*Purpose: Kidney allografts transplanted across certain mouse strain combinations (e.g. DBA/2 to B6) develop tolerance while other combinations do not (e.g. B6 to DBA/2). We have recently shown that a subset of infiltrating conventional and plasmacytoid dendritic cells in the accepted DBA/2 kidney allografts are cross-dressed. In certain tumor models, cross-dressing is observed in T cells and these cells are immunoregulatory. In this study, we looked at the frequency of recipient T cells cross-dressed with donor MHC in accepted kidney allografts.

*Methods: DBA/2 donor kidneys were transplanted into C57/BL6 recipients. Recipients were sacrificed at several timepoints post-transplant, kidney allografts were examined by H&E and immunohistochemistry staining. Some renal allografts were enzymatically digested and mononuclear cells were isolated by density centrifugation; flow cytometry and image flow cytometry was then used to quantify the percentage of H-2K^b+H-2K^d cross-dressed T cells. Preliminary Nanostring RNA analysis was performed on sorted cross-dressed (XD) and non-crossed (nXD) CD4+ and CD8+ T cells.

*Results: Transplantation of DBA/2J kidneys into B6 recipients resulted in the presence of cross-dressed T cells within the allograft as early as 6 days and persisting until 46 days post-transplantation shown by flow cytometry (Fig. 1) and further characterized by image flow cytometry (Fig. 2). Preliminary RNA Nanostring data show that XD CD4+ T cells contain exosomal transcripts (CD63, CD81, Lamp1, Lamp2), as well as specific endothelial and epithelial transcripts, while XD CD8+ T cells do not, suggesting that XD CD8+ T cells may have undergone cross-dressing via trogocytosis. Cross-dressing was not appreciatively observed in rejecting allografts.

*Conclusions: Our data suggest that cross-dressing of infiltrating recipients T cells correlates with kidney graft acceptance in a model of spontaneous tolerance.

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