Evolving Approaches in the Identification of Allograft-Reactive T and B Cells in Mice and Humans

Abstract

Whether a transplanted allograft is stably accepted, rejected or achieves immunological tolerance is dependent on the frequency and function of alloreactive lymphocytes, making the identification and analysis of alloreactive T and B cells in transplant recipients critical for understanding mechanisms, and the prediction of allograft outcome. In animal models, tracking the fate of graft-reactive T and B cells allows investigators to uncover their biology and develop new therapeutic strategies to protect the graft. In the clinic, identification and quantification of graft-reactive T and B cells allows for the early diagnosis of immune reactivity and therapeutic intervention to prevent graft loss. In addition to rejection, probing of T and B cell fate in vivo provides insights into the underlying mechanisms of alloimmunity or tolerance that may lead to biomarkers predicting graft fate. In this review, we discuss existing and developing approaches to track and analyze alloreactive T and B cells in mice and humans and provide examples of discoveries made utilizing these techniques. These approaches include mixed lymphocyte reactions (MLRs), trans-vivo delayed-type hypersensitivity (DTH), enzyme-linked immunospot (ELISpot) assays, the use of antigen receptor transgenic lymphocytes, and utilization of peptide-MHC complex (pMHC) multimers, along with imaging techniques for static multiparameter analysis or dynamic in vivo tracking. Such approaches have already refined our understanding of the alloimmune response and are pointing to new ways to improve allograft outcomes in the clinic.

Heterogeneity of memory B cells

“…Reports that memB cells specific for vaccine antigens can be detected in the blood, and that their frequency tracks with time post-antigen exposure (60, 61), suggest that an assay that accurately quantifies circulating donor-specific memB cells is feasible (46, 62). Such an assay will likely complement our ability to quantify DSA by providing an earlier diagnosis of acute or chronic AMR, as well as in identifying successful desensitization protocols and treatments of AMR.…”

Outstanding questions in transplantation: B cells, alloantibodies, and humoral rejection

“…Indeed, the need to quantify donor‐specific T and B cell responses posttransplant is self‐evident but is technically challenging given the diversity in HLA alleles in the human population. In murine experimental models, this can be achieved with MHC tetramers, which allow for donor‐reactive CD4 + conventional T cells and Tregs, CD8 + T cells, and B cells to be tracked simultaneously in an individual recipient . Efforts are being pursued to adapt these approaches to tracking alloreactive T and B cells in humans.…”

“…In murine experimental models, this can be achieved with MHC tetramers, which allow for donor-reactive CD4 + conventional T cells and Tregs, CD8 + T cells, and B cells to be tracked simultaneously in an individual recipient. [64][65][66][67][68] Efforts are being pursued to adapt these approaches to tracking alloreactive T and B cells in humans. In this regard, the repertoire of alloreactive T cells is being defined following in vitro mixed lymphocyte reaction and high-throughput T cell-receptor (TCR) sequencing, followed by statistical modeling to measure their diversity and frequency.…”

Expanding the Toolkit for the Study of Allospecific B and T Cell Responses

“…In the current issue of Transplantation, Young et al, 1 review a variety of methods that can be used to detect alloreactive B and T cells and place the methods in the context of the knowledge gained and potential future applications. Some of the methods will be familiar to readers of Transplantation as they have been used extensively in studies of transplantation, such as mixed lymphocyte reactions (MLR), in vivo mouse models including model antigens and/or T cells with transgenic (Tg) T cell receptors (TCRs), trans-vivo delayed type hypersensitivity, and quantification of donor-specific antibody.…”

“…In vivo, either direct or indirect pathways can be studied with Tg TCRs restricted to recognition of donor or recipient MHC or a specific peptide-MHC, respectively. Young et al, have included a table describing various TCR Tg mice that have been used in the study of alloreactivity, 1 which is a valuable resource for researchers contemplating studies of the alloimmune response. Another promising technology is retrogenic TCRs, 3 which are retrovirus expressing designated TCR rearrangements that can be used to transduce bone marrow progenitors.…”