Locking out illness
“The first step for any intracellular pathogen is getting inside the cell. And if you’re missing the doorway, then the virus can’t accomplish the first step in its life cycle,” Dr. Murphy says. “Getting inside is fundamental.”
Changes in cell-surface molecules can also make someone more likely to have an infection or severe disease. One such group of cell-surface molecules that have been linked to both increasing and decreasing the risk of various infections are histo-blood group antigens. The most familiar members of this group are the molecules that define blood types A, B, and O.
Scientists have also identified one example of total resistance to infection involving these molecules. In 2003, researchers showed that people who lack a functional copy of a gene known as FUT2 cannot be infected with Norwalk virus, one of more than 30 viruses in the norovirus family that cause illness in the digestive tract.
The gene FUT2 encodes an enzyme that determines whether or not blood group antigens are found in a person’s saliva and other body fluids as well as on their red blood cells.
“It didn’t matter how many virus particles we challenged an individual with, if they did not have that first enzyme, they did not get infected,” says researcher Lisa Lindesmith, a virologist at the University of North Carolina in Chapel Hill.
No norovirus
Norwalk is a relatively rare type of norovirus. But FUT2 deficiency also provides some protection against the most common strains of norovirus, known as GII.4, which have periodically swept across the world over the past quarter-century. These illnesses take an especially heavy toll on children in the developing world, causing malnutrition and contributing to infant and child deaths.
But progress in translating these insights about genetic resistance into drugs or other things that could reduce the burden of noroviruses has been slow.
“The biggest barrier here is lack of ability to study the virus outside of humans,” Lindesmith says.
Noroviruses are very difficult to grow in the lab, “and there’s no small animal model of gastrointestinal illness caused by the viruses.”
We are clearly making giant strides in improving those skills,” says Lindesmith. “But we are just not quite there yet.”
In the years before COVID-19 emerged, tuberculosis was responsible for the largest number of annual worldwide deaths from an infectious disease. It’s a lung disease caused by the bacterium Mycobacterium tuberculosis, and it has been a pandemic for thousands of years.
Some 85%-95% of people with intact immune systems who are infected with TB control the infection and never get active lung disease. And some people who have intense, continuing exposure to the bacterium, which is spread through droplets and aerosols from people with active lung disease, apparently never become infected at all.
Thwarting uberculosis
Understanding the ways of these different forms of resistance could help in the search for vaccines, treatments, and other ways to fight tuberculosis, says Elouise Kroon, MD, a graduate student at Stellenbosch University in Cape Town, South Africa.
“What makes it particularly hard to study is the fact that there is no gold standard to measure infection,” she says. “So, what we do is infer infection from two different types of tests” -- a skin test and a blood test that measure different kinds of immune response to molecules from the bacterium.
Dr. Kroon and other researchers have studied resistance to infection by following people living in the same household as those with active lung disease or people who live and work in crowded conditions in high-risk communities. But not all such studies have used the same definition of so-called resisters, documented exposure in the same way, or followed up to ensure that people continue to test negative over the long term.
The best clue that has emerged from studies so far links resistance to infection to certain variations in immune molecules known as HLA class II antigens, says Marlo Möller, PhD, a professor in the TB Host Genetics Research Group at Stellenbosch University.
“That always seems to pop up everywhere. But the rest is not so obvious,” she says. “A lot of the studies don’t find the same thing. It’s different in different populations,” which may be a result of the long evolutionary history between tuberculosis and humans, as well as the fact that different strains of the bacterium are prevalent in different parts of the world.
COVID-19 is a much newer infectious disease, but teasing out how it contributes to both severe illness and resistance to infection is still a major task.