Macrochimerism in Intestinal Transplantation: Association With Lower Rejection Rates and Multivisceral Transplants, Without GVHD

Bidirectional intragraft alloreactivity drives the repopulation of human intestinal allografts and correlates with clinical outcome

A paradigm in transplantation states that graft-infiltrating T cells are largely non-alloreactive "bystander" cells. However, the origin and specificity of allograft T cells over time has not been investigated in detail in animals or humans. Here, we use polychromatic flow cytometry and high throughput TCR sequencing of serial biopsies to show that gut-resident T cell turnover kinetics in human intestinal allografts are correlated with the balance between intra-graft host-vs-graft (HvG) and graft-vs-host (GvH) reactivities and with clinical outcomes. In the absence of rejection, donor T cells were enriched for GvH-reactive clones that persisted long-term in the graft. Early expansion of GvH clones in the graft correlated with rapid replacement of donor APCs by the recipient. Rejection was associated with transient infiltration by blood-like recipient CD28+ NKG2D^Hi CD8+ alpha beta T cells, marked predominance of HvG clones, and accelerated T cell turnover in the graft. Ultimately, these recipient T cells acquired a steady state tissue-resident phenotype, but regained CD28 expression during rejections. Increased ratios of GvH to HvG clones were seen in non-rejectors, potentially mitigating the constant threat of rejection posed by HvG clones persisting within the tissue-resident graft T cell population.

The Mononuclear Phagocyte System in Organ Transplantation

The mononuclear phagocyte system (MPS) comprises monocytes, macrophages and dendritic cells (DCs). Over the past few decades, classification of the cells of the MPS has generated considerable controversy. Recent studies into the origin, developmental requirements and function of MPS cells are beginning to solve this problem in an objective manner. Using high-resolution genetic analyses and fate-mapping studies, three main mononuclear phagocyte lineages have been defined, namely, macrophage populations established during embryogenesis, monocyte-derived cells that develop during adult life and DCs. These subsets and their diverse subsets have specialized functions that are largely conserved between species, justifying the introduction of a new, universal scheme of nomenclature and providing the framework for therapeutic manipulation of immune responses in the clinic. In this review, we have commented on the implications of this novel MPS classification in solid organ transplantation.