Biological sex has long been known to have some influence. “In general, males tend to have more severe infections than females,” says Camila Consiglio, a systems immunologist at Lund University in Sweden. “On the flip side, we know that females have better responses to vaccines.” But what wasn’t known before the pandemic is that an infection—such as Covid—can shift these sex differences, increasing the variation in immune responses across the population. In a paper recently published in Nature, a team led by John Tsang—a systems immunologist at Yale University—found that Covid can affect the immune response long after infection, changing the way people respond to a subsequent vaccine for a different disease. Specifically, they discovered that males who had recovered from a mild bout of Covid responded more robustly to a later flu vaccine compared with females who had recovered from Covid, and also more robustly than healthy controls—flipping the usual sex difference. The scientists think that these findings can help researchers build better vaccines and figure out why pathogens impact people so differently. Knowing whether you are more or less vulnerable to infections could help personalize your medical care in the future—making this information potentially lifesaving. Tsang’s group had already been investigating the long-term influence of infections on a person’s immune system—but the arrival of Covid in 2020 suddenly made studying this a lot easier. “We got this huge wave that happened around a very short time-window to a very large population,” Tsang says. Having so many people who simultaneously had experienced a mild infection and then recovered was a rare occurrence, and it created an excellent group to sample from. By following these otherwise relatively healthy patients, Tsang thought, they could reliably test to see how different people’s immune systems reacted to a vaccine in the wake of an infection. So, the team collected blood samples from 33 patients who had recovered from mild Covid, as well as from 40 healthy age- and sex-matched controls who hadn’t yet caught the virus. Later, all participants received the seasonal flu vaccine, and after this blood was collected again several points (with some samples taken as long as 100 days post-vaccination). The next step was to run all these samples through whole blood transcriptomics—a form of genetic sequencing that reveals all of the genes being used (or “expressed”) by blood cells to create substances that they need, including immune molecules. This provided a high-level overview of potential differences in immune activity between males and females. While there were some slight baseline differences prior to receiving the flu vaccine, the scientists found clearer ones immediately afterward. “You see significantly stronger inflammatory responses in the Covid-recovered males,” says Tsang, indicating that Covid-recovered males created more antibody-producing cells and antibodies in response to the vaccine. “That was a huge surprise. Often, you find that kind of response to be higher in females.” A particular type of cell seemed to be contributing to the response to the flu vaccine: effective-memory T cells (which are generated after an infection and can “remember” the specific pathogen they encountered) with a receptor called GPR56 expressed on their surface. As it turns out, Covid-recovered males had more of these cells compared to Covid-recovered females and healthy controls. But why would these cells, seemingly linked to Covid, respond to a flu vaccine? “The canonical assumption is that an infection will generate cells that are specific to the virus,” Tsang says. But as he explains, this isn’t necessarily the case. Other more broadly reactive immune cells can be activated too. Known as “bystander cells,” these react very quickly to vaccination, sending up alarm bells that lead to the immune system creating antibodies in response. Indeed, when the scientists examined the GPR56-positive T cells, they found that they shared similarities with bystander cells already known to be activated during acute Covid infection. Therefore, they hypothesized, these GPR56-positive cells probably acted in a way similar to bystander cells, persisting in the body after Covid, and kick-starting immune responses to other invaders—in this case, the flu vaccine. To prove this theory, the scientists needed to see how GPR56-positive T cells responded to something akin to infection or vaccination. When they isolated these T cells, cultured them in a petri dish, and stimulated them with small signaling molecules called cytokines known to be produced during infection or vaccination, the scientists found that the T cells secreted high levels of inflammatory proteins that had been found in the Covid-recovered males—providing evidence that this cell type could indeed have triggered the immune response that ultimately created more flu antibodies. They had found their smoking gun. Consiglio is curious to see, in the future, what effect these immune system differences between Covid-recovered males and females have when a person is actually infected by the flu or another virus. Looking at sex and prior infections also raises the question of how other factors might influence the immune response. Sabra Klein, a microbiologist at Johns Hopkins University, is interested in seeing how something like age might also factor into the equation and perhaps create a sliding scale of immune responses. “We often treat these kinds of variables as binary—you’re young or you’re old, you’re male or you’re female—and we often don’t do enough to interrogate these intersections,” Klein says. Ultimately, Tsang and his team hope that these results can help scientists design more effective vaccines or figure out ways to predict how a person might respond to an infection. They want to find out if those GPR-positive cells can be more efficiently primed to respond to a pathogen. On the flip side, the scientists are also curious about how these cells (and others) function during autoimmunity, where the immune system is overactive. Until then, they will continue to appreciate the complexities of the immune system—particularly how it has evolved during the pandemic. “We always thought of looking at the human immune system as a very diverse natural experiment,” Tsang says. Thanks to the pandemic, the opportunity to learn from this experiment was that much greater—meaning we now know more about why our immune systems are so different, and how they change over time.
