Alloantibodies: Basic and clinical concepts

Importance of T cells to accelerated rejection and acceptance of renal allografts in sensitized rat recipients

BACKGROUND

Sensitized recipients often experience fulminant allograft loss by yet ill-defined cellular and/or humoral immune mechanisms. In this study, we analyzed the contribution of cellular elements, in particular T cells, to the accelerated rejection of renal allografts in sensitized rats.

METHODS AND RESULTS

LEW rats sensitized with BN skin grafts died of uremia in 3.3+/-0.9 days after transplantation of a BN kidney, similarly to bilaterally nephrectomized animals. Adoptive transfer of 10(6) graft-infiltrating mononuclear cells as well as their CD25+ subset into otherwise normal LEW recipients accelerated rejection of BN test cardiac allografts (5.4+/-0.5 days to 6.6+/-0.4 days vs. 7.8+/-0.8 days in controls, P<0.0007), while the CD25- population was ineffective (8.0+/-0.6 days, NS). Furthermore, alpha/beta-T-cell receptor (TCR)-targeted therapy with R73 monoclonal antibody abrogated accelerated rejection, and produced long-term survival in sensitized animals treated before kidney engraftment (day -7 to day -1). Long-term survival was associated with an up-regulation of intragraft interleukin-4 and interleukin-10 expression in conjunction with depressed Th-1-type cytokines. In addition, alpha/beta-TCR-targeted therapy even in low subtherapeutic dose decreased IgM alloantibody levels, and prevented the switch from IgM to IgG alloantibody response.

CONCLUSIONS

This is the first report that documents the striking efficacy of alpha/beta-TCR-targeted therapy in sensitized rat renal transplant recipients. The results provide evidence for a critical role of T cells for both accelerated rejection and long-term graft survival. Up-regulation of Th2-type cytokine profile may, at least in part, contribute to the acquisition of immune unresponsiveness after alpha/beta-TCR-targeted therapy in this well-defined rat renal transplant model.

Determination of HLA-A,B residue mismatch acceptability for kidneys transplanted into highly sensitized patients: a report of a collaborative study conducted during the 12th International Histocompatibility Workshop

BACKGROUND

During the 12th International Histocompatibility Workshop, a collaborative study between 35 laboratories was conducted on a group of highly allosensitized patients who had received a kidney transplant from 1981 to 1995. The major goal of the study was to assess how serum screening against a large cell panel could determine donor HLA mismatch acceptability in relation to graft outcome.

METHODS

Twenty laboratories participated in an extensive screening of 92 high panel-reactive antibody (PRA) sera from patients at 29 transplant centers worldwide; each patient had received a kidney allograft from an HLA-A,B mismatched unrelated donor. Screening was done by complement-dependent lymphocytotoxicity and antihuman globulin augmentation techniques using a common protocol and shared standardized reagents. After an extensive quality-control assessment, we selected data from 14 participants who had screened the sera against a combined panel of 535 HLA-typed cells.

RESULTS

With the 2x2 table-based Multiscreen computer program, we could readily determine for virtually every patient the significant correlations between serum reactivity and the presence of panel cell markers, including private and public HLA-A,B epitopes and amino acid residues assigned from published sequencing data. Donor mismatch acceptability was assessed at the amino acid residue level. In the complement-dependent lymphocytotoxicity (n=49; PRA=84.1+/-12.1%) and antihuman globulin (n=60; PRA=92.5+/-5.8%) groups, the 3-month graft survivals were 28% and 30% lower for unacceptable residue mismatches.

CONCLUSIONS

These studies underscore the importance of a comprehensive serum screening analysis in the selection of appropriately mismatched donors for highly sensitized transplant patients.

Role of platelet activating factor in kidney transplant rejection in the rat

Platelet activating factor (PAF) is a potent lipid mediator with a broad range of biologic activities. Experimental evidence suggests that PAF plays a role in the pathogenesis of a variety of inflammatory processes including allograft rejection. In this study, we evaluated the effects of the PAF antagonist BN52021 on the course of renal allograft rejection in a rat model. Kidneys from ACI (RT1a) rats were transplanted into fully allogeneic PVG (RT1c) rat recipients. Animals received 60 mg/kg/day of the PAF antagonist or vehicle beginning immediately prior to the transplantation procedure. In rats treated with the PAF antagonist, allograft GFR and plasma flow were maintained at levels that were significantly greater than controls. Despite the improvement in renal allograft function, BN52021 had no effect on allograft histomorphology and both groups manifested intense inflammatory cell infiltration consistent with acute cellular rejection. PAF antagonism reduced urinary excretion of thromboxane metabolites and decreased thromboxane production by homogenates prepared from kidney allografts. The PAF antagonist had no effect on urinary excretion of peptidoleukotriene metabolites or on the production of LTB4 by allografts. These data support a role for PAF in the pathophysiology of acute renal allograft rejection, and they suggest that the hemodynamic effects of PAF during rejection may be mediated through stimulation of thromboxane A2. In view of the beneficial effects of PAF blockade in rejection as well as recent reports describing efficacy in models of cyclosporine nephrotoxicity, PAF antagonists may have clinical applications in human renal allograft recipients.

Rapid, comprehensive analysis of human cytokine mRNA and its application to the study of acute renal allograft rejection

Cytokine mRNA analysis was performed on human renal allograft needle core biopsies by a PCR-based assay. The assay was specifically developed to be capable of simultaneous analysis of multiple interleukin transcripts (IL-1-IL-12), as well as those of other relevant cytokines, by one person in less than 1 day from cultured cells or directly from tissue samples. It was initially used on preparations containing known amounts of plasmid DNA encoding individual cytokine cDNA sequences, confirming that the sensitivity of this technique was both well defined and comparable for all target sequences tested. Analysis of human PBLs prior to stimulation, after polyclonal stimulation with PHA and after simultaneous treatment with PHA and MP or CyA, was also performed to show a proportional relationship between mRNA levels measured by PCR and protein release measured by ELISA (R2 = 0.86). This correlation was not adversely altered by pharmacologic immunosuppression by MP or CyA. Thus, this method of PCR primer design and usage was appropriate for the clinical study of cytokine mRNA levels during allograft rejection. Direct study of cytokine mRNA in allograft biopsy tissue showed that IL-2 was specifically and significantly (p = 0.006) elevated during ACR when compared to other causes of graft dysfunction. Transcripts from the IFN-gamma and IL-6 genes were also increased in ACR (p = 0.001 and 0.017, respectively), whereas increased IL-8 mRNA was correlated with irreversible loss of graft function (p = 0.02). TNF-alpha, IL-1 beta, and IL-10 gene transcripts were also detected during ACR, but were not quantitatively increased compared to other forms of graft injury (p > 0.2). We conclude that acute cellular rejection is associated with intragraft mRNA from the IL-2 gene. Other transcripts, including those from the IFN-gamma, IL-6, and IL-8 genes, are detected in increased amounts during this process. Messenger RNA from the TNF-alpha, IL-1 beta and IL-10 genes is also detected during ACR, but the presence of these transcripts is not exclusive to this process.