Differing Mechanisms for Early Versus Persistent Donor T cell Chimerism in Peripheral Blood of Human Intestinal Transplant Recipients

Abstract

Donor T cell macrochimerism (>4%) often appears in blood following intestinal transplantation (ITx), usually without graft-versus-host disease[1]. Patients with donor T cell macrochimerism displayed less graft rejection, slower recipient T cell replacement within the graft and less donor-specific antibody (DSA) production[2]. However, the reasons for the development and persistence of donor T cell chimerism are unknown. We analyzed the repertoire, phenotype and origin of circulating donor T cells in ITx recipients using flow cytometry, mixed lymphocyte reaction (MLR) and high-throughput TCR sequencing. Expanded graft-versus-host (GvH) clones triggered by rapidly replaced recipient antigen-presenting cells were detected early in ileum biopsies (Figure 1A). Enrichment of GvH clones in the graft and absence of Class I DSA in the circulation were associated with donor T cell macrochimerism. Cumulative frequencies of GvH clones in blood early post-Tx correlated with the peak level of donor T cell chimerism (Figure 1B). Frequencies of host-versus-graft (HvG) clones subsequently declined in the blood of a patient with a high level of T cell chimerism (>38%), suggesting that lymphohematopoietic GvH responses may attenuate HvG responses (Figure 1C). Long-term donor T cells were markedly enriched for the naïve recent thymic emigrant phenotype compared with recipient cells regardless of donor age (Figure 2A). GvH clones were absent among long-term circulating naïve donor-derived T cells, which were enriched for CD31 expression and T-cell receptor excision circles (TRECs), suggesting they developed de novo in the recipient thymus. Long-lasting (POD > 200) circulating donor cells lacked GvH reactivity yet were immunocompetent in MLR, partially due to the presence of recipient-specific Tregs (Figure 2B). We also detected donor-derived hematopoietic stem cells (HSCs) and/or progenitors (HPs) in blood and stoma specimens post-Tx and in donor liver, ileum and perfusates pre-Tx. HSCs/HPs from ileum and bone marrow of each organ donor showed similar properties on CyTOF (mass cytometry) analysis, suggesting they may have similar functional phenotypes. Together, donor GvH-reactive T cells and HSCs/HPs carried within the graft promote and maintain donor T cell chimerism in blood, which correlates with better clinical outcomes and suggests an approach to reduce graft rejection and achieve tolerance post-ITx.FIGURE 1: (A) Enrichment of GvH clones in intestinal biopsies and absence of Class I DSA (POD <90) in the circulation were associated with donor T cell macrochimerismin blood. (B) Correlation between the cumulative frequency of GvH clones in blood early post-Tx and the peak level of donor T cell chimerism. (C) Cumulative frequency of HvG clones in blood post-Rx.FIGURE 2: (A) Circulating donor-derived naïve T cells display a recent thymic emigrant (RTE) phenotype. (B) Long-lasting (POD>200) circulating donor cells lacked GvH reactivity yet were immunocompetent in MLR, partially due to the presence of recipient-specific Tregs.Research reported in this publication was performed in the Columbia Center for Translational Immunology (CCTI) Flow Cytometry Core, supported in part by the Office of the Director, National Institutes of Health under awards S10RR027050 and S10OD020056. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.