Following infection, it doesn't take long for HIV to gain a foothold in the body, spreading rapidly by hitching a ride in our immune cells. But the details of these early phases of infection have remained hazy, so in an attempt to shed some light, scientists have now managed to develop a way to watch retroviruses like HIV spreading through the body in real time, inside a living organism. Achieved by a team at Yale, mice were first anesthetized and infected with fluorescently tagged viruses so that their movements could be stalked. They were specifically interested in how the viruses spread in the lymphoid tissue, since this encompasses the various components that are crucial to immune responses.
They then used snazzy laser microscopy to peer into the inner workings of the mouse, capable of penetrating about a millimeter deep. And what they visualized was pretty epic:
Our immune systems can be both a blessing and a curse. In HIV for example, cells that help initiate immunity to this virus also end up worsening the situation by inadvertently playing pass the parcel, transferring infectious particles to susceptible target cells that were actually attempting rectify the insult.
We know quite a lot about processes such as these from studying cells in a dish, which can be done in a rather sophisticated manner using fancy laser microscopes and fluorescent dyes that stain bits of the virus and host cells. While invaluable to our understanding of the process of infection, in vitro studies such as these are limited in that they don’t necessarily reflect what goes on in the body. For starters, your immune system is much more complex than one or two isolated cell types packed into a flask: there is a whole host of different components that interact and communicate in the body.
As described in Science, they found that the process of dissemination begins with viruses being captured by a type of immune cell called a macrophage, which usually scouts the body in search of invaders to gobble up. These virus-laden cells then form connections, called synapses, with a different type of immune cell called a B cell. It is through these spindly structures that the virus gets passed between the cells, a process known as trans-infection. These freshly-infected B cells then migrate into the lymph node, spreading the virus further and establishing infection.
This research is, of course, about more than making pretty pictures and videos. The scientists are hopeful that these insights could lead to a new way to block the spread of HIV, perhaps by using molecules to prevent the virus from attaching to macrophages in the first place.