A type of the immune system’s T cells known to fight against bacterial infections is strongly activated in people with moderate to severe COVID-19, according to a study which provides a better understanding of how the body responds to the novel coronavirus infection. Researchers, including those from the Karolinska Institute in Sweden, noted that this component of the immune system called MAIT cells make up about one to five percent of T cells in the blood of healthy people, and are primarily important for controlling bacteria, but can also be recruited to fight some viral infections. They explained that T cells are a type of white blood cells that are specialised in recognizing infected cells, and are an essential part of the immune system. In the current study, published in the journal Science Immunology, the scientists assessed the role played by MAIT cells in COVID-19 disease. They examined the presence and character of MAIT cells in blood samples from 24 patients admitted to Karolinska University Hospital with moderate to severe COVID-19 disease, and compared these with blood samples from 14 healthy controls and 45 individuals who had recovered from COVID-19. Four of the patients died in the hospital, the study noted. “To find potential treatments against COVID-19, it is important to understand in detail how our immune system reacts, and in some cases, perhaps contribute to worsening the disease,” said Johan Sandberg, a co-author of the study at Karolinska Institutet. According to the study, the number of MAIT cells in the blood decline sharply in patients with moderate or severe COVID-19, and the remaining cells in circulation are highly activated. Based on these results, the scientists suggested that the MAIT cells are engaged in the immune response against the novel coronavirus SARS-CoV-2. This pattern of reduced number and activation in the blood is stronger for MAIT cells than for other T cells, they said. The study also noted that pro-inflammatory MAIT cells accumulated in the airways of COVID-19 patients to a larger degree than in healthy people. “Taken together, these analyses indicate that the reduced number of MAIT cells in the blood of COVID-19 patients is at least partly due increased accumulation in the airways,” Sandberg said. The scientists added that the number of MAIT cells in the blood of convalescent COVID-19 patients recovered at least partially in the weeks after disease, which can be important for managing bacterial infections in individuals who have had COVID-19. They said the MAIT cells tended to be extremely activated in the patients who died. “The findings of our study show that the MAIT cells are highly engaged in the immunological response against COVID-19,” Sandberg said. The scientists believe the characteristics of MAIT cells make them engaged early on in both the systemic immune response, and in the local immune response in the airways to which they are recruited from the blood by inflammatory signals. “There, they are likely to contribute to the fast, innate immune response against the virus. In some people with COVID-19, the activation of MAIT cells becomes excessive and this correlates with severe disease,” Sandberg added.
