Depletion of Alloreactive T Cells by a Specific Anti–Interleukin-2 Receptor p55 Chain Immunotoxin Does Not Impair In Vitro Antileukemia and Antiviral Activity

Generation and ex vivo expansion of cytotoxic T lymphocytes directed toward different types of leukemia or myelodysplastic cells using both HLA-matched and partially matched donors

OBJECTIVE

Successful priming and in vitro expansion of anti-leukemia cytotoxic T lymphocytes (CTL) are preliminary conditions for designing approaches of adoptive immunotherapy in patients with hematological malignancies undergoing allogeneic hematopoietic stem cell transplantation (HSCT). In this study, we evaluated the possibility of generating and expanding in vitro CTL directed toward different types of either leukemia or myelodysplastic cells, using both HLA-matched and partially matched donors.

PATIENTS AND METHODS

Eleven donor/recipient pairs were enrolled; donor-derived dendritic cells, pulsed with patient blast cells, were used to generate CTL.

RESULTS

Anti-leukemia CTL lines were successfully obtained from 10 of 11 donors. After repeated rounds of stimulation, CTL lines showed, along with an increase in cytotoxic activity, a variable but continuous expansion of cultured cells. In order to increase the magnitude of CTL expansion, two anti-leukemia CTL lines were further stimulated using allogeneic feeder cells, anti-CD3, and low doses of interleukin-2 (IL-2). This stimulation gave rise to 150-fold to 270-fold expansion of the absolute number of cultured cells. Most cultures showed either absent or low reactivity of anti-leukemia CTL against patient non-leukemia cells. Three anti-leukemia CTL lines displayed a more pronounced cytotoxicity against nonmalignant recipient cells, which was always lower than that observed against leukemia blasts (LB). Spectratyping analysis of the TCR-Vbeta subfamilies revealed a preferential expansion of oligoclonal populations that persisted in CTL lines following repeated rounds of stimulation.

CONCLUSIONS

Results provide the biological background for designing protocols of adoptive immunotherapy for the control of minimal residual disease in patients with hematological malignancies given HSCT.

Transfer of allogeneic CD62L- memory T cells without graft-versus-host disease

The major challenge in allogeneic hematopoietic cell transplantation is how to transfer allogeneic T-cell immunity without causing graft-versus-host disease (GVHD). Here we report a novel strategy to selectively prevent GVHD by depleting CD62L(+) T cells (naive and a subset of memory T cells). In unprimed mice, CD62L(-) T cells (a subset of memory T cells) failed to proliferate in response to alloantigens (which the mice have never previously encountered) and were unable to induce GVHD in allogeneic hosts. CD62L(-) T cells contributed to T-cell reconstitution by peripheral expansion as well as by promoting T-cell regeneration from bone marrow stem/progenitor cells. CD62L(-) T cells from the animals previously primed with a tumor cell line (BCL1) were able to inhibit the tumor growth in vivo but were unable to induce GVHD in the third-party recipients. This novel technology may allow transfer of allogeneic recall antitumor and antimicrobial immunity without causing GVHD.

Selective depletion of donor alloreactive T cells without loss of antiviral or antileukemic responses

Poor immune reconstitution after haploidentical stem cell transplantation results in a high mortality from viral infections and relapse. One approach to overcome this problem is to selectively deplete the graft of alloreactive cells using an immunotoxin directed against the activation marker CD25. However, the degree of depletion of alloreactive cells is variable following stimulation with recipient peripheral blood mononuclear cells (PBMCs), and this can result in graft versus host disease (GVHD). We have refined this approach using recipient Epstein-Barr virus (EBV)-transformed lymphoblastoid cell lines (LCLs) as stimulators to activate donor alloreactive T cells. Our studies demonstrate that allodepletion with an anti-CD25 immunotoxin following stimulation with HLA-mismatched host LCLs more consistently depleted in vitro alloreactivity than stimulation with host PBMCs, as assessed in primary mixed lymphocyte reactions (MLRs). Allodepletion using this approach specifically abrogates cytotoxic T-cell responses against host LCLs. In interferon-gamma (IFN-gamma) enzyme-linked immunospot (ELISPOT) assays, antiviral responses to adenovirus and cytomegalovirus (CMV) were preserved following allodepletion. Likewise, using HLA-A2-pp65 tetramers, we have shown that the frequency of CMV-specific T cells is unaffected by allodepletion. Moreover, the donor anti-EBV response is partially retained by recognition of EBV antigens through the nonshared haplotype. Finally, we studied whether allodepletion affects the response to candidate tumor antigens in myeloid malignancies. Using HLA-A2-PR1 tetramer analysis, we found that the frequency of T cells recognizing the PR1 epitope of proteinase 3 was not significantly different in allodepleted and unmanipulated PBMCs from patients with chronic myeloid leukemia (CML) undergoing transplantation. Based on these data, we have embarked on a phase 1 clinical trial of addback of allo-LCL-depleted donor T cells in the haplo-identical setting.

Tissue-restricted T cell alloresponses across HLA barriers: selection and identification of leukemia-restricted CTL in HLA-mismatched stimulator-responder pairs

Exploiting the graft-versus-leukemia (GVL) effect in mismatched transplants requires its separation from graft-versus-host disease (GVHD). We generated leukemia-specific cytotoxic T lymphocytes (CTL) in three haplotype-mismatched, two class I-mismatched and two single HLA-A locus-matched stimulator-responder pairs. Six patients with chronic myelogenous leukemia and one patient with acute myeloid leukemia transformed from MDS were studied. CTL generated after 10 days stimulation with unselected leukemic peripheral blood mononuclear cells inhibited leukemic CFU-GM colony growth (>85% at 10:1 effector:target ratio) with no third-party colony inhibition. In five pairs, responders were cultured separately with leukemia cells, PHA-B or LCL from the stimulator. After 2-4 restimulations, the T cell repertoire was examined by flow analysis using Vbeta-specific antibodies. Test cultures (but not controls) showed preferential expansion of 1-4 Vbeta families either common to two or more stimulators or unique to a particular stimulator. Notably, we elicited leukemia-specific TCR Vbeta expansions on four out of five occasions. In two pairs, responder cells selected for the appropriate leukemia-specific Vbeta family were shown to have leukemia-specific cytotoxicity. These leukemia-restricted T-cells were CD8+ or CD4+ and CD25+ or CD57+. The results support the development of strategies to selectively deplete GVHD and conserve GVL reactivity in mismatched transplants.

A prospective comparison of immune reconstitution in pediatric recipients of positively selected CD34+ peripheral blood stem cells from unrelated donors vs recipients of unmanipulated bone marrow from related donors

Positively selected CD34(+) hematopoietic stem cells from unrelated donors (UD-HSCT) have been successfully transplanted, but little is known about immune reconstitution in this setting. Here we report a prospective comparison of immune reconstitution in recipients of UD-HSCT and of unmanipulated bone marrow from matched sibling donors (MSD-BMT). T-cell reconstitution occurred more than 100 days later in the UD-HSCT than in the MSD-BMT group. The first T cells after UD-HSCT were almost exclusively CD45RO(+) HLA-DR(+), whereas early-emerging T cells after MSD-BMT more frequently expressed CD62L, CD28, and CD25. In both groups, numbers of CD45RA(+) naive T cells increased after 180 days. After UD-HSCT, the T-cell-receptor (TCR)-repertoire was severely skewed and showed significantly reduced diversity during the first year, but only minor abnormalities were seen after MSD-BMT. TCR-diversity increased simultaneously with the number of naive T cells. In both groups, we observed transient expansions of gammadelta T cells. B cells were reconstituted more rapidly in UD-HSCT than in MSD-BMT recipients, whereas the rapidity of NK-cell reconstitution was similar in the two groups. In summary, T-cell reconstitution was slower after UD-HSCT than after MSD-BMT because of the delayed recovery of early memory-type T cells with reduced TCR-diversity, whereas naive T-, NK-, and B cells were reconstituted similarly in the two groups.

[Allogeneic adoptive immunotherapy]

Recognition of the importance of immune cells present in a hematopoietic graft has resulted in a significant change in the perception of allogeneic hematopoietic transplantation. Such a transplant modality is now perceived has a very efficient form of adoptive allogeneic immunotherapy unfortunately associated with significant toxicity.

Optimized clinical-scale culture conditions for ex vivo selective depletion of host-reactive donor lymphocytes: a strategy for GvHD prophylaxis in allogeneic PBSC transplantation

BACKGROUND

Ex vivo selective depletion (SD) is a strategy to prevent GvHD, in which host-reactive donor lymphocytes are selectively eliminated from a PBSC allograft while conserving useful donor immune function. Prior to testing this strategy in patients, our goal was to develop a clinical-scale SD process, which involves co-culture of donor lymphocytes and irradiated recipient cells, followed by the addition of an immunotoxin (IT) directed against the alpha-chain of the IL-2 receptor (CD25), expressed on activated donor T cells.

METHODS

Stimulator cells were generated from immunomagnetically selected and expanded recipient T lymphocytes. Donor PBMCs from G-CSF-mobilized peripheral blood were co-cultured for 72 h with irradiated stimulator cells. Alloreactive T cells were targeted for elimination by the addition of the anti-CD25 IT, RFT5-SMPT-dgA, and the IT enhancer, NH(4)Cl.

RESULTS

Stimulator-cell selection/expansion yielded > 2 x 10(10) highly enriched CD3(+) cells (98.9 +/- 2.2%). After SD, cell recovery was 68.5 +/- 23.3% and viability was 84.6 +/- 6.4%. This permitted a potential T-cell dose >/= 1 x 10(8) CD3(+) cells kg(-1) to transplant recipients. Although SD donor lymphocytes retained little proliferative capacity against the original stimulator cells (2.6 +/- 0.6%), responses were conserved against third party cells (107.6 +/- 18.6%), the bacterial superantigen staphylococcus enterotoxin B (108.2 +/- 4.2%), and CMV Ag (72.1 +/- 3.8%).

DISCUSSION

We have demonstrated that ex vivo SD is feasible in clinical-scale culture conditions. The ability of this strategy to prevent GvHD is the subject of an ongoing clinical trial, in which the SD lymphocyte product is transplanted in conjunction with a T cell-depleted PBSC allograft.

Definitive separation of graft-versus-leukemia- and graft-versus-host-specific CD4+ T cells by virtue of their receptor beta loci sequences

Although graft-versus-host (GVH) disease (GVHD) is usually associated with graft versus leukemia (GVL), GVL can occur in the absence of clinical GVHD. There is evidence to suggest that GVL and GVH are mediated by different clones of T cells. The objective of this study was to identify the two types of T cells based on their receptor sequences. To this end we used irradiated nonleukemic cells from recipients as stimulator cells in a primary mixed leukocyte reaction (MLR). The activated CD4(+) donor T cells that expressed CD25 were purified by cell sorting. To prepare GVL-specific T cells, alloreactive T cells in the primary MLR were first depleted with an anti-CD25 immunotoxin. The remaining T cells had negligible alloreactivity in a secondary MLR. The allodepleted cells were then stimulated by using purified leukemia cells from the same individual as stimulator cells, and the CD25(+)-activated cells were purified by cell sorting. The GVL- and GVH-specific T cells were analyzed for their T cell receptor (TCR) clonality by using anchored RT-PCR of all the TCRbeta locus complementarity-determining region 3 (CDR3) sequences. By comparing TCRbeta CDR3 sequences from transformed bacterial colonies, we were able to demonstrate that T cells mediating GVH were different from those mediating GVL in each of the eight HLA-mismatched and one HLA-matched donor/recipient pairs. By using the appropriate TCRbeta CDR3-specific primers and probes, the GVH- and GVL-specific clones were monitored in a recipient undergoing an allogeneic stem cell transplant from her HLA-matched related donor.

New Developments in Allotransplant Immunology

After allogeneic stem cell transplantation, the establishment of the donor’s immune system in an antigenically distinct recipient confers a therapeutic graft-versus-malignancy effect, but also causes graft-versus-host disease (GVHD) and protracted immune dysfunction. In the last decade, a molecular-level description of alloimmune interactions and the process of immune recovery leading to tolerance has emerged. Here, new developments in understanding alloresponses, genetic factors that modify them, and strategies to control immune reconstitution are described.

In Section I, Dr. John Barrett and colleagues describe the cellular and molecular basis of the alloresponse and the mechanisms underlying the three major outcomes of engraftment, GVHD and the graft-versus-leukemia (GVL) effect. Increasing knowledge of leukemia-restricted antigens suggests ways to separate GVHD and GVL. Recent findings highlight a central role of hematopoietic-derived antigen-presenting cells in the initiation of GVHD and distinct properties of natural killer (NK) cell alloreactivity in engraftment and GVL that are of therapeutic importance. Finally, a detailed map of cellular immune recovery post-transplant is emerging which highlights the importance of post-thymic lymphocytes in determining outcome in the critical first few months following stem cell transplantation. Factors that modify immune reconstitution include immunosuppression, GVHD, the cytokine milieu and poorly-defined homeostatic mechanisms which encourage irregular T cell expansions driven by immunodominant T cell–antigen interactions.

In Section II, Prof. Anne Dickinson and colleagues describe genetic polymorphisms outside the human leukocyte antigen (HLA) system that determine the nature of immune reconstitution after allogeneic stem cell transplantation (SCT) and thereby affect transplant outcomethrough GVHD, GVL, and transplant-related mortality. Polymorphisms in cytokine gene promotors and other less characterized genes affect the cytokine milieu of the recipient and the immune reactivity of the donor. Some cytokine gene polymorphisms are significantly associated with transplant outcome. Other non-HLA genes strongly affecting alloresponses code for minor histocompatibility antigens (mHA). Differences between donor and recipient mHA cause GVHD or GVL reactions or graft rejection. Both cytokine gene polymorphisms (CGP) and mHA differences resulting on donor-recipient incompatibilities can be jointly assessed in the skin explant assay as a functional way to select the most suitable donor or the best transplant approach for the recipient.

In Section III, Dr. Nelson Chao describes non-pharmaceutical techniques to control immune reconstitution post-transplant. T cells stimulated by host alloantigens can be distinguished from resting T cells by the expression of a variety of activation markers (IL-2 receptor, FAS, CD69, CD71) and by an increased photosensitivity to rhodamine dyes. These differences form the basis for eliminating GVHD-reactive T cells in vitro while conserving GVL and anti-viral immunity. Other attempts to control immune reactions post-transplant include the insertion of suicide genes into the transplanted T cells for effective termination of GVHD reactions, the removal of CD62 ligand expressing cells, and the modulation of T cell reactivity by favoring Th2, Tc2 lymphocyte subset expansion. These technologies could eliminate GVHD while preserving T cell responses to leukemia and reactivating viruses.

Human colon adenocarcinoma in the SCID/CB6 radiation chimera is susceptible to adoptive transfer of allogeneic human peripheral blood mononuclear cells

The aim of this study was to develop a murine model of human colon carcinoma (hCC) and to ascertain the potential of cellular immunotherapy in this model. Fragments of hCC obtained at surgery from 6 patients were transplanted under the kidney capsule of lethally irradiated CB6 mice radioprotected with severe combined immunodeficient (SCID) mice bone marrow. Tumor xenografts conserved their malignant behavior in the new environment, invading the mouse kidney parenchyma and expanding into the peritoneal cavity and adjacent tissues. Their growth was typically exponential, and they expanded to dimensions that allowed their subsequent fragmentation and passage to further preconditioned mice. Human carcinoembryonic antigen (hCEA) was detected on the implanted tumor and at occasionally spontaneous lung metastases. Most significantly, high levels of this tumor marker were detected in the sera of tumor-bearing mice, providing a useful tool, which allowed long-term experiments, monitoring of tumor progression, and its response to some treatment modalities. For instance, complete resection of the transplanted tumors, by means of nephrectomy, resulted in the disappearance of hCEA from mice sera within 2 weeks. Similarly, adoptive transfer of allogeneic human peripheral blood mononuclear cells (PBMC) into the peritoneum of tumor-bearing mice, resulted in their rapid engraftment, infiltration of tumor mass, and a significant drop of hCEA levels in mice serum, accounting for inhibition of tumor growth. We suggest that this novel model of human colon carcinoma affords the opportunity for in vivo evaluation of different preclinical treatment modalities, particularly, those involving manipulation with immune effector cells.

P-glycoprotein targeting: a unique strategy to selectively eliminate immunoreactive T cells

T lymphocytes have been found to harbor P-glycoprotein (Pgp) and to demonstrate modulation of its ion channel transporter function according to the state of activation of T lymphocytes. We hypothesized that cytotoxic chemicals that are extruded by Pgp could be used to specifically eliminate immunoreactive T-cell populations. In this study, we evaluated the capacity of 4,5-dibromorhodamine methyl ester (TH9402), a photosensitizer structurally similar to rhodamine, a dye transported by Pgp, and which becomes highly cytotoxic on activation with visible light to selectively deplete alloreactive T lymphocytes. Stimulation of T cells with mitogens or allogeneic major histocompatibility complex-mismatched cells resulted in the preferential retention of the TH9402 rhodamine-derivative in activated T cells, both CD4+ and CD8+. Photodynamic cell therapy of TH9402-exposed T cells led to the selective elimination of immunoreactive T-cell populations. In addition, this treatment preserved resting T cells and their capacity to respond to third-party cells. Inhibition of Pgp enhanced cellular trapping of the dye in nonactivated T cells and resulted in their depletion after exposure to light. Targeting of Pgp-deficient cells may therefore represent an appealing strategy for the prevention and treatment of graft-versus-host disease and other alloimmune or autoimmune disorders.

Immune reconstitution without graft-versus-host disease after haemopoietic stem-cell transplantation: a phase 1/2 study

BACKGROUND

Allogeneic haemopoietic stem-cell transplantation (HSCT) is the treatment of choice for many haematological malignancies and inherited disorders. When stem cells for transplantation come from a human leucocyte antigen matched unrelated donor, or from a partly mismatched related donor, ex-vivo T-cell depletion of the graft can prevent development of graft-versus-host disease, but lead in turn to a delay in immune reconstitution and a concordant increase in incidence of opportunistic infections and leukaemic relapses. We aimed to infuse T cells selectively depleted in allogeneic T cells that cause graft-versus-host disease using an ex-vivo procedure designed to eliminate alloactivated donor T cells, with an immunotoxin that reacts with a cell surface activation antigen, CD25.

METHODS

We did a phase 1/2 study, in which 1-8 x 10(5) allodepleted T cells/kg were infused between days 15 and 47 into 15 paediatric patients who had acquired or congenital haemopoietic disorders and who received HSCT on day 0. Occurrence of graft-versus-host disease and time to immune reconstitution were assessed. No treatment for graft-versus-host disease was given.

FINDINGS

Less than 1% residual anti-host alloreactivity was recorded in 12 of 16 procedures. Other immune responses were preserved by the allodepletion procedure in 12 cases. No cases of severe (greater than grade II) graft-versus-host disease arose. Evidence for early T-cell expansion was shown in three patients with continuing viral infections. Specific antiviral responses, such as strong cytolytic activity, were noted.

INTERPRETATION

Our results show that ex-vivo selective depletion of T cells that cause graft-versus-host disease is efficient and feasible, even in haploidentical settings.

Adenovirally transduced dendritic cells induce bispecific cytotoxic T lymphocyte responses against adenovirus and cytomegalovirus pp65 or against adenovirus and Epstein-Barr virus EBNA3C protein: a novel approach for immunotherapy

Cytomegalovirus (CMV), Epstein-Barr virus (EBV), and adenovirus (Ad) cause significant morbidity and mortality in immunocompromised patients undergoing allogeneic stem cell transplantation. We have established a procedure to generate polyclonal cytotoxic T lymphocyte (CTL) populations with specificity against Ad and CMV or against Ad and EBV. Healthy donor-derived dendritic cells (DCs) were transduced with recombinant adenovirus encoding either CMV pp65 or EBV EBNA3C and used to stimulate autologous T cells. Stimulated T lymphocytes displayed specific simultaneous cytotoxicity against CMV and adenovirus and to a lesser extent against adenovirus and EBV. Recombinant vaccinia virus encoding individual adenovirus proteins showed that the T cell response to the adenovirus was directed mainly against the capsid protein hexon. The frequency of IFN-gamma-secreting T cells was 0.02% for adenovirus alone, and 0.05 and 0.14% for adenoviruses encoding EBNA3C and pp65, respectively. pp65-specific CTLs killed autologous fibroblasts infected with the laboratory strain CMV AD169. The culture conditions were specific as alloreactive T cells were not expanded. Therefore, this approach could be considered in order to generate efficient virus cytolytic T cells to be used as adoptive immunotherapy in transplanted patients.

Prevention of graft-versus-host disease while preserving graft-versus-leukemia effect after selective depletion of host-reactive T cells by photodynamic cell purging process

In this study, we investigated the possibility of selective depletion of donor alloantigen-specific T cells from C57BL/6 (H-2(b)) mice to prevent graft-versus-host disease (GVHD). These cells were first activated with irradiated BALB/c (H-2(d)) host spleen cells in a 5-day mixed lymphocyte culture. Following this activation, a photoactive rhodamine derivative called 4,5-dibromorhodamine 123 (TH9402), was added. This compound is selectively retained in the mitochondria of activated host-reactive cells but not tumor- or third-party-specific resting cells. The treated cells were subsequently exposed to visible light (514 nm) to deplete the TH9402-enriched activated host-reactive cells. Treatment with photodynamic cell purging process (PDP) inhibited antihost responses measured by cytotoxic T lymphocytes (CTL) by 93%, and interferon-gamma production by 66%. By contrast, anti-BCL1 (BALB/c-origin leukemia/lymphoma) and anti-third-party C3H/HeJ (H-2(k)) responses were preserved. PDP-treated primed C57BL/6 cells were further tested in vivo. All lethally irradiated BALB/c mice inoculated with BCL1 cells and T-cell-depleted bone marrow cells developed leukemia by day +30, with 50% mortality by 100 days. All mice died of GVHD after addition of 5 x 10(6) untreated primed C57BL/6 cells. However, addition of same numbers of PDP-treated cells allowed 90% of the recipients to survive more than 100 days without detectable BCL1 tumor cells and free of GVHD. Moreover, PDP-treated primed C57BL/6 cells retained the ability to induce GVHD in the third-party C3H/HeJ mice. These data suggest that PDP can selectively deplete host alloantigen-specific T cells for GVHD prevention and immune and antileukemia function preserve.

Murine acute graft-versus-host disease can be prevented by depletion of alloreactive T lymphocytes using activation-induced cell death

Depletion of T lymphocytes from allogeneic bone marrow transplants successfully prevents the development of graft-versus-host disease (GvHD) but is associated with impaired engraftment, immunosuppression, and abrogation of the graft-versus-leukemia effect. We therefore explored the possibility of selectively eliminating alloreactive T cells by CD95/CD95L-mediated activation-induced cell death (AICD) in a major histocompatibility complex allogeneic murine model system. Activation of resting or preactivated T lymphocytes from C3H/HeJ (H-2(k)) mice was induced with irradiated BALB/cJ (H-2(d)) mouse-derived stimulators. Substantial decrease (> or = 80%) of proliferative and lytic responses by activated alloreactive T cells was subsequently achieved by incubating the mixed lymphocyte culture with an agonistic monoclonal antibody to CD95, and residual T cells recovered did not elicit alloreactivity upon challenge to H-2(d). Depletion of alloreactive T lymphocytes by AICD was specific because reactivity to an I-A(d)-restricted ovalbumin (OVA) peptide by OVA-specific CD4(+) T cells mixed into the allogeneic T-cell pool and subjected to induction of AICD in the absence of OVA peptide could be preserved. Adoptive transfer of donor-derived allogeneic T lymphocytes, depleted from alloreactive T cells by AICD in vitro, in the parent (C3H/He) to F(1) (C3H/He x BALB/c) GvHD model prevented lethal GvHD. The results presented suggest that alloreactive T cells can effectively be depleted from allogeneic T cells by induction of AICD to prevent GvHD and might introduce a new strategy for the separation of GvH-reactive T cells and T cells mediating antiviral and possibly graft-versus-leukemia effects.

Recombinant immunotoxins in targeted cancer cell therapy

Targeted cancer therapy in general and immunotherapy in particular combines rational drug design with the progress in understanding cancer biology. This approach takes advantage of our recent knowledge of the mechanisms by which normal cells are transformed into cancer cells, thus using the special properties of cancer cells to device novel therapeutic strategies. Recombinant immunotoxins are excellent examples of such processes, combining the knowledge of antigen expression by cancer cells with the enormous developments in recombinant DNA technology and antibody engineering. Recombinant immunotoxins are composed of a very potent protein toxin fused to a targeting moiety such as a recombinant antibody fragment or growth factor. These molecules bind to surface antigens specific for cancer cells and kill the target cells by catalytic inhibition of protein synthesis. Recombinant immunotoxins are developed for solid tumors and hematological malignancies and have been characterized intensively for their biological activity in vitro on cultured tumor cell lines as well as in vivo in animal models of human tumor xenografts. The excellent in vitro and in vivo activities of recombinant immunotoxins have lead to their preclinical development and to the initiation of clinical trail protocols. Recent trail results have demonstrated potent clinical efficacy in patients with malignant diseases that are refractory to traditional modalities of cancer treatment: surgery, radiation therapy, and chemotherapy. The results demonstrate that such strategies can be developed into a separate modality of cancer treatment with the basic rationale of specifically targeting cancer cells on the basis of their unique surface markers. Efforts are now being made to improve the current molecules and to develop new agents with better clinical efficacy. This can be achieved by development of novel targeting moieties with improved specificity that will reduce toxicity to normal tissues. In this review, the design, construction, characterization, and applications of recombinant immunotoxins are described. Results of recent clinical trails are presented, and future directions for development of recombinant immunotoxins as a new modality for cancer treatment are discussed.

Improving immune reconstitution while preventing graft-versus-host disease in allogeneic stem cell transplantation

Allogeneic hematopoietic stem cell transplantation (HSCT) is the treatment of choice for many hematologic malignancies or inherited disorders. Ex vivo T-cell depletion (TCD) of the graft and post-transplantation immunosuppression efficiently prevent the development of graft-versus-host disease (GVHD). However, the consequence of these nonspecific approaches is a long-lasting immunodeficiency associated with increased disease relapse, graft rejection, and reactivation of viral infections. Donor lymphocyte infusion, to treat leukemic relapse after allogeneic HSCT, can cause severe GVHD. Several strategies are being optimized to specifically inactivate anti-host T cells while preserving antileukemic or antimicrobial immunocompetence, based on ex vivo or in vivo elimination of anti-host T cells or on the modulation of their anti-host activity.

Immunotherapy to reconstitute immunity to DNA viruses

Defects in cytotoxic T-lymphocyte (CTL) function after hemopoietic stem cell transplantation (HSCT) are associated with an increased frequency and severity of viral diseases. Initial investigations of viral infections in immunosuppressed mice and subsequent clinical studies of cytomegalovirus (CMV) and Epstein-Barr virus (EBV) in human stem cell transplant patients have suggested that adoptive transfer of virus-specific T cells may restore protective immunity and control established infections. Current efforts focus on optimizing adoptive immunotherapy approaches and developing strategies for generating T cells specific for multiple viruses to provide broader protection.

Ex vivo priming for long-term maintenance of antileukemia human cytotoxic T cells suggests a general procedure for adoptive immunotherapy

Adoptive cellular immunotherapy has proven to be a successful approach in preventing and curing cytomegalovirus infection and Epstein-Barr virus-associated lymphomas after bone marrow transplantation. Translation of this approach for preventing leukemia relapse after bone marrow transplantation might require ex vivo priming and long-term maintenance of leukemia blast-specific T cells. To accomplish this goal, procedures were optimized for the in vitro priming of naive CD8 using dendritic cells activated by CD40 ligation, interleukin-12 (IL-12), and IL-7. Using T lymphocytes and dendritic cells obtained from HLA-matched allogeneic bone marrow transplantation donors and leukemia blasts as a source of tumor antigens, anti-acute myeloid leukemia cytotoxic T lymphocytes (CTLs) were induced. In these experiments, it was found that though it is possible to induce CTLs using immature dendritic cells, IL-12, and IL-7, obtaining long-term CTLs requires the presence of CD4 T cells in the priming phase. Using this approach, long-term antileukemia CTL lines could be generated from 4 of 4 bone marrow donors. Because this procedure does not require definition of the target antigen and because it selects responding cells from a virgin T-cell repertoire, its general application is suggested in adoptive immunotherapy and in the definition of tumor rejection antigens.

Coexistence of acute cellular rejection and lymphoproliferative disorder in a lung transplant patient

We report the case of a 37-year-old man who underwent bilateral lung transplantation for end-stage cystic fibrosis. Two months after his operation, a computed tomographic scan showed multifocal nodules throughout both lungs. Endobronchial biopsies revealed an Epstein-Barr virus-associated B-cell lymphoproliferation. Transbronchial biopsies revealed perivascular lymphoid infiltrates composed of predominantly small T lymphocytes. These perivascular infiltrates were retrospectively considered to be an acute cellular rejection rather than the periphery of the lymphoproliferative disorder. This opinion was based on several arguments: (a) a decrease in dosage of maintenance immunosuppression led to total regression of the lymphoproliferation but did not affect the perivascular lymphoid infiltrates; (b) the treatment of the acute cellular rejection temporarily induced the disappearance of the perivascular infiltrates; (c) the expression of Epstein-Barr virus was not detected in the perivascular infiltrates; and (d) on autopsy, performed 1 year later, severe obliterative bronchiolitis lesions were discovered, for which acute cellular rejection is the main risk factor. These observations point to the possibility that acute cellular rejection and an Epstein-Barr virus-associated lymphoproliferative disorder may coexist.

Recombinant immunotoxins for cancer therapy

Recombinant immunotoxins consist of Fv regions of tumour-selective antibodies fused to toxins found in bacteria, plants or fungi. These toxins must be modified to remove normal-tissue binding sites but to retain all other functions of cytotoxicity. The recombinant antibody fragments target the modified toxin to cancer cells which are killed, either by direct inhibition of protein synthesis, or by concomitant induction of apoptosis. Cells that are not recognised by the antibody fragment because they do not carry the tumour antigen, are spared. Many factors influence the in vivo antitumour activity of recombinant immunotoxins. Among them are considerations of which types of cancer may be the best targets for immunotoxin therapy as well as tumour specificity of the antigen that is targeted by the recombinant antibody. Other relevant issues are the affinity of immunotoxins and their ability to enter and penetrate into tissues and tumours, which in turn is dependent on the size of the protein. A great deal of protein-engineering is required to stabilise the recombinant antibody moiety of immunotoxins, since stability of the molecules is crucial for good clinical efficacy. Excellent activity and specificity can be observed for many recombinant immunotoxins in in vitro assays using cultured cancer cells as well as in animal tumour models. Ongoing clinical trials provide examples where the promising preclinical data correlate with successful results in experimental cancer therapy.

Intravenous injection of apoptotic leukocytes enhances bone marrow engraftment across major histocompatibility barriers

Cross-tolerization of T lymphocytes after apoptotic cell uptake by dendritic cells may be involved in self-tolerance maintenance. Furthermore, immunosuppressive properties are attributed to apoptotic cells. This study evaluated the consequences of apoptotic leukocyte administration in a restrictive engraftment model of murine bone marrow (BM) transplantation. Sublethally irradiated recipients received a limited number of allogeneic BM, with or without irradiated apoptotic leukocytes of different origins. No graft-versus-host disease was observed. Whereas only a low proportion of mice receiving BM cells alone engrafted, addition of apoptotic irradiated leukocytes, independently of the origin (donor, recipient, third-party mice, as well as xenogeneic peripheral blood mononuclear cells), significantly enhanced engraftment. Similar results were obtained after infusion of leukocytes rendered apoptotic by UVB irradiation or by anti-Fas monoclonal antibody stimulation, thus confirming the role of apoptotic cells in engraftment facilitation. Overall, these results suggest that apoptotic leukocytes can nonspecifically facilitate allogeneic BM engraftment. Such a simple approach could be of interest in BM transplantation settings involving an important HLA donor/recipient disparity, a T-cell-depleted graft, or reduced conditioning regimen intensity.

Human alloantigen-specific anergic cells induced by a combination of CTLA4-Ig and CsA maintain anti-leukemia and anti-viral cytotoxic responses

The success of allogeneic hematopoietic stem cell transplantation from HLA-disparate donors depends on the development of new strategies for graft-versus-host disease prevention able to target specifically donor antihost alloreactivity, while preserving GVL and antiviral immune surveillance. Recent experimental and clinical work has shown the feasibility of an approach based on induction of anergy to host alloantigens through blockade of B7/CD28 costimulatory signal in donor T cells, but data on the impact of this strategy on the recovery of the immune system are still lacking. We devised an ex vivo method for induction of host alloantigen-specific unresponsiveness based on treatment with the B7/CD28 blocking agent CTLA4-Ig associated with CsA. We then proceeded to assess the maintenance of an effective immune response towards viral pathogens and tumor cells after CTLA4-Ig/CsA treatment, by measuring the frequency of CTL precursors directed against CMV- and EBV-infected targets, and against autologous leukemic blasts. We demonstrated that (1) CTLA4-Ig and CsA can act synergistically in inducing a state of unresponsiveness to alloantigens; (2) the number of leukemia-reactive, EBV-specific and CMV-specific CTLp is not impaired by CTLA4-Ig/CsA treatment. Our data provide the first direct in vitro evidence that it is possible to preserve antiviral and antileukemia effector cells after blockade of CD28/B7 interaction during MLR.

Immunological aspects of haploidentical stem cell transplantation in children

Thirty-eight children with high-risk hematological malignancies underwent transplantation with megadoses of peripheral mobilized CD34+ cells from haploidentical parents (n = 24) or from matched unrelated donors (n = 14). The CD34+ cells were isolated to a purity of > 98% using magnetic-activated cell sorting. This high purity was associated with an almost complete depletion of T lymphocytes. No pharmacological prophylaxis for graft-versus-host disease (GvHD) was used, and significant primary GvHD was not seen. A final engraftment was seen in all patients. Sixteen patients are alive and disease-free with a median follow-up of 24 months. The immunological reconstitution was faster in the patients transplanted with CD34+ stem cells from the haploidentical donors compared to the matched unrelated donors, and the transplantation of large numbers of haploidentical CD34+ stem cells seems to be superior to that of the matched unrelated donors. The phenotypical and functional analysis of the immune reconstitution provided some insights into the biology of transplantation of highly purified CD34+ cells. In this article, we summarize our current results with the transplantation of highly purified stem cells and discuss possible implications for further antileukemic post-transplant therapeutic strategies.

Postgrafting administration of granulocyte colony-stimulating factor impairs functional immune recovery in recipients of human leukocyte antigen haplotype-mismatched hematopoietic transplants

In human leukocyte antigen haplotype-mismatched transplantation, extensive T-cell depletion prevents graft-versus-host disease (GVHD) but delays immune recovery. Granulocyte colony-stimulating factor (G-CSF) is given to donors to mobilize stem cells and to recipients to ensure engraftment. Studies have shown that G-CSF promotes T-helper (Th)-2 immune deviation which, unlike Th1 responses, does not protect against intracellular pathogens and fungi. The effect of administration of G-CSF to recipients of mismatched hematopoietic transplants with respect to transplantation outcome and functional immune recovery was investigated. In 43 patients with acute leukemia who received G-CSF after transplantation, the engraftment rate was 95%. However, the patients had a long-lasting type 2 immune reactivity, ie, Th2-inducing dendritic cells not producing interleukin 12 (IL-12) and high frequencies of IL-4- and IL-10-producing CD4(+) cells not expressing the IL-12 receptor beta(2) chain. Similar immune reactivity patterns were observed on exposure of donor cells to G-CSF. Elimination of postgrafting administration of G-CSF in a subsequent series of 36 patients with acute leukemia, while not adversely affecting engraftment rate (93%), resulted in the anticipated appearance of IL-12-producing dendritic cells (1-3 months after transplantation versus > 12 months in transplant recipients given G-CSF), of CD4(+) cells of a mixed Th0/Th1 phenotype, and of antifungal T-cell reactivity in vitro. Moreover, CD4(+) cell counts increased in significantly less time. Finally, elimination of G-CSF-mediated immune suppression did not significantly increase the incidence of GVHD (< 15%). Thus, this study found that administration of G-CSF to recipients of T-cell-depleted hematopoietic transplants was associated with abnormal antigen-presenting cell functions and T-cell reactivity. Elimination of postgrafting administration of G-CSF prevented immune dysregulation and accelerated functional immune recovery.

Immunotoxins

Immunotoxins are molecules which contain a protein toxin connected to a targeting antibody. The goal of therapy is for the molecule to bind selectively to cancer cells, or to cells mediating autoimmune disease, internalise and then for the toxin to kill the cell. Several immunotoxins meeting this definition are in preclinical and clinical development, but none are approved yet for use in general practice. One close relative of immunotoxins is the growth factor fusion toxin, wherein the targeting antibody is replaced with a growth factor that selectively binds and this ligand is fused in a recombinant fashion to a protein toxin. One such molecule, containing human interleukin-2 (IL-2) fused to truncated diphtheria toxin (DT), has recently been approved under the name Ontak, and others are under development. A newer class of immunotoxins, termed recombinant immunotoxins, contain the variable or antigen binding domains of an antibody fused in recombinant fashion to a toxin. Recombinant immunotoxins, like growth factor fusion toxins, can be produced efficiently from bacteria and have a defined structure with respect to the linkage between the toxin and the ligand. However, they can, like conventional immunotoxins, be directed to antigens other than growth factor receptors, including receptor subunits. Several recombinant immunotoxins are under clinical testing and major responses have been reported, particularly in haematological malignancies. Some of these molecules may enter clinical practice in the future as targeted therapy, which is a modality distinct from those of chemotherapy, surgery and radiation therapy.

Tolerance and hematopoietic stem cell transplantation 50 years after Burnet's theory

OBJECTIVE

In 1949, the original formulation of Burnet's theory on the mechanisms responsible for the capacity of the immune system to discriminate between foreign antigens (i.e., the "non-self") and the cells of its own body (i.e., the "self") was published. Since then, further refinements and reconsiderations of the basic concepts underlying the achievement of a state of tolerance toward a certain antigen have been reported. Here, we attempt to analyze critically new clinical and experimental strategies aimed at inducing alloantigen-specific unresponsiveness.

DATA SOURCES

The data discussed in this review are drawn from articles and abstracts published in journals covered by the Science Citation Index and Medline.

STATE OF THE ART

Induction of tolerance toward alloantigens still remains one of the most elusive goals of clinical immunology. Until now, nonspecific immunosuppressive drugs have been used to successfully perform both solid organ and hematopoietic stem cell transplantation. However, using this approach, patients given an allograft are exposed to the threat of infections, tumors, and other side effects. Moreover, in solid organ transplant recipients, permanent tolerance toward the graft's alloantigens is never achieved. Recently, considerable progress has been made in expanding our knowledge of transplant tolerance. The traditional model of central tolerance, derived from Burnet's concept, has been complemented by knowledge of mechanisms of peripheral tolerance.

CONCLUSIONS

New experimental and therapeutic trials based on the blockade of costimulatory molecules, as well as on generation and infusion of either regulatory or nonimmunogenic cells, have been recently proposed for inducing alloantigen-specific tolerance.The achievements obtained in understanding the mechanisms of unresponsiveness toward non-self antigens are fundamental prerequisites for successful allogeneic transplants, and they could open a new exciting era of specific, immunosuppressive therapies.

The role of peptides in T cell alloreactivity is determined by self-major histocompatibility complex molecules

By analyzing T cell responses against foreign major histocompatibility complex (MHC) molecules loaded with peptide libraries and defined self- and viral peptides, we demonstrate a profound influence of self-MHC molecules on the repertoire of alloreactive T cells: the closer the foreign MHC molecule is related to the T cell's MHC, the higher is the proportion of peptide-specific, alloreactive ("allorestricted") T cells versus T cells recognizing the foreign MHC molecule without regard to the peptide in the groove. Thus, the peptide repertoire of alloreactive T cells must be influenced by self-MHC molecules during positive or negative thymic selection or peripheral survival, much like the repertoire of the self-restricted T cells. In consequence, allorestricted, peptide-specific T cells (that are of interest for clinical applications) are easier to obtain if T cells and target cells express related MHC molecules.

Management of graft-versus-host disease in paediatric bone marrow transplant recipients

We discuss clinical strategies for the prophylaxis and treatment of both acute and chronic graft-versus-host disease (GVHD) with particular attention to children. Grades II to IV acute GVHD occur in 10 to 50% of patients given an allogeneic transplantation of haemopoietic stem cells (HSCT) from a genotypically HLA-identical donor. A significantly higher incidence and severity of the disease is reported in patients receiving transplants from partially matched family donors or unrelated volunteers. Younger individuals or patients receiving HSCT from younger donors develop GVHD less frequently than do older recipients. Severe acute GVHD is characterised by a significant decrease in survival probability, even though the graft-versus-leukaemia activity associated with both acute and chronic GVHD may reduce the risk of leukaemia relapse. Prophylaxis of acute GVHD usually consists of in vivo post-grafting immunosuppression with cyclosporin alone or in combination with methotrexate; methotrexate alone can be considered in leukaemia patients with a high risk of relapse. In recent years, tacrolimus is increasingly being used instead of cyclosporin, alone or in combination with methotrexate. In vitro T cell depletion in paediatric patients is usually reserved for those with transplants from partially matched family donors or unrelated volunteers. The treatment of patients with grades II to IV acute GVHD should be immediate and aggressive, as the quality and duration of the response directly correlates with survival. The overall response rate to treatment is often unsatisfactory, ranging from 40 to 50% of cases. First-line treatment usually consists of corticosteroids. In patients not responding to corticosteroids, antilymphocyte globulin and monoclonal antibodies directed towards lymphocytes and/or cytokines produced during GVHD are employed, but with variable success. Patients experiencing acute GVHD are also prone to develop chronic GVHD. whose classical treatment is based on the use of cyclosporin and corticosteroids. More recently, encouraging results in the treatment of patients with chronic GVHD have been reported with the use of extracorporeal photochemotherapy. Other drugs, such as ursodeoxycholic acid, etretinate and clofazimine, are under evaluation.

Donor lymphocyte infusions

The infusion of lymphocytes from the original marrow donor (donor lymphocyte infusions [DLI]) is remarkably effective in treating chronic myeloid leukemia in relapse after allogeneic stem cell transplantation. DLI are less effective in acute leukemia and other hematologic tumors, but the use of interleukin-2 in conjunction with DLI after allograft may increase the response rate. The use of DLI to treat certain solid tumors is under investigation. In contrast, the value of donor lymphocytes for treating infectious complications post-transplant and graft failure has been established. The major drawback of DLI remains graft-versus-host disease, but novel regimens of administration and/or selective manipulation of donor cells prior to infusion have reduced its incidence. Further progresses in this area will help to establish the role of nonmyeloablative conditioning for allografting.

Donor lymphocyte infusions for relapse of chronic myeloid leukemia after allogeneic stem cell transplant: where we now stand

The infusion of lymphocytes from the original marrow donor (donor lymphocyte infusion [DLI]) reinduces complete remission in a high percentage of patients with chronic myeloid leukemia (CML) who relapse after allogeneic stem cell transplant, and thus, is probably the best initial approach to their management. The major predictive factor for response is the disease stage at time of treatment, because patients in molecular or cytogenetic relapse fare better than those in hematologic relapse. Moreover, patients with a short interval between transplant and DLI have a higher probability of response than those with longer intervals. The durability of DLI-induced remissions has not yet been established, but they appear to be prolonged. The observation that DLI can be highly effective for patients in relapse has encouraged the recent development of new strategies designed to minimize the myeloablative regimen and exploit the immunotherapeutic component of the transplant. The principal complication associated with use of DLI is the occurrence of graft-versus-host disease (GVHD). Several approaches have been tested to reduce the incidence or impact of GVHD, based on the ex vivo depletion of alloreactive donor cells or the use of donor T cells transduced with a suicide gene. The incidence of GVHD can also be reduced by starting with low doses of donor cells and "escalating" subsequent doses as required. However, the identification of selective targets for leukemia-reactive immunity is probably the optimal strategy to resolve the problem of GVHD. Although currently minor histocompatibility antigens appear to be the most likely targets for DLI, several groups are focusing on the generation of leukemia-specific immunity. The results obtained by use of tumor-associated antigens presented by dendritic cells are encouraging and may lay the foundations for the use of adoptive immunotherapy in the autologous setting.

Immunotoxins in cancer therapy

Immunotoxins are composed of a protein toxin connected to a binding ligand such as an antibody or growth factor. These molecules bind to surface antigens (which internalize) and kill cells by catalytic inhibition of protein synthesis within the cell cytosol. Immunotoxins have recently been tested clinically in hematologic malignancies and solid tumors and have demonstrated potent clinical efficacy in patients with malignant diseases that are refractory to surgery, radiation therapy and chemotherapy - the traditional modalities of cancer treatment. This therapy is thus evolving into a separate modality of cancer treatment, capable of rationally targeting cells on the basis of surface markers. Efforts are underway to obviate impediments to clinical efficacy, including immunogenicity and toxicity to normal tissues. Immunotoxins are now being developed to new antigens for the treatment of cancer.