In vivo responses of alloreactive lymphocytes stimulated in vitro. Skin graft rejection mediated by MLR-Primed lymphocytes

Mouse lymphocytes that have been primed in vitro against alloantigens show a specific increase in cells reactive to the priming antigens in mixed lymphocyte response (MLR) and include cells that are specifically cytotoxic in vitro. The primed population also contains cells capable of causing rejection of skin grafts when injected into nude mice. Functional enrichment of cells capable of rejecting skin grafts bearing specific alloantigens and depletion of cells capable of rejecting a third-party graft have been shown. Priming the cells a second time in vitro may result in a moderate enrichment of cells capable of rejecting the specific graft and depletion of cells reactive to third-party skin compared with once-primed cells. These findings support the prediction that the MLR is an in vitro model of allograft responses in vivo.

Differences between the Fc receptors of two lymphocyte subpopulations of human peripheral blood

The Fc receptors of human peripheral blood lymphocytes bearing stable membrane Ig (B cells) and those bearing cytophilic membrane Ig (UL cells) were evaluated for binding avidity and interaction with human 'Ia-like' alloantigens. Titration experiments showed that binding of soluble antigen-antibody complexes to UL cells was readily detected at low concentations (5-10 microgram/ml), whereas high concentrations (400-800 microgram/ml) were necessary to detect binding to most B lymphocytes. Binding at all concentrations was dependent on an intact Fc portion of the antibody molecule within the antigen-antibody complex. F(ab')2 fragments of antibodies against human 'Ia-like' antigens inhibited binding of complexed Ig to B cells but not UL cells. These differences are compatible with the possibility that the Fc receptors of the two cell populations are distinct molecular entities or, alternatively, are the same molecules and differ in quantity, distribution, or mobility on the surface of the two cell types.

Specific suppression of MLC and CML by anti-idiotypic antibodies in the mouse

Rabbit antisera directed against idiotypic determinants of alloreactive mouse CBA anti-C57BL/6 T blasts were raised in the following manner: first, a rabbit serum directed against nonspecific CBA blasts cells was prepared by injecting CBA concanavalin A blasts three times at monthly intervals into a rabbit. Second, specific CBA anti-C57BL/6 T lymphoblasts were induced in a mixed lymphocyte culture (MLC), were purified by gravity sedimentation through a fetal calf serum gradient, and, finally, were incubated with the anti-blast serum from the first step. During this incubation, presumably all epitopes of the blast cell population were blocked by anti-blast antibodies, except for the greatly amplified set of CBA anti-C57BL/6 alloreactive idiotypes. The mixture was then injected into fresh rabbits, which were boosted with similar mixtures after 3 and 6 weeks. Blood samples were removed 10 days after each injection. Such sera, when used together with complement, inhibited specifically the stimulation of CBA cells by C57BL/6 antigens in MLC and the CBA anti-C57BL/6 killer cells.

Relationships between beta2-microglobulin and alloantigens coded for by the major histocompatibility complexes of the rabbit and the guinea pig

Treatment of rabbit and guinea pig lymphocytes with Fab' fragments of anti-beta2-microglobulin completely inhibited the cytotoxic effects of alloantisera to RLA or GPLA antigens, respectively. Aggregation of beta2-microglobulin on the lymphocyte surface by successive incubations with goat anti-beta2-microglobulin on the lymphocyte surface by successive incubations with goat anti-beta2-microglobulin and F(ab')2 fragments of rabbit anti-goat IgG also made rabbit lymphocytes resistant to lysis by anti-RLA, and guinea pig lymphocytes resistant to lysis by anti-RLA, and guinea pig lymphocytes resistant to lysis by anti-GPLA. The two kinds of pretreatment of guinea pig lymphocytes did not affect the cytotoxicity of antisera directed against guinea pig Ia antigens. These results in conjunction with previous findings in the mouse and in man suggest that beta2-microglobulin on the lymphocyte surface in mammals is generally associated with major serologically defined histocompatibility antigens but not with I-region-associated antigens.

Accumulation of allo-MHC cross-reactive memory T cells in bone marrow

The T cells in the bone marrow (BM) have recently been shown to be enriched with memory T cells. We investigated in this study the reactivity of minor-antigen specific memory cytotoxic T lymphocytes (CTLs) induced from the BM of in vivo primed mice using two different antigen systems. The antigen-specific CTLs could be efficiently induced from the BM of immunized mice. This CTL activity was not observed with naïve control mice, indicating that the activity was largely attributable to the memory T cells. Notably, these minor antigen specific CTLs showed cross-reactivity to allo-MHC antigens. Cold target inhibition analyses revealed that the same CTL populations were responsible for both anti-minor antigen and anti-allo-MHC reactivity. Taken collectively, these results not only confirmed functionally the enrichment of memory CTLs in the BM, but also indicated that such memory cells could cross-react with allo-MHC antigens. The possible role of these BM-resident memory T cells in the development of graft-versus-host disease (GVHD) is also discussed.

Suppression of islet allogeneic immune response by indoleamine 2,3 dioxygenase-expressing fibroblasts

Success of transplantation of pancreatic islets which is a promising way for restoring efficient insulin regulation in type 1 diabetes depends on lifelong use of immunosuppressive drugs. To eliminate the use of systemic immunosuppressive drugs for islet transplantation, we examined the potential use of a local immunosuppressive factor, indoleamine 2,3-dioxygenase (IDO). Thus, the aim of this study was to determine whether local expression of IDO in bystander syngeneic fibroblasts could prevent islet allogeneic immune response in vitro. C57BL/6 (B6) mouse fibroblasts were induced to express IDO by either IFN-gamma treatment or transduction with an adenoviral vector and were co-cultured with B6 mouse lymphocytes and BALB/c mouse pancreatic islets in the presence or absence of an IDO inhibitor. Proliferation of lymphocytes were then assessed using [(3)H]-thymidine incorporation assay. IDO-expression by co-cultured syngeneic fibroblasts resulted in a five-fold decrease in lymphocyte proliferation rate upon stimulation of lymphocytes by allogeneic mouse pancreatic islets (21.9% +/- 5.3 and 22.1% +/- 4.9 in the preparations with IFN-gamma treated and genetically modified IDO-expressing fibroblasts, respectively vs. 100% in control groups, P < 0.01). Allogeneic response was restored when IDO inhibitor was added to the culture indicating that suppression was due to IDO. In conclusion, this study shows that local expression of IDO by syngeneic bystander fibroblasts can suppress in vitro proliferation of lymphocytes in response to stimulation with allogeneic pancreatic islets. This local immunosuppressive function of IDO may be employed for development of a novel alternative strategy for preventing allogeneic islet graft rejection.

Blockade of chronic graft-versus-host disease by alloantigen-induced CD4+CD25+Foxp3+ regulatory T cells in nonlymphopenic hosts

CD4(+)CD25(+) regulatory T cells (Tregs) are well known to suppress immunopathology induced in lymphopenic animals following T cell reconstitution, including acute graft-versus-host disease (GVHD) post-bone marrow transplantation. The regulatory potential of this subset in nonlymphopenic hosts and in chronic, Th2-mediated GVHD is less clear. We have generated alloantigen-specific cells from CD4(+)CD25(+) populations stimulated with MHC-disparate dendritic cells and found them to express a stable Treg forkhead box p3(+) phenotype with enhanced suppressive activity mediated by cell contact. When transferred into nonlymphopenic F1 hosts, nonspecific Tregs proliferated as rapidly as CD4(+)CD25(-) cells but displayed distinct growth kinetics in vitro. Tregs, expanded in response to alloantigen in vitro, displayed greatly enhanced suppressive activity, which was partially antigen-specific. They were effective inhibitors of chronic GVHD, blocking donor cell engraftment, splenomegaly, autoantibody production, and glomerulonephritis. CD25(+) and CD25(-) cells were equally susceptible to inhibition by immunosuppressive drugs targeting TCR signaling and rapamycin, but Tregs were resistant to inhibition by dexamethasone. The data indicate that alloantigen-driven expansion, rather than homeostatic proliferation, is key to the effectiveness of CD4(+)CD25(+) Tregs in GVHD and suggest that cellular therapy with alloantigen-induced Tregs in combination with glucocorticoid treatment would be effective in prevention of chronic GVHD after immune reconstitution.

Allorecognition and the alloresponse: clinical implications

The artificial transfer of tissues or cells between genetically diverse individuals elicits an immune response that is adaptive and specific. This response is orchestrated by T lymphocytes that are recognizing, amongst others, major histocompatibility complex (MHC) molecules expressed on the surface of the transferred cells. Three pathways of recognition are described: direct, indirect and semi-direct. The sets of antigens that are recognized in this setting are also discussed, namely, MHC protein products, the MHC class I-related chain (MIC) system, minor histocompatibility antigens and natural killer cell receptor ligands. The end product of the effector responses are hyperacute, acute and chronic rejection. Special circumstances surround the situation of pregnancy and bone marrow transplantation because in the latter, the transferred cells are the ones originating the immune response, not the host. As the understanding of these processes improves, the ability to generate clinically viable immunotherapies will increase.

T cell therapies following hematopoietic stem cell transplantation: surely there must be a better way than DLI?

Advances in the past few years have significantly improved adoptive immunotherapy strategies available following autologous and allogeneic hematopoietic stem cell transplantation (HSCT). Minimal residual disease, relapsed disease and viral infections remain a significant cause of mortality in patients undergoing HSCT. Novel therapies are critically needed to overcome these management dilemmas, while sparing the graft-versus-tumor effect and avoiding graft-versus-host disease. This review focuses on the T-cell strategies currently available to allay disease while minimizing toxicities in patients who have undergone HSCT.

Human interferon lambda-1 (IFN-lambda1/IL-29) modulates the Th1/Th2 response

Interferon lambda-1 (IFN-lambda1/IL-29) is a member of the Type-III interferon family, which contains three ligands: IFN-lambda1, 2 and 3. These three ligands use the same unique heterodimeric receptor composed of CRF2-12 (IFN-lambda-R1/IL-28Ralpha) and CRF2-4 (IL10-R-beta) chains. Like their close relatives, the Type-I interferons, IFN-lambda1, 2 and 3, promote the phosphorylation of STAT1 and STAT2, induce the ISRE3 complex, elevate OAS and MxA expression and exhibit antiviral activity in vitro. Their use of the IL10-R-beta chain and their ability to phosphorylate STAT3, STAT4 and STAT5 suggested that they may also exhibit immunomodulatory activity; their antiviral action led us to hypothesize that this activity might be directed toward the Th1/Th2 system. Here, we have demonstrated that IFN-lambda1 altered the activity of Th cells in three separate experimental systems: (i) mitogen stimulation, (ii) mixed-lymphocyte reaction (MLR) and (iii) stimulation of naive T cells by monocyte-derived dendritic cells (mDC). In Con-A stimulation assays, the inclusion of IFN-lambda1 consistently led to markedly diminished levels of secreted interleukin (IL-13) with occasional coincident, modest elevation of secreted IFN-gamma. IL-13 secretion was 100-fold more sensitive to IFN-lambda1 than was IFN-gamma secretion. These observations were also made in the allogeneic two-way MLR. IFN-lambda1 was able to alter cytokine-mediated Th biasing and when naive T cells were exposed to allogeneic mDC that had been matured in the presence of IFN-lambda1, secreted IL-13 was again markedly and consistently reduced, whereas secreted IFN-gamma was largely unaltered. These functions were independent of IL-10. Our data support a hitherto unsuspected role for IFN-lambda1 in modulating the development of Th1 and Th2 cells, with an apparent emphasis on the diminution of IL-13 secretion.

Murine bone marrow stromal progenitor cells elicit an in vivo cellular and humoral alloimmune response

Stromal progenitor cells (SPC) exhibit immunosuppressive effects in vitro that have led to speculation regarding their capacity to evade host immune recognition and to treat autoimmune diseases and gravt-versus-host disease. However, there is little in vivo experimental data to support these immunologic claims. To assess immune recognition of SPC in vivo, we evaluated the immune response of animals transplanted with SPC. C57BL/6 (B6) or Balb/c adult, murine, bone marrow-derived SPC (AmSPC) were administered by intraperitoneal injection into B6 recipients. T cell proliferation and alloantibody response was assessed from spleens and peripheral blood harvested from transplanted animals and analyzed by cell proliferation assay and flow cytometry. To assess tolerance induction, transplanted animals also received allogeneic skin grafts. Animals injected with allogeneic AmSPC mounted an accelerated CD4 response to alloantigen compared to syngeneic AmSPC injected and uninjected controls. Allogeneic AmSPC-injected animals also demonstrated high titers (> or =1:1000) of antibody directed against allogeneic AmSPC targets. Animals primed with donor or host-matched AmSPC also failed to induce tolerance, and all animals exhibited rejection of allogeneic skin grafts (n = 7, P < .0001). In contrast to their in vitro behavior, our data demonstrate that AmSPC are recognized by the host immune system in vivo, elicit a cellular and humoral immune response, and fail to induce tolerance. These findings have significant implications for all allogeneic SPC-based therapeutic strategies.

Regulatory T cells and T cell depletion: role of immunosuppressive drugs

Allogeneic immune responses are modulated by a subset of host T cells with regulatory function (Treg) contained within the CD4(+)CD25(high) subset. Evidence exists that Treg expand after peritransplantation lymphopenia, inhibit graft rejection, and induce and maintain tolerance. Little, however, is known about the role of Treg in the clinical setting. IL-2 and activation by T cell receptor engagement are instrumental to generate and maintain Treg, but the influence of immunosuppressants on Treg homeostasis in humans in vivo has not been investigated. This study monitored Treg phenotype and function during immune reconstitution in renal transplant recipients who underwent profound T cell depletion with Campath-1H and received sirolimus or cyclosporine (CsA) as part of their maintenance immunosuppressive therapy. CD4(+)CD25(high) cells that expressed FOXP3 underwent homeostatic peripheral expansion during immune reconstitution, more intense in patients who received sirolimus than in those who were given CsA. T cells that were isolated from peripheral blood long term after transplantation were hyporesponsive to alloantigens in both groups. In sirolimus- but not CsA-treated patients, hyporesponsiveness was reversed by Treg depletion. T cells from CsA-treated patients were anergic. Thus, lymphopenia and calcineurin-dependent signaling seem to be primary mediators of CD4(+)CD25(high) Treg expansion in renal transplant patients. These findings will be instrumental in developing "tolerance permissive" immunosuppressive regimens in the clinical setting.

Tolerogenic response: Allorecognition pathways

The induction of immune tolerance in transplant recipients has been sought for many years but only a fundamental understanding of the immunological mechanisms underlying graft rejection will allow manipulation of the anti-graft immune response. In general, acute rejection is better understood and treated than chronic rejection, as they occur through partially different mechanisms. It is now generally accepted that recognition of same-species, non-self antigens (allorecognition) occurs through at least two different mechanisms, the direct and indirect pathways. In the direct pathway, donor MHC molecules on the surface of donor antigen-presenting cells (APCs) are recognised directly by the recipient's T cells. This mechanism is so immediate that it seems to be primarily involved in acute graft rejection. Since APCs of donor origin are depleted with time a second mechanism, the indirect pathway, takes over to cause chronic rejection, in which foreign MHC molecules are internalised, partially digested and presented as peptides to recipient T cells. Nonetheless, a number of studies are only fully understood when a third proposed allorecognition mechanism is taken into account. This is the semi-indirect pathway, as discussed in this short report.

Do NK-cell receptors and alloreactivity affect solid organ transplantation?

Although natural killer cells lyse targets without pre-sensitization, and in an MHC-unrestricted manner, they can also respond to healthy allogeneic cells of different MHC type. Such alloreactivity is a consequence of NK cells using clonally distributed, inhibitory MHC class I receptors to achieve tolerance to healthy autologous cells. Absence of an appropriate MHC class I ligand on an allogeneic cell erroneously informs the NK cell that the allogeneic cell has lost MHC class I expression and should be killed. Potential NK-cell allo-reactivities are common in non-HLA-identical hematopoietic cell transplants and can have both beneficial and detrimental effects. Less is known of NK-cell allo-reactivities in solid organ transplantation. In animal models NK cells are neither necessary nor sufficient for acute transplant rejection, but they can make a contribution by helping activate T cells. Genes encoding NK-cell receptors for polymorphic MHC class I molecules are also highly polymorphic, contributing to variability of the NK-cell repertoire and response in the human population. These receptors could represent intrinsic patient factors that influence the success of their transplanted solid organs.

Optical imaging of experimental arthritis using allogeneic leukocytes labeled with a near-infrared fluorescent probe

PURPOSE

The purpose of this study was to assess the feasibility of inflammation detection in an antigen-induced arthritis model using fluorescent leukocytes and optical imaging.

METHODS

Antigen-mediated monoarthritis was induced in the right knee of 12 Sprague-Dawley rats. Six rats remained untreated and six rats were treated with cortisone. All rats received ex vivo fluorescent-labeled rat leukocytes. Optical images of both knees were acquired before and at 5 min, 1 h, 4 h, and 24 h after cell injection. Images were evaluated qualitatively and quantitatively by calculating signal intensity ratios between the right arthritic (A) and contralateral normal (N) knee. A/N ratios were tested for significant differences between baseline values and values after cell injection using a paired t test as well as between the untreated and cortisone-treated group using an unpaired t test. Synovial specimens were processed and evaluated for labeled cells with fluorescence microscopy.

RESULTS

At 4 h and 24 h p.i., the A/N ratios of untreated arthritic knees showed a significant signal increase compared with baseline values (p<0.05) and a significant difference compared with A/N ratios of cortisone-treated animals (p<0.05). Fluorescent microscopy confirmed the presence of labeled cells in the arthritic synovium.

CONCLUSION

Inflammation in antigen-induced arthritis can be detected with ex vivo labeled allogenic leukocytes and optical imaging.

Host MHC class II+ antigen-presenting cells and CD4 cells are required for CD8-mediated graft-versus-leukemia responses following delayed donor leukocyte infusions

Following bone marrow transplantation, delayed donor leukocyte infusions (DLIs) can induce graft-versus-leukemia (GVL) effects without graft-versus-host disease (GVHD). These antitumor responses are maximized by the presence of host hematopoietic antigen-presenting cells (APCs) at the time of DLI. Using a tumor-protection model, we demonstrate here that GVL activity following administration of DLIs to established mixed chimeras is dependent primarily on reactivity to allogeneic MHC antigens rather than minor histocompatibility or tumor-associated antigens. CD8(+) T-cell-dependent GVL responses against an MHC class II-negative tumor following delayed DLI require CD4(+) T-cell help and are reduced significantly when host APCs lack MHC class II expression. CD4(+) T cells primed by host APCs were required for maximal expansion of graft-versus-host reactive CD8(+) T cells but not their synthesis of IFN-gamma. In contrast, the GVL requirement for CD4(+) T-cell help was bypassed almost completely when DLI was administered to freshly irradiated recipients, indicating that the host environment is a major factor influencing the cellular mechanisms of GVL.

Capture of antigen-specific T lymphocytes from human blood by selective immortalization to establish long-term T-cell lines maintaining primary cell characteristics☆

To establish long-term, antigen-specific T-cell lines and clones, we selectively immortalized antigen-responsive T cells from human peripheral blood mononuclear cells (PBMCs). PBMCs were stimulated with either alloantigen or soluble antigen, then infected with a murine leukemia virus-based retroviral vector carrying an immortalizing gene, either the Tax gene from human T-cell leukemia virus type 1, or the human telomerase-reverse transcriptase gene. Since such vectors can only integrate in dividing cells, only antigen-activated T cells are efficiently transduced. This approach generated immortalized antigen-specific CD4+ and CD8+ T-cell lines that maintained strictly IL-2-dependent growth and HLA-restricted, antigen-specific responsiveness, some of which have been in continuous culture for longer than 1 year, far in excess of the survival of parallel control non-immortalized cultures. Clones derived from these lines showed antigen-specific proliferation with induced cytokine and chemokine production, and, in the case of a CD8+ T-cell clone, antigen-specific cytolytic activity. This approach provides a convenient, reproducible means for generating a stable, continuously renewable source of antigen-specific T lymphocytes for a variety of studies of T cell biology.

Bortezomib induces selective depletion of alloreactive T lymphocytes and decreases the production of Th1 cytokines

We explored the ability of the proteasome inhibitor bortezomib, which prevents nuclear factor kappaB (NF-kappaB) activation, to block T-cell activation, proliferation, and survival within alloreactive compared with resting T cells. For this purpose, T cells were stimulated with PHA, alphaCD3/alphaCD28, or allogeneic dendritic cells or through mixed lymphocyte cultures. NF-kappaB expression increased in activated T lymphocytes compared with resting T cells. Of interest, the higher the NF-kappaB expression, the more intense the proliferative blockade induced by bortezomib. Moreover, after mixed lymphocyte reaction (MLR) cultures, alloreactive T cells were 2 logs more sensitive to bortezomib-induced apoptosis than the resting T-cell counterpart. This effect was due to a selective induction of apoptosis among activated T cells that was related to caspase activation and cleavage of the antiapoptotic bcl-2 protein and was partially abolished by the addition of the pancaspase inhibitor Z-VAD-FMK. In addition, after secondary MLR, the number of activated T cells was significantly reduced among T lymphocytes previously cultured with bortezomib when cells from the same donor were used as stimulating cells. By contrast, when third-party donor cells were used as stimulating cells, no significant differences were observed between T lymphocytes previously exposed or not to the drug, indicating a highly specific depletion of T lymphocytes alloreactive against primary donor antigens. The addition of bortezomib decreased not only the proliferation and viability of activated T lymphocytes but also the levels of IFNgamma and IL-2, which were significantly decreased among activated T cells cultured with bortezomib at doses ranging from 10 to 100 nM. In conclusion, at concentrations reached in the clinical setting, bortezomib induces selective apoptosis and decreases Th1 response among alloreactive T lymphocytes while it barely affects unstimulated T cells. These results establish the basis for the clinical use of bortezomib in the management of graft-versus-host disease (GVHD).

How TCRs bind MHCs, peptides, and coreceptors

Since the first crystal structure determinations of alphabeta T cell receptors (TCRs) bound to class I MHC-peptide (pMHC) antigens in 1996, a sizable database of 24 class I and class II TCR/pMHC complexes has been accumulated that now defines a substantial degree of structural variability in TCR/pMHC recognition. Recent determination of free and bound gammadelta TCR structures has enabled comparisons of the modes of antigen recognition by alphabeta and gammadelta T cells and antibodies. Crystal structures of TCR accessory (CD4, CD8) and coreceptor molecules (CD3epsilondelta, CD3epsilongamma) have further advanced our structural understanding of most of the components that constitute the TCR signaling complex. Despite all these efforts, the structural basis for MHC restriction and signaling remains elusive as no structural features that define a common binding mode or signaling mechanism have yet been gleaned from the current set of TCR/pMHC complexes. Notwithstanding, the impressive array of self, foreign (microbial), and autoimmune TCR complexes have uncovered the diverse ways in which antigens can be specifically recognized by TCRs.

Human monocytes as intermediaries between allogeneic endothelial cells and allospecific T cells: a role for direct scavenger receptor-mediated endothelial membrane uptake in the initiation of alloimmunity

Recipient monocytes, T cells, and donor endothelial cells (ECs) are recognized as critical components of allograft rejection. We have recently shown that human monocytes infiltrate vascularized allografts before clinical rejection and have thus hypothesized that monocytes, rather than costimulation-poor ECs, initiate an alloimmune response. However, the nature of the interactions between ECs, monocytes, and T cells has been incompletely defined. Specifically, it is not clear whether these cells interact in a hierarchical manner, nor is it apparent what constitutes an interaction. We therefore studied human ECs, monocytes, and T cells in various isolated in vitro combinations to define the salient features of their contact and to determine whether their interactions were sequential in nature. We find that T cells proliferate poorly to allogeneic ECs and autologous monocytes but well to autologous monocytes following allogeneic EC contact. We show that monocytes gain their stimulatory capacity by phagocytizing allogeneic but not autologous EC membranes in a process governed by scavenger receptors. This process facilitates the subsequent presentation of intact donor HLA molecules to T cells (semidirect presentation). Moreover, monocytes are receptive to T cell help only after exposure to ECs and require CD4+ T cells to optimally express costimulatory molecules and foster Ag presentation. Our results indicate that monocytes engage allogeneic ECs through scavenger receptors and are then primed to facilitate T cell activation in a codependent manner. This reciprocal codependence allows for monocytes to serve as a regulated bridge between the allograft and T cells.