Cytotoxic T lymphocyte recognition of a xenogeneic major histocompatibility complex antigen expressed in transgenic mice

Immunogenicity of two FMDV nonameric peptides encapsulated in liposomes in mice and the protective efficacy in guinea pigs

It has been predicted that nonameric peptides I (VP1_26–34, RRQHTDVSF), II (VP1_157–165, RTLPTSFNY) and III (VP1_45–53, KEQVNVLDL) from the VP1 capsid protein of the foot-and-mouth disease virus (FMDV) are T cell epitopes. To investigate whether these peptides have immunological activity, BALB/c mice were immunized with peptide I, II or III conjugated with immunostimulating complexes (ISCOMs). A cytotoxic T lymphocyte assay was used to evaluate the cytotoxic activity induced by peptides along with by measuring peptide-specific T-cell proliferation and CD8⁺ T lymphocyte numbers in whole blood and interferon (IFN)-γ production in peripheral blood mononuclear cells induced by peptides. To further identify the protective efficacy of peptides, an FMDV challenge assay was done in guinea pigs. Peptides I and II stimulated significant increases in T-cell proliferation, CD8⁺ T lymphocytes, and IFN-γ secretion and cytotoxic activity compared to controls. The FMDV challenge assay indicated peptides I and II can protect over 60% of animals from virus attack. The results demonstrate that peptides I and II encapsulated in liposomes should be CTL epitopes of FMDV and can protect animals from virus attack to some extent.

Porcine neural xenografts in the immunocompetent rat: immune response following grafting of expanded neural precursor cells

Intracerebral neural xenografts elicit a host immune response that results in their rapid rejection. This forms a key barrier to the therapeutic use of xenogeneic tissue transplantation for conditions such as Parkinson's disease. The current study sought to provide insight into the cellular components of donor cell suspensions that are important in stimulating the host rejection response and thereby to suggest rational manipulations of xenogeneic donor tissue that might ultimately enhance its clinical utility. The neural stem cell mitogens, epidermal growth factor and fibroblast growth factor-2, have been used to isolate and expand populations of primordial neural precursor cells from the embryonic pig brain. The immune response elicited by these cells on transplantation into the non-immunosuppressed rat has been fully characterised. In the first experiments, expanded neural precursors were grafted into the hemi-parkinsonian, non-immunosuppressed Sprague-Dawley rat and graft status and host response examined 10, 21, 35 and 60 days post-transplantation. While equivalent primary tissue grafts were completely eliminated at 35 days, grafts of expanded neural precursors with healthy neurofilament-positive projections were present at all time-points, and two large grafts remained even at 60 days. Some grafts appeared to elicit minimal host immune responses at the time-points they were examined, although most did appear to be undergoing a rejection process since a co-ordinated response involving host cytotoxic T-lymphocytes, microglia/macrophages, immunoglobulin M and complement could be demonstrated to varying degrees. Subsequent experiments went on to demonstrate further that expanded precursor populations and primary tissue suspensions differed in their immunogenic profile. Firstly, when primary tissue was injected intraperitoneally into immunocompetent rats a vigorous primary humoral response was generated. No such response was detected following injection of expanded neural precursors. Secondly, flow cytometric analysis revealed small but significant levels of class II porcine major histocompatibility complex expression in primary cell suspensions but no such expression in expanded precursor populations.The results of this study therefore demonstrate that the immunogenicity of porcine neural cell suspensions used for intracerebral grafting is reduced when neural stem cell mitogens are used to expand precursor cells. The implications of these findings in the development of novel xenogeneic cellular therapies for neurodegenerative conditions such as Parkinson's disease are discussed.

A novel mode of immunoprotection of neural xenotransplants: masking of donor major histocompatibility complex class I enhances transplant survival in the central nervous system

To determine the role of major histocompatibility complex (MHC) class I in immunological rejection of neural xenotransplants, F(ab')2 fragments of a monoclonal antibody to porcine MHC class I were used to mask this complex on porcine fetal striatal cells transplanted into rat striata previously lesioned with quinolinic acid. Presence of MHC class I on the surface of porcine striatal cells was confirmed by fluorescence-activated cell sorting prior to F(ab')2 treatment. At three to four months post-transplantation, survival of F(ab')2-treated xenografts was assessed by means of donor-specific immunostaining and compared to that of untreated xenografts in non-immunosuppressed rats and in rats immunosuppressed with cyclosporine A. In this study, masking of donor MHC class I by F(ab')2 treatment resulted in enhanced xenografts survival compared to the non-immunosuppressed controls (graft survival rates, 52% and 7%, respectively; P < 0.005) at survival times up to four months. While xenograft survival in F(ab')2-treated animals was not significantly different from that in cyclosporine-treated rats (74% graft survival), mean graft volume in F(ab')2-treated animals was smaller than that in cyclosporine-treated animals (1.07 +/- 0.30 mm3 versus 3.14 +/- 0.51 mm3; P < 0.005). The cytoarchitectonic organization of the xenografts was similar in F(ab')2- and cyclosporine-treated animals, and grafts in both groups exhibited long distance target-directed axonal outgrowth. The pattern of immunoreactivity to porcine MHC class I in the xenografts corresponded to the regional distribution of donor glia. In xenografts undergoing rejection, infiltration with host inflammatory cells was restricted to necrotic graft remnants and spared the nearby host structures. We conclude that MHC class-I-restricted immune mechanisms play an important role in neural xenograft rejection and that masking of this complex on donor cells may provide a useful strategy for immunoprotection of neural xenografts.

Neural xenotransplantation: reconstruction of neuronal circuitry across species barriers

Selective replacement of degenerated neurons in the adult brain with allogeneic fetal neuroblasts is a promising therapeutic modality for human neurodegenerative diseases, but is confounded with practical and potential ethical problems. To evaluate the potential of xenogeneic donors as a cell source for neural transplantation, we have critically examined the available experimental evidence in animal models pertaining to the survival, integration and function of xenogeneic fetal neuroblasts in the host brain. A statistical meta-analysis across multiple studies revealed that immunologically-related transplantation parameters (immunosuppression and donor-host phylogenetic distance) were the main determinants of neural xenograft survival. The immunological basis for xenograft rejection is reviewed in the context of novel immunoprotection strategies designed to enhance xenograft survival. Furthermore, the evidence for behavioral recovery based on anatomical and functional integration of neural xenografts in the host brain is examined with an awareness of developmental considerations. It is concluded that neural xenotransplantation offers a unique opportunity for effective neuronal replacement with significant potential for clinical use.

Leukocyte infiltration and glial reactions in xenografts of mouse brain tissue undergoing rejection in the adult rat brain. A light and electron microscopical immunocytochemical study

Neural mouse xenografts undergoing rejection in the adult recipient rat brain were characterized with regard to infiltrating host leukocytes and reactions of graft and host astro- and microglial cells. Rejection occurred within 35 days with infiltration of the grafts by in particular macrophages and T-cells as well as blood-brain barrier (BBB) leakage for IgG. In the surrounding host brain microglial cells showed increased histochemical staining for nucleoside diphosphatase (NDPase) and increased immunocytochemical expression of complement receptor type 3 (CR3), while astroglial cells displayed an increased immunoreactivity for glial fibrillary acidic protein (GFAP). Light microscopic findings of rat major histocompatibility complex (MHC) antigen class I on microglial cells, endothelial cells and leukocytes were confirmed at the ultrastructural level and extended to include a few astrocytes. Rat and mouse MHC antigen class II was only detected on leukocytes and activated microglia. We suggest that host macrophages and activated host and xenograft microglial cells act in situ as immunostimulatory cells on T-helper cells, and that increased levels of donor MHC antigen class I may further enhance the killer activity exerted by host T-cytotoxic cells.

HLA expression and function in single and double HLA-B27-transgenic mice

The expression and function of HLA antigens in mice single transgenic for HLA-B27.2 (sTGM-B27.2) or double transgenic (dTGM) for HLA-B27.2 and human beta 2-microglobulin (h beta 2m) were compared. B27.2 could be well detected on the cell membrane of lymphocytes of sTGM. However, the expression in sTGM was much lower than in dTGM mice. Nevertheless, also in sTGM mice, the B27-transgene product possessed all functional properties of a class I HLA molecule. This was shown by the recognition and induction of antibodies and cytotoxic T cells, by the induction of "allo"-immunity, including skin graft rejection, and by the ability to present viral antigens. In dTGM, the expression of B27 on peripheral blood lymphocytes, spleen and lymphnode cells was comparable to H-2. However, on thymocytes, a relatively lower expression of HLA than H-2 was observed. This low expression of B27 on thymocytes is in concert with the observation that B27 is expressed only in the medulla of the thymus and not detectable in the cortex.

Recognition of xeno-(HLA, SLA) major histocompatibility complex antigens by mouse cytotoxic T cells is not H-2 restricted: a study with transgenic mice

Cytotoxic T lymphocytes (CTLs) recognize antigens in the context of major histocompatibility complex (MHC) class I gene products. The T-cell receptor (TCR) that mediates this MHC-restricted antigen recognition recognizes short peptide fragments rather than the intact antigen. Presentation of peptides to the TCR may thus be a major function of the MHC. An intriguing question emerging from this model is whether peptide presentation also applies to foreign MHC antigens and which of the available MHC molecules can present preferentially the peptides of the foreign MHC molecule. Allo- and xenoreactive CTLs might either recognize native MHC class I molecules or peptides presented by self MHC or by the foreign class I MHC itself. The finding that synthetic peptides corresponding to MHC class I regions are recognized by allo- and xenoreactive CTLs suggests that recognition of foreign MHC by CTLs might involve degraded fragments presented by syngeneic class I molecules. We used MHC transgenic mice as a tool to study these questions. The CTL responses against human (HLA) antigen B27 were analyzed by using HLA-B27 transgenic mice with various H-2 haplotypes. We report here that mouse xeno-MHC-specific (anti-B27) CTLs are perfectly able to kill human and mouse cells expressing the appropriate xenoantigen and that in primary and secondary responses to xeno-MHC, the mouse T-cell repertoire does not use self-H-2 as a restriction element. Absence of H-2 restriction was confirmed by the lack (less than 1/10(6] of H-2-restricted HLA-specific CTL precursors. Therefore, H-2-restricted recognition of xeno-MHC antigens cannot be generalized as part of a classical MHC class I-specific response. These results indicate that xenoreactive CTLs usually recognize intact MHC molecules or MHC peptides preferentially presented by their native MHC molecule. We suggest the latter possibility.

Immunological reaction and blood-brain barrier in mouse-to-rat cross-species neural graft

Pieces of brainstem tissue from mouse embryos were transplanted into the cerebellar vermis of 49 adult rats, which had or which had not been treated with Cyclosporin A (10 mg/kg/day). With no treatment of immunosuppressants survival rates of xenografts were low. However, when Cyclosporin A was administered, the rates increased from 40% (4/10) to 67% (8/12) 2 weeks after grafting and from 25% (3/12) to 60% (9/15) 4 weeks after grafting, although immunological reactions of varying severities were noted in all of the surviving grafts. The present immunocytochemical study elucidated the composition of cell infiltrations frequently seen in the grafts. The results showed that a large number of cytotoxic/suppressor T lymphocytes appeared, while the numbers of helper/inducer T lymphocytes were relatively small. In addition, increased staining of astrocytes and microglia was observed in areas of cell infiltration. These activated cells might play a certain role in the process of graft rejection in the brain. Formation of the blood-brain barrier in the xenografts was examined by means of peroxidase cytochemistry and immunohistochemistry. In brains containing surviving grafts limited leakage of peroxidase, following its injection into the host systemic circulation 30-75 min prior to sacrifice, was detectable at the graft-host interface and at the operation scar near the pial surface. In brains containing rejected grafts extensive extravasation of peroxidase was detected. The severity of the immunological reaction was correlated with the intensity of the rupture in the blood-brain barrier. The findings suggested that the immunological reaction contributed to the transendothelial permeability changes in the vessels of brains containing rejected grafts.

Immunological function of HLA-C antigens in HLA-Cw3 transgenic mice

The human major histocompatibility complex encodes three classical class I antigens, HLA-A, -B, and -C. Of these HLA-A and -B act as strong transplantation antigens and as restriction molecules for recognition of foreign antigen by cytotoxic T lymphocytes. In contrast, little is known about HLA-C and it is not clear whether HLA-C has the same functional properties as HLA-A and -B. Transgenic C57BL/6 mice expressing the HLA-Cw3 gene were established. Functional studies demonstrated that transgenic skin was rapidly rejected by normal C57BL/6 mice and that cytotoxic T lymphocytes generated by immunization of the Cw3 transgenic mice with influenza and Sendai virus were restricted by the Cw3 molecule. These data suggest that HLA-Cw3 has immunological functions comparable to those of HLA-A and -B.