Human liver allograft acceptance and the "tolerance assay": in vitro anti-donor T cell assays show hyporeactivity to donor cells, but unlike DTH, fail to detect linked suppression

Human allograft acceptance is associated with immune regulation, characterized by donor-antigen-linked suppression of delayed-type hypersensitivity (DTH). We wished to determine if "classical" in vitro assays of alloreactivity could also detect linked suppression and thus be useful in the clinical diagnosis of active immune regulation. We analyzed peripheral blood mononuclear cells from a group of eight liver transplant recipients, one of whom had stopped all immunosuppression 4.5 years ago yet continues to have good graft function (graft acceptor). The regulator phenotype was defined as the ability to suppress a DTH response to a recall antigen in the presence of donor antigen. Using the trans vivo DTH test, we identified four regulators, and four nonregulators. When we tested two of the regulators for in vitro mixed lymphocyte culture (MLC) and cytotoxic T lymphocyte (CTL) responses to B-lymphoblastoid cell lines (B-LCL), we found both patients to be specifically hyporesponsive to donor compared with third-party B-LCL stimulators. However, in contrast to the linked suppression of DTH seen when a given B-LCL expressed donor-type HLA-B antigens, there was no evidence of linked suppression in vitro, either in CTL, proliferative, or interferon-gamma cytokine release assays. The primary CTL hyporesponsiveness to donor B-LCL could not be reversed by neutralizing antibodies to transforming growth factor beta or interleukin-10, which could restore a strong DTH response to donor B-LCL. We conclude that DTH analysis can readily detect donor antigen-linked suppression in liver transplant recipients. CTL and MLC tests failed to do so. These findings may be relevant to the development of a tolerance assay suitable for use in clinical trials.

Soluble donor HLA class I and beta 2m-free heavy chain in serum of lung transplant recipients: steady-state levels and increases in patients with recurrent CMV infection, acute rejection episodes, and poor outcome

We determined the concentration of donor sHLA/beta(2)m and total beta(2)m-free heavy chain (HC) in the serum of lung transplant recipients with ELISA assays. While we were unable to detect specific donor beta(2)m-free HCs due to a lack of available antibodies, we could determine if events that led to an increase in the release of beta(2)m-free HC also led to an increase in the release of donor sHLA/beta(2)m, particularly the 36 kDa, proteolytically cleaved form. We found that lung transplants constituitively release donor sHLA/beta(2)m at ng/ml levels. The levels (both of donor sHLA/beta(2)m and total beta(2)m-free HC) were significantly increased in CMV-sero-negative recipients (but not in CMV-sero-positive recipients) at the onset of post-transplant CMV disease. Acute rejection episodes were also associated with an increased release of donor sHLA/beta(2)m, but not of beta(2)m-free HC. However, in patients with particularly poor outcome (i.e., graft loss within 1 year) there was a significant release of beta(2)m-free HC. Analysis of one such patient showed a predominance of 36 kDa forms of donor-sHLA/beta(2)m. Our data are consistent with the hypothesis that the metalloproteinase that cleaves beta(2)m-free HC is active during uncontrolled CMV infection and acute rejection. However, recall responses to CMV and controlled immune responses to donor may result in little or no activation of sHLA class I release.

Peptide-conformed beta2m-free class I heavy chains are intermediates in generation of soluble HLA by the membrane-bound metalloproteinase

Molecular mechanisms of soluble HLA-release by a membrane-bound metalloproteinase (MPase) are not defined. We have investigated the possibility that certain beta2-microglobulin (beta2m)-free heavy chains (HC) retain peptide-induced conformations before and after the cleavage by using mutant HLA-A2.242K HC with reduced affinity for beta2m. We show that dissociation of HC/beta2m complexes on the surface of C1R lymphoblastoid cells generates both conformed and non-conformed beta2m-free HC recognized by conformation-dependent antibodies. Conformed HC, having bound the HLA-A2-specific peptide HTLV-1 tax 11-19, can retain their proper conformations after dissociation of beta2m. Further, conformed and non-conformed surface beta2m-free HC are cleaved by the MPase, and some released HC preserve their conformations. Exogenous beta2m binds only to conformed HC, and protects them from cleavage as effectively as the MPase inhibitor BB-2116. We propose that soluble HLA-release requires generation of peptide-conformed beta2m-free HC intermediates on the cell surface, which are then cleaved by the MPase and in solution may reassociate with beta2m. Given the role of soluble HLA in the indirect allorecognition, the activity of this MPase may be important in transplant rejection.

The metalloproteinase-mediated pathway is essential for generation of soluble HLA class I proteins by activated cells in vitro: proposed mechanism for soluble HLA release in transplant rejection

We and others have found donor-derived soluble beta2m-associated HLA class I proteins (sHLA/beta2m) in the serum of allograft recipients with acute and chronic rejection. Whether appearance of sHLA/beta2m and upregulated expression of donor cell-bound HLA/beta2m during allograft rejection are related events is unknown. Activation-induced upregulation of in vitro HLA/beta2m expression correlates with the surface expression of another form of HLA class I, namely beta2m-free HLA heavy chains (beta2m-free HC). We have shown that beta2m-free HC, but not beta2m-associated HC, are then cleaved by a specific membrane-bound metalloproteinase and released into supernatants as soluble 36 kDa proteins. We show now that activated peripheral blood lymphocytes produce predominantly the 36 kDa form of sHLA proteins which is present in supernatants as both beta2m-free HC and sHLA/beta2m. Importantly, the metalloprotease inhibitor BB-94 blocked not only the release of soluble beta2m-free HC, but also the appearance of sHLA/beta2m in cell supernatants. Low levels of 36 kDa beta2m-free HC were also present in human plasma of healthy donors. These data suggest an important role for the HLA class I-specific metalloproteinase in vivo in healthy individuals and during allograft rejection in the generation of soluble beta2m-free and beta2m-associated HLA proteins.

Soluble HLA class I in epithelial lining fluid of lung transplants: associations with graft outcome

We hypothesized that the small amounts of donor HLA-A and HLA-B proteins detected in the serum during organ allograft rejection are indicative of higher local releases within the graft itself. We determined the concentrations of total HLA class I (HLA-I) and, in selected cases, specific donor and host HLA-A and HLA-B proteins, in the epithelial lining fluid (ELF) sampled by bronchoalveolar lavage (BAL) of lung transplant recipients (n = 37) and of normal controls (n = 25). We found that 1) HLA-I proteins were enriched in the lung ELF relative to other proteins; 2) the concentration of HLA-I in the ELF of well-functioning transplants was similar to that in normal lungs; 3) HLA-I proteins and total proteins were elevated in the ELF of patients who developed chronic rejection or refractory acute rejection; 4) the concentration of HLA-I was correlated with the percentage of neutrophils but not with the percentage of lymphocytes in the ELF of transplanted lungs; and 5) only the percentage of lymphocytes was elevated in the ELF of transplant patients with active CMV infections. Total HLA-I from the ELF was found to contain a mixture of both donor- and recipient-type HLA-A and HLA-B proteins and the donor-type HLA-A2 was found to be highly enriched in the ELF relative to serum.

Donor-derived human leukocyte antigen class I proteins in the serum of heart transplant recipients

BACKGROUND

Human leukocyte antigen class I proteins are expressed on most cell types in all organ allografts but are constitutively secreted only by certain organs, for example, the liver. We hypothesized that detectable levels of donor-derived human leukocyte antigen proteins would be released from transplanted cardiac allografts only when the allograft was immunologically stimulated, that is, during rejection and perhaps during viral infection. If so, then the release of donor human leukocyte antigen might be a noninvasive monitor of these events.

METHODS

We used an enzyme-linked immunosorbent assay to detect donor-derived human leukocyte antigen-A2 in the serum of 21 human leukocyte antigen-A2 negative recipients of human leukocyte antigen-A2-positive heart transplants. The level of donor human leukocyte antigen-A2 during the first 100 days after transplantation was correlated with the clinical status of the patient.

RESULTS

We found little or no donor human leukocyte antigen in the serum of heart transplant recipients whose postoperative clinical course was unremarkable for infection or rejection. We did find donor-derived human leukocyte antigen in the serum of heart transplant recipients transiently in the week immediately after transplantation, continuously from patients in whom chronic rejection was developing, during cytomegalovirus infection, and during some, but not all, acute rejection episodes as determined by endomyocardial biopsy.

CONCLUSIONS

These findings are consistent with the hypothesis that the donor human leukocyte antigen serum level reflects vascular diseases, rather than myocardial disease in the transplanted heart. Therefore, the serum level of donor human leukocyte antigen cannot be used as a monitor of cellular infiltration and myocyte damage as currently assessed by endomyocardial biopsy but may be an early indicator of the development of vascular disease such as chronic rejection.

Monitoring of kidney and simultaneous pancreas-kidney transplantation rejection by release of donor-specific, soluble HLA class I

Using an HLA-A2-specific ELISA we monitored daily pretransplantation and posttransplantation sera from five kidney and eight simultaneous pancreas-kidney HLA-A2-negative recipients of HLA-A2-positive transplants during hospitalization. We found that, unlike liver transplants, neither kidney nor simultaneous pancreas-kidney transplants continuously secreted donor HLA proteins. However, three of four rejection episodes in kidney recipients and seven of seven rejection episodes in simultaneous pancreas-kidney recipients were accompanied by elevated serum levels of donor sHLA-A2 (> 5 ng/ml). In only one kidney patient was there a release of donor antigen without evidence of rejection, but in the simultaneous pancreas-kidney group most patients had at least one time point of detectable sHLA-A2 without strong evidence of kidney rejection. While total sHLA levels were also elevated during rejection, the rise in donor-specific sHLA was more dramatic when compared to pretransplantation background levels. We hypothesized that the release of donor sHLA class I proteins by transplanted organs might be a systemic indication of rejection in both pancreas and kidney allografts. The detection of donor sHLA in recipient sera could be an important noninvasive monitor of rejection, especially in the pancreas, which is currently difficult to monitor as a single-organ transplant.