Influence of I-E expression on induction of neonatal transplantation tolerance

Tolerance and immunity following in utero transplantation of allogeneic fetal liver cells: the cytokine shift

Although in utero transplantation (IUT) has resulted in donor-specific tolerance to posnatal solid organ transplantation, the mechanisms of this tolerance remain poorly understood. Our recent findings demonstrate that under specific conditions prenatal injection of allogeneic cells may lead to allosensitization instead of tolerance. These laboratory observations were supported by clinical findings as well, and therefore suggested that, depending on the conditions of prenatal transplantation, tolerance or immunity may develop. The present study explored the role of CD4 cells, cytokines, and I-E superantigen in developing tolerance vs. immunity after in utero transplantation. Sixteen animals survived IUT (40-60% survival rate) and were free from any signs of graft-versus-host disease (GVHD). Mice were considered tolerant when their antidonor and antihost CTL responses were similar, sensitized when antidonor responses were significantly higher than antihost and anti-third-party responses, and nontolerant when antidonor responses in transplanted and control mice were similar. The TH1 --> TH2 shift was associated with tolerance and TH2 --> TH1 shift with allosensitization. Our results showed that tolerant BALB/c (H-2d, I-E+) --> CS7BL/6 (H-2b, I-E-) (2/7) mice showed higher IL-4 (p < 0.05) in antidonor MLR, and partial deletion of recipient I-E-reactive T cells (CD3Vbeta11) (p < 0.045). On the other hand, nontolerant animals (5/7) demonstrated high production of IFN-gamma (p < 0.05) without deletion of CD3Vbeta11 T cells. In C57CBL/6 (H-2b, I-E-) --> C3H (H-2k, I-E+) mice CD3Vbeta11 T cells do not play any role in tolerance induction because they are deleted in the C3H background. Tolerant mice (4/9) showed an overproduction of IL-4 (p < 0.05) in antidonor MLR whereas allosensitized animals (5/9) demonstrated high level of IFN-gamma (p < 0.05). Suppressor cells seem to play no role in tolerant C57BL/6 --> C3H as demonstrated by suppressor assay. Hence, a shift from TH1 --> TH2 or TH2 --> TH1 cytokines may determine whether tolerance or immunity develops.

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

BACKGROUND

Immunological tolerance to foreign antigen is most easily achieved during the neonatal period. Although deletion of T cells has been demonstrated in neonatal tolerance models in which donor and recipient express different MHC class II molecules, the requirement for deletion in MHC class I-disparate models is less clear. To address this issue, we used as recipient the T cell receptor (TCR) transgenic mouse (TgM) strain 2C in which the majority of CD8+ T cells express a single alpha/beta TCR alloreactive to H-2Ld, thus facilitating direct monitoring of the class I alloreactive population.

METHODS

Newborn (less than 24 hr of age) 2C TgM received injections i.v.with syngeneic C57BL/6J (H-2b) (B6) or semiallogeneic (B6xDBA)F1 (H-2bd; H-2Ld+) splenocytes. Adults were subsequently analyzed in terms of tolerance, deletion of 2C+ T cells, and chimerism.

RESULTS

The results showed that semiallogeneic-, but not syngeneic-, injected neonates were unresponsive as adults to H-2Ld-expressing target cells in vitro and the majority of these mice accepted H-2Ld+ skin grafts. Delaying the injection to 72 hr after birth or reducing the number of cells injected essentially abolished in vivo unresponsiveness in 2C recipients. Thus, the 2C TCR Tg model demonstrates the characteristics typical of neonatal tolerance. Injection of 2C neonates within 24 hr of birth with semiallogeneic versus syngeneic cells led to more than a 12-fold reduction of CD8+ 2C+ T cells in adult spleen and LNCs. In contrast, deletion of CD8+ 2C+ cells in adult thymus was not consistently observed. Based on MHC class II expression to distinguish donor (I-E+) and recipient (I-E-) cells, semiallogeneic-injected mice were chimeric in spleens and lymph nodes (LNs).

CONCLUSIONS

These results demonstrate that neonatal MHC class I tolerance in the adult is associated with low level hematopoietic chimerism and extrathymic deletion of alloreactive CD8+ T cells.

Neonatal transplantation tolerance is associated with a systemic reduction in memory cells, altered chimeric cell phenotype, and modified eicosanoid and cytokine production

Certain B10 background mice are resistant to tolerance induction following a neonatal inoculation of semiallogeneic class I/II MHC-disparate cells despite early thymic clonal deletion of alloreactive cells. The emergence of memory T cells and persistence of particular chimeric cells in the thymus has an association with this resistance. In these studies, we utilized a hemisplenectomy technique to examine systemic cell populations of adult Bl0.S (H2s, H2E-) mice that received (Bl0.S x B10.A)F1 cells at birth and before and following application (and rejection or acceptance) of Bl0.A (H2k/d, H2E+) skin grafts. Prior to skin graft challenge, tolerant mice had reduced splenic levels of memory (CD45hi, PgP-1hi, Mel-14neg) T cells as compared with the rejecting recipients and following B10.A graft challenge, the nontolerant mice showed a further increase in these cells. Elevated pretransplant levels of donor H2Kk+ cells coexpressing B220, CD11b, or CD3 were seen in the tolerant mice. Following skin grafting, splenic chimerism was reduced with differing chimeric cell phenotypes between the tolerant and nontolerant mice. In vitro production of PGE2 in a MLC was delayed in the tolerant mice with minimal production of IL-2 and IL-4. Nontolerant mice made high levels of TxB2 and heightened, early production of IL-2 and IL-4 during the MLC. Thus, tolerance induction is associated with increased numbers of particular chimeric cells, fewer peripheral lymphoid immunocompetant memory T cells, impaired eicosanoid secretion, and reduced alloreactivity and alloantigen-driven IL-2/IL-4 production. It appears that alloreactive cells necessary to break tolerance are generated when fewer class II+ (e.g., B220+, CD11b+) chimeric cells are present and that there is a coexistence of effector and regulatory T cell subpopulations in the nontolerant mice. By comparison, tolerance acquisition does not appear associated with the presence or generation of a predominant subtype of T cell but rather is likely more dependent upon clonal deletion processes.

MHC class II tolerant T cells undergo apoptosis upon re-exposure to tolerogen in vivo

Tolerance of MHC class II alloantigens can be achieved by intravenous injection of semiallogeneic hematopoietic cells into neonatal mice. Lymphoid cells of tolerant mice fail to proliferate or secrete interleukins IL-2 or IL-4 when stimulated in vitro with tolerogen. Since the lymphoid organs of B10.T(6R) tolerant mice contain normal levels of I-E reactive (V beta 11+) CD4+ T cells, deletion of alloreactive T cells does not appear to be the mechanism involved in the tolerance induction. To test whether T cells from tolerant animals can become activated under conditions that do not involve alloantigen stimulation, we stimulated these cells with immobilized anti-V beta 11 antibodies. Spleen cells from grafted tolerant and rejector mice proliferated in response to anti-V beta 11+ antibodies, suggesting they were not inert. We then tested whether V beta 11+ T cells from grafted mice can be induced to proliferate following stimulation with alloantigen in vivo. We adoptively transferred T cells from grafted tolerant and rejector mice into irradiated (B10.AQR x B10.T(6R))F1 mice and harvested the lymphoid organs after 65 h. Cells from both grafted tolerant and rejector mice underwent blast transformation, but only cells from rejector mice proliferated when exposed to immobilized anti-V beta 11 antibodies. The failure of V beta 11+ cells from tolerant mice to proliferate after in vivo stimulation may be because they are apoptotic. To test this hypothesis, spleen cells from naive or neonatally tolerized (with (B10.AQR x B10.T(6R))F1 cells) B10.T(6R) mice were adoptively transferred into irradiated (B10.AQR x B10.T(6R))F1 mice and bcl-2 expression was analysed in harvested V beta 11+ cells. Large cells recovered from recipients of naive 6R cells expressed bcl-2 mRNA. By contrast, large cells harvested from recipients of tolerized 6R cells did not express bcl-2 mRNA, suggesting bcl-2 mRNA expression was downregulated in these mice. Moreover, in another experiment, large V beta 11+ cells from grafted tolerant animals recovered after transfer into irradiated (B10.AQR x B10.T(6R))F1 mice did not express the bcl-2 protein as determined by flow cytometry, and contained fragmented DNA as assessed by the TUNEL method. Taken together, these data suggest that MHC class II tolerant T cells undergo apoptosis upon re-exposure to tolerogen in vivo.

Split tolerance induced by orthotopic liver transplantation in mice

Spontaneous orthotopic liver allograft acceptance associated with microchimerism in mice induces tolerance to subsequent skin or heart transplants from the donor but not third-party animals. Despite in vivo hyporesponsiveness, in vitro MLC and CTL assays showed continuing antidonor reactivity. Cells isolated from recipients' spleens and grafted livers, when tested in MLC and CTL assays, were antidonor reactive out to 3 months to the same degree as splenocytes obtained from either naive or presensitized (with skin or heart) mice. Nevertheless, passive transfer of splenocytes or liver lymphocytes from liver tolerant mice, but not naive or sensitized donor strain mice, were able to prolong skin graft survival significantly in naive irradiated recipients. By using a strain combination in which the donor but not the recipient expressed the stimulatory endogenous super-Ag (Mlsf), it was possible to determine whether super-Ag-reactive T cells bearing V beta 5 and V beta 11 were deleted or anergic. Phenotypic analysis of cells isolated from recipients' spleens and grafted livers (up to 90 days after transplant), when compared with naive animals, showed no significant difference in V beta 5 and V beta 11 TCR expression. Additionally, when these isolated spleen cells were tested for antibody-mediated stimulation, both anti-V beta 5 and V beta 11 TCR mAb led to marked proliferation of cells obtained from naive and liver-transplanted recipients, but as expected, proliferation was very low in cells from naive donors. These results suggest that liver transplantation induces donor-specific tolerance in vivo, which may not be reflected in in vitro proliferative and cytotoxicity assays (split tolerance). Furthermore, this tolerance does not seem to be induced by clonal deletion or anergy of minor-lymphocyte-stimulating-antigen-reactive T cells in the recipients.

Ontogeny of tolerogen-responsive lymphocytes following neonatal inoculation of class II disparate semiallogeneic cells

Spleens of adult mice of the A strain background that were rendered tolerant as neonates of class II alloantigens only (A.TH tolerant of A.TL, A.TL tolerant of A.TH) contain large numbers of tolerogen-responsive T cells, many of which secrete IL-4, but not IL-2. Since these spleens also contain suppressor cells that can adoptively transfer skin allograft acceptance in vivo and can prevent generation of class II-specific cytotoxic T cells in vitro, it is important to determine the origins during postnatal development of these cells. Class II disparate, semiallogeneic haematopoietic cells were injected into newborn A.TH and A.TL mice. Periodically thereafter (1 to 60 days post-injection, but prior to challenge with a tolerogen-bearing skin graft), thymocytes and splenocytes from these mice were examined in vitro for tolerogen-specific reactivity in mixed lymphocyte reactions during which proliferation and IL-2 and IL-4 production were assayed. Within 24 hours of neonatal injection, the thymus and spleens of injected mice were profoundly depleted of tolerogen-responsive T cells. However, there was no commensurate loss of I-E-related V beta 5+ cells in the thymus of A.TH mice that received neonatal inoculations of I-E-bearing A.TL cells. During the ensuing weeks, tolerogen-responsive proliferative and IL-4-secreting T cells were detected in thymus and spleen. However, not until after the mice were more than 60 days of age were tolerogen-responsive cells able to secrete IL-2. Since physical clonal deletion of tolerogen-related V beta 5+ cells is a characteristic of neither neonatal nor adult A.TH and A.TL mice that received injections of semiallogeneic cells at birth, and since tolerogen-responsive IL-4 producing cells exist in adult mice that have permanently accepted (A.TH x A.TL)F1 skin grafts, our results imply that the tolerogen-responsive T cells detected in adult tolerant mice are descendants of the novel IL-4-producing T cells that arise in the thymus almost immediately after the tolerance conferring inoculum of semiallogeneic cells. The possible mechanisms responsible for generation of IL-4-producing, tolerogen-responsive T cells and the role of these cells in maintenance of tolerance of class II alloantigens are discussed.

Alloreactivity against IE-encoded antigens: evidence of the discrepancy between graft rejection and reactivity of IE-reactive T cells

Participation of IE antigens (Ag) in immune response as the transplantation Ag was examined. IE- B10.A(4R)(4R; Kk, IAk, IE-, Db) mice could not reject skin graft from IE Ag alone-disparate B10.A(2R) (2R; Kk, IAk, IEk, Db) mice despite intravenous (iv) injection of 2R spleen cells (SC) before or after skin grafting, indicating that graft rejection could not be caused across IE Ag-barrier alone. Furthermore, 4R SC could not induce lethal graft-versus-host disease (GVHD) in supralethally (950 rad) irradiated 2R mice. On the other hand, infiltration of lymphoid cells was observed at the site of transplanted 2R skin in 4R mice. SC of 4R mice unprimed or primed with 2R skin or 2R SC showed the capability to proliferate in vitro in response to 2R Ag. In immunofluorescence analysis of lymph node cells (LNC) of 4R mice injected iv with 2R SC 7 days earlier, IE-reactive CD4+Vbeta 11+ T cells did not change in number, but slightly increased the expression of interleukin-2 receptor (IL-2R). In 2R mice irradiated with 670 rad and injected iv with 4R SC 7 days earlier, 4R-derived CD4+V beta 11+ T cells proliferated, changed to blastoid form, and showed a markedly increased expression of IL-2R. To further investigate the influence of IE alloantigens on transplantation immunity, IL-2 production and anti-class I CTL activity were assayed. The 4R SC capable of recognizing IEk and Dk Ag of B10.BR (Kk, IAk, IEk, Dk) generated levels of both IL-2 and CTL activities higher than those of 2R SC capable of recognizing Dk Ag alone. These results strongly suggest that IE alloantigens indirectly act as the transplantation Ag by the stimulation of IE-reactive CD4+ helper T cells resulting in the differentiation of class I-restricted CD8+ T cells.

Transplantation tolerance

New experimental protocols for the induction of transplantation tolerance continue to be developed. In the past year, encouraging data have been reported from a clinical trial using a protocol specifically designed to induce tolerance to the histocompatibility antigens of the kidney donor. Progress has also been made in our understanding of the mechanisms responsible for the induction and maintenance of tolerance to alloantigens in vivo; it is becoming increasingly clear that more than one mechanism can be involved, particularly at different phases in the response.