Donor T cell macrochimerism (>4%) often appears in blood without graft-versus-host disease (GVHD) following intestinal transplantation (ITx), and is associated with less graft rejection and donor-specific antibody (DSA) production^[1][2]. Blood chimerism is also observed in other lymphoid and myeloid lineages^[1]. We investigated the mechanisms underlying multilineage chimerism in blood.

We analyzed the repertoire, phenotype and origin of graft-resident and circulating donor T cells in ITx recipients using flow cytometry, mixed lymphocyte reaction and high-throughput TCR sequencing to identify and track alloreactive T cells.

Expanded graft-versus-host (GvH) clones triggered by rapidly replaced recipient antigen-presenting cells were detected early in intestinal biopsies (Figure 1A). Enrichment of graft GvH clones and absence of Class I DSA in the circulation were associated with donor T cell macrochimerism. The cumulative frequency of GvH clones in blood early post-Tx was correlated with peak donor T cell chimerism levels (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 a lymphohematopoietic GvH response (without GVHD) may attenuate HvG responses (Figure 1C). While donor ileum lymphocytes display a tissue-resident memory (Trm) phenotype, circulating donor T cells include naïve and other non-Trm phenotypes (Figure 2A-B). Long-term circulating donor T cells were markedly enriched for the recent thymic emigrant phenotype and for T-cell receptor excision circles (TRECs) compared with recipient cells, suggesting they were recent thymic emigrants (Figure 2C-D). Donor circulating B cells and γδ T cells were also enriched for naïve phenotype compared to recipient cells. Myeloid lineage chimerism recurred at low levels late post-Tx in patients with macrochimerism. Consistent with the possibility that blood chimerism originated from progenitors in the graft, we detected donor-derived hematopoietic stem cells (HSCs) and/or hematopoietic progenitors (HPs) in human intestinal mucosa.

Our findings suggest that locally expanded GvH-reactive donor T cells in the graft enter the recipient circulation and attack host hematopoietic cells, which may allow the engraftment of graft-derived HSCs/HPs, thereby promoting sustained mixed chimerism. This pathway may help to reduce graft rejection and achieve tolerance after ITx.

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. .

[1] Zuber J et al. Am J Transplant. 2015; 15: 2691–2703.
[2] Zuber J et al. Science Immunology. 2016; 1: eaah3732.