*Purpose: After lung transplantation (LuTx), a transient chimerism of donor cells exists in the blood of recipients due to the migration of lymphocytes from the transplanted lung into the periphery. We aimed to characterize the phenotype of donor CD8+ and CD4+ T and NK cells and to investigate whether they might represent tissue-resident memory (TRM) cells.

*Methods: Lymphocyte dynamics in recipient blood were determined in 97 lung transplant patients directly (T0), 24 hours (T24) and 3 (wks) weeks after LuTx using flow cytometry with lineage-, tissue-, and donor HLA class I allele-specific mAb. The same makers were used to determine the phenotype of lymphocytes present in organ storage solution (perfusate, n=102), recipient explanted lung parenchyma (n=28) and donor trachea (n=17).

*Results: In peripheral blood of all recipients, donor-derived CD4⁺ and CD8⁺ T and CD56^dim NK cells were detected at T0, T24 and 3 wks after LuTx and had higher CD69 expression compared to recipient cells (p=0.01 to 0.04), were mostly CCR7- memory cells and CD25 negative. This phenotype was similar to T and NK cells in corresponding perfusates. In recipient parenchyma and donor trachea, most CD69⁺ T and NK cells showed coexpression of other tissue residency markers such as CD103, CD49a and PD-1 with significant enrichment in trachea (p<0.05). In contrast, these markers were not found in circulating donor lymphocytes and perfusates, indicating that they represent distinct memory T and NK subsets. Donor T and NK cells showed higher IFN-g production with and w/o PMA/Iono stimulation compared to recipient cells (p<0.05). The presence of these particular TRM cells did not have an impact on the development of PGD 24h after transplantation. However, patients with high frequencies of donor T cells showed a trend towards a CLAD-free survival 2 years post LuTx, although no statistical significance was reached (p=0.15).

*Conclusions: Our results demonstrate that donor T and NK cells found in the periphery of lung transplant recipients are a distinct subset from circulating lymphocytes and TRM cells present in lung tissue, since they express CD69 but lack expression of other classical TRM markers. They display a higher functional capacity despite the onset of immunosuppression. Donor T cells might be clinically relevant for tolerance induction and long-term survival after transplantation due to their unique features.

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