22.2 Memory T cells

Unlike B cells which produce prolonged antibody, the effector T cells are short lived and their cytotoxicity is seen only in presence of antigens. Prolonged exposure of effector T cells is avoided because of its capability to damage tissues by secreted cytokines. The population of T cells not exposed to antigens (naïve T cell) is in constant circulation between the lymphoid tissue and blood. They differentiate into effector cells only after exposure of an antigen. Their population reaches to the peak following a week of infection when cytotoxic T lymphocyte population starts playing role to clear the infection. Once the infection is cleared, the majority of the T cell population is destroyed by apoptosis. The apoptosis of T cells is modulated by CD95L- CD95 interaction. The survivor of this event becomes long lasting memory T cell population.

Memory T cells can be distinguished from other lymphocytes in terms of their cytokines secretion and by the expression of surface molecules. Memory T cells, express high levels of CD44 over their surface and hence are CD44+. In addition, memory T cells express IL-2Rβ which is a receptor for IL-2 and IL-15. The expression of IL-2Rβ and its binding with IL-15 helps memory T cells for its prolonged survival and slow division. Another cytokine important for the survival of CD4+ T helper cell is IL-7. In absence of IL-15, the memory cells undergo apoptosis. Memory T cells secretes large amount of adhesion molecules that helps in their firm binding with the antigen presenting cells. Memory T cells also secretes large amount of cytokines such as IL-4 and interferon-γ that helps in modulating the T cell immune response. In humans the half life of memory cells are 8-20 years depending upon the health condition and nutritional status of an individual.

Interestingly, human memory T cells express TLR-2 after exposure to a lipoprotein antigen (TLR-2 substrate) under the influence of IL-2 and IL-15. This phenomenon suggests that a bacterial pathogen associated molecular pattern may induce the production of memory T cell response. It is amazing to think about all this aspect in terms of evolution and how the memory response gets amplified in higher organism and their ancestor form in lower organism. It is quite obvious that the immune system in its primitive forms is less diverse as compared to higher organism and its complexity increases as we are going up in the hierarchy.