Clinical Relevance of Antibodies to Non-HLA Antigens in Organ Transplantation

The outcome of the endothelial precursor cell crossmatch test in lymphocyte crossmatch positive and negative patients evaluated for living donor kidney transplantation

The presence of human leukocyte antigen (HLA) and non-HLA antibodies (Abs) in kidney transplant recipients is associated with graft rejections. This study reports the results of an endothelial precursor cell crossmatch (EPCXM) test for detection of non-HLA Abs and its correlation to lymphocyte crossmatch (LXM) test results, the degree and type of sensitization, and transplantation (Tx) outcome in patients evaluated for living donor (LD) kidney transplantation (KTx). Patients were tested before any pre-transplantation (pre-Tx) treatment and at Tx. Pre-Tx treatments included B cell depletion and Ab removal. Patient records were reviewed for assessment of renal graft function, results of biopsies, and identification of complications affecting the graft. Pre-Tx sera from 32% of the LD patients had IgG and/or IgM-binding donor EPCs. Twenty-five percent of the patients were EPCXM IgM+. Of the patients with negative LXM tests, 25% had EPC Abs mainly of IgM class not reactive with HLA. There was no difference in rejection frequency or serum creatinine levels between the EPCXM+ and EPCXM- groups. The pre-Tx EPCXM+ group had significantly more patients with delayed graft function. Prospective studies with appropriate control groups are needed to establish whether pre-treatments aiming at removing anti-endothelial cell antibodies, as detected by the EPCXM pre-Tx, have a beneficial effect on short-term and long-term graft survival.

A flow cytometric crossmatch test for simultaneous detection of antibodies against donor lymphocytes and endothelial precursor cells

Complement-dependent cytotoxicity or flow cytometric lymphocyte crossmatch (LXM) tests may fail to detect clinically significant antibodies (Abs) against non-human leukocyte antigen (HLA). A flow cytometric endothelial precursor cell crossmatch (EPCXM) test (XM-ONE) is available for detection of Abs against donor endothelial precursor cells (EPCs). We showed that lymphocytes co-purified with EPCs can be used in LXM tests allowing simultaneous detection of Abs reactive with donor EPCs and lymphocytes. The lymphocyte population co-purified with EPCs on anti-Tie-2 Ab-coupled magnetic beads contained CD 8(+) and CD 4(+) T-cells, B-cells, and natural killer (NK)- and natural killer T (NKT)-cells. HLA class I antigen expression was slightly higher on CD 3(+) lymphocytes co-purified on Tie-2 Ab beads than on unseparated lymphocytes, whereas HLA class I and II antigen levels on CD 19(+) lymphocytes were not significantly different. Sera from 10 patients with panel-reactive Abs were tested on cells from nine donors using flow cytometric LXM and EPCXM tests. There was a very good correlation (R(2) = 0.94) between the channel shift values obtained on unseparated and Tie-2 Ab bead-isolated T-lymphocytes, whereas the correlation between the channel shift values obtained on the two B-lymphocyte populations was lower (R(2) = 0.71). T- and B-lymphocytes co-purified with EPCs can be used in LXM tests enabling simultaneous detection of donor lymphocyte- and EPC-reactive Abs in a single-tube XM-ONE assay.

Multicenter evaluation of a novel endothelial cell crossmatch test in kidney transplantation

BACKGROUND

Despite their clinical importance, clinical routine tests to detect anti-endothelial cell antibodies (AECA) in organ transplantation have not been readily available. This multicenter prospective kidney transplantation trial evaluates the efficacy of a novel endothelial cell crossmatch (ECXM) test to detect donor-reactive AECA associated with kidney allograft rejection.

METHODS

Pretransplant serum samples from 147 patients were tested for AECA by a novel flow cytometric crossmatch technique (XM-ONE) using peripheral blood endothelial progenitor cells as targets. Patient enrolment was based on acceptance for transplantation determined by donor lymphocyte crossmatch results.

RESULTS

Donor-reactive AECA were found in 35 of 147 (24%) patients. A significantly higher proportion of patients with a positive ECXM had rejections (16 of 35, 46%) during the follow-up of at least 3 months compared with those without AECA (13 of 112, 12%; P<0.00005). Both IgG and IgM AECAs were associated with graft rejections. Mean serum creatinine levels were significantly higher in patients with a positive ECXM test at 3 and 6 months posttransplant.

CONCLUSIONS

XM-ONE is quick, easy to perform on whole blood samples and identifies patients at risk for rejection and reduced graft function not identified by conventional lymphocyte crossmatches.

Endothelial cells as targets of allograft rejection

Endothelial cells harbor many antigenic determinants that may be targets for antibodies and, as such, induce acute or chronic antibody-mediated rejections. In certain cases of organ transplantation, antibodies reacting with endothelial cells, but not with ABO or human leukocyte antigens, can be observed and these are probably responsible for the rejection of the graft. The antigenic targets, however, are still poorly defined and the mechanisms of action of such antibodies would appear to be diverse, leading to the lack of a relevant in vitro assay for the detection of those antibodies. Increasing data suggest that, apart from direct alloimmune responses, autoimmune mechanisms might be triggered by alloreactivity and also play a significant role in the pathogenesis of the vascular lesions, the so-called chronic rejection, observed in organ allografts.

Antigraft antibody-mediated expression of metalloproteinases on endothelial cells. Differential expression of TIMP-1 and ADAM-10 depends on antibody specificity and isotype

Endothelial cell (EC) interaction with antigraft antibodies (Abs) mediates EC injury and activation involved in vascular graft rejection. The aim of this study was to identify EC genes regulated in response to antigraft Ab binding that contribute to the endothelium alterations implicated in graft rejection or survival. By means of RNA differential display, 13 cDNA fragments corresponding to genes differentially expressed in ECs incubated with antigraft Abs were identified. Among these cDNAs were found the tissue inhibitor of metalloproteinase-1 (TIMP-1) and a desintegrin and metalloproteinase (ADAM-10). We demonstrated that TIMP-1 and ADAM-10 mRNA and protein expression was rapidly upregulated in ECs in response to antigraft Ab binding. Our data showed that TIMP-1 was upregulated in response to human IgG but not IgM and anti-galactosyl (Gal) alpha1-3Gal human xenogeneic Abs. In contrast, upregulation of ADAM-10 in ECs was shown to be mostly mediated by anti-Galalpha1-3Gal IgM Abs. Specific effects of human IgG and IgM xenogeneic Abs on endothelial transcripts indicate that different isotypes and specificities of Abs may mediate different EC changes. Our results suggest that interaction of ECs with antigraft Abs, according to their specificity, selectively induces synthesis and release of metalloproteinases and inhibitors, controlling proteolytic processes and immunological events that respectively contribute to graft rejection or survival.

Chronic humoral rejection: identification of antibody-mediated chronic renal allograft rejection by C4d deposits in peritubular capillaries

The pathogenesis of chronic renal allograft rejection (CR) remains obscure. The hypothesis that a subset of CR is mediated by antidonor antibody was tested by determining whether C4d is deposited in peritubular capillaries (PTC) and whether it correlates with circulating antidonor antibodies. All cases (from January 1, 1990, to July 31, 1999) that met histologic criteria for CR and had frozen tissue (28 biopsies, 10 nephrectomies) were included. Controls were renal allograft biopsies with chronic cyclosporine toxicity (n = 21) or nonspecific interstitial fibrosis (n = 10), and native kidneys with end-stage renal disease (n = 10) or chronic interstitial fibrosis (n = 5). Frozen sections were stained by two-color immunofluorescence for C4d, type IV collagen and Ulex europaeus agglutinin I. Antidonor HLA antibody was sought by panel-reactive antibody analysis and/or donor cross matching in sera within 7 wk of biopsy. Overall, 23 of 38 CR cases (61%) had PTC staining for C4d, compared with 1 of 46 (2%) of controls (P < 0.001). C4d in PTC was localized at the interface of endothelium and basement membrane. Most of the C4d-positive CR tested had antidonor HLA antibody (15 of 17; 88%); none of the C4d-negative CR tested (0 of 8) had antidonor antibody (P < 0.0002). The histology of C4d-positive CR was similar to C4d-negative CR, and 1-yr graft survival rates were 62% and 25%, respectively (P = 0.05). Since August 1998, five of six C4d-positive CR cases have been treated with mycophenolate mofetil +/- tacrolimus with a 100% 1-yr graft survival, versus 40% before August 1998 (P < 0.03). These data support the hypothesis that a substantial fraction of CR is mediated by antibody (immunologically active). C4d can be used to separate this group of CR from the nonspecific category of chronic allograft nephropathy and may have the potential to guide successful therapeutic intervention.