*Purpose: Cardiac allograft vasculopathy (CAV) is the leading cause of heart graft recipient mortality after five years post-transplant. A major hurdle to improving outcomes is the poor understanding of the mechanisms underlying CAV development. Immune infiltrates, including B cells and plasma cells (PC), are commonly observed in grafts during CAV and may contribute to its pathophysiology through local antibody responses. We sought to characterize these responses at the single cell level.

*Methods: Coronary arteries and surrounding tissue from three cardiac allograft explants with CAV were collected prospectively. Dissociated specimens were subjected to single-cell RNA sequencing for transcriptomic and immune repertoire profiling. Recombinant antibodies (rAbs) from highly expanded clonotypes from one explant were generated and binding of rAbs to various antigens was assessed. Immune complexes (ICs) generated from rAbs were used to stimulate cytokine production in immune cells.

*Results: Immune infiltrates were composed of CD4+ and CD8+ T cells, NK cells, macrophages, B cells and PC in all three cases. Immune cells and endothelial cells expressed IgG Fc receptors. B cell V(D)J sequencing revealed expanded clonotypes within the PC clusters consisting mainly of the IgG isotype. Two out of the three rAbs generated from the most prolific clonotype sequences exhibited polyreactive binding profiles including reactivity to apoptotic cells, insulin, heart tissue lysate and malondialdehyde. In contrast, none of the rAbs exhibited reactivity to HLA class I or II. In addition, ICs prepared with rAbs derived from PC expanded in situ stimulated TNF-a secretion in PBMCs and IFN-g secretion in NK cells.

*Conclusions: Polyreactive PC are present in graft infiltrates in CAV. ICs derived from polyreactive antibodies can stimulate immune cells to produce proinflammatory cytokines, suggesting a potential role for local polyreactive antibody production in CAV.

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