Prolidase deficiency causes spontaneous T cell activation and lupus-like autoimmunity

Prolidase Deficiency (PD) is a multi-system disorder caused by mutations in the PEPD gene, which encodes a ubiquitously-expressed metallopeptidase essential for the hydrolysis of dipeptides containing C-terminal proline or hydroxyproline. PD typically presents in childhood with developmental delay, skin ulcers, recurrent infections and in some patients, autoimmune features which can mimic systemic lupus erythematosus (SLE). The basis for the autoimmune association is uncertain, but might be due to self-antigen exposure with tissue damage, or indirectly driven by chronic infection and microbial burden. Here we address the question of causation and show that Pepd null mice have increased anti-nuclear autoantibodies and raised serum IgA, accompanied by kidney immune complex deposition, consistent with an SLE-like disease. These features are associated with an accumulation of CD4 and CD8 effector T cells in the spleen and liver. Pepd deficiency leads to spontaneous T cell activation and proliferation into the effector subset, which is cell intrinsic and independent of antigen receptor specificity or antigenic stimulation. However, an increase in KLRG1+ effector CD8 cells is not observed in mixed chimeras, in which the autoimmune phenotype is also absent. Our findings link autoimmune susceptibility in PD to spontaneous T cell dysfunction, likely to be acting in combination with immune activators that lie outside the haemopoietic system but result from the abnormal metabolism or loss of non-enzymatic prolidase function. This knowledge provides insight into the role of prolidase in the maintenance of self-tolerance and highlights the importance of treatment to control T cell activation. Overall design: Splenic T cells of 4 replicates of CD45.2+ WT or 4 replicates CD45.2+ Pepd-/- T cells isolated from WT:WT or WT:Pepd-/- mixed chimeric mice were subjected to 3' scRNAseq in combination with ADT T cell markers. Samples were multiplexed by hashtagging, WT are B0301-B0304, Pepd-/- are B0305-B0308.