Characterization and manipulation of T cell immunity to skin grafts expressing a transgenic minor antigen

rFliC prolongs allograft survival in association with the activation of recipient Tregs in a TLR5-dependent manner

Allorejection remains an obstacle for successful organ transplantation. Although different types of immunosuppressive agents are effective for controlling rejection and prolonging graft survival, drug treatment is limited because of side effects and toxicity. Therefore, it is necessary and urgent to identify new candidate drugs for inducing allotolerance. Recently, it has been reported that bacterial flagellin induces the immunosuppressive activity of regulatory T cells (Tregs) in humans in vitro. In the present study, we analyzed the effects of recombinant flagellin (rFliC) on allograft survival and explored the underlying mechanisms associated with the activation of recipient Tregs in a murine skin allotransplantation model. The results showed that rFliC administration (3 mg/kg, once per day for 3 days, i.p.) prolonged allograft survival (mean survival time: 18.4±1.1 days) compared to the control group (10±0.7 days, P<0.01). Additionally, higher positive expression of Toll-like receptor 5 (TLR5) was detected within the allograft administered with rFliC. The frequency of CD4(+)CD25(+)Foxp3(+) Tregs; the expression of Treg-related factors TLR5, Foxp3, TGF-β1 and IL-10; and the proliferation and suppression of Tregs were increased following rFliC administration compared to the control. Moreover, the increased expression of tolerance-related molecules and the proliferation of Tregs induced by rFliC were attenuated by an anti-TLR5 blocking antibody both in vivo and in vitro. In conclusion, rFliC administration prolongs the survival of allografts, which is associated with the activation of recipient Tregs in a TLR5-dependent manner. rFliC may be a new candidate for anti-allorejection therapy.

Targeting T-cell memory: where do we stand?

PURPOSE OF REVIEW

The need to control donor-reactive T cell memory for successful organ transplantation is widely acknowledged. Alloreactive memory T cells are present in many human transplant recipients prior to transplantation and are less susceptible to the effects of currently used immunosuppression than naïve T cells. This review brings together recent advances in various fields of immunology that are potentially applicable for targeting memory T cells in sensitized transplant patients.

RECENT FINDINGS

The topics of the discussion are evoked by the latest findings on immunobiology of memory T cells and include functional diversity of T-cell memory, characteristic features of memory T-cell homeostasis and signaling, costimulatory requirements of memory T cells and their susceptibility to regulation.

SUMMARY

Despite apparent resistance of memory T cells to currently used therapies, recent findings suggest that recall responses by memory T cells can be controlled at many different levels. Use of this information may facilitate development of future tools managing T-cell memory in transplant settings.

Evaluation of 131I-anti-MIF mAb as a reporter for allograft rejection

Finding a specific agent will be useful for monitoring allorejection in clinic. The macrophage migration inhibitory factor (MIF) was reported to be one of the major cytokines involved in allorejection. In this study, we evaluated whether (131)I-anti-MIF mAb could be an efficient imaging reporter for monitoring allorejection. (131)I-anti-MIF mAb or control (131)I-IgG was injected to skin allotransplantation mice and T/NT ratios were evaluated. The imaging changes of grafts were dynamically displayed by whole-body images. The results showed that up-regulation of MIF expression was found in allografts but not in isografts. During the whole progression of rejection, the T/NT ratio in the (131)I-anti-MIF mAb group was significantly higher than that in the (131)I-IgG group and markedly increased on the top of rejection. The graft-rejection could also be shown more clearly in the (131)I-anti-MIF mAb group by whole-body imaging. These results implied that (131)I-anti-MIF mAb may be a valid method for facilitating the development of protocols to monitor allorejection.

Successful tapering of immunosuppression to low-dose monotherapy steroids after living-related human leukocyte antigen-identical renal transplantation

BACKGROUND

Living-related (LR) human leukocyte antigen (HLA)-identical renal transplant (RTx) recipients often receive standard immunosuppression, despite the absence of mismatched major HLA-antigens and the known complications of long-term use of immunosuppression. No data are available on the need for immunosuppression for these specific patients. We wondered whether their immunosuppressive load could be radically reduced.

METHOD

Between November 1982 and November 2005, 83 LR HLA-identical RTx were performed in our center. Their unadjusted graft survival was 74% at 10 years. In 29 patients (median time after transplantation 5.6 [range 1.0-21.4] years) with stable uncompromised renal function, we tapered their immunosuppression from triple or dual therapy to prednisolone 5 mg/day. Follow up on prednisolone monotherapy was at least 24 months.

RESULTS

In 27 of 29 patients reduction of immunosuppression to prednisolone monotherapy was uneventful. One patient, using dual therapy, developed JC-virus nephropathy resulting in graft loss. One refused further discontinuation of his medication. Four (15%) of the 27 patients on monotherapy developed biopsy-proven recurrence of their original disease. Only one of them showed a transient decline in renal function. One additional patient developed minor proteinuria and a rise in serum creatinine level, as a result of chronic urinary tract infections. The remaining 23 of 27 patients (85%) had an uneventful follow up during 24 months prednisolone monotherapy.

CONCLUSION

We conclude that HLA-identical LR RTx recipients who are at least 1 year after transplantation might be treated with low-dose steroid monotherapy. Close surveillance of patients for recurrence of their original disease is recommended to allow for potential early therapeutic intervention.

Donor deficiency of decay-accelerating factor accelerates murine T cell-mediated cardiac allograft rejection

Decay-accelerating factor (DAF) is a cell surface regulator that accelerates the dissociation of C3/C5 convertases and thereby prevents the amplification of complement activation on self cells. In the context of transplantation, DAF has been thought to primarily regulate antibody-mediated allograft injury, which is in part serum complement-dependent. Based on our previously delineated link between DAF and CD4 T cell responses, we evaluated the effects of donor Daf1 (the murine homolog of human DAF) deficiency on CD8 T cell-mediated cardiac allograft rejection. MHC-disparate Daf1(-/-) allografts were rejected with accelerated kinetics compared with wild-type grafts. The accelerated rejection predominantly tracked with DAF's absence on bone marrow-derived cells in the graft and required allograft production of C3. Transplantation of Daf1(-/-) hearts into wild-type allogeneic hosts augmented the strength of the anti-donor (direct pathway) T cell response, in part through complement-dependent proliferative and pro-survival effects on alloreactive CD8 T cells. The accelerated allograft rejection of Daf1(-/-) hearts occurred in recipients lacking anti-donor Abs. The results reveal that donor DAF expression, by controlling local complement activation on interacting T cell APC partners, regulates the strength of the direct alloreactive CD8(+) T cell response. The findings provide new insights into links between innate and adaptive immunity that could be exploited to limit T cell-mediated injury to an allograft following transplantation.

A critical precursor frequency of donor-reactive CD4+ T cell help is required for CD8+ T cell-mediated CD28/CD154-independent rejection

Ag-specific precursor frequency is increasingly being appreciated as an important factor in determining the kinetics, magnitude, and degree of differentiation of T cell responses, and recently was found to play a critical role in determining the relative requirement of CD8(+) T cells for CD28- and CD154-mediated costimulatory signals during transplantation. We addressed the possibility that variations in CD4(+) T cell precursor frequency following transplantation might affect CD4(+) T cell proliferation, effector function, and provision of help for donor-reactive B cell and CD8(+) T cell responses. Using a transgenic model system wherein increasing frequencies of donor-reactive CD4(+) T cells were transferred into skin graft recipients, we observed that a critical CD4(+) T cell threshold precursor frequency was necessary to provide help following blockade of the CD28 and CD154 costimulatory pathways, as measured by increased B cell and CD8(+) T cell responses and precipitation of graft rejection. In contrast to high-frequency CD8(+) T cell responses, this effect was observed even though the proliferative and cytokine responses of Ag-specific CD4(+) T cells were inhibited. Thus, we conclude that an initial high frequency of donor-reactive CD4(+) T cells uncouples T cell proliferative and effector cytokine production from the provision of T cell help.

Transfusion-related immunomodulation by platelets is dependent on their expression of MHC Class I molecules and is independent of white cells

BACKGROUND

Transfusion-related immunomodulation (TRIM) has been correlated with the presence of white cells (WBCs) in blood transfusions, but the role of components such as platelets (PLTs) in mediating TRIM has not been extensively examined. We designed a murine PLT transfusion model to study whether leukoreduced PLTs mediate TRIM effects.

STUDY DESIGN AND METHODS

CBA recipient mice were administered four weekly transfusions of either fresh (4 hr) or aged (24 and 72 hr) donor leukoreduced PLTs from allogeneic BALB/c mice and then transplanted with skin grafts from donor-matched mice. TRIM was measured by comparing the times to graft rejection and these were correlated with immunoglobulin G (IgG) antibody development measured by flow cytometry.

RESULTS

Compared with nontransfused control recipients, four transfusions of fresh, extremely leukoreduced (<0.05 WBCs/mL), allogeneic PLTs significantly (p < 0.002) reduced the recipient's ability to reject donor-matched skin grafts (survival >49 days compared with <14 days in nontransfused controls) despite the presence of high-titered serum IgG donor antibodies. In contrast, however, aged PLTs or fresh PLTs devoid of MHC Class I molecules were unable to affect skin graft survival nor stimulate antibody production. The PLT age-related inability to induce TRIM was shown to be due to loss of PLT-associated MHC Class I molecules; soluble supernatant MHC molecules that were transfused were unable to induce TRIM.

CONCLUSION

These results suggest that fresh PLTs can induce TRIM independently of WBCs due to their MHC antigen expression whereas aging results in loss of MHC and ability to mediate TRIM. The findings support the concept that either active MHC removal from fresh PLTs or passive removal by, for example, storage, may reduce any deleterious effects of TRIM in transfusion recipients.

A direct comparison of rejection by CD8 and CD4 T cells in a transgenic model of allotransplantation

Introduction:

The relative contributions of CD4⁺ and CD8⁺ T cells to transplant rejection remain unknown. The authors integrated a previous model of CD4-mediated graft rejection with a complementary model of CD8-mediated rejection to directly compare the function of graft-reactive CD4⁺ and CD8⁺ lymphocytes in vivo in a model where rejection requires transgenic T cells. These studies allow direct comparison of CD4 and CD8 T cell responses to the same antigen without the confounding effects of T cell depletion or homeostatic proliferation.

Materials and Methods:

Clone 4 and TS1 mice possess MHC class I- and II-restricted CD8⁺ and CD4⁺ T cells, respectively, which express transgenic T cell receptors that recognize the influenza hemagglutinin antigen (HA). We compared the in vivo response of CFSE-labeled, HA-specific transgenic CD8⁺ and CD4⁺ T cells after adoptive transfer into syngeneic BALB/c mice grafted with HA-expressing skin.

Results:

As in the authors’ CD4⁺ model, HA104 skin was consistently rejected by both Clone 4 mice (n=9, MST: 14.2) and by 5×10⁵ Clone 4 lymphocytes transferred to naive BALB/c hosts that do not otherwise reject HA⁺ grafts. Rejection correlated with extensive proliferation of either graft-reactive T cell subset in the draining lymph nodes, and antigen-specific CD4⁺ and CD8⁺ cells acquired effector function and proliferated with similar kinetics.

Conclusions:

These data extend the authors’ unique transgenic transplantation model to the investigation of CD8 T cell function. The initial results confirm fundamental functional similarity between the CD4 and CD8 T cell subsets and provide insight into the considerable redundancy underlying T cell mechanisms mediating allograft rejection.

Human bullous pemphigoid antigen 2 transgenic skin elicits specific IgG in wild-type mice

Bullous pemphigoid antigen 2 (BPAG2) is targeted by autoantibodies in patients with bullous pemphigoid (BP), and absent in patients with one type of epidermolysis bullosa (OMIM #226650). A keratin 14 promoter construct was used to produce transgenic (Tg) mice appropriately expressing human BPAG2 (hBPAG2) in murine epidermal basement membrane (BM). Grafts of Tg skin placed on gender-matched, syngeneic wild type (Wt) or major histocompatibility complex I (MHC I)-/- mice elicited IgG that bound human epidermal BM and BPAG2. Production of such IgG in grafted mice was prompt (detectable within 16+/-2 days), robust (titer > or = 1,280), durable (present > or = 380 days), and correlated with the involution and loss of Tg skin grafts. MHC II-/- mice grafted with Tg skin did not develop anti-hBPAG2 IgG or graft loss indicating that MHC II:CD4+ T cell interactions were crucial for these responses. Tg skin grafts on Wt mice developed neutrophil-rich infiltrates, dermal edema, subepidermal blisters, and deposits of immunoreactants in epidermal BM. This model shows fidelity to alterations seen in patients with BP, has relevance to immune responses that may arise in patients with epidermolysis bullosa following BPAG2 gene replacement, and can be used to identify interventions that may block production of IgG against proteins in epidermal BM.

Progression of alloresponse and tissue-specific immunity during graft coronary artery disease

Chronic rejection remains the major obstacle for long-term transplant survival. Both indirect alloresponse and tissue-specific autoimmunity have been implicated in its pathogenesis. The interrelationship between these two types of host anti-graft response remains poorly understood. We have developed an immunosuppression-free mouse model of graft coronary artery disease (GCAD), in which all FVB (H-2(q)) cardiac allografts placed into minor Ag (mHC)-mismatched DBA/1 (H-2(q)) hosts survived more than 112 days, and developed GCAD. We then examined the kinetics of both anti-mHC alloresponse and host autoimmunity against heart-specific antigen, cardiac myosin (CM). At 8 days post-transplantation, recipient mice showed minimal intragraft inflammation and apoptosis, and limited expansion of allo-specific T cells. In addition, we observed early production of anti-myosin IgG1 autoantibodies, which occurred in the absence of activated CM-specific T lymphocytes. By day 56, GCAD indices, the numbers of mHC- and CM-reactive T cells, and the levels of circulating allo- and CM-specific antibodies were all significantly increased. While host alloresponse was exhausted at 112 days post-transplant, T-cell reactivity against CM persisted. Our data suggest that both allo- and tissue-specific immunity might contribute to the induction of GCAD. They indicate that continual autoimmune response against graft tissue antigens may provide for GCAD sustenance.

Suramin has adjuvant properties and promotes expansion of antigen-specific Th1 and Th2 cells in vivo

Aluminum hydroxide and incomplete Freund's adjuvant (IFA) are the only adjuvants approved for human use. Both are T helper 2 (Th2) adjuvants, however, T helper 1 (Th1) immunity is induced if microbial products such as mycobacteria, CpG's, or bacterial toxins are included in the adjuvant preparation. The usefulness of bacterial toxins, such as Pertussis toxin (PT) or Cholera toxin (CT), as adjuvants for human vaccination is limited by toxic side effects and high immunogenicity. Hence, we asked whether or not the adjuvant activity of bacterial toxins on Th1 and Th2 immunity could be mimicked by chemical compounds of small molecular weight and less immunogenicity. In the present study, we show that Suramin, a small molecular weight naphthylurea, which mainly acts on G-proteins and on P2X/P2Y receptors, promotes expansion of hen eggwhite lysozyme (HEL)-specific Th1 and Th2 cells upon immunization of BALB/c mice with HEL in aluminum hydroxide (alum). The results indicated that the adjuvant effects of Suramin on T cell responses were mediated by enhancing the expression of MHC class II and costimulatory molecules on antigen presenting cells (APCs), and by increasing their pro-inflammatory cytokine production. Together, the results suggest that small molecular weight compounds such as Suramin could be used as alternative vaccine adjuvants.

Effects of T cell frequency and graft size on transplant outcome in mice

The features that determine whether graft-reactive T lymphocytes develop into effector cells capable of mediating organ destruction are not well understood. To investigate potential factors involved in this process, we first confirmed that female recipient mice acutely rejected minor Ag-disparate male skin, but not heart transplants. Despite this difference in outcome, heart and skin transplantation induced antidonor T cell responses of similar magnitude, specificity, and cytokine profile. The heart-graft-primed T cells transiently infiltrated the graft and ultimately induced the development of chronic transplant vasculopathy. Increasing the frequency of donor-reactive T cells by presensitization or by using TCR (CD8+ antimale)-transgenic recipients did not mediate acute rejection but accelerated the pace and severity of the vasculopathy. Surprisingly, decreasing the tissue mass of the donor heart by 50% resulted in acute rejection of these smaller grafts without increasing the frequency of antidonor effector T cells in the recipients. In complementary studies, placement of one or two male skin grafts on a single recipient did not affect the frequency or cytokine profile of the induced antimale T cell repertoire. Nonetheless, the recipients of single grafts acutely rejected the transplanted skin while the recipients of two skin grafts did not. These results provide new insight into the pathogenesis of transplant vasculopathy and provide an explanation for the difference in outcome between murine skin and heart transplants by highlighting the novel concept that the efficiency of transplant-reactive T cell immunity is heavily influenced by the tissue burden it encounters at the effector stage.

Indirect Minor Histocompatibility Antigen Presentation by Allograft Recipient Cells in the Draining Lymph Node Leads to the Activation and Clonal Expansion of CD4+T Cells That Cause Obliterative Airways Disease

Ag recognition by OVA-reactive OT-II (I-Ab restricted) and DO11.10 (I-Ad restricted) TCR-Tg CD4+ T cells after heterotopic transplantation of OVA transgene-expressing tracheal grafts was examined as a model of minor histocompatibility Ag (mHAg)-induced chronic allograft rejection. In response to airway allotransplantation with grafts expressing the OVA transgene, these TCR-Tg CD4+ T cells expressed the activation markers CD69 and CD44, demonstrated evidence of blastogenesis, underwent multiple rounds of cell division leading to their clonal expansion in the draining lymph node, and proceeded to differentiate to a effector/memory T cell phenotype based on a reduction in the expression of CD45RB. These mHAg-specific TCR-Tg CD4+ T cells responded equally well to fully MHC-mismatched tracheas and to class II-deficient allografts, demonstrating that donor mHAg recognition by recipient CD4+ T cells does not rely on Ag presentation by donor-derived APC. The activation of mHAg-specific TCR-Tg CD4+ T cells after their adoptive transfer into recipient mice given MHC-matched, but mHAg-disparate, airway allografts was associated with their movement into the allograft and the near uniform destruction of the transplanted airway tissue secondary to the development of obliterative airways disease. These results demonstrate that an activation of mHAg-reactive CD4+ T cells in the draining lymph node by recipient APC that indirectly express graft mHAg-derived peptide/class II MHC complexes precedes responder T cell proliferation and differentiation, and leads to the eventual migration of these alloreactive T cells to the transplanted airway tissue and the promotion of chronic graft rejection.

BAL levels of interleukin-18 do not change before or during acute rejection in lungtransplant recipients

STUDY OBJECTIVE

Acute rejection (AR) of the allograft is a major clinical problem after lungtransplantation. Repeated episodes of AR increase the risk of developing obliterative bronchiolitis, the main cause of mortality in this patient group. It is believed that AR is caused by T-lymphocytes reacting to donor antigens and in turn activating antigen presenting cells (APC) such as alveolar macrophages. Hypothetically, the interferon-gamma inducing cytokine IL-18 released from activated macrophages can play a role in the development of AR by modulating cytotoxic T-lymphocytes.

DESIGN

To determine whether IL-18 may serve as a marker of AR, we retrospectively analysed the concentration of soluble IL-18 protein and inflammatory cells in bronchoalveolar lavage fluid (BAL) from lungtransplant recipients.

PATIENTS

To minimize confounding factors, eight pairs of patients were matched for age, gender, pre-op diagnosis, type of operation, absence of infection and time post transplant.

METHODS

BAL levels of IL-18 (ELISA) and BAL cell differentials were analysed before, during and after an episode of AR and compared with the matched control group.

CONCLUSION

We found no changes in IL-18 concentration in BAL associated with AR. IL-18 in BAL did not correlate with BAL lymphocyte percentage. We conclude that change in soluble IL-18 protein does not constitute a useful marker of acute rejection in lung allograft recipients.

Development of a novel transgenic mouse for the study of interactions between CD4 and CD8 T cells during graft rejection

The goal of this study was the development of a system in which the cooperative interactions between CD4 and CD8 T cells specific for defined peptides from a single minor histocompatibility antigen could be studied. A transgenic mouse strain that expresses chicken ovalbumin (Act-mOVA) on the surface of all cells in the body was produced as a source of tissues containing such an antigen. Skin grafts from Act-mOVA donors were rapidly and completely rejected by wild-type recipients, but only when both CD4 and CD8 T cells were present. CD4 T cells by themselves caused an incomplete form of rejection characterized by rapid but partial contraction of Act-mOVA grafts. CD8 T cells alone caused complete rejection of Act-mOVA skin grafts but only after a long delay. Adoptively transferred ovalbumin-specific TCR-transgenic CD4 and CD8 T cells were stimulated by Act-mOVA graft antigens and CD8 T-cell accumulation in the grafts was enhanced by specific CD4 T cells. These findings, together with the fact that the ligand for ovalbumin peptide-specific CD8 T cells can be detected in Act-mOVA tissues with an MHC-restricted antibody, make this an ideal system for the study of cooperation between CD4 and CD8 T cells.

Critical role of the Toll-like receptor signal adaptor protein MyD88 in acute allograft rejection

The Toll-like receptors (TLRs) are recently discovered germline-encoded receptors on APCs that are critically important in innate immune recognition of microbial pathogens. However, their role in solid-organ transplantation is unknown. To explore this role, we employed a skin allograft model using mice with targeted deletion of the universal TLR signal adaptor protein, MyD88. We report that minor antigen-mismatched (HY-mismatched) allograft rejection cannot occur in the absence of MyD88 signaling. Furthermore, we show that the inability to reject these allografts results from a reduced number of mature DCs in draining lymph nodes, leading to impaired generation of anti-graft-reactive T cells and impaired Th1 immunity. Hence, this work demonstrates that TLRs can be activated in a transplant setting and not solely by infections. These results link innate immunity to the initiation of the adaptive alloimmune response.

The relative contribution of direct and indirect antigen recognition pathways to the alloresponse and graft rejection depends upon the nature of the transplant

In this study, we measured direct and indirect T-cell alloresponses mediated by CD4(+) and CD8(+) T cells in three mouse transplantation models: skin, cornea, and retina. We show that the contribution of direct and indirect antigen recognition pathways to the alloresponse to fully allogeneic grafts varies depending upon the nature of the tissue/organ transplanted. The implications of this finding for understanding the cellular mechanisms by which rejection is mediated in different transplant models are discussed.

Skin graft rejection elicited by beta 2-microglobulin as a minor transplantation antigen involves multiple effector pathways: role of Fas-Fas ligand interactions and Th2-dependent graft eosinophil infiltrates

Beta(2)-microglobulin (beta(2)m)-derived peptides are minor transplantation Ags in mice as beta(2)m-positive skin grafts (beta(2)m(+/+)) are rejected by genetically beta(2)m-deficient recipient mice (beta(2)m(-/-)). We studied the effector pathways responsible for the rejection induced by beta(2)-microglobulin-derived minor transplantation Ags. The rejection of beta(2)m(+/+) skin grafts by naive beta(2)m(-/-) mice was dependent on both CD4 and CD8 T cells as shown by administration of depleting mAbs. Experiments performed with beta(2)m(-/-)CD8(-/-) double knockout mice grafted with a beta(2)m(+/+) MHC class I-deficient skin showed that sensitized CD4 T cells directed at beta(2)m peptides-MHC class II complexes are sufficient to trigger rapid rejection. Rejection of beta(2)m(+/+) grafts was associated with the production of IL-5 in vitro, the expression of IL-4 and IL-5 mRNAs in the grafted tissue, and the presence within rejected grafts of a considerable eosinophil infiltrate. Blocking IL-4 and IL-5 in vivo and depleting eosinophils with an anti-CCR3 mAb prevented graft eosinophil infiltration and prolonged beta(2)m(+/+) skin graft survival. Lymphocytes from rejecting beta(2)m(-/-) mice also displayed an increased production of IFN-gamma after culture with beta(2)m(+/+) minor alloantigens. In vivo neutralization of IFN-gamma inhibited skin graft rejection. Finally, beta(2)m(+/+) skin grafts harvested from B6(lpr/lpr) donor mice, which lack a functional Fas molecule, survived longer than wild-type beta(2)m(+/+) skin grafts, showing that Fas-Fas ligand interactions are involved in the rejection process. We conclude that IL-4- and IL-5-dependent eosinophilic rejection, IFN-gamma-dependent mechanisms, and Fas-Fas ligand interactions are effector pathways in the acute rejection of minor transplantation Ags.

In vivo blockade of macrophage migration inhibitory factor prevents skin graft destruction after indirect allorecognition

BACKGROUND

The effector mechanisms that ultimately destroy transplanted tissues are poorly understood. In particular, it is not clear how CD4+ T cells primed to donor-derived determinants expressed on recipient MHC molecules (the indirect pathway) can mediate graft destruction in the absence of cognate recognition of peptide: MHC on the graft cells themselves. Macrophage migration inhibitory factor (MIF) inhibits macrophage movement and is a proinflammatory and regulatory cytokine known to be essential for development of delayed-type hypersensitivity reactions.

METHODS

To test whether MIF participates in graft destruction following indirect recognition, we studied rejection of MHC-II-deficient skin grafts placed on allogeneic SCID recipients adoptively transferred with naïve CD4+ T cells, and the recipients were treated with neutralizing anti-MIF monoclonal antibody or isotype control IgG. In this model graft rejection can only occur indirectly as the graft cells lack MHC II for recognition by the recipient CD4+ T cells.

RESULTS

We found that in vivo blockade of MIF inhibited indirect CD4+ cell-mediated skin graft destruction, and markedly reduced detectable macrophages within the grafts. The neutralizing anti-MIF antibody significantly inhibited alloreactive DTH but did not prevent T cell priming or interferon-gamma release by primed T cells.

CONCLUSIONS

The results strongly implicate MIF as an active participant in skin graft destruction after indirect recognition and suggest that this effect is mediated through an inhibition of macrophage migration and/or function.

Tolerance or immunity to a tumor antigen expressed in somatic cells can be determined by systemic proinflammatory signals at the time of first antigen exposure

Mice transgenic for the E7 tumor Ag of human papillomavirus type 16, driven from a keratin 14 promoter, express E7 in keratinocytes but not dendritic cells. Grafted E7-transgenic skin is not rejected by E7-immunized mice that reject E7-transduced transplantable tumors. Rejection of recently transplanted E7-transgenic skin grafts, but not of control nontransgenic grafts or of established E7-transgenic grafts, is induced by systemic administration of live or killed Listeria monocytogenes or of endotoxin. Graft recipients that reject an E7 graft reject a subsequent E7 graft more rapidly and without further L. monocytogenes exposure, whereas recipients of an E7 graft given without L. monocytogenes do not reject a second graft, even if given with L. monocytogenes. Thus, cross-presentation of E7 from keratinocytes to the adaptive immune system occurs with or without a proinflammatory stimulus, but proinflammatory stimuli at the time of first cross-presentation of Ag can determine the nature of the immune response to the Ag. Furthermore, immune effector mechanisms responsible for rejection of epithelium expressing a tumor Ag in keratinocytes are different from those that reject an E7-expressing transplantable tumor. These observations have implications for immunotherapy for epithelial cancers.

A role for TGFbeta and B cells in immunologic tolerance after intravenous injection of soluble antigen

BACKGROUND

Intravenous injection of soluble antigen has been reported to induce immunologic tolerance through a variety of mechanisms including T-cell deletion, anergy, and suppression. To clarify the reported discrepancies, we studied mechanisms of intravenous tolerance to a defined transgenic minor transplantation antigen in mice.

METHODS

Wild-type C57BL/6 (B6) mice or congenic B6 B-cell knockout mice were made tolerant to beta-galactosidase (beta-gal). Clinical tolerance was assessed by placement of B6 beta-gal transgenic (tg) and third-party skin grafts. In vitro analysis of T- and B-cell immunity and in vivo treatment with anti-TGFbeta antibodies were used to define mechanisms of induced tolerance.

RESULTS

Intravenous injection of beta-gal induced true immunologic tolerance to beta-gal tg skin in wild-type but not in B-cell-deficient recipients, suggesting that antigen presentation by B cells was required for the effect. The tolerogenic manipulation primed a population of CD4+, beta-gal-specific, TGFbeta-producing T cells. Although evidence for both anergy and suppression were observed, subsequent data demonstrated that TGFbeta was a critical immunoregulatory mediator of the tolerant state: neutralizing anti-TGFbeta antibodies fully prevented the induction of tolerance to B6 beta-gal tg skin grafts. Second male beta-gal tg grafts placed onto female recipients that were previously made tolerant to female beta-gal tg skin were rapidly rejected, however, suggesting that this TGFbeta-induced tolerance could not be linked to additional antigenic determinants.

CONCLUSIONS

The studies demonstrate a critical role for TGFbeta in mediating tolerance after intravenous injection of antigen but additionally raise concerns about the stability of this tolerant state.

Potential of costimulation-based therapies for composite tissue allotransplantation

Clinical success has not been routinely achieved for composite tissue allotransplantation (CTA). Although most of the technical details of CTA have been overcome, the immunological aspects of these procedures have proved complex. Many traumatic conditions requiring CTA contraindicate acute global immunosuppression. Moreover, the risk of long-term immunosuppression is difficult to reconcile with non-life-threatening defects that can be adequately palliated. Recently, several successful immunomodulating strategies have been introduced for solid organ transplantation. They include therapies that alter costimulatory signals at engraftment. One approach, using treatment with a monoclonal antibody directed against CD154, has shown promise in rodent and nonhuman primate models and is discussed as a potential strategy for CTAs.

Heterogeneity of T cell clones specific for a single indirect alloantigenic epitope (I-Ab/H-2Kd54-68) that mediate transplant rejection

BACKGROUND

One of the complexities of solid organ allograft rejection is the inherent diversity of the specific T cell antigenic epitopes that participate in this response, including the role of direct alloantigen recognition and indirect recognition of donor-derived peptides in recipient antigen-presenting cells. To probe the role of distinct T cell receptor (TCR) avidity differences and the role of cytokine expression patterns of different effector T cells that may participate in allograft rejections, we have identified a dominant allopeptide derived from the H-2Kd molecule, recognized by H-2b CD4 T cells in the context of syngeneic I-Ab.

METHODS

To identify a stimulatory peptide derived from the H-2Kd molecule, a panel of synthetic overlapping peptides was screened for immunogenicity and a panel of T cell clones established. These clones were characterized for TCR Vbeta usage by mAb staining and/or reverse transcribed-polymerase chain reaction analysis, peptide dose sensitivity as a marker of TCR avidity, cytokine expression phenotype in vitro, and their ability to mediate rejection of a vascularized cardiac allograft after adoptive transfer to immunodeficient mice.

RESULTS

The H-2Kd54-68 peptide was identified as a dominant stimulatory peptide by the ability of T cells from C57BL/6 (H-2b) mice primed by a combination of allogeneic spleen cell injection and mixed peptide immunization to mount an in vitro proliferative response and interferon-gamma production by peptide stimulation. Furthermore, direct immunization with synthetic H-2Kd54-68 peptide of normal C57BL/6 mice resulted in accelerated rejection of both skin and cardiac allografts from B10.D2 (H-2d) mice, but not 3rd party B10.BR (H-2k) grafts. A panel of 15 distinct CD4+ clones specific for H-2Kd54-68 peptide were established and shown to utilize a variety of TCR Vbeta and different apparent TCR avidities to H-2Kd54-68 peptide when stimulated in vitro. To characterize these clones further, two clones were chosen based on the difference of avidity to H-2Kd54-68 peptide. The cytokine expression pattern was determined and indirect alloantigen specificity confirmed by analysis of responses to purified peptide and B10.D2 spleen cells using normal H.2b and I-Abeta chain knockout mice as APC donors. Both of these T cell clones were able to mediate rejection of B10.D2, but not B10.BR hearts, in immunodeficient mice, but the morphological pattern of T cell infiltration was distinct.

CONCLUSIONS

These results demonstrate the potential importance of fine dissection of the alloantigeneic response to solid organ transplants and provide unique insights into the role of TCR avidity and cytokine expression patterns in different morphological patterns of transplant rejection.

Comprehensive assessment of determinant specificity, frequency, and cytokine signature of the primed CD8 cell repertoire induced by a minor transplantation antigen

T cell immunity is often focused on one peptide segment of a complex protein Ag, with other epitopes inducing weaker, low frequency responses or no responses at all. Such determinant hierarchy has been well characterized for MHC class II-restricted CD4 cell immunity, but is less well understood for class I-restricted CD8 cell responses. We studied class I determinant recognition in a skin transplant model with beta-galactosidase (beta-gal) as a minor transplantation Ag. CD8 T cells from C57BL/6 mice that rejected congenic C57BL/6 beta-gal transgenic skin were tested in enzyme-linked immunospot assays for recall responses to single-step, overlapping, 9-mer peptides that spanned a 94-aa region of the beta-gal sequence. This approach provided every possible class I-restricted peptide for CD8 cell recognition, allowing us to define the in vivo frequency of CD8 cells specific for each of the 86 individual peptides. While four peptides were predicted to bind to the Kb or Db molecules, only one (beta-gal96-103) actually induced an immune response. No peptides outside of the motifs were recognized. Tolerization to beta-gal96-103 significantly prolonged beta-gal transgenic skin graft survival, confirming its immune dominance. Therefore, single-determinant dominance characterized this CD8 cell response. The data demonstrate the feasibility of large-scale, comprehensive, class I determinant mapping, an approach that should be indispensable in measuring CD8 cell immunity in humans.