The shared epitope phenomenon-A potential impediment to virtual crossmatch accuracy.

With the implementation of the new kidney allocation system (KAS), there is increased reliance on a virtual crossmatch/histocompatibility risk assessment (vXM) for evaluating potential presence, as well as strength, of HLA antibodies against a potential donor. The accuracy of such an assessment depends on the precision in the identification of the recipient's antibody profile and the potential donor's HLA typing. While the development of the single antigen bead (SAB) multiplex assay has improved the sensitivity and specificity of HLA antibody detection, several limitations of the assay (specific to certain sensitized patients) can complicate accurate interpretation of results. In this report, we focus on the "shared-epitope" phenomenon, a condition in which antibody strength can be underrepresented, or its presence completely missed, due to binding of the antibody to competing targets on multiple antigens (beads), effectively "diluting" the resulting MFI readout. Here, we provide a relevant background to understand this phenomenon and present a couple of case studies illustrating how it can be investigated, leading to a more accurate histocompatibility consultation.

Can solid phase assays be better utilized to measure efficacy of antibody removal therapies?

Antibody removal therapies are used for patients with antibody-mediated-rejection or those requiring desensitization to become transplantable. Accurate measurement of antibody levels prior to, and during treatment, are required to choose the best therapeutic approach, and to provide measure of treatment efficacy. Currently, the FDA does not regard solid-phase assays for HLA-antibody identification as a reliable surrogate-marker for treatment efficacy. Serum samples from 40 patients (58 assays; >2200 positive data points) undergoing antibody-removal-therapies were tested as sample-pairs, pre- and post-treatment. MFI values of IgG and C1q single-antigen-bead assays were compared with antibody titer values (serial dilutions). Antibody reduction was tracked and the differences in pre-to-post-treatment values were calculated as delta-reduction of antibody levels. Dynamic patterns of titration studies reduced effects of serum-inherent inhibitory factors (prozone-like); eliminated over-saturation limitations, and provided better estimation of antibody-binding strength compared with the other methods. Moreover, delta-reduction of antibody values using titration studies was significantly more uniform compared with either IgG or C1q tests. Analyzing antibody results using only C1q positive or only higher MFI values did not change the overall magnitude of results. Overall, titration studies provided better estimate of responsiveness to treatment and thus can serve as companion to monitoring efficacy of antibody-removal therapies.

The presence of HLA-directed antibodies after heart transplantation is associated with poor allograft outcome.

BACKGROUND: The clinical significance of HLA-directed antibodies newly detected after transplantation (HT) is controversial. METHODS: Seventy-one HT recipients consented to enroll. Mean follow-up time was 28 months (range 6-48). Panel reactive antibody (PRA) analysis was performed on posttransplant sera (2 weeks, 1, 2, 3, 6, and 12 months and annually thereafter) using Flow-PRA. A mean of 6.9+/-1.2 serum samples per patient were obtained. Severity of cellular rejection was measured using the ISHLT grading system. Coronary angiography and intravascular ultrasound (IVUS) studies were performed annually to evaluate severity of allograft vasculopathy. RESULTS: Twenty-five recipients had newly detected HLA-directed antibodies during the first year postHT. HLA class I antibodies were detected in 18 patients (25.4%), and class II in 11 patients (15.5%). The majority of donor recipient pairs were HLA mismatched (4.6+/-1.2 of the six major HLA antigens). Only mismatches at HLA-A locus had significant association with de novo posttransplant antibody formation. Length of ischemia time was correlated with early and sustained presence of de novo HLA-directed antibodies postheart transplant. Importantly, an association between de novo HLA-directed antibodies and cellular rejection was notes (P=0.0002). De novo HLA class II directed antibodies are also associated with IVUS documented vasculopathy (P<0.002). Finally, death due to allograft failure is associated with the presence of de novo formed HLA class II directed antibodies (P=0.008). CONCLUSIONS: Identifying the formation of de novo HLA-directed antibodies following heart transplantation may predict allograft outcome. This, in turn, may serve as a tool for individualization of immunosuppression protocols in heart transplant recipients.

Donor-specific hyporesponsiveness in ELISPOT assay is associated with early recurrence of hepatitis C in liver transplant recipients.

Recurrence of hepatitis C in liver transplant recipients is a common event that often leads to loss of the allograft. There are no means to prevent, or even predict, those patients who are more prone to early aggressive recurrence. Therefore there is an increased need for tailored immunosuppression protocols specific to this patient population. Fifteen liver transplant recipients (eight hepatitis C virus [HCV]+; 7 HCV-) were followed for 12-24 months after transplantation. The frequency of donor-specific interferon (IFN)-gamma- or interleukin-10-producing lymphocytes was monitored using ELISPOT assays. Of the eight HCV+ recipients, six experienced recurrence within the first year after transplant. All six patients had very low (negligible) frequency of donor-specific IFN-gamma precursors; in most cases not higher than the nonstimulated (spontaneous) secretion rate. The other two patients who did not recur exhibited donor-specific IFN-gamma reactivity. A significant difference in the frequency of alloantigen-specific T cells was observed between HCV+ recipients and patients with other indications for transplantation. The results of this preliminary study suggest that posttransplant monitoring of the frequency of donor-specific IFN-gamma-producing precursors may differentiate a subset of patients at risk for early recurrent hepatitis C and therefore may help to devise treatment strategies for HCV+ liver recipient after transplantation.

Mycophenolic acid inhibits maturation and function of human dendritic cells and B cells.

Mycophenolic acid (MPA) is considered an immunosuppressive compound mainly because of its inhibitory effects on lymphocyte proliferation. Here we studied specifically the effects of MPA on the ability of dendritic cells (DCs) to activate T cells via the indirect pathway and on the maturation and function of B-lineage cells. We demonstrated that DC cell-surface receptors, associated with antigen uptake and antigen processing and presentation (CD83 and CD205), were differentially downregulated in the presence of MPA, translating into a decreased uptake of alloantigens and reduced stimulation of T cells with decreased cytokine secretion (interleukin (IL)-1Ra and transforming growth factor (TGF)-alpha). Similarly, MPA significantly inhibited B-cell differentiation into memory and plasma cells in vitro and decreased secretion of TNF-alpha, IL-1Ra, and IL-10. We further demonstrated for the first time that not only the amount of antibody secretion was significantly lowered in the presence of MPA but also the total number of antibody-producing cells was reduced. Importantly, we provide direct evidence that HLA-specific antibody secretion was also affected using a newly developed HLA antibody-specific B-cell enzyme-linked immunospot assay. Our data indicate additional pathways by which MPA downregulates the immune system. This in turn may lead to improved conditions for allograft tolerance and control of allograft rejection.