Researchers uncover genetic machinery answerable for EBV-induced B cell growth transformation

B cells or B lymphocytes are a crucial a part of the body’s immune system. When healthy B cells are infected with the Epstein-Barr virus (EBV), they undergo growth transformation, a process that immortalizes B cells, resulting in their uncontrolled proliferation. The induction of immortalized B cells is step one in the event of posttransplant lymphoproliferative disorder (PTLD), which may evolve to lymphoma and other lymphoproliferative disorders. One key feature of B cell growth transformation is the enlargement of cells and nuclei. Nevertheless, the massive query remains-;what are the precise molecular mechanisms regulating EBV-induced B cell growth transformation?

A recent study by Japanese researchers, published online in Microbiology Spectrum on 6 July 2023, has now provided the reply. Through the use of primary B cells from healthy donors as a substitute of cell lines, the team uncovered the genetic machinery answerable for inducing growth transformation following EBV infection. Explaining the rationale behind the study, principal investigator Prof. Takayuki Murata commented, “Insights from EBV research that uses cell lines has been limited, because cell lines are already in an immortalized state. To beat this roadblock, we used primary B cells from healthy donors after which infected them with EBV. This allowed us to watch the step-by-step growth transformation of B cells and analyze the mechanisms involved.”

Step one involved the careful remark of primary B cells infected with wild-type EBV using electron microscopy and immunostaining. As early as two days after infection, the morphology of B cells showed significant alterations. An enlargement of the nucleolus (a region throughout the nucleus that produces ribosomes, the cell’s protein-producing machinery) was observed, together with a rise within the variety of nucleoli. Interestingly, nucleolar enlargement was followed by an enlargement of both-;the nuclei and the cells.

To know the transcriptional changes that occurred in infected B cells, RNA sequencing was performed. “Among the many genes showing significantly altered expression levels, one named IMPDH2 stood out, because it had previously been linked to similar morphological changes in glioblastoma (one other kind of cancer). Careful evaluation showed that the degrees of the IMPDH2 peaked two days after infection-;coinciding with the timing of nucleolar enlargement. This suggested that we were on the fitting path,” explained Dr. Atsuko Sugimoto from Fujita Health University School of Medicine, who was also an element of the research team.

Interestingly, changes similar to IMPDH2 induction and nucleolar enlargement could possibly be seen when primary B cells were activated using inflammatory signals, even within the absence of EBV infection. Finally, the inhibition of IMPDH2 using silencer RNAs and the drug mycophenolic acid (MPA) prevented the expansion transformation of primary B cells after EBV infection, producing smaller nucleoli, nuclei, and cells. This confirmed that IMPDH2 induction played a key role in the expansion transformation of EBV-infected B cells.

The subsequent step involved understanding how EBV prompts IMPDH2 expression. Three key viral genes-;EBNA2 and LMP1-;were tested due to their known role in EBV-induced B cell transformation. Interestingly, when EBV lacking EBNA2 was used for infection, IMPDH2 induction following primary EBV infection was blocked. This effect was not observed with LMP1 knockout. “This very clearly demonstrated that EBV induces IMPDH2 expression via EBNA2–dependent mechanisms. As well as, cellular transcription factor MYC was also involved within the IMPDH2 induction,” clarified Dr. Sugimoto.

With several key pieces of evidence on their plate, the researchers finally set out to search out the ultimate piece of the puzzle. To spotlight the clinical significance of their findings, they examined whether the drug mycophenolate mofetil (MMF) could prevent B cell transformation and PTLD. Prof. Murata elaborated, “Like MPA, which we tested in the sooner a part of our study, MMF is an IMPDH2 inhibitor. More importantly, MMF is already a clinically approved immunosuppressant. That’s the reason it was useful to check if it could possibly be applied for the clinical prevention of PTLD.” As expected, the administration of MMF in a pre-clinical mouse xenograft model led to improved survival and reduced splenomegaly (enlargement of the spleen, indicating reduced B cell proliferation). These observations confirmed that the usage of MMF can inhibit PTLD development.

This study is the primary to display that IMPDH2 activation and nucleolar hypertrophy are essential for EBV-induced B cell transformation and that IMPDH2 inhibition can suppress PTLD. It may lead to the adoption of MMF as an agent for the prevention of EBV-positive PTLD, providing significant relief to transplant patients.