Fig 7. Visualizing the occurrence of age-dependent characteristics of T cell activation.

The requirement of activation by the TCR/CD3 complex is less dependent on costimulation by CD28 in CB naive CD4⁺ T cells compared to that seen in adult cells. The intensity of Ca²⁺ influx, NFATc2 expression and IL-2 response are all age-dependent. A dramatic shift is seen in the naive T cell response at the age of 2 months. The cells’ capacity to produce high amounts of IL-2 is suddenly abrogated. Ca²⁺ influx declines to the lowest values observed in life. At 6 months of age, the IL-2 response starts to improve slowly. This “reprogramming” of T cells takes place as the passively transferred maternal Abs in the infant are beginning to decline. Limited T-cell responses likely contribute to the high risk of infants to suffer from infections and infection-related pathologies such as Sepsis and SIDS [43,44] during the first months of life.