Naive T cell

A naive T cell (Th0 cell) is a T cell that has differentiated in bone marrow, and successfully undergone the positive and negative processes of central selection in the thymus. Among these are the naive forms of helper T cells (CD4+) and cytotoxic T cells (CD8+). A naive T cell is considered mature and, unlike activated or memory T cells, has not encountered its cognate antigen within the periphery. A naive T cell (Th0 cell) is a T cell that has differentiated in bone marrow, and successfully undergone the positive and negative processes of central selection in the thymus. Among these are the naive forms of helper T cells (CD4+) and cytotoxic T cells (CD8+). A naive T cell is considered mature and, unlike activated or memory T cells, has not encountered its cognate antigen within the periphery. Naive T cells are commonly characterized by the surface expression of L-selectin (CD62L) and C-C Chemokine receptor type 7 (CCR7); the absence of the activation markers CD25, CD44 or CD69; and the absence of memory CD45RO isoform. They also express functional IL-7 receptors, consisting of subunits IL-7 receptor-α, CD127, and common-γ chain, CD132. In the naive state, T cells are thought to require the common-gamma chain cytokines IL-7 and IL-15 for homeostatic survival mechanisms. While naive T cells are regularly regarded as a developmentally synchronized and fairly homogeneous and quiescent cell population, only differing in T cell receptor specificity, there is increasing evidence that naive T cells are actually heterogeneous in phenotype, function, dynamics and differentiation status, resulting in a whole spectrum of naive cells with different properties. For instance, some non-naive T cells express surface markers similar to naive T cells (Tscm, stem cell memory T cells; Tmp, memory T cells with a naive phenotype), some antigen-naive T cells have lost their naive phenotype, and some T cells are incorporated within the naive T cell phenotype but are a different T cell subset (Treg, regulatory T cells; RTE, Recent Thymic emigrant). It is important to appreciate these differences when assessing naive T cells. Majority of human naive T cells are produced very early in life when infant's thymus is large and functional. Decrease in naive T cell production due to involution of the thymus with age is compensated by so called 'peripheral proliferation' or 'homeostatic proliferation' of naive T cells which have emigrated from the thymus earlier in life. The homeostatic proliferation causes change to naive T cell gene expression and i.e. is manifested by acquisition of CD25 surface protein expression. Naive T cells can respond to novel pathogens that the immune system has not yet encountered. Recognition by a naive T cell clone of its cognate antigen results in the initiation of an immune response. In turn, this results in the T cell acquiring an activated phenotype seen by the up-regulation of surface markers CD25+, CD44+, CD62Llow, CD69+ and may further differentiate into a memory T cell.