Extrathymic deletion of CD8+ alloreactive T cells in a transgenic T cell receptor model of neonatal tolerance

Immune tolerance: mechanisms and application in clinical transplantation

The achievement of immune tolerance, a state of specific unresponsiveness to the donor graft, has the potential to overcome the current major limitations to progress in organ transplantation, namely late graft loss, organ shortage and the toxicities of chronic nonspecific immumnosuppressive therapy. Advances in our understanding of immunological processes, mechanisms of rejection and tolerance have led to encouraging developments in animal models, which are just beginning to be translated into clinical pilot studies. These advances are reviewed here and the appropriate timing for clinical trials is discussed.

Induction of Xenogeneic Neonatal Tolerance to Transgenic Human Leukocyte Antigen Class I Grafts

BACKGROUND

The immune response against xenografts is vigorous and poorly controlled with conventional immunosuppressants. Therefore, success in xenotransplantation will depend on developing additional approaches such as induction of immunologic unresponsiveness or tolerance. Although classic protocols of neonatal tolerance induction in mice are very tolerogenic in many allogeneic models, they have generally failed in xenogeneic models. The purpose of these studies was to determine whether failure results from an intrinsic property of xenogenic major histocompatibility complex (MHC) molecules themselves or, instead, is caused by some limitation in species-specific molecular interactions distinct from the polymorphic domains of xenogenic MHC molecules.

METHODS

Our approach was to test the ability of lymphoid cells from a transgenic (Tg) mouse donor expressing a xeno-MHC class I molecule encoding the polymorphic alpha1/alpha2 for human leukocyte antigen (HLA)-B7 to induce neonatal tolerance in non-Tg syngeneic C57BL/6 recipients. Because the donor and recipient strains are genetically identical (C57BL/6, H-2b) except for Tg human MHC HLA-B7, any species-specific molecular incompatibility in this mouse anti-human class I xeno-combination that could potentially interfere with induction of tolerance has been eliminated.

RESULTS

Our results show that HLA-B7 Tg-, but not C57BL/6 syngeneic-, injected neonates were unresponsive as adults to HLA-B7-expressing target cells in vitro and specifically accepted HLA-B7-expressing Tg skin grafts. In addition, neonatal injection of donor cells resulted in peripheral chimerism.

CONCLUSIONS

These experiments demonstrate that, as long as species-specific molecular interactions are maintained, recognition of the polymorphic domains of xenogeneic MHC does not represent a barrier to neonatal tolerance induction.

Tick-borne flaviviruses

Tick-borne encephalitis (TBE), one of the most dangerous neuroinfections in Europe and Asia, is caused by tick-borne encephalitis virus (TBEV) and currently involves approximately 11,000 human cases annually, mostly in Russia. This chapter describes the main problems associated with the epidemiology, ecology, pathogenesis, and control of this disease. We have attempted to review the factors that influence the incidence and distribution of TBE, and to discuss possible reasons for the different clinical manifestations including most commonly observed asymptomatic infections, fever forms, acute encephalitis, and the less frequently registered biphasic milk fever and chronic encephalitis. Epidemiologic data concerning the other tick-borne flaviviruses, namely Louping ill virus, Langat virus, and Powassan virus that also produce encephalitis on a smaller scale, are also presented. Here we describe the history and current epidemiological role of Omsk hemorrhagic fever virus and Kyasanur forest disease virus, two viruses that are genetically closely related to TBEV, but produce hemorrhagic fever instead of encephalitis, and provide possible explanations for these differences. The other viruses in the tick-borne flavivirus group are also included despite the fact that they do not play an essential epidemiologic role in humans. This chapter contains a brief history of vaccination against TBE including the trials with live attenuated vaccine and reviews the modern trends in development of vaccine virus strains.

Crossroads of extrathymic lymphocytes maturation pathways

The majority of T cells located in peripheral lymphoid organs are dependents on the thymus for regular differentiation and function. Only a minority of T lymphocytes are thymus-independent. These cells pass by extrathymic maturation processes and become mature T lymphocytes. Some data suggest that mechanism of extrathymic lymphocytes maturation (eTLM) includes migration, proliferation, differentiation and selection of lymphocytes as well as thymic pathway. With aging and progression of thymic involution or in accidental thymic involution, pathway of eTLM derives emphasis. T cells from extrathymic pathway probably can polarize action of thymic-dependent T cells or participate in immune reaction in antigen-destructive or antigen-protective manners. Consequently, extrathymic pathways can be a source of self-reactive T cells or cells which participate in mechanisms of trophoblast or tumor escape. Results of eTLM probably are not presets, already depend upon many factors and microenvironmental snapshots. Factors like cytokines, prostaglandine, microbes, MHC molecules, hormones, Fas ligand, heat shock proteins, phenotypes of dendritic cells and APCs, probably can be polarizing courses of eTLM pathway. Definitive to the course of extrathymic-derived cells action, presumably is resultant of microenvironmental relations and interactions of foregoing factors. Hypothesis that microbes, especially viruses, can be promoters of extrathymic (self)antigen-reactive lymphocytes maturation is real as well as hypothesis that extrathymic lymphocytes selection and products of selected lymphocytes can be included in mechanisms of tumor, trophoblast and transplant rejection or escape.

Neonatal tolerance in the absence of Stat4- and Stat6- dependent Th cell differentiation

Neonatal tolerance to specific Ag is achieved by nonimmunogenic exposure within the first day of life. The mechanism that regulates this tolerance may provide the basis for successful organ transplantation and has recently been thought to be immune deviation from the inflammatory Th1 response to a Th2 response. To test the importance of Th2 cells in the establishment of neonatal tolerance, we examined neonatal tolerance in Stat4- and Stat6-deficient mice, which have reduced Th1 and Th2 cell development, respectively. Neonatal tolerance of both the T and B cell compartments in Stat4- and Stat6-deficient mice was similar to that observed in wild-type mice. Cytokine production shifted from a Th1 to a Th2 response in wild-type mice tolerized as neonates. In contrast, tolerance was observed in Stat6-deficient mice despite maintenance of a Th1 cytokine profile. These results suggest that cells distinct from Stat6-dependent Th2 cells are required for the establishment of neonatal tolerance.