Structural basis for autoantibody recognition of phosphatidylserine-beta 2 glycoprotein I and apoptotic cells.

Apoptotic cells contain nuclear autoantigens that may initiate a systemic autoimmune response. To explore the mechanism of antibody binding to apoptotic cells, 3H9, a murine autoantibody with dual specificity for phospholipids and DNA, was used. H chain mutants of 3H9 were constructed, expressed as single-chain Fv (scFv) in Escherichia coli, and assessed for binding to phosphatidylserine, an antigen expressed on apoptotic cells. Both 3H9 and its germline revertant bound to dioleoyl phosphatidylserine in ELISA, and binding was enhanced by beta 2 glycoprotein I (beta 2GPI), a plasma protein that selectively binds to apoptotic cells. Higher relative affinity for DOPS-beta 2GPI was achieved by the introduction of Arg residues into the 3H9 H chain variable region at positions previously shown to mediate DNA binding. Specificity of the two structurally most diverse scFv for apoptotic cells was shown by flow cytometry, and two populations of scFv-bound cells were identified by differences in propidium iodide staining. The results suggest that, in autoimmunity, B cells with Ig receptors for apoptotic cells and DNA are positively selected, and that the antibodies they produce have the potential to affect the clearance and processing of apoptotic cells.

Increased CD95/Fas-induced apoptosis of HIV-specific CD8(+) T cells.

Why HIV-specific CD8(+) T cells ultimately fail to clear or control HIV infection is not known. We show here that HIV-specific CD8(+) T cells exhibit increased sensitivity to CD95/Fas-induced apoptosis. This apoptosis is 3-fold higher compared to CMV-specific CD8(+) T cells from the same patients. HIV-specific CD8(+) T cells express the CD45RA(-)CD62L(-) but lack the CD45RA(+)CD62L(-) T cell effector memory (T(EM)) phenotype. This skewing is not found in CMV- and EBV-specific CD8(+) T cells in HIV-infected individuals. CD95/Fas-induced apoptosis is much higher in the CD45RA(-)CD62L(-) T(EM) cells. However, cytotoxicity and IFNgamma production by HIV-specific CD8(+) T cells is not impaired. Our data suggest that the survival and differentiation of HIV-specific CD8(+) T cells may be compromised by CD95/Fas apoptosis induced by FasL-expressing HIV-infected cells.

Alpha beta TCR+ T cells play a nonredundant role in the rejection of heart allografts in mice.

BACKGROUND: Although the transplantation of solid organs and cellular grafts is a clinical routine, the morbidity and mortality associated with immunosuppression is significant. This could be avoided by the induction of donor-specific tolerance. To develop targeted antirejection strategies and regimens to induce donor-specific tolerance, cell populations in the recipient-mediating rejection of solid organ and cellular grafts must be defined. In this study we examined the role of alpha beta-TCR+ cells in the rejection of allogeneic heart grafts, by use of knockout (KO) mice deficient in the production of alpha beta-TCR+ T cells. METHODS: C57BL/6-TcrbtmlMom (alpha beta-KO) and C57BL6/J (B6) recipient mice were transplanted with B10.BR/SgSnJ (B10.BR) or BALB/c heart allografts. Animals also received bone marrow from normal B10.BR donors, followed by donor-specific or third-party heart transplants. RESULTS: Naive B6 control mice rejected B10.BR and BALB/c grafts within 16 days. In striking contrast, B10.BR and BALB/c heart allografts were indefinitely accepted in unmanipulated alpha beta-KO mice. The immune responsiveness was restored after bone marrow transplantation from normal donors. After bone marrow transplantation major histocompatibility-disparate BALB/c third-party heart grafts were rejected, whereas donor-specific grafts were still accepted. CONCLUSIONS: alpha beta-TCR+ T cells play a nonredundant role in the rejection of heart allografts in mice. Bone marrow chimerism is associated with donor-specific transplantation tolerance.

Clinical applications of mixed chimerism.

Bone marrow transplantation (BMT) is currently a procedure that is associated with high morbidity and mortality. Thus, the clinical application of this technique is limited to the treatment of life-threatening hematopoietic malignancies. The morbidity and mortality of BMT is mainly related to graft-versus-host disease (GVHD), failure of engraftment, and toxicity related to fully myeloablative conditioning. GVHD can be prevented by T-cell depletion. However, T-cell depletion increases the risk of failure of engraftment. With the identification of a facilitating cell population that enables engraftment of hematopoietic stem cells across major histocompatibility barriers, the dichotomy between GVHD and failure of engraftment has been resolved. If one could overcome the toxicity of conditioning with the development of partially ablative conditioning strategies, BMT could be used for the treatment of a variety of nonmalignant diseases, as well as in the induction of donor-specific transplantation tolerance. This review outlines the development and advantages of partially ablative conditioning strategies and illustrates possible applications of the technique. Forty years ago E.D. Thomas discussed the potential of BMT for treating immunodeficiencies and for the induction of transplantation tolerance. BMT can be viewed as a natural form of gene therapy to replace a defective cell or enzyme with a functional and normally regulated one.

Mixed allogeneic chimerism to induce tolerance to solid organ and cellular grafts.

Transplantation of solid organs and cellular grafts has become clinical routine in the last 30 years. However, the requirement for life-long immunosuppression is associated with infections, malignancies and end-organ toxicity. Moreover, the treatment fails to prevent chronic rejection. The induction of donor-specific transplantation tolerance would solve these problems, but has remained an elusive goal. One approach to achieve transplantation tolerance is through hematopoietic chimerism. This review outlines different concepts of hematopoietic chimerism focusing on macrochimerism. Mixed allogeneic chimerism, also known as macrochimerism, is defined as engraftment of hematopoietic stem cells achieved by bone marrow transplantation (BMT). It discusses the advantages and limitations of the BMT as well as approaches to overcome these limitations in the future.

Xenotransplantation: application of disease resistance.

1. Organ transplantation is now clinically routine for patients with end-stage organ failure. One major limitation in transplantation is chronic rejection involving the loss of the graft despite the use of immunosuppressive agents. Haematopoietic stem cell (HSC) chimerism, achieved through bone marrow transplantation (BMT), induces donor-specific tolerance to transplanted organs and prevents chronic rejection. 2. A second major limitation to organ transplantation is the donor shortage. Xenotransplantation, the transplantation of organs between different species, would have the ability to increase the availability of donor organs. 3. Current immunosuppressive therapies do not prevent the rejection of xenografts. Therefore, the only reliable method for achieving donor-specific tolerance to xenografts may require HSC chimerism. 4. In order to justify the use of BMT to induce transplantation tolerance in patients with non-life-threatening diseases, the morbidity and mortality associated with current conditioning regimens must be addressed. 5. The use of partial conditioning regimens to promote engraftment of xenogeneic HSC and the development of donor-specific tolerance may eventually make xenotransplantation in humans a clinical reality. 6. Additional advantages of xenotransplantation are the ability to genetically engineer the donor xenograft and resistance of some xenografts to infection by human viruses because of the species specificity of most viruses. 7. The clinical application of disease resistance for HIV and hepatitis B virus is the focus of the present review.