*Purpose: Lymphoid aggregates (LAs) have been identified within lung allografts post lung transplant (LTx). However, it remains controversial whether they represent benign lymphoid tissue or immunologically-active tertiary lymphoid organs (TLOs) and whether they participate in rejection processes, tolerance or both. We aimed to assess in detail the B and T cell component and clarify the specific molecular expression of LAs within lungs affected by chronic lung allograft dysfunction (CLAD).

*Methods: We restrospectively reviewed 46 patients who underwent a 2nd LTx due to CLAD between March 2008 and Aug 2018 at our institution. We cut sequential 5 um sections of formalin-fixed paraffin-embedded explanted lung tissue for hematoxylin-eosin, immunohistochemistry (CD21, BCL6, Lymphotoxin-beta; LTb) and immunofluorescence (CD3, CD20, PNAd) staining (Figure).

*Results: The time interval between 1st LTx and CLAD was 38.1 ± 37.9 months and time from 1st to 2nd LTx was 64.1 ± 51.7 months. We assessed 600 intra-pulmonary LAs >= 1.0×10⁴ um² (13.3 ± 17.2 LAs per CLAD patient). Compared with 10 healthy donor lung samples, LAs were found much more frequently in CLAD lungs (28.2 ± 38.4 x 10³ n/mm² compared to 2.5 ± 4.2 x 10³ n/mm²; p=0.0003). While the sizes of LAs were similar between CLAD and healthy lungs, LAs in CLAD lungs were less dense compared to healthy lungs (7.5 ± 3.0 x 10³ cells/mm² vs. 10.6 ± 3.9 x 10³ cells/mm², p=0.0395). A reticular CD21⁺ follicular dendritic cell staining pattern was positive in 21.4% of CLAD lung LAs compared to 60% in healthy lungs. Within CLAD lungs, compared to CD21^– LAs, the CD21⁺ LAs had a higher percentage of CD20⁺ B cells, as well as cells staining positive for LTb and BCL6, molecules involved in follicular B and T cell development. CLAD lung CD21^– LAs were assessed for PNAd⁺ high endothelial venules (HEV), characteristic of TLOs. The majority of CLAD lung LAs were CD21^–PNAd^– (72.8%) and a minority CD21^–PNAd⁺ (5.9%). CD21^–PNAd⁺ CLAD lung TLOs contained greater proportions of CD3⁺ T, less CD20⁺ and less LTb⁺ cells than CD21^–PNAd^–. Within CD21^– LAs, positive BCL6 staining correlated with more T cells.

*Conclusions: CLAD lung LAs can be broadly categorized into CD21⁺, CD21^–PNAd⁺, and CD21^–PNAd^– LAs. The CD21⁺ B cell-rich LAs have strong LTb and BCL6 staining, consistent with highly-organized active lymphoid structures. CD21^–PNAd⁺ LAs have the highest proportion of T cells. Future studies should assess whether these distinct LAs play differential roles in post-LTx immune responses.

« Back to 2019 American Transplant Congress