PCV2 Infection Represses the Differentiation of Light Zone Germinal Center B Cells by Inhibiting Their Interaction with Helper Cells

Porcine circovirus 2 (PCV2) is one of the most widespread immunosuppressive viruses, impairing the protective efficacy of vaccines in pig herds. Previous studies have shown that PCV2 infection reduces the generation of immune memory and antibody secretion induced by vaccination in hosts. In this study, we used single-cell mRNA sequencing of mice splenic cells to show that PCV2 infection decelerates the differentiation of light zone germinal center (GC) B cells into memory B cells and plasma cells. We found that, although PCV2 infection led to lymphocyte depletion in the spleens of mice, the remaining splenic B cells were activated by the infection. The percentage of naïve B cells in PCV2-infected mice decreased mainly due to differentiation rather than death. Meanwhile, the percentages of memory B cells and plasma cells increased without significant enhancement of functional gene expression. Focusing on the GC B cells, we found that PCV2 infection activated the proliferation of dark zone GC B cells, but not the differentiation of light zone GC B cells. Furthermore, the transcriptional level of Prdm1 was not significantly altered by PCV2 infection, and the level of Bach2 was dramatically reduced. Further analysis showed that the interactions between light zone GC B cells and dendritic cells, macrophages, and follicular helper T cells were weakened in the spleens of PCV2-infected mice. In conclusion, this study found that PCV2 infection impairs the differentiation of B cells into functional memory B cells and plasma cells. This may be an important and previously unrecognized reason why PCV2 infection impairs vaccine efficiency.