GPR18 plays an atheroprotective role by regulating humoral and effector T-cell responses
A Prabhu, M Geiger, Y Jansen, J Cyster, S Bortoluzzi, S SteffensAbstract
Background and Aims
The GPR18-resolvin D2 (RvD2) axis is a key mediator of pro-resolving signalling that counteracts inflammation in atherosclerosis. While studies investigating its role in atherosclerosis have focused on innate immune cells, the contribution of GPR18-RvD2 to the regulation of adaptive immune responses in this context is largely unknown.
Methods
Mice lacking apolipoprotein E (Apoe-/-) were crossed with mice lacking Gpr18, and resulting Apoe-/-Gpr18-/- mice as well as Apoe-/- controls were fed with Western Diet (WD). Atherosclerotic plaques and splenic architecture were assessed by histology. Immunophenotyping was performed by flow cytometry analysis of spleens and para-aortic lymph nodes, ELISA, and Luminex assay. In vitro plasmablast differentiation assays were performed to assess the antibody-producing capacity of B cells.
Results
Apoe-/-Gpr18-/- mice exhibited increased plaque burden in the aortic root after 4 to 16 weeks WD compared to corresponding Apoe-/- controls (females: n=6, p= 0.017; males: n=7, p= 0.029), which was also observed in the aortic arch after 16 weeks WD (females: n=6, p= 0.028; males: n=7, p= 0.043). The proatherogenic phenotype in mice lacking Gpr18 was accompanied by a disruption of the splenic follicular structure and a more pronounced humoral response after 4 weeks WD, with increased germinal centre B cells and elevated IgM (n=10, p=0.004) and IgG levels (n=10, p=0.007). This shifted towards a class-switched profile with lower IgM, but persistently high IgG levels after 16 weeks WD. In vitro stimulation of Apoe-/- splenocytes with RvD2 reduced the expansion of plasmablasts and IgM release (n=5, p=0.031). Furthermore, Apoe-/-Gpr18-/- mice showed a more pronounced expansion of CD8+ effector T cells (n=10, p=0.026), T follicular helper cells (n=10, p=0.029), as well as elevated Th1 and Th17 cytokine plasma levels compared to Apoe-/- controls.
Conclusion
Collectively, these findings suggest that GPR18 acts as a central checkpoint restraining humoral and effector T cell responses in atherosclerosis.Graphical abstractFor image description, please refer to the figure legend and surrounding text.