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Blood chimerism has been reported sporadically among visceral transplant recipients, mostly in association with graft-vs-host disease (GVHD). We hypothesized that a higher degree of mixed chimerism would be observed in multivisceral (MVTx) than in isolated intestinal (iITx) and isolated liver transplant (iLTx) recipients, regardless of GVHD. We performed a longitudinal prospective study investigating multilineage blood chimerism with flow cytometry in 5 iITx and 4 MVTx recipients up to one year post-transplant. Although only one iITx patient experienced GVHD, T-cell mixed chimerism was detected in 8 out of 9 iITx/MVTx recipients. Chimerism was significantly lower in the four subjects who displayed early moderate to severe rejection. Pre-formed high titer donor-specific antibodies, bound in vivo to the circulating donor cells, were associated with an accelerated decline in chimerism. Blood chimerism was also studied in 10 iLTx controls. Among non-sensitized patients, MVTx recipients exhibited greater T and B-cell chimerism than either iITx and iLTx recipients. Myeloid lineage chimerism was present exclusively among iLTx and MVTx (6/13) recipients, suggesting that its presence required the hepatic allograft. Our study demonstrates, for the first time, frequent T cell chimerism without GVHD following visceral transplantation and a possible relationship with reduced rejection rate in MVTx recipients.

We examined tolerance mechanisms in patients receiving HLA-mismatched combined kidney and bone marrow transplantation (CKBMT) that led to transient chimerism under a previously-published non-myeloablative conditioning regimen (Immune Tolerance Network study ITN036). Polychromatic flow cytometry (FCM) and high throughput sequencing of TCRβ hypervariable regions of DNA from peripheral blood T regulatory cells (Tregs) and CD4 non-Tregs revealed marked early enrichment of regulatory T cells (CD3+CD4+CD25highCD127lowFoxp3+) in blood that resulted from peripheral proliferation (Ki67+), possibly new thymic emigration (CD31+) and, in one tolerant subject, conversion from non-Tregs. Among recovering conventional T cells, central memory CD4+ and CD8+ cells predominated. A large fraction of the T cell clones detected in post-transplant biopsy specimens by TCR sequencing were detected in the peripheral blood and were not donor-reactive. Our results suggest that enrichment of Tregs by new thymic emigration and lymphopenia-driven peripheral proliferation in the early post-transplant period may contribute to tolerance following CKBMT. Furthermore, most conventional T cell clones detected in immunologically quiescent post-transplant biopsies appear to be circulating cells in the microvasculature rather than infiltrating T cells.

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