Following the recognition of antigens and costimulators, T cells express proteins that are involved in their proliferation, differentiation, and effector functions (Fig. 1). Naive T cells that have not encountered antigen have a low level of protein synthesis. Within minutes of antigen recognition, new gene transcription and protein synthesis are seen in the activated T cells. These newly expressed proteins mediate many of the subsequent responses of the T cells. The expression of these proteins is a consequence of signal transduction pathways emanating from the TCR complex and costimulatory receptors.

Fig1. Proteins produced by antigen-stimulated T cells. Antigen recognition by T cells results in the synthesis and expression of a variety of proteins, examples of which are shown. The kinetics of production of these proteins (A) are approximations and may vary in different T cells and with different types of stimuli. The possible effects of costimulation on the patterns or kinetics of gene expression are not shown. The functions of some of the surface proteins expressed on activated T cells are shown in (B). CD69 is a marker of T cell activation involved in cell migration; the interleukin-2 receptor (IL-2R) receives signals from the cytokine IL-2 that promotes T cell survival and proliferation; CD40 ligand is an effector molecule of T cells; CTLA-4 is an inhibitor of immune responses. c-Fos (shown in A) is a transcription factor. TCR, T cell receptor.

Antigen recognition activates several biochemical mechanisms that lead to T cell responses, including the activation of enzymes such as kinases, recruitment of adaptor proteins, and production or activation of functional transcription factors (Fig. 2). These bio chemical pathways are initiated when TCR complexes and the appropriate coreceptor are brought together by binding to MHC-peptide complexes on the surface of APCs. In addition, there is an orderly movement of proteins in both the APC and T cell membranes at the region of cell-to-cell contact, such that the TCR com plex, CD4/CD8 coreceptors, and CD28 coalesce to the center and the integrins move to form a peripheral ring. This redistribution of signaling and adhesion molecules is required for optimal induction of activating signals in the T cell. The region of contact between the APC and T cell, including the redistributed membrane proteins, is called the immune synapse. Although the synapse was first described as the site of delivery of activating signals from membrane receptors to the cell’s interior, it may serve other functions. Some effector molecules and cytokines may be secreted through this region, ensuring that they do not diffuse away but are targeted to the cell in contact with the T cell. Enzymes that degrade or inhibit signaling molecules are also recruited to the synapse, so it may be involved in terminating lymphocyte activation as well.

Fig2. Signal transduction pathways in T lymphocytes. Antigen recognition by T cells induces early signaling events, which include tyrosine phosphorylation of molecules of the T cell receptor (TCR) complex and the recruitment of adaptor proteins to the site of T cell antigen recognition. These early events lead to the activation of several biochemical intermediates, which in turn activate transcription factors that stimulate transcription of genes whose products mediate the responses of the T cells. The possible effects of costimulation on these signaling pathways are not shown. These signaling pathways are illustrated as independent of one another, for simplicity, but may be interconnected in more complex networks. AP-1, Activating protein 1; APC, antigen-presenting cell; GTP/GDP, guanosine triphosphate/diphosphate; ITAM, immunoreceptor tyro sine-based activation motif; mTOR, mammalian target of rapamycin; NFAT, nuclear factor of activated T cells; PKC, protein kinase C; PLCγ1, γ1 isoform of phosphatidylinositol-specific phospholipase C; PI-3, phosphatidy linositol-3; ZAP-70, zeta-associated protein of 70 kD.

The cytoplasmic tails of the CD4 and CD8 coreceptors have a constitutively attached protein tyrosine kinase called Lck. several transmembrane signaling proteins are associated with the TCR, including the CD3 and ζ chains. CD3 and ζ contain motifs, each with two tyrosine residues, called immunoreceptor tyrosine-based activation motifs (ITAMs), which are critical for signaling. Lck, which is brought near the TCR complex by the CD4 or CD8 molecules, phosphorylates tyrosine residues contained within the ITAMs of the CD3 and ζ proteins, and this is the event that launches signal transduction in the T cells. The importance of the coreceptors is that by binding to MHC molecules, they bring the kinase close to its critical substrates in the TCR complex. The phosphorylated ITAMs of the ζ chain become docking sites for a tyrosine kinase called ZAP-70 (zeta-associated protein of 70 kD), which also is phosphorylated by Lck and thereby made enzymatically active. The active ZAP-70 then phosphorylates various adaptor proteins and enzymes, which assemble near the TCR complex and mediate additional signaling events.

The major signaling pathways linked to TCR com plex activation are the calcium-NFAT pathway, the Ras– and Rac–MAP kinase pathways, the PKCθ–NF-κB pathway, and the PI-3 kinase pathway:

• Nuclear factor of activated T cells (NFAT) is a transcription factor present in an inactive phosphorylated form in the cytosol of resting T cells. NFAT activation and its nuclear translocation depend on the concentration of calcium (Ca2+) ions in the cytosol. This signaling pathway is initiated by phosphorylation and activation of an enzyme called phospholipase Cγ (PLCγ) by a kinase, Itk, that becomes attached to one of the adaptor proteins in the signaling complex. Activated PLCγ catalyzes the hydrolysis of a plasma membrane phospholipid called phosphatidylinositol 4,5-bisphosphate (PIP2). One by-product of PLCγ-mediated PIP2 breakdown, called inositol 1,4,5-triphosphate (IP3), binds to IP3 receptors on the endoplasmic reticulum (ER) membrane and the mitochondria and initiates release of Ca2+ into the cytosol. In response to the loss of calcium from intracellular stores, a plasma membrane calcium channel is opened, leading to the influx of extracellular Ca2+ into the cell, which further increases the cytosolic Ca2+ concentration and sustains this for hours. The elevated cytosolic Ca2+ leads to activation of a phosphatase called calcineurin. This enzyme removes phosphates from cytoplasmic NFAT, enabling the transcription factor to migrate into the nucleus, where it binds to and activates the promoters of several genes, including the genes encoding the T cell growth factor IL-2 and components of the IL-2 receptor. Calcineurin inhibitors (cyclosporine and tacrolimus) are drugs that block the phosphatase activity of calcineurin, and thus suppress the NFAT-dependent production of cytokines by T cells. These drugs are widely used as immunosuppressants to prevent graft rejection and other T cell–mediated inflammatory conditions .

 • The Ras/Rac–MAP kinase pathways include the guanosine triphosphate (GTP)-binding Ras and Rac proteins, several adaptor proteins, and a cascade of enzymes that eventually activate one of a family of mitogen-activated protein (MAP) kinases. These pathways are initiated by ZAP-70–dependent phosphorylation and accumulation of adaptor proteins at the plasma membrane, leading to the recruitment of Ras or Rac, and their activation by exchange of bound guanosine diphosphate (GDP) with GTP. Ras•GTP and Rac•GTP, the active forms of these proteins, initiate different enzyme cascades, leading to the activation of distinct MAP kinases. The terminal MAP kinases in these pathways, called extracellular signal regulated kinase (ERK) and c-Jun amino-terminal (N-terminal) kinase (JNK), respectively, induce the expression of a protein called c-Fos and the phosphorylation of another protein called c-Jun. c-Fos and phosphorylated c-Jun combine to form the transcription factor activating protein 1 (AP-1), which enhances the transcription of several T cell genes.

• Another major pathway involved in TCR signaling consists of activation of the θ isoform of the serine-threonine kinase called protein kinase C (PKCθ), which leads to activation of the transcription factor NF-κB. PKC is activated by diacylglycerol, which, like IP3, is generated by PLC-mediated hydrolysis of mem brane inositol lipids. PKCθ acts through adaptor proteins recruited to the TCR complex to activate NF-κB.

• TCR signal transduction also involves a lipid kinase called PI-3 kinase, which phosphorylates the mem brane phospholipid PIP2 to generate phosphatidyl inositol (3,4,5)-trisphosphate (PIP3). PIP3 is required for the activation of a number of targets, including a serine-threonine kinase called Akt, or protein kinase B, which has many roles, including stimulating expression of antiapoptotic proteins and thus promoting survival of antigen-stimulated T cells. The PI-3 kinase/Akt pathway is triggered not only by the TCR but also by CD28 and IL-2 receptors. Akt activates mTOR (mammalian target of rapamycin), a serine-threonine kinase that is involved in stimulating protein translation and promoting cell survival and growth. Rapamycin, a drug that binds to and inactivates mTOR, is used to treat graft rejection.

The various transcription factors that are induced or activated in T cells, including NFAT, AP-1, and NF-κB, stimulate transcription and subsequent production of cytokines, cytokine receptors, cell cycle inducers, and effector molecules such as CD40L (see Fig. 1). All of these signals are initiated by antigen recognition, because binding of the TCR and coreceptors to pep tide-MHC complexes is necessary to bring together critical enzymes and substrates in T cells.

As stated earlier, recognition of costimulators, such as B7 molecules, by their receptor CD28 is essential for full T cell responses. The biochemical signals transduced by CD28 on binding to B7 costimulators are less well defined than are TCR-triggered signals. CD28 engagement likely amplifies some TCR signaling pathways that are triggered by antigen recognition (signal 1) and may induce other signals that complement TCR signals.

Lymphocyte activation is associated with a pro found change in cellular metabolism. In naive (resting) T cells, low levels of glucose are taken up and used to generate energy in the form of adenosine triphosphate (ATP) by mitochondrial oxidative phosphorylation. Upon activation, glucose uptake increases markedly, and the cells switch to aerobic glycolysis. This process generates less ATP but facilitates the synthesis of more amino acids, lipids, and other molecules that provide building blocks for organelles and for producing new cells. As a result, it is possible for activated T cells to more efficiently manufacture the cellular constituents that are needed for their rapid increase in size and for producing daughter cells.

Having described the stimuli and biochemical path ways in T cell activation, we now discuss how T cells respond to antigens and differentiate into effector cells capable of combating microbes.