A team of researchers led by Dr. Michal Schwartz of Prof. Noam Stern-Ginossar’s lab in Weizmann’s Molecular Genetics Department addressed this question by focusing on human cytomegalovirus, a member of the herpes family that infects most of the population. Like other herpes viruses, cytomegalovirus awaits silently in the bodies of carriers, although it might cause an onset of symptomatic illness anytime later in life. The virus often raises its ugly head when a patient is immunosuppressed, following an organ transplant or chemotherapy. Pregnant women contracting the virus can pass it to the fetus, which sometimes suffers from serious illness as a result.
The scientists monitored the process of infection in two groups of immune cells – macrophages and their precursor cells – for 144 hours. At several points in time after infection, they sequenced RNA molecules from individual cells. Since RNA molecules carry the recipes for protein production, sequencing them revealed which proteins are produced, and in what quantities, at each stage of infection.
“As expected, shortly after infection the cells still only produced their own proteins,” says Schwartz. “However, a few hours later, the cells split into two groups. As some kept making their own proteins, others started assembling viral proteins, a step that initiates the multiplication of the viral genome and its spread throughout the body. We found this step to be irreversible: From the moment cells expressed just two initial viral proteins, we couldn’t stop the viral takeover.”