Thread by @vipintukur

During COVID pandemic, many were surprised by the apparent lack of illness in infants.
Experts believed that a newborn's immune system was an immature version of an adult's, but a new work shows that newborns' T cells outperform those of adults at fighting off infections 1/

For example, adult T cells outperform newborn T cells at tasks including recognizing antigens, forming immunological memory and responding to repeat infections, which has led to the belief that infant's T cells were just a weaker version of the adult ones. 2/

The researchers discovered that newborn T cells are not deficient: Instead, they are involved in a part of the immune system that does not require antigen recognition: the innate arm of the immune system. 3/

While adults T cells use adaptive immunity, newborn T cells are activated by proteins associated with innate immunity, the part of the immune system that offers rapid but nonspecific protection against microbes the body has never encountered. 4/

Neonatal T cells can participate in the innate arm of immune system. This enables newborn T cells to do something that most adult T cells cannot: respond during the very first stages of an infection and defend against a wide variety of unknown bacteria, parasites and viruses. 5/

So, it is not possible to say adult T cells are better than neonatal T cells or neonatal T cells are better than adult T cells. They just have different functions. 6/

Although T cells are primarily activated in response to antigen recognition via their T cell receptor (TCR), they can also undergo TCR-independent “bystander” activation in response to certain cytokines. 7/

The researchers found that neonatal CD8+ T cells undergo a distinct and robust program of innate-like bystander activation controlled by a balance of Bach2 and AP-1 transcription factor activity. 8/

Neonatal CD8+ T cells could protect against bacterial, viral, and parasitic pathogens in a TCR-independent manner. Innate-like CD8+ T cells were also present in both adult mice and humans, indicating that multiple layers of defense may exist within the adult naïve T cell pool. 9/

These findings provide support for the layered immune hypothesis and indicate that the CD8+T cell compartment is more functionally diverse than previously thought. 10/10

science.org/doi/10.1126/sc…