Digital cytometry of leukocyte landscapes reveals shared correlates of rejection across organ transplants

Background. Improved understanding of the cellular mechanisms of organ transplant rejection is needed for developing targeted therapies to improve outcomes. We used digital cytometry to estimate the digital leukocyte entities (DLE) shared by rejection in biopsies of kidney, heart, lung, and liver organ transplants. Methods. We used CIBERSORTx with the LM22 leukocyte reference to estimate associations between 22 DLEs with rejection in 8422 transplant biopsies - 4898 kidney, 2361 heart, 652 lung, and 538 liver. Results. In all organs, rejection correlated with an increase in DLEs designated as classically activated macrophages and a decrease in DLEs designated regulatory T cells. T cell-mediated rejection (TCMR) was associated with a massive increase in otherwise rare T cell DLEs designated memory-activated CD4 and Tγδ, as well as follicular-helper CD4 and CD8. TCMR also had increased DLE designated plasma cells, compatible with a role for the B cell lineage in antigen presentation in TCMR. Antibody-mediated rejection (ABMR) in kidney and heart transplant biopsies correlated with increased DLEs representing activated mast cells, activated NK cells, and monocytes. Some findings were validated by single-cell/nucleus RNAseq data from kidney and liver biopsies. Conclusion. The shared correlations of DLEs with rejection in all organs - e.g. CD4ma with TCMR and activated mast cells with ABMR - suggest novel aspects of rejection. These findings can guide the search for the in vivo leukocytes that correspond with the digital entities, with potential clinical applications. Trial registration. ClinicalTrials.gov NCT01299168, NCT02670408, NCT02812290, NCT03193151. Funding. Genome Canada and a grant from Natera, Inc.

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