*Purpose: Humoral alloimmune memory is the main barrier for successful transplantation and is generated by a complex and compartmentalized B-cell immune response. Understanding the role of different cell subsets is key in order to establish effective therapeutic strategies. Here we aim at describing the role of distinct key cellular compartments such as bone marrow (BM) and periphery (blood and spleen) driving alloantigen-specific immune responses in the context of kidney transplantation.

*Methods: In order to characterize the presence and kinetics of humoral alloimmune memory of distinct B-cell counterparts, we evaluated the numbers and function of distinct B-cell subsets in main lymphoid compartments, both in the BM and periphery (spleen and peripheral blood), in an alloantigen-specific manner in two distinct settings: in a highly sensitized experimental rat model after two sequential skin transplants from MHC(RTL-1) mismatch Wistar-Agouti to Lewis rats and in a group of highly HLA sensitized patients in the waiting list for kidney transplantation. Analyses of circulating DSA, spleen and peripheral blood MHC/HLA-specific IgG-secreting mBC and plasmablast numbers and frequencies, as well as LLPC responses in the BM were tracked using solid-phase assays, flow cytometry and MHC/HLA-specific B-cell FluoroSpot assays.

*Results: Sensitized animals showed a peak of DSA after 10 days and its presence was sustained until month 3, when they progressively diminished. High frequencies of MHC(RTL1)-specific IgG-secreting short-live PC in the spleen were detected at day 14 and were no longer detectable after month 1. Conversely, RTL1-specific mBC and LLPC both were detectable after month 1 and until month 3 of follow-up. Interestingly, RTL1-specific IgG-secreting mBc were also observed in the BM until month 3. Highly sensitized patients showed a 100% cPRA with high MFI values. High frequencies HLA-specific IgG-secreting CD138+LLPC and CD19+CD27+mBc were detected in the BM and peripheral blood, respectively. Notably, half of the patients assessed, did also show high frequencies of HLA-specific mBC, after a 6-day polyclonal stimulation, in the CD138- cell fraction of the BM.

*Conclusions: Our study describes the biological complexity of MHC/HLA-specific alloimmune memory in sensitized individuals, where distinct functionally active B-cell subsets play an active role within different but interconnected immune compartments. Multilevel therapeutic strategies should be conceived for successful humoral alloimmune memory control.

« Back to 2020 American Transplant Congress