At least two signals are necessary to activate a naive lymphocyte before it can recognize and target a particular antigen (Figure 1).¹ In order to develop an immune response to an antigen, T-cells must not only recognize their cognate antigen presented on major histocompatibility complex (MHC) molecules but they must also receive non–antigen-specific stimulatory signals in the absence of negative regulatory signals. According to one model, this two-signal activation process "holds in check" the highly sensitive T-cell receptor (TCR), ensuring that activation occurs only when the antigen is presented by the appropriate antigen-presenting cell (APC).²

Figure 1. T-Cell Activation Requires Signaling Through the TCR (Signal 1) and Costimulatory Pathways (Signal 2).

Activated APCs acquire an improved ability to process antigens and subsequently express high levels of costimulatory molecules; thus, they are primed to provide both signals needed for T-cell activation.² Previous chapters have discussed how proteins are processed into smaller peptides via proteosomes and then presented onto the surface of APCs within the groove of class II MHC, or in class I MHC on the surface of any nucleated cell. This antigen-MHC complex interacts with the TCR on naive T-lymphocytes thus enabling immune education for an antigen-specific cellular response. Binding of the TCR and antigen-MHC complex represents the first activation signal. This chapter focuses on the costimulatory proteins that provide the second signal.

Costimulation Allows for T-Cell Activation

Although many different molecules are known to provide costimulatory signals to T-cells, the B7 molecules, B7-1 (also known as CD80) and B7-2 (also known as CD86), are the best characterized. These proteins are expressed at low levels on resting APCs, but their expression is upregulated once these cells are activated.^2,3 For example, B-cell expression of B7 is upregulated by crosslinking of B-cell receptors, CD40 signaling, and/or exposure to interleukin (IL)-4 or interferon (IFN)-γ.⁴

Ligation of CD28 by B7 on the naive T-cell provides the costimulatory signal needed to trigger T-cell activation.⁵ (In contrast, memory T-cells can be activated without costimulation and can respond to lower concentrations and shorter durations of exposure to antigens.⁶) CD28 is found on nearly all CD4+ T-cells and about half of CD8+ T-cells.³

Specifically, CD28 ligation in the presence of a TCR signal results in:^2,3,7-9

• T-cell response to even low levels of TCR ligation, leading to T-cell proliferation and expansion
• Facilitation of IL-2 gene expression, which requires both CD28 and TCR signaling
• Increased production of IL-2, a cytokine that promotes T-cell proliferation and primes the cells for subsequent apoptosis after Fas ligation
• Upregulation of IL-2 receptors
• Increased transcription of multiple cytokine genes, including IL-2, IFN-γ, and tumor necrosis factor (TNF), as well as stabilization of cytokine gene transcripts
• Expression of Bcl-xL, which regulates and suppresses apoptosis

B7 binding to CD28 on T-cells also appears to play a role in determining T-helper-cell differentiation. According to one theory, B7-1 preferentially promotes a Th1 phenotype, whereas B7-2 preferentially promotes the Th2 phenotype.³ According to another hypothesis, it is the strength of the signal rather than the type of B7 that dictates the pathway, with a strong signal promoting Th2 differentiation and a weaker signal promoting Th1 differentiation.⁴

Furthermore, although CD28 ligation largely is thought to play a role in stimulating T-cell activation and preventing apoptosis, it has been shown to play the opposite role in the presence of high levels of TCR signal. At a certain threshold level of TCR signal, if CD28 is ligated, the T-cell will be activated.¹⁰ If the threshold is exceeded, however, then CD28 ligation will trigger apoptosis.

The mechanism by which costimulation facilitates activation is still unclear, but it appears to enhance TCR signaling.² Dual signaling from CD28 and TCR is thought to recruit lipid rafts containing downstream signaling molecules (eg, leukocyte-specific protein tyrosine kinase [Lck], linker for activation of T-cells [LAT]) to the T-cell membrane.¹¹ Aggregation of these rafts colocalizes these signaling molecules with the TCR, thereby facilitating TCR signaling.¹² Experienced T-cells have more rafts on their surfaces than do naive T-cells, so that reactivation does not require strong costimulation.¹¹ In addition, there is evidence that CD28 ligation produces second messenger signaling involving activation of the phosphatidylinositol-3 kinase (PI3K) pathway,² which ultimately leads to inhibition of apoptosis, thereby enhancing T-cell survival.²

Although B7 ligation of CD28 is thought to be the main costimulatory pathway for T-cells, other costimulatory pathways have been identified. TCR binding or CD28 coligation upregulates another important costimulation receptor called inducible costimulator (ICOS).¹³ The ICOS receptor binds to another B7 family protein called ICOS ligand or B7RP-1. Macrophages, monocytes, immature dendritic cells, and naive B-cells express ICOS ligand, as do nonhematopoietic cells (eg, fibroblasts, epithelial cells) that have been exposed to inflammatory cytokines.¹³ ICOS has been hypothesized to serve as the second signal of T-cell activation in tissues where APCs are not found.² ICOS signaling also promotes expression of IL-10, but not IL-2.¹³

A number of receptors in the TNF family have been found to provide costimulatory signals that can activate T-cells. These include CD137 (4-1BB), CD27, CD30, glucocorticoid-induced TNF receptor (GITR), and CD134 (OX40).^2,8 OX40 is noteworthy in that it may trigger antitumor activity in CD4+ and natural killer T-cells.¹⁴ In addition, IL-12 may substitute for CD28 costimulation in activating cytotoxic T-lymphocytes.¹⁵

Although tumor cells may present antigenic peptides on MHC, they commonly lack costimulatory proteins necessary to fully activate T-cells.¹⁶ This suggests that introduction of costimulatory molecules into tumor cells (eg, via vaccines), may increase their immunogenicity.^4,16,17 Similarly, introduction of IL-12 into the tumor microenvironment may activate cytotoxic T-cells responsive to tumor antigens.¹⁵