*Purpose: After lung transplantation (LTx), the development of early donor HLA-specific antibodies (eDSA) is associated with antibody-mediated rejection (AMR) and poor graft survival. Since 2013, patients with eDSA within the first month after LTx are treated with IgA/IgM enriched intravenous immunoglobulins (IgGAM), occasionally with anti-CD20 antibody (Rituximab). We hypothesise that the composition of naïve and memory B cell subsets differs between eDSA-positive and negative patients already before and directly after LTx.

*Methods: In a pilot study of 58 LTx recipients, B cell subsets were analysed by flow cytometry using CD19, CD20, IgD, CD24, CD27antibodies pre, post (T0), 24 hours (T24) and 3 weeks after LTx. The proportions of B cell subsets were compared between eDSA-positive (n=13, 22,4%) and -negative (n=45, 77,6%) patients at discharge three weeks after transplantation.

*Results: Within the first 24h, patients without eDSAs showed a significant increase in IgD+ CD27- naïve B cells (p<0,0001) accompanied by a significant decrease in IgD- CD27+ memory B cells, independently of the presence of eDSA (all, p<0,0007). Of note, eDSA-positive patients constantly showed higher frequencies of IgD⁺CD27^–CD24^hi naïve and lower frequencies of IgD^–CD27^–CD24^lo memory B cell subsets compared to eDSA-negative patients. A transient increase in IgD⁺CD27⁺ switch memory B cells was detected at T0 in both groups, returning to baseline levels already at T24.

*Conclusions: Lung transplant recipients show remarkable dynamics of naïve, memory and switch memory B cells within the first 24 hours characterized by a shift from naïve towards memory B cells, which was more pronounced in the eDSA group. Based on these preliminary data, a refined B cell monitoring may be able to identify patients with a higher risk for eDSA development in the future and pave the way to more specific treatment options to prevent eDSA.

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