In another slide Dr. Pereira shared a picture of one cross-section of an entire villus, while she stressed that there is a lot of work that goes into any one section that is analyzed. She added, “This is the model that we felt confident in by studying isolated primary cells from the placenta and also we had already begun to characterize their pattern of infection at the uterine placental interface.”
Seeing this, Dr. Pereira and her colleagues sought to find the answer to another question: What is the real target for the Zika virus? The researchers compared a Ugandan prototype strain (MR766) of the virus (from their studies
) with an epidemic Nicaraguan strain of Zika virus. After further exploration, she found that the patterns pertaining to the cytotrophoblasts and the Hofbauer cells
“are consistently reproducible in all of the tissues.” She added that there are functional differences manifested in cytotrophoblasts infected by the Nicaraguan strain of the virus and those infected by the phototype strain. “We see that Zika-infected cytotrophoblasts downregulate a marker of cell proliferation [Ki67] and we think that the [villus] explant model is useful for testing drugs to [fight] Zika virus that are suitable for pregnancy,” she said.
Taking a closer look at the Ugandan prototype strain versus the Nicaraguan strain, the researchers began to see the differences “which we had hints at but didn’t really know until we counted all of the migrating cells and measured everything.” Dr. Pereira showed pictures illustrating the Ugandan prototype strain and how only a few cells had migrated out at 3 days’ post-infection. Another picture illustrated how the cells in the Nicaraguan strain actually traveled four times the distance than what was seen in the prototype strain; they both expressed ZIKV E and NS3 proteins that produce infectious virus.
In order to prove it, the researchers took a closer look at the other cell type that was infected: Hofbauer cells in the villus core. She explained that the Hofbauer cells “express E [and] NS3 [proteins]. They are in the villus core [that border proximal cell columns]. There are no infected cytotrophoblasts around them, so they are independently infected from the cytotrophoblasts. They have to be close to an area of cell columns and somehow prepare the area for Hofbauer cells to become infected. We don’t know what that is, but that connection happens.”
As the villi branch, first they sprout, and then make a new villus; this actually works to prepare the villus core to “allow the virus to reach the Hofbauer cells inside.” The cytotrophoblasts outside are not infected. Dr. Pereira also stressed that the Hofbauer cells surround the villus blood vessel so closely that, at first, the researchers couldn’t discern if they were actually a part of the blood vessel edge. They also found that, “A Hofbauer cell being infected could bring virus close to this blood vessel and infect the fetal bloodstream.”