Abstract Body (Do not enter title and authors here): Background: Numerous studies indicate an association between cardiac dilation and autoimmune processes. Still, a deeper understanding of the mechanisms linking immune regulation and pathophysiological processes in the heart is required. Programmed cell death protein 1 (PD-1) is an immune checkpoint regulator. PD-1 knockout (PD-1 KO) in mice leads to heart dilation in only some mice.
Purpose: This study investigated differences between PD-1 KO mice with and without a cardiac phenotype.
Methods: Female and male mice aged 3-14 weeks were included, n=12 PD-1 KO mice showing cardiac dilation (PD-1 dil), n=16 with non-dilated hearts (PD-1 ctrl) and n=11 BALB/c mice (ctrl). B cell activating factor (BAFF) levels were measured in serum via ELISA. Heart weights were normalized. Proteome analysis and gene expression profiling were conducted using the apical third of the heart, significant alterations (fold change of ≥|1.5|, adjusted p-value <0.05) were analysed.
Results: Cardiac dilation occurred in 6% of PD-1 KO mice. Heart weights and BAFF levels of PD-1 dil were significantly higher compared to PD-1 ctrl and ctrl (heart weight: 12.9/6.9/6.8 mg/mm; BAFF: 25.9/19.7/15.2 ng/ml). Proteomics analysis revealed more than 1,100 significantly altered proteins (PD-1 dil vs. PD-1 ctrl: 504↓, 623↑; PD-1 dil vs. ctrl: 676↓, 702↑). Only 19 proteins were differentially abundant comparing both ctrls (4↓, 15↑). Principal component analysis clearly distinguished PD-1 dil from both ctrl groups, which in turn strongly overlapped. Activation z-score analysis showed an inhibition of mainly mitochondrial functions (z≤-2) in PD-1 dil vs. PD-1 ctrl and in PD-1 dil vs. ctrl. In these comparison groups, some of the most significantly enriched terms associated with cardiovascular diseases were cardiac fibrosis, dilation and inflammation. A regulation of proteins associated with immunological diseases was observed in both comparisons. Moderate correlation between datasets for proteome and transcriptome was observed (PD-1 dil vs. PD-1 ctrl: r=0.5, p<0.0001; PD-1 dil vs. ctrl: r=0.41, p<0.0001).
Conclusion: PD-1 KO mice show an inconsistent phenotype. Thereby PD-1 dil and PD-1 ctrl display significant differences in immune regulation and cardiac pathways, underscoring immunological influences on the development of cardiac pathologies. Future studies will focus on identifying molecular mechanisms that lead to the described phenotype.