Early Lymphoid Aggregates in Protocol Kidney Allograft Biopsies: Molecular Characterization and Association with Long-Term Kidney Allograft Outcome

BACKGROUND: The presence of lymphoid tissues and their impact on long-term kidney graft outcome remains unclear. This study investigates the occurrence, molecular and phenotypic characteristics of lymphoid aggregates (LA) in protocol biopsies of clinically stable kidney transplant recipients, and their correlation with long-term clinical outcome.

METHODS: We retrospectively analyzed 414 protocol biopsies from a multicenter cohort of 278 stable kidney transplant recipients taken at either 6-, 12- or 24-months post-transplant. On a subset of 16 biopsies at 6 months post-transplant, mRNA gene expression analysis was performed using Nanostring B-HOT panel alongside phenotypic analysis with immunostaining for developmental staging of LA. Finally, the presence of LA was correlated to various clinical kidney allograft outcomes.

RESULTS: Stage I LA were characterized by upregulation of genes linked to immune regulation (LAG3, CTLA4, FOXP3), T-cell signaling, and chemotaxis pathways, with enrichment of regulatory T cells (Tregs), memory T cells, T follicular helper cells (Tfh), and dendritic cells. Immunohistochemistry confirmed higher numbers of FOXP3+ Tregs and dendritic cells in stage I LA, while high endothelial venules were absent. Over follow-up, participants with stage I LA showed slightly higher eGFR values and a trend toward better graft survival compared with those without LA or with stage II LA, although most differences did not reach statistical significance.

CONCLUSIONS: In stable kidney transplant recipients, stage I LA displayed molecular and phenotypic features consistent with a regulated immune environment. The molecular and immunohistochemical analysis of stage I LA suggests a more regulatory molecular phenotype and composition, which may contribute to better long-term graft survival observed in this cohort of functionally stable transplant recipients. Observed trends toward more favorable outcomes suggest that early LA may not be deleterious and could represent local immune regulation, though larger prospective studies are needed to confirm these findings.