The frequency of pretransplant donor cytotoxic T cell precursors with anti-host specificity predicts survival of patients transplanted with bone marrow from donors other than HLA-identical siblings

Acute graft-versus-host disease of the kidney in allogeneic rat bone marrow transplantation

Allogeneic hematopoietic cell or bone marrow transplantation (BMT) causes graft-versus-host-disease (GVHD). However, the involvement of the kidney in acute GVHD is not well-understood. Acute GVHD was induced in Lewis rats (RT1l) by transplantation of Dark Agouti (DA) rat (RT1(a)) bone marrow cells (6.0 × 10(7) cells) without immunosuppression after lethal irradiation (10 Gy). We examined the impact of acute GVHD on the kidney in allogeneic BMT rats and compared them with those in Lewis-to-Lewis syngeneic BMT control and non-BMT control rats. In syngeneic BMT and non-BMT control rats, acute GVHD did not develop by day 28. In allogeneic BMT rats, severe acute GVHD developed at 21-28 days after BMT in the skin, intestine, and liver with decreased body weight (>20%), skin rush, diarrhea, and liver dysfunction. In the kidney, infiltration of donor-type leukocytes was by day 28. Mild inflammation characterized by infiltration of CD3(+) T-cells, including CD8(+) T-cells and CD4(+) T-cells, and CD68(+) macrophages to the interstitium around the small arteries was noted. During moderate to severe inflammation, these infiltrating cells expanded into the peritubular interstitium with peritubular capillaritis, tubulitis, acute glomerulitis, and endarteritis. Renal dysfunction also developed, and the serum blood urea nitrogen (33.9 ± 4.7 mg/dL) and urinary N-acetyl-β-D-glucosaminidase (NAG: 31.5 ± 15.5 U/L) levels increased. No immunoglobulin and complement deposition was detected in the kidney. In conclusion, the kidney was a primary target organ of acute GVHD after BMT. Acute GVHD of the kidney was characterized by increased levels of urinary NAG and cell-mediated injury to the renal microvasculature and renal tubules.

Predicting alloreactivity in transplantation

Human leukocyte Antigen (HLA) mismatching leads to severe complications after solid-organ transplantation and hematopoietic stem-cell transplantation. The alloreactive responses underlying the posttransplantation complications include both direct recognition of allogeneic HLA by HLA-specific alloantibodies and T cells and indirect T-cell recognition. However, the immunogenicity of HLA mismatches is highly variable; some HLA mismatches lead to severe clinical B-cell- and T-cell-mediated alloreactivity, whereas others are well tolerated. Definition of the permissibility of HLA mismatches prior to transplantation allows selection of donor-recipient combinations that will have a reduced chance to develop deleterious host-versus-graft responses after solid-organ transplantation and graft-versus-host responses after hematopoietic stem-cell transplantation. Therefore, several methods have been developed to predict permissible HLA-mismatch combinations. In this review we aim to give a comprehensive overview about the current knowledge regarding HLA-directed alloreactivity and several developed in vitro and in silico tools that aim to predict direct and indirect alloreactivity.

Translating in vitro prediction of cytotoxic T cell alloreactivity to hematopoietic stem cell transplantation outcome

INTRODUCTION

Previously we developed a weighted amino acid (AA) mismatch score predictive for cytotoxic T cell (CTL) alloreactivity (in vitro CTLp assay) based on the structure of the HLA class I molecule. The aim of this study is to confirm the clinical relevance of the CTLp assay and to validate the AA mismatch score as an alternative and easy to use tool to predict permissible mismatches in hematopoietic stem cell transplantation (HSCT).

METHODS

We selected patients transplanted with a 9/10 single HLA class I mismatched graft (n=171) at three Dutch HSCT centers. A CTLp assay was performed in 73 donor-recipient pairs. As a control we selected 168 10/10 HLA matched pairs that were matched to the 9/10 single HLA class I mismatched pairs for HSCT year, donor type, patient age and diagnosis.

RESULTS

We observed that pairs with negative a CTLp assay had statistically significant decreased incidence of mortality after HSCT comparable to that of 10/10 HLA matched pairs. However, the weighted AA mismatch score did not significantly predict any HSCT end point of interest.

CONCLUSION

Further investigation is needed to unravel the mechanisms involved in causing the beneficial effect of a negative CTLp assay, before other alternative tools to predict HSCT outcome may be developed.

Differential Immunogenicity of HLA Class I Alloantigens for the Humoral versus the Cellular Immune Response: “Towards Tailor-Made HLA Mismatching”

The immunogenicity of an individual human leukocyte antigen (HLA) class I mismatch is different for the cellular and the humoral alloimmune responses. The consequence is that the same antigen can induce a strong antibody response and no cytotoxic T lymphocyte reactivity, but the reverse can occur also. Exact knowledge of the immunogenicity of an HLA mismatch for an individual patient can lead to a strategy of tailor-made HLA mismatching if no HLA identical donor is available. Depending on the clinical situation, one should select a donor with HLA mismatches according to the humoral or cellular mismatch algorithm.

Conditioning including antithymocyte globulin followed by unmanipulated HLA-mismatched/haploidentical blood and marrow transplantation can achieve comparable outcomes with HLA-identical sibling transplantation

The outcomes of 293 patients with leukemia undergoing HLA-identical sibling (n = 158) or related HLA-mismatched (n = 135) hematopoietic cell transplantation (HCT) performed during the same time period were compared. Patients received BUCY2 in HLA-identical sibling HCT or BUCY2 + ATG in mismatched HCT as conditioning regimens, followed by unmanipulated marrow and/or peripheral blood (PB) transplantation. All patients achieved full engraftment. The cumulative incidences of grades II to IV acute graft-versus-host disease (aGVHD) in the matched and mismatched cohorts were 32% (CI, 25%-39%) versus 40% (CI, 32%-48%, P = .13), respectively, with the relative risk (RR) = 0.64 (95% CI, 0.43-0.94), P = .02. The incidence of chronic GVHD did not differ significantly between the cohorts (P = .97). Two-year incidences of treatment-related mortality and relapse for matched versus mismatched were 14% (range, 9%-20%) versus 22% (range, 15%-29%) with P = .10 and 13% (range, 8%-19%) versus 18% (range, 10%-27%) with P = .40, respectively. Two-year adjusted leukemia-free survival (LFS) and overall survival were 71% (range, 63%-78%) versus 64% (range, 54%-73%) with P = .27 and 72% (range, 64%-79%) versus 71% (range, 62%-77%) with P = .72, respectively. Multivariate analyses showed that only advanced disease stage and a diagnosis of acute leukemia had increased risk of relapse, treatment failure, and overall mortality. In summary, HCT performed with related HLA-mismatched donors is a feasible approach with acceptable outcomes.

Differential immunogenicity of HLA mismatches in clinical transplantation

Although HLA matching is beneficial in clinical transplantation, it is not feasible to select a completely HLA matched donor for every potential recipient because of the enormous polymorphism of the HLA system. As a consequence, the majority of the recipients will be transplanted with a mismatched donor organ or hematopoietic stem cell transplant. For this large group of patients it is important to take advantage of the differential immunogenicity of HLA mismatches and to select for them a donor with HLA mismatches of low immunogenicity, the so-called acceptable mismatches. The differential immunogenicity of HLA mismatches can be determined by either retrospective analysis of graft survival data or by in vitro assays measuring T-cell and B-cell alloreactivity. A recently developed computer algorithm (HLAMatchmaker) can be instrumental in selecting donors with HLA mismatches, which do not lead to alloantibody formation. The theoretical background and practical implications of this acceptable mismatch approach are discussed.

Microchimerism after liver transplantation: absence of rejection without abrogation of anti-donor cytotoxic T-lymphocyte-mediated alloreactivity

Microchimerism (MC) is defined by the persistence of <1% circulating donor cells resulting from cell migration from the graft; MC may play a role in the induction of unresponsiveness to allogeneic tissues, or may be merely the consequence of the graft's acceptance following immunosuppression. To analyze early MC (7 patients) and late MC (12 patients) following a liver transplantation, we designed a sensitive and semiquantitative nested polymerase chain reaction (PCR) protocol based on the detection of incompatible human leukocyte antigen (HLA)-DRB1 donor alleles. MC was measured in multiple PCR samples and expressed as percent positive PCRs / time point. The detection level was 1 donor cell / 10(5) patient cells. All patients had detectable early MC, ranging from 5 to 100% positive PCRs in the 1st 3 months after transplantation. The kinetic analysis demonstrated that MC decreased during the 1st year in 6 of 7 patients. All of the 4 patients with the lowest MC had rejection episodes, vs. none among the 3 patients with MC >50%. However, cytotoxic T-lymphocyte reactivity (CTL) against HLA class I donor antigens could be demonstrated 1 year posttransplant in 2 patients with a high level of early MC. MC is a dynamic process, which is easily detectable <3 months after liver transplantation. In conclusion, a correlation between the level of early MC and the absence of rejection episodes was observed. However, high levels of early MC did not abrogate the persistence of an alloreactive response measured in vitro 1 year after transplantation, which suggests that MC did not lead to clonal deletion of donor-specific CTL.

Antirecipient helper and cytotoxic T-cell frequencies in bone marrow transplantation

We assayed helper T-lymphocyte precursor frequencies (HTLPf), interferon (IFN)-gamma-producing cell frequencies (IFN-gammaPf) and CTL precursor frequencies (CTLPf) to see if they could predict the severity of acute graft-versus-host disease (aGVHD) and disease relapse after transplantation. In all, 48 bone marrow transplantation (BMT) patients and their HLA-identical sibling (n=29) or matched unrelated donors (MUD) (n=19) were recruited. HTLPf, IFN-gammaPf and CTLPf were measured using a limiting dilution assay (LDA). Patients were followed prospectively to assess the severity of aGVHD and the status of the primary disease after BMT. High (>5 x 10(-6)) HTLPf, CTLPf and IFN-gammaPf were significantly associated with the occurrence and severity of aGVHD in patients who received transplants from HLA-identical sibling. Among patients receiving BMT from MUD, HTLPf and CTLPf, but not IFN-gammaPf, were associated with aGVHD. Five patients had disease relapse post-BMT and the risk was not significantly associated with HTLPf, CTLPf or IFN-gammaPf. Patients with high (>5 x 10(-6)) HTLPf, IFN-gammaPf or CTLPf before BMT are at higher risk of developing aGVHD after transplantation from both matched sibling donors and MUD. Whether these parameters can predict disease relapse would have to be investigated with a larger cohort of patients.

Combining altered levels of effector transcripts in circulating T cells with a marker of endothelial injury is specific for active graft-versus-host disease

Cytotoxic T lymphocytes (CTLs) are important effector cells of graft-versus-host disease (GVHD) and vascular endothelial cells are target cells of allospecific CTL. A combined assessment of T-cell activation and endothelial injury should result in a specific and sensitive test for GVHD. We examined circulating T lymphocytes for effector molecules involved in CTL-mediated endothelial injury. We analyzed CD4 and CD8 T lymphocytes of 24 long-term survivors of allogeneic stem cell transplantation with or without GVHD, and nine healthy, age-matched controls for signs of CTL activation and endothelial injury. IFN-gamma transcript levels in CD8 T cells were significantly elevated in SCT recipients with GVHD compared to patients without GVHD (767 CD3epsilon units/T cell (376-2050) vs 211 CD3epsilon units/T cell (159-274), P=0.01). Fas ligand transcript levels in CD4 T cells were significantly elevated in SCT recipients without GVHD compared to patients with GVHD (20 CD3epsilon units/T cell (0-78) vs 0 CD3epsilon units/T cell (0-0), P=0.01). Von Willebrand factor plasma levels were high in patients with GVHD, but normal in patients without GVHD (209 (186-254) vs 120 (100-141), P=0.0005). This assessment of T-cell activation and endothelial injury results in a sensitive and specific test to identify patients with active chronic GVHD.

IL-13 production by donor T cells is prognostic of acute graft-versus-host disease following unrelated donor stem cell transplantation

Despite the success of human leukocyte antigen (HLA) typing in allogeneic stem cell transplantation (SCT) it is rare to find an unrelated donor that is perfectly matched, making identification of "permissive" mismatches of paramount importance. Here, we describe novel associations between donor T-cell cytokine production during donor-antipatient mixed lymphocyte reactions (MLRs) and acute graft-versus-host disease (aGVHD). The data reveal positive correlations between both Th1-type and Th2-type cytokine production and GVHD and the assay established could potentially represent a useful tool for identification of permissible unrelated SCT donors. Associations between interleukin 13 (IL-13) levels and aGVHD were by far the strongest predictor of a GVHD (P =.0002). All patients suffering severe (grade III) aGVHD following SCT had donors who produced very high pretransplantation IL-13 responses, while those developing little or no aGVHD (grades 0-I) produced no IL-13 at all. IL-13 levels were independent of all other cytokines measured as well as cytotoxic T-lymphocyte precursor (CTLp) frequencies. The cytokines IL-5, interferon gamma (IFN-gamma), and tumor necrosis factor alpha (TNF-alpha) also predicted development of aGVHD (P <.05 for all 3), appearing to be coproduced in the assay and correlating with estimated CTLp frequencies. The data challenge the notion that aGVHD is purely a Th1-type cytokine-driven response, high-lighting a novel and highly significant link between the Th2-type cytokine IL-13 and aGVHD.

Use of a reduced-intensity conditioning regimen for allogeneic transplantation in patients with chronic myeloid leukemia

Reduced-intensity conditioning that harnesses the potential of a graft-versus-tumor (GVT) effect has been proposed as an alternative to conventional myeloablative allogeneic stem cell transplantation. The primary aim is engraftment and this can be achieved with minimal immunosuppression. In this report, we describe the use of such regimens for CML in 17 patients who received human leukocyte antigen (HLA)-matched sibling allografts. Conditioning was with fludarabine, antithymocyte globulin (ATG) and busulfan for the first 11 patients, whereas fludarabine, busulfan and TBI were used for the remaining six patients. Engraftment was prompt in most of the cases. Complications and need for supportive therapy in the immediate post-transplant period were reduced drastically. Only two patients (both in the TBI group) died within the first 100 days. Acute graft-versus-host disease (GVHD) grade II-IV was seen in seven patients. Complications occurred later on. Chronic GVHD was observed in 11/17 patients. Lung infection and GVHD were the major killers. In surviving patients, after a median follow-up of 30 months (range 37-21 months), 6/17 (35.3%) are alive. Five are disease free and one patient is still in relapse even after a second donor lymphocyte infusion. Total treatment time and cost were more than with conventional transplants. We conclude that reduced-intensity transplantation still requires further refinement.

Predictive parameters for in vivo alloreactivity

Despite optimal HLA matching and a negative serological crossmatch, confrontation with allogeneic cells by organ- or stem-cell transplantation or platelet transfusion, can lead to an alloimmune response resulting in graft rejection, graft vs. host disease or platelet refractoriness. It would be attractive to be able to select beforehand those donor/recipient combinations, that do not lead to a destructive alloimmune response and exclude transplantation or transfusion with donors, that induce a strong alloimmune reaction. Many predictive parameters have been identified on the basis of retrospective analysis of graft survival data, and the results of in vitro tests to measure T and B cell alloreactivity. However, most of these parameters have shown to be relevant when a statistical analysis is performed on the population level but do not have a direct impact for the individual patient. An exception is a molecularly based algorithm, called HLA matchmaker, which seems to predict which HLA mismatches do not lead to alloantibody formation in a particular individual. Prediction of T cell alloreactivity is more difficult and will need the development of additional in vitro tools or adaptation of the HLA matchmaker program. Although the direct clinical implication of NK cell mediated allorecognition is not clear yet, this may be a complicating factor when establishing solid parameters for the prediction of an alloimmune reaction in vivo.

The role of in vitro alloreactive T-cell functional tests in the selection of HLA matched and mismatched haematopoietic stem cell donors

Acute graft vs. host (GVH) disease and graft rejection are most frequently caused by undetected or disregarded genetically based disparities between the donor and recipient of bone marrow derived haematopoietic stem cells (HSC). Incompatibilities in extremely polymorphic human leukocyte antigens (HLA), and in certain cases also minor histocompatibility antigens, represent the most important driving force of such unwanted events, threatening the successful outcome of haematopoietic stem cell transplantation (HSCT). The complexity of HLA polymorphism can be precisely and elegantly detected at the genomic level by several polymerase chain reaction (PCR) based techniques that have strongly backed up its predecessor, the far less informative classical serological typing. By applying these modern technologies, we gain the deepest insight into HLA allelic specificities and thus the possibility to, for example, trace and recruit unrelated histocompatible donors for a given patient. In the case when exclusively related intrafamilial HSC donors are being considered, we are confined to the fact that only 25-30% of patients can expect a completely HLA identical donor to be found within core or extended family members. The number of related as well as unrelated donors can be increased if certain HLA mismatches are accepted. When doing so, the precise definition of disparate histocompatibilty antigens between the patient and a possible donor should be carried out. But this does not give us the information about the functional immunogenicity of such differences. Therefore, in vitro functional assays, quantitating the alloreactive potential of lymphocyte T subsets, the central immunocompetent cells, are more than necessary. By evaluating mixed lymphocyte reaction (MLR), the analysis of helper T cell precursor (HTLp) and cytotoxic T cell precursor (CTLp) frequencies, the allogeneic impact of class II and class I HLA mismatches between a donor and graft recipient can be assessed and permissive disparities defined.

Separating graft-versus-tumor from graft-versus-host reactions

The full therapeutic potential of allogeneic stem cell transplantation, through its immunologically mediated graft-versus-tumor effect, in patients with hematologic malignancies is greatly compromised by the occurrence of graft-versus-host disease. Unfortunately, the use of non-selective immunosuppressive agents to reduce the incidence and severity of graft-versus-host disease is associated with severe immune compromise of the host and most likely a greater relapse risk of the underlying malignancy. Many attempts have been made to clinically separate these two effects. A critical overview of the published experience is the focus of this report. As the effector cells responsible for the two reactions are largely unknown, the limited success of the various approaches used is not surprising. A more thorough understanding of the antigenic stimuli involved in the initiation of the two reactions and of the molecular pathways through which the cytotoxic effects of T-cells are mediated is essential for abrogating graft-versus-host disease while preserving the graft-versus-tumor effect.

Cytotoxic T-lymphocyte precursor frequency (CTLp-f) as a tool for distinguishing permissible from non-permissible class I mismatches in T-cell-depleted allogeneic bone marrow transplantation

Matching for HLA has been the gold standard in bone marrow donor selection. But, with the ever increasing number of identified HLA alleles, it is becoming more difficult to find a fully HLA-identical donor other than a sibling. Retrospective analysis revealed that HLA mismatches do not necessarily give rise to acute graft-versus-host-disease (GVHD). However, we have no means of defining these 'permissible' mismatches before bone marrow transplantation (BMT). Thus, we set out to establish whether functional matching by means of helper and cytotoxic T-lymphocyte precursor frequency analysis (HTLp-f and CTLp-f respectively) can be applied to this end. Fifty-five recipient-donor pairs other than HLA-identical siblings, the recipient of which received a T-cell-depleted graft, were analysed by high-resolution HLA typing and/or HTLp-f/CTLp-f analysis. The predictive value of the CTLp-f assay for development of acute GVHD was confirmed. More importantly, our data indicate that the CTLp-f assay was able to discriminate permissible from non-permissible HLA-A, -B or -Cw mismatches, but not for DRB/DQB mismatches. The absolute number of alloreactive CTLs present in the graft correlated with the risk of acute GVHD. Although HTLp-f and CTLp-f together had a high negative predictive value, HTLp-f outcome by itself was not correlated with acute GVHD. As we have no evidence yet that HTLp-f or CTLp-f can identify permissible DRB/DQB mismatches, high-resolution matching for these antigens remains the best option. The combination of high-resolution DRB/DQB typing and the CTLp-f assay would enable the accurate prediction of the risk of acute GVHD while extending the pool of potential donors. Furthermore, it would enable adjustment of the number of T- cells in the graft accordingly to improve clinical outcome.

Cytotoxic T lymphocyte precursor frequency as a predictor of acute graft-versus-host disease in bone marrow transplantation from HLA-identical siblings

The measurement of precursor frequencies of donor anti-recipient cytotoxic T lymphocytes (CTL-p) has been shown to predict the incidence and the severity of acute graft-versus-host disease (aGVHD) in unrelated donor bone marrow transplantation (BMT). In HLA-identical sibling BMT, where aGVHD is most likely caused by minor histocompatibility antigen mismatches, this assay did not appear to be sensitive enough to provide similar predictive information. In this study, the CTL-p frequencies and the incidence and severity of aGVHD in 51 onco-hematological patients transplanted from HLA-identical siblings were compared. Sibling donors were selected on the basis of HLA identity using serological typing for HLA-A, B, C antigens, whereas HLA-DRB was tested by molecular analysis. Sibling identity was also confirmed by DNA heteroduplex analyses. Fifteen out of 21 (71%) patients with high precursor frequency (>1:100 x 10(3)) and 12 out of 30 (40%) with low precursor frequency (<1:100 x 10(3)) experienced clinically significant (II-IV) aGVHD. A significant correlation (P = 0.04) between CTL-p frequency and severe aGVHD was demonstrated. Moreover there was a positive trend for a high frequency response according to an increasing grade of aGVHD, which was statistically significant (P = 0.04). In our experience the CTL-p assay is a helpful predictive test for aGVHD in HLA-identical sibling BMT, indicating high risk patients possibly requiring additional prophylaxis.

T-cell frequency analysis does not predict the incidence of graft-versus-host disease in HLA-matched sibling bone marrow transplantation

BACKGROUND

Graft-versus-host disease (GVHD) is a major and sometimes fatal complication of allogeneic bone marrow transplantation (BMT). The prediction of GVHD remains an important issue in preventing morbidity and mortality after allogeneic BMT. In the past 10 years, there has been great interest in using the frequency analysis of alloreactive helper and cytotoxic T lymphocyte precursors (HTLp and CTLp) to detect recipient-specific alloreactivity and thus predict GVHD in HLA-matched related and unrelated BMT. However, the results remain controversial. The intention of the present study was to investigate whether alloreactive HTLp and CTLp frequencies measured in donor peripheral blood before BMT would be a useful predictor for the occurrence of acute GVHD after HLA-matched sibling BMT.

METHOD

A combined limiting dilution assay was used to determine alloreactive HTLp and CTLp frequencies for 42 HLA-matched sibling patient/donor pairs. The pretransplantation host-reactive HTLp and CTLp frequencies were then correlated with post-transplantation clinical outcomes of acute GVHD. The association between HTLp/CTLp frequencies and the incidence of acute GVHD was determined using the Fisher's exact test.

RESULTS

The mean values of HTLp and CTLp frequencies for this cohort of HLA-matched sibling patient/donor pairs were 1:321,322 (range 1:71,000 to 1:1000,000) and 1:195,260 (range 1:3,717 to 1:1000,000), respectively. Acute GVHD (> or =II) was observed in one of four patients with high (>1:100,000) HTLp frequencies and 20 of 36 patients with low (<1:100,000) HTLp frequencies. Similarly, 6 of 10 patients with high (>1: 100,000) CTLp frequencies and 14 of 29 patients with low (<1:100,000) CTLp frequencies developed acute GVHD (> or =II). The overall correlation between hostreactive HTLp/CTLp frequencies and the incidence of acute GVHD in this cohort of patients was 42.5% and 53.9%, respectively. There was no significant difference in the incidence of acute GVHD between the patients with either high or low host-reactive HTLp/ CTLp frequencies (P=0.331 and 0.716, respectively). The data were also analyzed separately for the adult patient group based on GVHD prophylaxis with either cyclosporine alone or the combination of cyclosporine and methotrexate. Within these two prophylaxis groups, neither HTLp nor CTLp frequencies correlated with acute GVHD.

CONCLUSION

Host-reactive HTLp and CTLp frequency analysis did not provide informative prediction for the occurrence of acute GVHD after HLA-matched sibling BMT.

T helper frequencies in peripheral blood reflect donor-directed reactivity in the graft after clinical heart transplantation

We describe the usefulness of a fast (48-h) limiting dilution assay (LDA) for the enumeration of human alloreactive helper T lymphocytes (HTL) in the peripheral blood, in relation to histologically defined rejection grades after heart transplantation. HTL frequencies (HTLf) in pretransplant samples varied from patient to patient, ranging from 106 to 625 HTL/106 peripheral blood mononuclear cells (PBMC). In the first week after heart transplantation (HTx), when immunosuppression was instituted, HTLf were significant lower (range 30-190 HTL/106). The level of HTL in the first week after HTx when rejection grade was 0 or 1A (ISHLT score) was considered to be the baseline frequency. This frequency did not correlate with the number of subsequent rejection episodes. During rejection (grade 3), donor-specific HTLf were increased above their baseline frequencies (P = 0.01). Expressed as percentage of baseline frequencies, HTLf increased significantly during acute rejection (AR) compared with 1-2 weeks before rejection (P = 0.003). The increase was specific, since viral infections did not result in a rise of donor-specific HTL, while also HTLf specific for third party HLA antigens were not elevated during rejection. Monitoring HTLf in peripheral blood with a shortened (48-h) assay may serve as a non-invasive method for detecting intragraft immunological reactivity. Demonstrating absence of donor-specific reactivity may limit the number of invasive endomyocardial biopsy (EMB) procedures and allow tapering of immunosuppressive treatment.

An in vivo model of human skin acute graft-versus-host disease: transplantation of cultured human epidermal cells and dermal fibroblasts with human lymphocytes into SCID mice

The ability of mixed epidermal cell-lymphocyte reactions to detect allogeneic reactivities in an in vivo model was investigated by developing an in vivo model of acute graft-versus-host disease (GVHD), using SCID mice with a C.B-17 background in which human skin structures were generated by transplantation of cultured human epidermal cells (HEC) with dermal fibroblasts (HDFC). Suspensions containing cultured HEC and HDFC from a single donor were mixed with autologous peripheral blood mononuclear cells (PBMNC) or with PBMNC from unrelated individuals, and were injected into the flanks of C.B-17-SCID mice. Ten and 21 days after injection, subcutaneous nodules generated in the mice were examined histologically and immunohistochemically. Cystic structures developing after injection of HEC and HDFC without human PBMNC showed normal epidermislike tissue. Human skin generated in SCID mice injected with HEC and HDFC with auto-PBMNC showed no graft-versus-host reaction (GVHR) histologically, whereas those mice injected with PBMNC from siblings that shared an HLA haplotype showed mild GVHR. Human skin in SCID mice injected with HEC and HDFC with histoincompatible unrelated PBMNC showed moderate to severe GVHR. The severity of GVHR paralleled the dose of unrelated PBMNC, and GVHR was prevented by peroral treatment with cyclosporine A. Immunohistochemically, inflammatory cells infiltrating human cutaneous tissue formed in the SCID mice were stained by an anti-human CD45RO antibody that reacts with human T cells but not with murine lymphocytes, and most T cells were stained by an anti-human CD8 antibody recognizing HLA class I antigens. These findings are similar to those in clinical skin graft-versus host disease (GVHD) observed in patients undergoing allogeneic bone marrow transplantation. This experimental system should be useful as an in vivo model of human skin GVHD.

Allogeneic transplantation from donors other than HLA-identical siblings

The decade of the 1990s has witnessed impressive advances in the use of alternative donors for allogeneic transplantations. Through funded collaborative, and single-center research efforts, there is great promise that complete and equal access to allogeneic transplantation will be realized. In the next decade, universal donor availability should maximize the patient's chance of overcoming otherwise fatal disease.

Molecular Typing Shows a High Level of HLA Class I Incompatibility in Serologically Well Matched Donor/Patient Pairs: Implications for Unrelated Bone Marrow Donor Selection

In comparison with HLA-matched sibling bone marrow transplants, unrelated donor transplants are associated with increased graft-versus-host disease and graft failure. This is likely in part due to HLA incompatibilities not identified by current matching strategies. High resolution DNA-based typing methods for HLA class II loci have improved donor selection and treatment outcome in unrelated donor bone marrow transplantation. By using DNA-based typing methods for HLA-A and -B on a cohort of 100 potential bone marrow donor/patient pairs, we find that serological typing for HLA class I is limited in its ability to identify incompatibilities in unrelated pairs. Furthermore, the incompatibilities identified are associated with the presence at high frequency of alloreactive cytotoxic T-lymphocyte precursors. DNA typing also indicates that HLA-C mismatches are common in HLA-A and -B serologically matched pairs. Such mismatches appear to be significantly less immunogenic with respect to cytotoxic T-lymphocyte recognition, but are expected to influence natural killer cell activity. Thus, improved resolution of HLA class I shows many previously undisclosed mismatches that appear to be immunologically functional. Use of high resolution typing methods in routine matching is expected to improve unrelated donor selection and transplant outcome.

Molecular Typing Shows a High Level of HLA Class I Incompatibility in Serologically Well Matched Donor/Patient Pairs: Implications for Unrelated Bone Marrow Donor Selection

In comparison with HLA-matched sibling bone marrow transplants, unrelated donor transplants are associated with increased graft-versus-host disease and graft failure. This is likely in part due to HLA incompatibilities not identified by current matching strategies. High resolution DNA-based typing methods for HLA class II loci have improved donor selection and treatment outcome in unrelated donor bone marrow transplantation. By using DNA-based typing methods for HLA-A and -B on a cohort of 100 potential bone marrow donor/patient pairs, we find that serological typing for HLA class I is limited in its ability to identify incompatibilities in unrelated pairs. Furthermore, the incompatibilities identified are associated with the presence at high frequency of alloreactive cytotoxic T-lymphocyte precursors. DNA typing also indicates that HLA-C mismatches are common in HLA-A and -B serologically matched pairs. Such mismatches appear to be significantly less immunogenic with respect to cytotoxic T-lymphocyte recognition, but are expected to influence natural killer cell activity. Thus, improved resolution of HLA class I shows many previously undisclosed mismatches that appear to be immunologically functional. Use of high resolution typing methods in routine matching is expected to improve unrelated donor selection and transplant outcome.

Predicting graft-versus-host disease in HLA-identical bone marrow transplant: a comparison of T-cell frequency analysis and a human skin explant model

BACKGROUND

Graft-versus-host disease (GVHD) occurring after HLA-identical sibling bone marrow transplantation (BMT) is considered to be mainly caused by minor histocompatibility antigen (mHag) disparities between the recipient and donor. In our laboratory, a human skin explant model has been successfully used to predict acute GVHD in HLA-identical sibling BMT. More recently, the frequency analysis of host-reactive helper and cytotoxic T lymphocyte precursors (HTLp and CTLp, respectively) has been shown to have potential application for predicting GVHD. In the present study, HTLp and CTLp frequency analysis and the skin explant model were directly compared for their ability to predict acute GVHD in HLA-identical sibling BMT.

METHODS

Host-reactive HTLp and CTLp frequencies were determined using a combined limiting dilution assay. A human skin explant model was used to detect graft-versus-host reactions in vitro. The results from the skin explant model (graft-versus-host reaction grades I-IV) and T cell frequency analysis (>/< 1:100,000) were correlated with posttransplant GVHD outcome, respectively.

RESULTS

The skin explant model correctly predicted GVHD outcome in 77% of cases (P=0.03). HTLp frequencies were very low in all patient/donor pairs tested. None of them exceeded 1:100,000, although 9/18 recipients developed GVHD (> or =clinical grade II) after transplant. In all patients tested, the relationship between either high (>1:100,000) or low (<1:100,000) CTLp frequency and occurrence of GVHD appeared to be random (P=1.0).

CONCLUSIONS

HTLp and CTLp frequency analysis did not predict the occurrence of acute GVHD after HLA-identical sibling BMT. The human skin explant model, however, remained an accurate indicator of acute GVHD and probably detects mHag disparities.

HA-1 and the SMCY-derived peptide FIDSYICQV (H-Y) are immunodominant minor histocompatibility antigens after bone marrow transplantation

BACKGROUND

Allogeneic bone marrow donors can be incompatible at different levels. Even HLA-identical pairs will be still incompatible for numerous minor histocompatibility antigens (mHag). Nevertheless, some incompatibilities are found to be associated with an increased risk of graft-versus-host disease (GVHD), which could be related to the way the immune system recognizes these antigens.

METHODS

We determined the specificity of cytotoxic T-cell clones isolated during acute GVHD or during bone marrow graft rejection in patients (n=14) transplanted with marrow from donors who were histoincompatible for different minor and/or major histocompatibility antigens.

RESULTS

We found a clear hierarchy among the different types of histoincompatibilities. In three combinations mismatched for a class I allele, all 27 clones isolated during GVHD were specific for the incompatible HLA molecule. In the 11 class I-identical combinations, 14 different mHags were recognized. The mHag HA-1, known to have a significant impact on the development of GVHD, was recognized in the two HA-1-incompatible combinations. In one of these combinations, which was sex mismatched, all 56 clones analyzed were directed against HA-1, demonstrating the dominance of this mHag. In the four HA-1-compatible, sex-mismatched combinations, the anti-H-Y response was directed against one immunodominant epitope rather than against multiple Y-chromosome-encoded epitopes. All male specific cytotoxic T lymphocytes (n=15) recognized the same high-performance liquid chromatography-purified peptide fraction presented by T2 cells. Moreover, all cytotoxic T lymphocytes tested (n=6) were specific for the SMCY-derived peptide FIDSYICQV, originally described as being the H-Y epitope recognized in the context of HLA-A*0201.

CONCLUSIONS

Some histocompatibility antigens are recognized in an immunodominant fashion and will therefore be recognized in the majority of mismatched combinations. Only for such antigens, correlations between mismatches and the occurrence of GVHD or graft rejections will be found.

Improved IL-2 detection for determination of helper T lymphocyte precursor frequency in limiting dilution assay

In the context of allogeneic bone marrow transplantation, an accurate estimate of the risk of developing graft-versus-host disease (GVHD) is of major interest. The pre-transplant frequency of donor's helper T-lymphocyte precursors (HTLp) directed against host's antigens may be helpful in predicting this risk. This technique relies on an indirect measurement of interleukin-2 (IL-2) secreted by the HTLp, as assessed by the proliferation of an IL-2 dependent cell line. Many authors use the murine CTLL-2 cell line in this assay, but these cells do not respond to the presence of minute amounts of IL-2 in the culture medium, and thus do not discriminate between the absence or the presence of very low levels of IL-2. We therefore decided to compare CTLL-2 with another IL-2 dependent cell line, the murine A9.12 cell line. A comparison was made using serial dilutions of recombinant human IL-2, limiting dilutions of baby hamster kidney (BHK) cells transfected with human IL-2 gene and in the context of clinical tests performed for the detection of pre-transplant HTLp. Both the sensitivity and reliability of the tests were better using A9.12. We conclude that the A9.12 cell line might be a more suitable tool for pre-transplant HTLp determinations before allogeneic bone marrow transplantation or whenever low IL-2 levels are to be measured.

Measurement of cytotoxic T lymphocyte precursor frequencies reveals cryptic HLA class I mismatches in the context of unrelated donor bone marrow transplantation

BACKGROUND

In this large, two-center study, 260 cytotoxic T lymphocyte precursor (CTLp) frequency assays, performed to assess patient-donor compatibility, were analyzed in relation to the degree of HLA matching.

METHODS

While the tissue-typing techniques used at the Royal Postgraduate Medical School (RPMS) and Anthony Nolan Bone Marrow Trust (ANBMT) differ, the results of the analyses on the two sites are analogous, with high CTLp frequencies (>1:100,000) in 42% and 41% of recipient-donor pairs, respectively.

RESULTS

Recipient-donor combinations with class I mismatches and class II identity were associated with high CTLp frequencies (collectively 83% vs. 17% low CTLp). This correlation was not as strong in pairs where class II mismatches were demonstrated (61% high vs. 39% low). Despite using different matching procedures, the RPMS and ANBMT both show that 32% of the "perfectly" matched pairs (i.e., where no mismatch was detected by any of the techniques used here) had high frequencies of recipient-specific CTLp.

CONCLUSIONS

The failure of conventional methods to identify such a level of histoincompatibilities indicates that the CTLp assay has an important role in the selection of unrelated donors for bone marrow transplantation.

Recognition of major histoincompatibilities after transplantation with marrow from HLA closely matched donors

BACKGROUND

To determine the extent to which major histoincompatibilities are recognized after bone marrow transplantation, we characterized the specificity of the cytotoxic T lymphocytes isolated during graft-versus-host disease. We studied three patients transplanted with marrow from donors who were histoincompatible for different types of HLA antigens.

METHODS

Patient 1 was mismatched for one "ABDR-antigen" (HLA-A2 versus A3). Two patients were mismatched for antigens that would usually not be taken into account by standard selection procedures: patient 2 was mismatched for an "HLA-A subtype" (A*0213 versus A*0201), whereas patient 3 was mismatched for HLA-C (HLA-C*0501 versus HLA-C*0701). All three HLA class I mismatches were detected by a pretransplant cytotoxic precursor test.

RESULTS

Analysis of the specificity of the cytotoxic T lymphocyte clones isolated after transplantation showed that the incompatibilities detected by the pretransplant cytotoxic precursor assay were the targets recognized during graft-versus-host disease.

CONCLUSIONS

Independent of whether the incompatibility consisted of a "full" mismatch, a "subtype" mismatch, or an HLA-C mismatch, all clones recognized the incompatible HLA molecule. In addition, some of these clones had undergone antigen selection and were clearly of higher specificity than the ones established before transplantation, indicating that they had been participating directly in the antihost immune response.

Hematopoietic stem cell transplants from unrelated donors

Transplantation of hematopoietic stem cells is now well established as life-saving therapy for patients with several different genetic and acquired diseases. This has been possible largely because of the identification of HLA as the human major histocompatibility system and the application of new technologies for improving the accuracy of HLA typing and matching. Clinical advances including the development of more effective immune suppression therapy and improvements in supportive care have also been critically important. The lack of HLA-identical sibling donors for most patients and the extensive polymorphism of HLA genes have necessitated establishing very large registries of HLA-typed volunteers. With more than 3 million donors now available worldwide, the number of patients identifying HLA matches has increased substantially. Despite a higher incidence of graft-versus-host disease (GVHD), the results of unrelated donor transplants are favorable when performed under optimal conditions with a well-matched donor. In order to provide transplants for all patients, improved methods for GVHD prophylaxis are needed that will make it possible to safely perform transplants even across limited degrees of HLA disparity.

Clinical significance of selective decline of donor-reactive IL-2-producing T lymphocytes after renal transplantation

Limiting dilution analysis technique was used to enumerate the circulating precursor frequency of donor and third-party-reactive helper T lymphocytes (HTLpf) in 28 renal allograft recipients before (pre-tx) and at three intervals (T1: 60-90 days, T2: 120-180 days, T3: 360-1620 days) after transplantation (post-tx). Two patterns of responses were identified, in group 1 (n = 12), a five to 31-fold reduction of donor-reactive HTLpf (ranging from 1/19231-1/62500) occurred within 90-1620 days post-tx, while in group 2(n = 16), no significant changes of donor-reactive HTLpf were seen. In both groups, the third-party-reactive HTLpf in most of these patients remained largely unchanged throughout the study period. The number of HLA-DR mismatches, total number of rejection episodes, serum creatinine levels, and biopsy findings at T3 were compared in both groups using Fisher's exact probability, and the Mann-Whitney test. We found that 11 patients (92%) in group 1 were HLA-DR compatible with donors, while nine (56%) patients in group 2 were HLA-DR compatible with donors, p = 0.04. In group 1 eight rejection episodes occurred in five (41.6%) patients during the study period, compared to 33 in 13 (81%) patients in group 2, p = 0.03. Group 1 had a significantly lower serum creatinine level (at T3); median: 136 vs 165 mumol/l for group 2, p = 0.03. Biopsy indicated no rejection (at T3) in eight (66%) patients in group 1 as compared to three (18%) patients in group 2, p = 0.03. Taken together, these results indicate that the frequency of circulating HTLpf correlate with the clinical status of the graft. Therefore monitoring of HTLpf in the peripheral blood could be useful in predicting graft outcome and selecting patients for reducing immunosuppression.

Molecular characterization of HLA-C incompatibilities in HLA-ABDR-matched unrelated bone marrow donor-recipient pairs. Sequence of two new Cw alleles (Cw*02023 and Cw*0707) and recognition by cytotoxic T lymphocytes

While the influence of HLA-AB and -DRB1 matching on the outcome of bone marrow transplantation (BMT) with unrelated donors is clear, the evaluation of HLA-C has been hampered by its poor serological definition. Because the low resolution of standard HLA-C typing could explain the significant number of positive cytotoxic T lymphocyte precursor frequency (CTLpf) tests found among HLA-AB-subtype, DRB1/B3/B5-subtype matched patient/donor pairs, we have identified by sequencing the incompatibilities recognized by CD8+ CTL clones obtained from such positive CTLpf tests. In most cases the target molecules were HLA-C antigens that had escaped detection by serology (e.g. Cw*1601, 1502 or 0702). Direct recognition of HLA-C by a CTL clone was demonstrated by lysis of the HLA class I-negative 721.221 cell line transfected with Cw*1601 cDNA. Because of the functional importance of Cw polymorphism, a PCR-SSO oligotyping procedure was set up allowing the resolution of 29 Cw alleles. Oligotyping of a panel of 382 individuals (including 101 patients and their 272 potential unrelated donors, 5 related donors and 4 platelet donors) allowed to determine HLA-C and HLA A-B-Cw-DRB1 allelic frequencies, as well as a number of A-Cw, B-Cw, and DRB1-Cw associations. Two new HLA-Cw alleles (Cw*02023 and Cw*0707) were identified by DNA sequencing of PCR-amplified exon 2-intron 2-exon 3 amplicons. Furthermore, we determined the degree of HLA-C compatibility in 287 matched pairs that could be formed from 73 patients and their 184 potential unrelated donors compatible for HLA-AB by serology and for HLA-DRB1/ B3/B5 by oligotyping. Cw mismatches were identified in 42.1% of these pairs, and AB-subtype oligotyping showed that 30% of these Cw-incompatible pairs were also mismatched for A or B-locus subtype. The degree of HLA-C incompatibility was strongly influenced by the linkage with B alleles and by the ABDR haplotypes. Cw alleles linked with B*4403, B*5101, B18, and B62 haplotypes were frequently mismatched. Apparently high resolution DNA typing for HLA-AB does not result in full matching at locus C. Since HLA-C polymorphism is recognized by alloreactive CTLs, such incompatibilities might be as relevant as AB-subtype mismatches in clinical transplantation.

Factors influencing the outcome of bone marrow transplants using unrelated donors

The use of unrelated donors for bone marrow transplantation (BMT) is associated with an increased morbidity and mortality when compared with HLA-identical donors, primarily due to an increased rate of graft-versus-host disease, but also to increased susceptibility to infections and graft failure. HLA matching for donors and recipients is the single most important factor influencing the outcome of BMT. However, unrelated donor selection generally relies on matching only for HLA-A, -B and -DR antigens without considering potential incompatibility for other HLA loci, such as HLA-C, -DQ and -DP. In addition, other factors that affect the outcome of BMT need to be taken into consideration in selecting the best unrelated donor. In this review, we will focus on the effects of HLA-associated factors in determining the result of a transplant procedure. We will also mention other relevant factors, drawing on our experience of laboratory studies performed at The Anthony Nolan Research Institute and clinical studies at the Hammersmith Hospital in London.

A future for HLA matching in clinical transplantation

The arguments for and against minimizing HLA antigen mismatching in clinical transplantation have been debated for the past 30 years (Terasaki & Cecka, 1994), but at least two major kidney exchange programmes (EURO- and UK-Transplant), with waiting lists totalling 17 thousand patients, prioritize the least HLA mismatched recipients. In the USA, in the United Network for Organ Sharing (UNOS) zero HLA-A,-B, DR mismatch for exchange of kidneys is mandatory. There is now a clear understanding of the in vivo function of HLA molecules, which have a central role in antigen presentation to T cells and thus initiate the immune response both in the normal individual and in transplant recipients. In the latter case, direct recognition of foreign HLA molecules by recipient T cells is also possible (Shoskes & Wood, 1994). With such a clear understanding of the central functional role for HLA in the immune response, it is not surprising that reports of increased kidney transplant survival correlate with reduced HLA mismatching, and it is of concern that some centres choose to allocate kidneys for transplant ignoring HLA. Reports of benefits of minimizing HLA mismatching in heart (Smith et al., 1995) and lung (Iwaki et al., 1993) transplantation are limited to retrospective studies, and no centre has claimed to allocate these organs taking HLA mismatch into account. Consequently, HLA mismatched thoracic organ transplants predominate, but nevertheless recipients of completely mismatched organs have decreased survivals (Opelz & Wujciak, 1994) and more rejection (Baan et al., 1991; Jarcho et al., 1994). Since transplantation of hearts and lungs is essentially only effective on one occasion, all effort should be made to maximize survival and incorporation of HLA mismatches in allocation criteria for these organs is now overdue (Disesa et al., 1990; Morris, 1994). In this brief review we choose not to cover the extensive literature on the role of HLA mismatching in organ transplantation, but consider the field at the present time with particular emphasis on realistic, practical application of HLA typing to the clinical situation and, in particular, the development of allocation procedures that will best suit the needs of individual patients.

The role of cell-mediated cytotoxicity in acute GVHD after MHC-matched allogeneic bone marrow transplantation in mice

The role of cell-mediated cytotoxicity in the complex pathophysiology of graft-versus-host disease (GVHD) has remained poorly defined for several decades. We transplanted T cells from Fas-ligand (FasL)-defective and perforin-deficient mutant donor mice into lethally irradiated MHC-matched allogeneic recipient mice to characterize the role of cell-mediated cytotoxicity in GVHD. Although recipients of allogeneic FasL-defective donor T cells underwent severe GVHD-associated cachexia, they exhibited only minimal signs of hepatic and cutaneous GVHD pathology. Recipients of perforin-deficient allogeneic donor T cells developed signs of acute GVHD, but the time of onset was significantly delayed. These findings demonstrate that Fas-mediated anti-recipient cytotoxicity may be critical for the development of hepatic and cutaneous GVHD, but is not required for GVHD-associated cachexia. In addition, perforin-mediated anti-recipient cytotoxicity appears to play an important role in the kinetics of GVHD pathophysiology, but is not required for GVHD-associated tissue damage.

Frequency of donor cytotoxic T cell precursors does not correlate with occurrence of acute graft-versus-host disease in children transplanted using unrelated donors

Bone marrow transplantation (BMT) from unrelated volunteers is frequently associated with both increased incidence and increased severity of acute graft-versus-host disease (GVHD). In the last years, it has been suggested that the analysis of the frequency of cytotoxic T lymphocyte precursors (CTLp) of unrelated HLA-matched donors can be used to detect disparity for HLA class I antigens and as a predictive test for development of severe GVHD. In this report, we summarized our experience regarding 20 pediatric patients affected by various hematological disorders, receiving allogeneic BMT from unrelated donors. HLA class I and II antigens of donor/recipient pairs were determined by means of serological typing, whereas molecular typing of HLA-class II antigens of patients and their potential donors was performed using PCR-SSP and PCR-fingerprinting techniques. CTLp values, estimated using limiting dilution analysis, were high (range, 1:7000-1:40,000) in 9 of 20 patients, while the other 11 children had low or undetectable levels (< 1:100,000) of CTL precursors. CTLp frequency was compared with the incidence and severity of GVHD observed after BMT. Our data demonstrate that the frequency of donor CTLp does not statistically correlate either with the occurrence of clinically significant acute GVHD or with disparity for HLA-class II molecular typing between donor and recipient. In particular, 4 of the 10 evaluable patients with an undetectable CTLp frequency developed grade IV, III, II, and IV acute GVHD, respectively. Although the limited number of subjects studied does not allow us to draw any firm conclusion, our data suggest a certain caution in considering this test suitable for the selection of potential donors.

Predicting graft-versus-host disease

Graft-versus-host disease (GVHD) is a serious complication that occurs after allogeneic bone marrow transplantation and contributes to transplant-related morbidity and mortality. Recent advances in the measurement of recipient-specific alloreactivity before marrow grafting have led to the development of in vitro assays that sufficiently predict the occurrence of moderate to severe acute GVHD. This predictive information provides the basis for attempts to minimize, or prevent, GVHD by prospective donor selection and individualized immunotherapeutic strategies.

Oligotyping of HLA-A2, -A3, and -B44 subtypes. Detection of subtype incompatibilities between patients and their serologically matched unrelated bone marrow donors

We have set up a simple PCR-SSO oligotyping procedure that is able to discriminate ten HLA-A2 (2 PCR/11 probes), two HLA-A3 (1 PCR/1 probe), and two HLA-B44 subtypes (1 PCR/2 probes). The frequency of these subtypes has been determined in a large panel of local blood donors and leukemic patients in combination with their unrelated potential donors. A*0201 and A*0301 were the predominant subtypes (> 95%) for A2 and A3, respectively. B*4402 occurred twice as frequently as B*4403. A2 and B44 subtype mismatches were analyzed in a group of 30 patients and their 116 unrelated potential donors who were matched serologically (low-stringency matching: AB without splits, DR1-10). For seven patients (23%) at least one A2- or B44-subtype-mismatched donor was found. For two of these patients (7%), the subtype-mismatched donor would have been considered as compatible on the basis of high stringency matching (AB splits, DRB1 subtypes, DRB3/B5). For one patient of Mediterranean origin, all five donors recruited from a north European registry (matched with high stringency) appeared to be subtype incompatible (A*0201/A*0205). The rather low percentage of A2- and B4-subtype mismatches in DRB1/B3/B5 matched combinations confirms the significance of linkage disequilibria of HLA antigens. Because unrelated donor selection is done through international registries, however, class I subtyping might be necessary when individuals originate from different geographic areas.

What can be done to prevent graft versus host disease?

Graft versus host disease presents a major obstacle to the widespread application of allogeneic bone marrow transplantation despite improvements in drug prophylaxis. Although animal models are providing an understanding of the biology of the process, opportunities for exploitation of that knowledge for therapeutic purposes are still limited. One way to overcome this impasse is to build on the positive benefits of T-cell purging of marrow, by combining it with intelligent correction of the known negative effects.

HLA-B35-subtype mismatches in ABDR serologically matched unrelated donor-recipient pairs

We have characterized HLA incompatibilities in a group of 17 B35-positive patients who were ABDR matched (AB serology and oligotyping for DR1-14) with their 28 (unrelated) potential bone marrow donors. High-resolution oligotyping for DR subtypes disclosed that nine combinations were in fact DR mismatched. Cytotoxic T-lymphocyte (CTL) activity was detected in nine combinations (32%). In the group matched for DR subtypes, three (16%) of 19 combinations were CTL positive. Patient-specific cytotoxic activity appeared to be directed against HLA C (two cases) or against a subtype of B35. In the group of DR-subtype-mismatched combinations, CTL activity was found in six (67%) of nine pairs. In all four cases that were studied in detail, however, CTL reactivity appeared to be directed against a variant subtype of B35. We have studied the B35 incompatibilities recognized in five different combinations by specificity analysis of the B35-specific CTLs and by partially sequencing of relevant segments of B35 exon 3. Preliminary data show that, within this relatively small Caucasoid group, at least five B35-variant subtypes could be distinguished. This would make B35 an antigen that will be frequently subtype mismatched, in particular when DR matching is done with low resolution (DR1-14) only.

High-resolution histocompatibility testing of a group of sixteen B44-positive, ABDR serologically matched unrelated donor-recipient pairs. Analysis of serologically undisclosed incompatibilities by cellular techniques, isoelectrofocusing, and HLA oligotyping

We have characterized HLA incompatibilities in a group of 16 B44-positive patients who were serologically ABDR matched with their 23 (unrelated) potential bone marrow donors. After analysis with a combination of cellular techniques, IEF for HLA-A/B and oligotyping for class II and HLA-B44, 44% of the patients revealed one or more HLA incompatibility with at least one of their potential donors. CTL activity was detected in 12 of the 22 combinations tested. CTL incompatibility occurred more frequently in DR subtype-mismatched combinations, but CTL reactivity was always directed against class I. To characterize these incompatibilities between matched unrelated individuals, we analyzed the specificity of T-cell clones from seven primary CTL cultures. In three combinations, CTL reactivity was directed against a subtype of B44. In two combinations, the CTL reactivity was directed against a non-B44 class I subtype. In two of seven combinations, the CTLs recognized an antigen that, though unconditionally associated with B4403, was expressed by 60% of the B4403+ cells only. Because all 12 of these B4403+ targets recognized could be typed for one HLA-C allele only (Cwl-Cw8), we believe that this alloreactivity might be directed against a serologically undefined Cw antigen.

Replacement of HLA class II serology by the HLA-DR microtitre plate oligotyping assay: a one-year experience in unrelated bone marrow donor selection

Because the selection of unrelated bone marrow donors requires a more accurate HLA-typing technique than is used for sibling donors, we have replaced class II serology by a rapid and highly discriminative HLA-DR DNA oligotyping assay on microtitre plates. We present here our 1-year experience with class II typing for 112 potential donors identified for 33 consecutive leukaemic patients. Although the donors were selected to be HLA-A, -B, -DR serologically identical, the microtitre plate oligotyping assay detected DR incompatibilities in 52.7% of the patient/donor pairs. One third was due to incorrect or incomplete typing of DR11 to DR16, the others to mismatches for the serologically indistinguishable subtypes of DR1, 11, 13 and 15, known to be relevant for T cell recognition.