Identification of New Minor Histocompatibility Antigens (mHags) in Patients with Chronic Graft Versus Host Disease (GvH) after Allogeneic Stem Cell Transplantation by SEREX.

Background: Minor histocompatibility antigens are antigenic peptides derived from normal cellular molecules which are presented in the context of major histocompatibility antigens (MHC class I and MHC class II). After an allogeneic hematopoietic stem cell transplantation (aHSCT) recipient-derived mHags can be recognized by T-cells of the transplanted immune system an mediate both a graft-versus-host disease and graft-versus- leukemia reaction. All mHags known to date have been identified by allo-reactive T-cells. Due to complex logistics needed for the establishment of such T-cell clones only a few mHags have been molecularly defined to date. Since patients with GvH develop antibodies against mHags and the presence of mHag antibodies has been shown to correlate with GvH and maintenance of remission [Miklos et al. (2005) Blood 105:2973-9] we set out to identify new mHags using SEREX the serological identification of antigens using recombinant expression cloning [Sahin et al. (1995) PNAS 92:11810-3].

Methods: Fibroblasts obtained by skin biopsies from patients with chronic GvH were propagated in vivo and used as a source to establish a cDNA library. cDNA was expressed by lambda phage in E.coli and expressed clones were screened for reactivity with antibodies in the serum of patients with chronic GvH. Positive clones were monoclonized and sequenced. The sequences were compared with known sequences using BLAST data bank. Reactive sera were compared with patient’s pre transplant serum and the serum from the donor to exclude pre-existing antibodies against (auto-)antigens expressed by the fibroblasts. Only neo-antigens recognized by the transplanted immune system were analyzed further.

Results: cDNA libraries obtained from 7 patients with chronic GvH after HLA-identical stem cell transplantation were screened with the sera of these 7 patients by SEREX. Antibodies from one patient with chronic GvH were absent in the donor and the patients pre-transplant serum, reacted with c1ORF107. This constellation proves that the c1ORF107 antibodies in the patient’s serum developed after transplant and recognized c1ORF as a neo-antigen. The immunogenic c1ORF107 of the patient differed at previously described polymorphic position 275 (rs585627) with G in the recipient leading to glutamate and C in the donor leading to glutamine. The definition of the c1ORF107 epitopes eliciting a T-cell response by “reverse T-cell immunology” is currently underway.

Conclusion: This study proves the principle that the analysis of the humoral immune response in patients with chronic GvH by SEREX allows for the definition of new mHags. SEREX in combination and followed by reverse T-cell immunology is a straight-forward approach which will expand considerably the number of molecularly defined mHags causing GvH in patients after allogeneic stem cell transplantation. Supported by Jose Carreras Foundation Germany