21.3.6  Autoimmune Disease

Additionally, autoimmune diseases focused on other tissues can cause ocular complications. The mechanism underlying inflammation and immune pathologies is at least as complicated as angiogenesis. Additionally, like angiogenesis, there exists a rather large armamentarium of therapeutics currently in use. Here, we will consider some of the more common targets for immune therapy in mechanistic detail.

21.3.6.1  Pathophysiology

Lymphocyte Activation

Lymphocytes which mediate chronic autoimmune conditions such as Behcet’s disease or VKH only live for approximately 10 days to 2 weeks, requiring a constant source of new activated lymphocytes to maintain the autoimmune condition. Hence lymphocyte activation pathways have provided many useful targets for therapeutic intervention. T cell activation requires two signals. Signal 1 is antigen, which is presented by MHC class II on the surface of APC. Both antigen and MHC class II engage the T cell receptor (TCR). The TCR undergoes recombination during development to produce a population of approximately 108 T cells, each expressing several thousand copies of a unique TCR. Once the appropriate antigen is presented to the appropriate TCR, the TCR begin to cluster and generate a complex set of intracellular signals that result in the activation of kinase cascades such as the MAPK cascade as well as the activation of transcription factors such as NF-kB and NFAT (Podojil and Miller 2009). In the presence of signal 1 alone, the T cell generally does not activate, but rather, enters into a nonresponsive (anergic) state. If however, the APC also provides signal 2, the T cell undergoes activation. Signal 2 (also called co-stimulation) can be provided by several different receptors; however, the best characterized is the CD28 receptor which binds to the ligands B7.1 and B7.2. Typically, the APC engulfs an invading pathogen, and components of the pathogen activate a family of Toll-like receptors (TLR) found on and within the APC. The APC then upregulates the surface expression of B7.1 and B7.2. These provide context to the presentation of antigen to the T cell. Engagement of CD28 by the B7 ligands results in a second intracellular signaling cascade involving the activation of members of the rho-GTPase family along with the inhibition of the phosphatases Cbl-b (Podojil and Miller 2009). Normally, T cells do not activate in the presence of self-antigen because of a series of mechanisms collectively referred to as tolerance. The mechanism by which immune tolerance is broken leading to lymphocyte activation is beyond the scope of this chapter but have been recently reviewed (von Boehmer and Melchers 2010). Nevertheless, T cell activation in the context of autoimmune disease still requires both signal 1 and signal 2 and both have provided targets for therapeutic intervention. Once activated, the T cell enters the cell cycle. A progenitor population is established over the course of several weeks termed memory T cells and it is likely that this constitutes the population from which the continuous supply of effector T cells arises during the course of the autoimmune disease. The requirement for a constant supply of new lymphocytes is mirrored by a constant requirement for DNA synthesis. Lymphocytes are somewhat restricted in

their source of material for the biosynthesis of DNA components. Thus, blockade of DNA synthesis has also been used as a more general therapeutic target to downregulate the autoimmune process. At present the antigens driving autoimmune disease with ocular complications are not yet known and thus cannot be used to target the disease.

21.3.6.2  Therapeutics Either in Current Use or in Clinical Trials

The previous section has provided a short overview of the signal transduction events which occur during the activation of the T cell. In this section, we will cover the methods currently in use to block this process.

Corticosteroids:  As described earlier, corticosteroids inhibit the activity of the transcription­ factors NF-kB and AP-1. Upon TCR engagement the T cell activates these transcription factors and one of the genes that is immediately produced is interleukin-2 (IL-2). IL-2 is secreted and binds to receptors in an autocrine fashion. Engagement of the IL-2 receptor is an essential step in the T cell activation process and corticosteroids block this process by blocking the transcription of IL-2.

CD28 signaling:  Azathioprine has been used for decades as an immunosuppressive therapy for transplants (Calne 1969), autoimmune disease (Corley et al. 1966), and ocular disease (Perkins 1974). It is a mercaptopurine derivative that was originally thought to function by inhibiting purine ring biosynthesis (Lennard 1992). However, it has recently become apparent that the mechanism involves the blockade of co­ -stimulation via CD28. In essence, azathioprine has been shown to be converted to 6-thioguanine (6-TG), which in turn, is converted to 6-thio-GTP. This GTP analog­ can bind to GTPase proteins in place of GTP but will not allow activation. CD28 engagement normally results in the activation of the transcription factors NF-kB and STAT3 via the activation of the GTPase Rac1. By binding to Rac1, 6-thio-GTP blocks this activation pathway (Tiede et al. 2003; Tuosto et al. 2000).

TCR signaling:  Cyclosporin-A (CSA) is derived from fungi and acts to inhibit calcineurin­ . Calcineurin is required for the activation of T cells. It dephosphorylates and activates the transcription factor NFAT which is required to initiate the gene transcription program necessary for T cell activation. It is somewhat more specific than the antimetabolites and thus can be safer. CSA is considered a cytostatic agent rather than a cytotoxic agent. The consequence of this is that the patient is not permanently­ immunosuppressed. Additionally, its onset is rapid. For this reason, CSA is becoming a mainstay of treatment for ocular disease associated with Behcet’s disease when conservative therapies such as colchicine, corticosteroids, or AZA have failed (Evereklioglu 2005). CSA has also been used effectively in the treatment of uveitis (Nussenblatt et al. 1985) and scleritis (Hakin et al. 1991).

DNA synthesis inhibitors:  A number of researchers classify azathioprine as a DNA synthesis inhibitor; however, as described earlier, azathioprine is more likely to function as an inhibitor of CD28 signal transduction.