Low incidence of IgA isotype of HLA antibodies in alloantigen exposed individuals

The Transplant Bellwether: Endothelial Cells in Antibody-Mediated Rejection

Ab-mediated rejection of organ transplants remains a stubborn, frequent problem affecting patient quality of life, graft function, and grant survival, and for which few efficacious therapies currently exist. Although the field has gained considerable knowledge over the last two decades on how anti-HLA Abs cause acute tissue injury and promote inflammation, there has been a gap in linking these effects with the chronic inflammation, vascular remodeling, and persistent alloimmunity that leads to deterioration of graft function over the long term. This review will discuss new data emerging over the last 5 y that provide clues into how ongoing Ab-endothelial cell interactions may shape vascular fate and propagate alloimmunity in organ transplants.

Correlation of Anti-HLA IgA Alloantibodies and Fc Receptor Motives with Kidney Allograft Survival

Immunoglobulin A (IgA) is the most abundant antibody isotype in humans and anti-HLA IgA was found in sera of transplant recipients. Focusing on patients awaiting kidney re-transplantation, we tested the impact of anti-HLA-class I/II IgA antibodies on graft survival. We analyzed 276 patients with and 238 without allograft failure. Eight motives of the Fcα receptor (FCAR) and Fcγ receptor were analyzed in patients with allograft failure. The distribution of anti-HLA IgA1/A2 and IgG antibodies differed significantly (p < 0.0001) between both patient groups, and IgA1 plus IgA2 antibodies were more abundant in patients with allograft failure. Allograft survival was significantly impaired if anti-HLA-class I plus II IgA was present, in the first 105 months (9 years) of follow-up (median of 43 vs. >105 months, p = 0.007). Patients with anti-HLA IgA and IgG vs. anti-HLA IgG only had a significantly shorter allograft survival within that follow-up period (88 vs. >105 months, p = 0.008). Moreover, allograft survival was shorter (p = 0.02) in carriers of GG vs. AA + AG genotypes of FCAR rs16986050. Thus, the presence of anti-HLA IgA plus IgG vs. IgG only was associated with shorter kidney allograft survival and FCAR motives may impact on graft survival.

Transcriptomic heterogeneity of antibody mediated rejection after heart transplant with or without donor specific antibodies

BACKGROUND

Antibody mediated rejection (AMR) is an increasingly studied cause of graft failure after heart transplantation. AMR diagnosis previously required the detection of circulating donor specific antibodies (DSA); however, the most recent criteria only require pathological findings. This classification defined a subset of patients with AMR, yet without known antibodies. Here, we sought to evaluate differences in the transcriptome profile associated with different types of AMR.

METHODS

RNA sequencing was used on endomyocardial biopsies to analyze and compare transcriptomic profiles associated with different subtypes of AMR defined by immunopathological and histopathological findings, as well as the presence or absence of DSA. Gene expression profiles were characterized for each diagnostic group.

RESULTS

The most divergent gene expression profiles were observed between patients with or without DSA. AMR subtypes associated with DSA showed expression of signature genes involved in monocyte activation and response to interferon. There was also substantial difference between the transcriptomic profiles of AMR defined by histopathological and immunopathological findings, the latter being associated with expression of mucin genes. In contrast, there was no differential RNA expression between patients with pAMR1i without DSA and those without AMR. Likewise, no differential expression was observed between patients with pAMR1h with DSA and pAMR2.

CONCLUSIONS

Overall, our studies reveal different expression profiles in endomyocardial biopsies in relation to some key criteria used to diagnose AMR. These findings support the view that the diagnosis of AMR encompasses several phenotypes that may rely on distinct mechanisms of injury.

Low incidence of IgA isotype of HLA antibodies in alloantigen exposed individuals

Human leukocyte antigen (HLA) antibodies are induced by pregnancy, transfusion, or transplantation. Serum from transplant recipients is regularly screened for IgG HLA antibodies because of their clinical relevance for transplant outcome. While other isotypes of HLA antibodies, such as IgA may also contribute to the alloimmune response, validated detection assays for IgA HLA antibody detection are lacking. Therefore, we modified the commonly used luminex screening assay for IgG HLA antibody detection (IgG-LMX) into an IgA HLA antibody screening assay (IgA-LMX). Optimization and validation was performed with IgG, IgA1, and IgA2 isotype variants of HLA-specific human recombinant monoclonal antibodies (mAbs). Reactivity patterns of IgA1 and IgA2 isotype HLA-specific mAbs in IgA-LMX were identical to those of the IgG isotype. Cross-reactivity with IgG and IgM antibodies and nonspecific binding to the beads were excluded. Further assay validation showed the absence of IgA HLA antibodies in serum from individuals without alloantigen exposure (n = 18). When the IgA-LMX assay was applied to sera from 289 individuals with known alloantigen exposure through pregnancy (n = 91) or kidney transplantation (n = 198), IgA HLA antibodies were detected in 3.5% of individuals; eight patients on the kidney retransplant waitlist and two women immunized through pregnancy. The majority (90%) of IgA HLA antibodies were directed against HLA class II and were always present in conjunction with IgG HLA antibodies. Results of this study show that this validated IgA-LMX method can serve as a screening assay for IgA HLA antibodies and that the incidence of IgA HLA antibodies in alloantigen exposed individuals is low.