning six days prior to transplantation, which limits its relevance only to living donor transplant recipients. To extend the clinical applicability of this approach, we developed a novel regimen “delayed tolerance,” with which the recipient initially undergoes organ transplantation with conventional immunosuppression, followed by conditioning and donor bone marrow transplantation (DBMT) at a later date. This approach might be likened to “planting flowers in a battle field.” That is, the recipient’s immunologic environment after organ transplantation is like a battlefield filled with hostile innate and adaptive immune-responses directed against donor antigeneic specificities. Implanting fragile donor hematopoietic progenitors into this environment and encouraging them to bloom in this vicious field requires special treatments

Innate Immunity and Resistance to Tolerogenesis in Allotransplantation

The development of immunosuppressive drugs to control adaptive immune responses has led to the success of transplantation as a therapy for end-stage organ failure. However, these agents are largely ineffective in suppressing components of the innate immune system. This distinction has gained in clinical significance as mounting evidence now indicates that innate immune responses play important roles in the acute and chronic rejection of whole organ allografts. For instance, whereas clinical interest in natural killer (NK) cells was once largely confined to the field of bone marrow transplantation, recent findings suggest that these cells can also participate in the acute rejection of cardiac allografts and prevent tolerance induction. Stimulation of Toll-like receptors (TLRs), another important component of innate immunity, by endogenous ligands released in response to ischemia/reperfusion is now known to cause an inflammatory milieu favorable to graft rejection and abrogation of tolerance. Emerging data suggest that activation of complement is linked to acute rejection and interferes with tolerance. In summary, the conventional wisdom that the innate immune system is of little importance in whole organ transplantation is no longer tenable. The addition of strategies that target TLRs, NK cells, complement, and other components of the innate immune system will be necessary to eventually achieve long-term tolerance to human allograft recipients

High-resolution quantitative imaging of mammalian and bacterial cells using stable isotope mass spectrometry

Background: Secondary-ion mass spectrometry (SIMS) is an important tool for investigating isotopic composition in the chemical and materials sciences, but its use in biology has been limited by technical considerations. Multi-isotope imaging mass spectrometry (MIMS), which combines a new generation of SIMS instrument with sophisticated ion optics, labeling with stable isotopes, and quantitative image-analysis software, was developed to study biological materials. Results: The new instrument allows the production of mass images of high lateral resolution (down to 33 nm), as well as the counting or imaging of several isotopes simultaneously. As MIMS can distinguish between ions of very similar mass, such as ^{12}C^{15}N^{-} and ^{13}C^{14}N^{-}, it enables the precise and reproducible measurement of isotope ratios, and thus of the levels of enrichment in specific isotopic labels, within volumes of less than a cubic micrometer. The sensitivity of MIMS is at least 1,000 times that of ^{14}C autoradiography. The depth resolution can be smaller than 1 nm because only a few atomic layers are needed to create an atomic mass image. We illustrate the use of MIMS to image unlabeled mammalian cultured cells and tissue sections; to analyze fatty-acid transport in adipocyte lipid droplets using ^{13}C-oleic acid; to examine nitrogen fixation in bacteria using ^{15}N gaseous nitrogen; to measure levels of protein renewal in the cochlea and in post-ischemic kidney cells using ^{15}N-leucine; to study DNA and RNA co-distribution and uridine incorporation in the nucleolus using ^{15}N-uridine and ^{81}Br of bromodeoxyuridine or ^{14}C-thymidine; to reveal domains in cultured endothelial cells using the native isotopes ^{12}C, ^{16}O, ^{14}N and ^{31}P; and to track a few ^{15}N-labeled donor spleen cells in the lymph nodes of the host mouse. Conclusion: MIMS makes it possible for the first time to both image and quantify molecules labeled with stable or radioactive isotopes within subcellular compartments

Signalisation cellulaire par pontage bipolaire (mono- ou bicellulaire) au moyen d'anticorps specifiques : influence de l'isotype sur la nature du signal

SIGLECNRS T Bordereau / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Editorial: Allorecognition by Leukocytes of the Adaptive Immune System

The term allorecognition refers to the series of mechanisms used by an individual’s immune system to distinguish its own cells and tissues from those of another individual belonging to the same species. During evolution, different cells and molecules of both innate and adaptive immune systems have been selected to recognize and respond to antigens expressed by allogeneic cells, but not autologous cells (alloantigens). This research topic focuses on allorecognition by lymphocytes of the adaptive immune system and its involvement in rejection or tolerance of allogeneic transplants. T and B cells recognizing alloantigens via specific receptors become activated and undergo proliferation and differentiation into different types of effector and memory cells. Allorecognition by lymphocytes occurs regularly during pregnancy upon trafficking of both maternal and fetal cells. In this setting, allorecognition triggers an alloresponse that is protective towards the fetus thus preventing abortion. Protective alloimmunity is mediated through cooperation between different lymphocytes and antigen presenting cells (APCs), as well as regulatory mediators and receptors. Likewise, certain transplants placed in organs and tissues called immune-privileged sites such as the eye, the central nervous system and the testis elicit protective rather than destructive adaptive immune responses. Therefore, under certain circumstances, allorecognition by regulatory lymphocytes (Tregs and Bregs) can lead to tolerance of alloantigens. In contrast, allorecognition by T cells in non-immune privileged sites and under inflammatory conditions leads to a destructive immune response. Indeed, after transplantation of most allogeneic organs and tissues, activation of pro-inflammatory T cells (TH1 and TH17), which recognize donor MHC proteins (direct pathway) or peptides derived from donor MHC and minor antigens (indirect pathway), leads to graft rejection. This inflammatory response leads to the differentiation of allospecific cytotoxic T cells as well as production of donor specific antibodies by B cells, both of which contribute to the destruction of the transplant. In this Research Topic, we describe the different pathways of allorecognition by T cells involved in allograft rejection, as well as the role of different antigen presenting cells and graft-derived microvesicles (exosomes) involved in this process. Another aspect of this Research Topic addresses the essential role of alloreactive memory T cells in allograft rejection and resistance to transplant tolerance induction in laboratory rodents, as well as non-human primates and patients. Indeed, it has become evident that laboratory mice display very few memory alloreactive T cells pre-transplantation, essentially due to the fact that they are raised in pathogen-free facilities. In contrast, primates display high frequencies of alloreactive memory T cells, either generated through prior exposure to allogeneic MHC molecules or via cross-reactivity with microbial antigens. We and others have provided ample evidence showing that this feature accounts for differences in terms of tolerance susceptibility between laboratory rodents and patients. This implies that further investigation of tolerance protocols in laboratory mice should be performed using “dirty mice” i.e., mice raised in non-sterile conditions. In summary, this Research Topic addresses key aspects of allorecognition by lymphocytes and alloantigen presentation by dendritic cells, and specifically how these processes shape our immune system and govern the rejection or tolerance of allogeneic tissues and organs

Blockade of CD40-CD154 Costimulatory Pathway Promotes Survival of Allogeneic Corneal Transplants

purpose. To determine the effect of systemic anti-CD154 monoclonal antibody on the survival of orthotopic murine corneal transplants. methods. BALB/c mice were used as recipients of syngeneic, multiple minor histocompatability (H)–disparate, or major histocompatibility complex MHC-mismatched corneal transplants. Recipient beds were either avascular (normal risk) or neovascularized (high risk). Mice were randomized to receive either anti-CD154 antibody or control immunoglobulin by intraperitoneal injection at surgery and once weekly after surgery. After orthotopic corneal transplantation, all grafts were evaluated for signs of rejection by slit lamp biomicroscopy over 8 weeks. The high-risk transplants were continuously observed until week 18 after the therapy was discontinued at week 8. Allospecific delayed-type hypersensitivity (DTH) was evaluated after transplantation in high-risk graft recipients. Frequency of interferon (IFN)-γ–secreting T cells in the hosts was measured by enzyme-linked immunospot (ELISPOT) assay. results. In normal-risk transplantation, the 8-week survival rate improved from 25% in control mice to 88% in anti-CD154–treated hosts of minor H–disparate grafts (P = 0.0087) and from 78% in control mice to 100% in anti-CD154–treated recipients of MHC-mismatched transplants (P = 0.177). Of particular significance, in high-risk transplantation, anti-CD154 therapy dramatically enhanced the survival of both minor H– and MHC-disparate corneal transplants to 100% (P = 0.0001) and 92% (P = 0.0002), respectively. In addition, the anti-CD154–treated mice did not exhibit allospecific immunity. However, termination of anti-CD154 led to some loss in graft survival, especially among high-risk minor H–disparate grafts. The frequency of IFN-γ–producing T cells was significantly reduced in anti-CD154–treated hosts. conclusions. Continuous suppression of the CD40-CD154 costimulatory pathway promotes the acceptance of corneal transplants, regardless of the degree of allodisparity or preoperative risk. The beneficial effect of anti-CD154 treatment may be due in part to inhibition of Th1-mediated responses

Role of Memory T Cells in Allograft Rejection and Tolerance

Memory T cells are characterized by their low activation threshold, robust effector functions, and resistance to conventional immunosuppression and costimulation blockade. Unlike their naïve counterparts, memory T cells reside in and recirculate through peripheral non-lymphoid tissues. Alloreactive memory T cells are subdivided into different categories based on their origins, phenotypes, and functions. Recipients whose immune systems have been directly exposed to allogeneic major histocompatibility complex (MHC) molecules display high affinity alloreactive memory T cells. In the absence of any prior exposure to allogeneic MHC molecules, endogenous alloreactive memory T cells are regularly generated through microbial infections (heterologous immunity). Regardless of their origin, alloreactive memory T cells represent an essential element of the allograft rejection process and a major barrier to tolerance induction in clinical transplantation. This article describes the different subsets of alloreactive memory T cells involved in transplant rejection and examine their generation, functional properties, and mechanisms of action. In addition, we discuss strategies developed to target deleterious allospecific memory T cells in experimental animal models and clinical settings

A Paradigm Shift on the Question of B Cells in Transplantation? Recent Insights on Regulating the Alloresponse

B lymphocytes contribute to acute and chronic allograft rejection through their production of donor-specific antibodies (DSAs). In addition, B cells present allopeptides bound to self-MHC class II molecules and provide costimulation signals to T cells, which are essential to their activation and differentiation into memory T cells. On the other hand, both in laboratory rodents and patients, the concept of effector T cell regulation by B cells is gaining traction in the field of transplantation. Specifically, clinical trials using anti-CD20 monoclonal antibodies to deplete B cells and reverse DSA had a deleterious effect on rates of acute cellular rejection; a peculiar finding that calls into question a central paradigm in transplantation. Additional work in humans has characterized IL-10-producing B cells (IgM memory and transitional B cells), which suppress the proliferation and inflammatory cytokine productions of effector T cells in vitro. Understanding the mechanisms of regulating the alloresponse is critical if we are to achieve operational tolerance across transplantation. This review will focus on recent evidence in murine and human transplantation with respect to non-traditional roles for B cells in determining clinical outcomes