The Fascinating System of Eye-induced Immune Regulation

Historical Background

The immune privilege of the eye is a well-known but often oversimplified concept. Scientists first noticed the eye’s unusual immune properties in the 19th century. Van Dooremaal observed that mouse skin grafts survived much longer when transplanted into the anterior chamber (AC) of a dog’s eye.

Medawar later coined the term ocular “immune privilege”. He saw that foreign grafts in the AC survived far longer than similar grafts placed elsewhere in the body. Almost 30 years later, Kaplan et al. showed that alloantigenic cells placed in the AC actually escape the eye. These cells trigger a distinct immune response: serum alloantibodies form, while systemic cell-mediated immune responses are suppressed in an antigen-specific way.

Later studies in mice confirmed this effect, called AC-associated immune deviation (ACAID), and showed it is an important part of the eye’s immune privilege.

Figure 1. Organ systems involved in the induction of ACAID.
A brief description of the complex cellular-interplay, that causes ACAID (From Jerry Niederkorn’s review in Nature Immunology 7, 354 – 359;2006). Removal of the thymus, eye or spleen within 72 h of injection of antigen into the anterior chamber prevents the induction of ACAID. Chemical sympathectomy before anterior chamber injection of antigen also prevents the induction of ACAID. IL-, interleukin; BCR, B cell receptor. 

How ACAID Works

Several labs have confirmed that antigens placed in the AC trigger this distinct immune response. ACAID suppresses classical Th1 responses, such as delayed-type hypersensitivity (DTH) and complement-fixing antibodies. Still, it allows the mouse to generate non-complement-fixing IgG1 antibodies.

ACAID involves several cell types working together to create this antigen-specific immune suppression. Briefly: antigen injected into the ocular AC is taken up by circulating F4/80+ cells (a type of dendritic cell). These cells travel to the spleen and thymus.

Within 3 days of entering the thymus, these F4/80+ cells trigger the formation of CD4-CD8-NK1.1+ thymocytes. These cells are believed to enter the bloodstream and travel to the spleen, where they help generate splenic regulatory cells.

The spleen is the final organ involved in ACAID. There, several cell types interact – F4/80+ cells, natural killer T (NKT) cells, NK1.1 cells, gamma delta T cells, and B cells. Together, they produce CD4+ and CD8+ regulatory T cells (T regs) specific to the antigen originally injected into the AC. These T regs are the key drivers of the antigen-specific immune suppression.

ACAID may also occur in humans: people with acute retinal necrosis develop antibodies, but not cell-mediated immunity, against Varicella zoster.

Testing ACAID Against Autoimmune Disease

AC antigen injection induces different types of Tregs, and Tregs are already known to suppress autoimmunity. Building on this, Bhowmick et al. (2011) tested whether splenic regulatory T cells induced by an intracameral injection of MOG35-55 peptide could regulate MOG35-55-induced EAE (Experimental Autoimmune Encephalomyelitis). EAE is the animal model of human Multiple Sclerosis.

In this model, immunizing with MOG35-55 (a component of myelin protein) triggers an immune response against myelin. This causes inflammation and neurodegeneration similar to human Multiple Sclerosis. Bhowmick et al. found that injecting MOG35-55 peptide into the ocular AC could suppress MOG35-55-induced EAE, both:

• As a cure – when injected after disease onset
• As prevention – when injected before disease induction

This suppression was antigen-specific: injecting an unrelated antigen, such as ovalbumin, into the AC had no effect on EAE.

CD4 vs. CD8 Regulatory T Cells

The researchers then isolated the AC-injection-induced splenic regulatory T cells. Using adoptive transfer experiments, they found that:

• AC-induced CD4+ regulatory T cells could only suppress disease at an early stage (the priming phase)
• AC-induced CD8+ regulatory T cells could only suppress an already-established disease (the chronic phase)

Neither cell type was effective at the other’s stage.

This was likely the first study to distinguish between the priming and chronic phases of EAE. It also showed that different regulatory T cell populations can suppress autoimmune responses effectively at each stage. Additionally, it revealed that CD8+ regulatory T cell suppression of EAE depends on TGF-β, while CD4+ T reg suppression does not use TGF-β.

Distinct Suppression Mechanisms

Earlier studies established that ACAID-CD4 Treg cells do not express FoxP3, and that ACAID’s regulatory T cell induction and activity are independent of CD4+FoxP3+ regulatory T cells.

AC-induced CD8+ regulatory cells suppress IFN-γ production both in vitro and in vivo. They also suppress the T cells responsible for a DTH reaction in immunized mice. These CD8+ regulatory cells are restricted by Qa-1 antigens on effector T cells. The non-classical MHC class I molecule Qa-1 is only expressed on activated cells. Because of this, AC-induced CD8+ regulatory T cells specifically suppress activated T cells – meaning they target the effector, or chronic, phase of an autoimmune disease like EAE.

In short, ACAID suppresses both the induction and the activity of effector T cells, through distinct populations of regulatory T cells. Recently, researchers showed that Type II collagen (CII) – a key antigen in Rheumatoid Arthritis – can induce a similar CII-specific immune suppression via ACAID (Farooq et al., 2012). This opens up the possibility of testing this system in an arthritis model.

Implications for Autoimmune Disease Treatment

Together, this data suggests that adoptive transfer of regulatory T cells may only suppress an ongoing autoimmune disease in one case: those regulatory T cells must be specific to the disease-causing antigen. Alternatively, transferring polyclonal regulatory T cells may only cure ongoing disease in lymphopenic hosts. In these hosts, massive expansion of regulatory T cells could generate enough antigen-specific regulatory T cells.

If this also holds true in humans, it could be a significant limitation for using regulatory T cells clinically in autoimmune disease. That’s because human self-antigen-specific regulatory T cells have not yet been successfully expanded outside the body. This is where Anterior Chamber Associated Immune Deviation (ACAID) could be especially useful, since it can generate antigen-specific CD8+ and CD4+ regulatory T cells.

Arijit Bhowmick is currently a postdoctoral researcher at the Immunology institute of the Mount Sinai Medical Center, NY. He received his PhD in structural immunology from the National Institute of Immunology, New Delhi. His current research interests encompass autoimmunity, Th17 cells and structure based inhibitor designing.