Naive T cell

A naive T cell or Th0 cell^[1][2][3] is a T cell that has differentiated in bone marrow, and successfully undergone the positive and negative processes of central selection in the thymus. A naive T cell is considered mature and unlike activated T cells or memory T cells it has not encountered its cognate antigen within the periphery.

Phenotype

Naive T cells are commonly characterized by the surface expression of L-selectin (CD62L); the absence of the activation markers CD25, CD44 or CD69; and the absence of memory CD45RO isoform.^[4] They also express functional IL-7 receptor, consisting of subunits IL-7 receptor-α, CD127, and common-γ chain, CD132. In the naive state, T cells are thought to be quiescent and non-dividing, requiring the common-gamma chain cytokines IL-7 and IL-15 for homeostatic survival.^{[citation needed]}

Function

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⁺, CD62L^low, CD69⁺ and may further differentiate into a memory T cell.

Having adequate numbers of naive T cells is essential for the immune system to continuously respond to unfamiliar pathogens.

Mechanism of activation

When a recognized antigen binds to the T cell antigen receptor (TCR) located in the cell membrane of Th0 cells, these cells are activated through the following "classical" signal transduction cascade:^[5]

• the tyrosine kinase Lck is activated, which results in phosphorylation of
• the CD3 coreceptor complex and ζ-chains of the TCR and activation of the ζ-chain- associated protein Zap70
• activated Zap70 in turn phosphorylates the membrane adaptor Lat, which subsequently recruits several Src homology domain–containing proteins, including phospholipase C-γ1 (PLC-γ1)
• activation of PLC-γ1 results in the hydrolysis of phosphatidylinositol 4,5-bisphosphate to inositol 3,4,5-triphosphate and diacylglycerol
• inositol 3,4,5-triphosphate triggers release of Ca²⁺ from intracellular stores and diacylglycerol activates protein kinase C and RasGRP
• RasGRP in turn activates the mitogen-activated protein kinase cascade which ultimately upregulates gene expression and causes entry into the cell cycle

An alternative "non-classical" pathway involves activated Zap70 directly phosphorylating the p38 MAPK that in turn induces the expression of the vitamin D receptor (VDR). Furthermore, the expression of PLC-γ1 is dependent on VDR activated by calcitriol.^[5] Naive T cells have very low expression of VDR and PLC-γ1. However activated TCR signaling through p38 upregulates VDR expression and calcitriol activated VDR in turn upregulates PLC-γ1 expression. Hence the activation of naive T cells is crucially dependent on adequate calcitriol levels.^[5]

In summary, activation of T cells first requires activation through the non-classical pathway to increase expression of VDR and PLC-γ1 before activation through the classical pathway can proceed. This provides a delayed response mechanism where the innate immune system is allowed time (~48 hrs) to clear an infection before the inflammatory T cell mediated adaptive immune response kicks in.^[5]

See also

• Immune system
• Memory T cells

References

1. ^ Leitenberg D, Bottomly K (August 1999). "Regulation of naive T cell differentiation by varying the potency of TCR signal transduction". Semin. Immunol. 11 (4): 283–92. doi:10.1006/smim.1999.0184. PMID 10441214. 
2. ^ Weninger W, von Andrian UH (October 2003). "Chemokine regulation of naïve T cell traffic in health and disease". Semin. Immunol. 15 (5): 257–70. doi:10.1016/j.smim.2003.08.007. PMID 15001175. 
3. ^ Takada K, Jameson SC (December 2009). "Naive T cell homeostasis: from awareness of space to a sense of place". Nat. Rev. Immunol. 9 (12): 823–32. doi:10.1038/nri2657. PMID 19935802. 
4. ^ De Rosa SC, Herzenberg LA, Herzenberg LA, Roederer M (February 2001). "11-color, 13-parameter flow cytometry: identification of human naive T cells by phenotype, function, and T-cell receptor diversity". Nat. Med. 7 (2): 245–8. doi:10.1038/84701. PMID 11175858. 
5. ^ ^{a b c d} von Essen MR, Kongsbak M, Schjerling P, Olgaard K, Odum N, Geisler C (April 2010). "Vitamin D controls T cell antigen receptor signaling and activation of human T cells". Nat. Immunol. 11 (4): 344–9. doi:10.1038/ni.1851. PMID 20208539. http://www.microbio.uab.edu/CMIJournalClub/March17.pdf.