*Purpose: Antibody-mediated rejection (AMR) typically presents with elevated titers of circulating donor specific antibodies (DSA) and increased mean fluorescence intensity (MFI), a surrogate marker for antibody titers. However, patients with AMR may rarely have scant DSAs in circulation partly due to the “sponge effect” related to DSAs binding to human leukocyte antigen (HLA) molecules within the lung.

*Methods: We report the case of an 18-year-old female lung transplant recipient with cystic fibrosis who ultimately required retransplantation and developed circulating DSA directed toward the first allograft, but detected in circulation only after retransplantation.

*Results: Figure 1 depicts the MFI of post-transplant DSA to donor HLA. Two months after primary transplant, the patient developed de novo-DSA requiring monthly intravenous immunoglobulin infusions. Pulmonary function remained otherwise stable through the first post-transplant year, but declined insidiously over the following few months. At 19 months post-transplant, she was admitted for acute hypoxemic respiratory failure, necessitating prolonged hospital stay. Redo bilateral lung transplant was performed at 23 months from the primary operation. Following retransplant, serum DSA analysis showed persistent DSAs to mismatched HLA of both the first and second donor allograft. She was treated for probable AMR, and despite aggressive intervention, succumbed to RAS phenotype CLAD.

*Conclusions: The present case draws attention to a rare finding of sponge effect in a patient with AMR leading to allograft failure. We conclude that the absence of circulating DSAs does not rule out AMR. We also postulate that circulating DSAs directed toward a first allograft may drive AMR after retransplant due to cross-reactive epitopes.

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