| Lack of Costimulation Produces Anergy
Stimulation of the TCR in the absence of a second costimulatory signal not only fails to produce activation but is one method of inducing anergy, a long-term state of nonresponsiveness to the target antigen.2,18,19 This serves as a protection against inappropriate T-cell responses against self-antigens.16 Notably, T-cells thus stimulated remain in an anergic state long after the TCR stimulus is gone.2 If the anergic cell is exposed to IL-2 or stimulation of CD2 receptors when it next encounters its cognate antigen-MHC complex, the anergy may be reversed.8
Although many tumors present antigens recognized by T-cells, T-cell response is often attenuated by the lack of costimulatory ligands on tumor cells.20 Rather, initiation of a T-cell response requires presentation of tumor antigens by APCs capable of providing costimulation. Induction of B7 expression in tumor cells is therefore an attractive strategy for inducing a T-cell response to these tumors. Therapies capable of reversing anergy, returning T-cells to antigen-reactive states, also could theoretically be of benefit in treating cancer.21
Some Secondary Signaling Proteins Inhibit Effector Functions of T-Cells
Activated T-cells are extremely potent at destroying pathogen-infected host cells. Additional mechanisms are employed to further ensure that their activity is directed only toward appropriate targets and then "turned off" when infection is resolved. One such mechanism involves receptors that signal for T-cell inhibition, rather than activation. Such receptors often contain immunoreceptor tyrosine-based inhibitory motifs (ITIMs), which, when phosphorylated, recruit enzymes that block T-cell activation pathways.2
CTLA-4, one of the best understood T-cell inhibitory receptors, is expressed by both CD4+ and CD8+ T-cells. Following antigen-specific T-cell priming, the production and expression of CTLA-4 are increased. Although CTLA-4 mRNA transcription occurs fairly rapidly, expression of cell surface CTLA-4 is delayed, not reaching maximum levels until about 2 to 3 days after exposure to antigen.4,22,23 The CTLA-4 protein is highly homologous to CD28, and their genes colocalize to the same region of chromosome 2.24 However, CTLA-4 binds to B7 on the APC with about 20-fold greater affinity than does CD28.23,24 As shown in figure 2, CTLA-4 interaction with B7 results in: 2,8
- Downregulation of T-cell proliferation
- Blockade of IL-2 receptor expression
- Suppression of IL-2 production
- Inhibition of stimulatory signals from CD28 and the TCR
- Induction of anergy or apoptosis
GL1, expressed by activated B-cells, also serves as a ligand for CTLA-4.25
|
Figure 2. CTLA-4 Inhibits T-Cell Activation.
|
Click to Enlarge
|
(A) Stimulation of T-cells via TCR (by antigen/MHC) and CD28 (by B7-1 and B7-2) results in numerous biochemical signaling events, leading ultimately to clonal expansion and effector cell function.
(B) CTLA-4 competes for binding to B7-1 and B7-2, precluding CD28-mediated activating signals. Engagement of CTLA-4 also induces inhibitory signals, including changes in the spectrum of mitogen-activated protein kinases that are activated and recruitment of phosphatases that counteract the effects of the TCR-stimulated protein tyrosine kinases.
|
|
With permission from Lippincott Williams & Wilkins.2
|
The physiologic importance of CTLA-4 signaling to the immune system is best revealed by the fact that CTLA-4-deficient mice succumb at a relatively young age to a lymphoproliferative disorder characterized by dramatic enlargement of secondary lymphoid organs.2,26,27
The exact mechanism by which CTLA-4 downregulates T-cell responses remains poorly understood. One model suggests that CTLA-4 competes with CD28 binding, thereby increasing the TCR/CD28 threshold for T-cell activation.20,23 An alternate or additional pathway appears to involve direct signaling by CTLA-4 to actively dampen T-cell responses.2 In addition, CTLA-4 may function to limit proliferation of already activated T-cells, thus acting as an attenuation signal.20
Blockade of CTLA-4 is an attractive intervention for tumor immunology. Inhibition of CTLA-4 may augment the activity of existing tumor-reactive T-cells by reducing the necessary signal strength for activation and permitting expansion of already activated T-cells.20 In essence, this could provide greater numbers of antitumor high-affinity T-cells. This therapeutic approach is discussed in more detail in Chapter 4.
A fundamental concept of increasing importance in tumor immunology is the role of immunoregulatory cells. Studies have demonstrated the infiltration of CD4+CD25+FoxP3+ cells in a variety of solid tumor metastases.28-30 Activity of these regulatory T-cells may be dependent upon expression of X-linked forkhead/winged helix transcription factor FoxP3, disruption of which causes a lethal lymphoproliferative disorder in mice.31 In addition, regulatory T-cell subsets, specifically CD4+CD25+ cells, express high levels of CTLA-4, which has led to the question of whether blockade of CTLA-4 alters the function of regulatory cells. This mechanism of action of CTLA-4 on immune regulatory cells has been shown to some extent in animal models32 but effects on endogenous T-cells in cancer patients indicate that CTLA-4 blockade does not deplete CD4+FoxP3+ regulatory T-cells.33 Thus, effects of CTLA-4 are most likely mediated through its inhibition of effector T-cell activation.
Other less well-characterized inhibitory receptors have also been found on T-cells. For example, PD-1 binds to two members of the B7 family: PD-L1 and PD-L2. While PD-1 is known to have a T-cell inhibitory effect, the mechanism by which it does so is not yet clear.2 Dampening of immune response by PD-1 is a contributing factor in tumor evasion of destruction by the immune system. This is illustrated by the association between PD-L1 or PD-L2 expression in renal cell carcinoma, breast cancer, gastric carcinoma, bladder carcinoma, ovarian cancer, and pancreatic cancer and poorer prognosis.34 Immunotherapies that interfere with PD-1/PD-L1 interactions are being developed.21 Other receptors found to promote T-cell tolerance, and which therefore may be therapeutic targets, include B7-H3 and B7S1.18
Activation-Induced Cell Death
Finally, some other cell surface receptors orchestrate signaling cascades leading to apoptosis. For example, Fas (CD95), one of the best known "death receptors," is present at low levels in naive T-cells but is upregulated in activated cells. Binding of CD95 ligand (FasL) to these receptors leads to apoptosis of the activated T-cell if the cell has previously been primed by IL-2.35 This is an important mechanism for eliminating activated T-cells when they are no longer needed.2
In Summary
Positive and negative second signals play an important role in determining whether a T-cell becomes activated, anergic, or inhibited after encountering its cognate antigen-MHC complex. T-cell costimulation pathways offer numerous potential targets for development of immunotherapies to treat cancer. Some of the furthest along in clinical development are the CTLA-4 inhibitors. These therapies will be discussed in more detail in the remaining two chapters.
|
