Ординатура / Офтальмология / Английские материалы / Ocular Periphery and Disorders_Dartt, Bex, Amore_2011

The author suggests that these inflammatory processes normally are prevented by the same strategy that prevents immune responses to food antigens, soluble autoantigens, and cellular debris. There may be particular challenges to maintaining immunoregulation in the lacrimal glands, where vigorous traffic through the transcytotic apparatus releases an exceptionally heavy burden of potential autoantigens into the stromal space, and in the ocular surface tissues, where environmental stresses may induce release of inflammatory mediators that abrogate the local signaling milieu’s ability to generate regulatory antigen presenting cells. Additional challenges may result from the epithelia’s dependence on systemic hormones to maintain expression of the local signaling milieu.

See also: Adaptive Immune System and the Eye: T CellMediated Immunity; Conjunctiva Immune Surveillance; Defense Mechanisms of Tears and Ocular Surface; Lacrimal Gland Overview; Tear Film.

Further Reading

Brandtzaeg, P. and Johansen, F.-E. (2005). Mucosal B cells: Phenotypic characteristics, transcriptional regulation, and homing properties. Immunological Reviews 206: 32–63.

Cain, C. and Phillips, T. E. (2008). Developmental changes in conjunctiva-associated lymphoid tissue of the rabbit. Investigative Ophthalmology and Visual Science 49: 644–649.

Evans, E., Zhang, W., Jerdeva, G., et al. (2008). Direct interaction between Rab3D and the polymeric immunoglobulin receptor and trafficking through regulated secretory vesicles in lacrimal

gland acinar cells. American Journal of Physiology. Cell Physiology

294: C662–C674.

Franklin, R. M, Kenyon, K. R., and Tomasi, T. B. (1973). Immunohistologic studies of human lacrimal gland: Localization of

immunoglobulins, secretory component and lactoferrin. Journal of Immunology 110: 984–992.

Kaetzel, C. S. (2005). The polymeric immunoglobulin receptor: Bridging innate and adaptive immune responses at mucosal surfaces.

Immunological Reviews 206: 83–99.

Kim, H., Fariss, R. N., Zhang, C., et al. (2008). Mapping of the neonatal Fc receptor in the rodent eye. Investigative Ophthalmology and Visual Science 49: 2025–2029.

Knop, E., Knop, N., and Claus, P. (2008). Local production of secretory IgA in the eye-associated lymphoid tissue (EALT) of the normal human ocular surface. Investigative Ophthalmology and Visual Science 49: 2322–2329.

Macpherson, A. J., McCoy, K. D., Johansen, F.-E., and Brandtzaeg, P. (2008). The immune geography of IgA induction and function.

Mucosal Immunology 1: 11–22.

Meagher, C. K., Liu, H., Moore, C. P., and Phillips, T. E. (2005). Conjunctival M cells selectively bind and translocate Maackia amurensis leukoagglutinin. Experimental Eye Research

80: 545–553.

Mircheff, A. K., Wang, Y., de Saint Jean, M., Ding, C., and Schechter, J. E. (2007). Lacrimal epithelium mediates hormonal influences on APC and lymphocyte cycles in the ocular surface system. In: Zierhut, M., Rammensee, H. G., and Streilein, J. W. (eds.) Antigen Presenting Cells and the Eye, pp. 93–119. New York: Informa.

Rose, C. M., Qian, L., Hakim, L., et al. (2005). Accumulation of catalytically active proteases in lacrimal gland acinar cell endosomes during chronic ex vivo muscarinic receptor stimulation.

Scandinavian Journal of Immunology 61: 36–50.

Seder, R. A., Marth, T., Sieve, M. C., et al. (1998). Factors involved in the differentiation of TGF-beta-producing cells from naive CD4+

T cells: IL-4 and IFN-g have opposing effects, while TGF-b positively regulates its own production. Journal of Immunology 160: 5719–5728.

Spiekermann, G. M., Finn, P. W., Ward, E. S., et al. (2002). Receptor-mediated immunoglobulin G transport across mucosal barriers in adult life: Functional expression of FcRn in the mammalian lung. Journal of Experimental Medicine

196: 303–310.

Wang, Y., Chiu, C. T., Nakamura, T., et al. (2007). Traffic of endogenous, over-expressed, and endocytosed prolactin in rabbit lacrimal acinar cells. Experimental Eye Research 85: 749–761.

Weiner, H. L. (2001). Induction and mechanism of action of transforming growth factor-b-secreting Th3 regulatory cells. Immunological Reviews 182: 207–214.

Glossary

Antigen – A molecule recognized by receptors expressed by T cells (TCR) and B cells (immunoglobulin).

Complement – A system of serum proteins and cell surface proteins that combine to form the membrane attack complex that perforates cell membranes.

Component proteins of the complement cascade have enzymatic activity and generate inflammatory mediators.

Cytokines – Soluble molecules that modulate immune responses.

Inflammation – The hallmark of an immune response characterized by edema, redness, and pain. Inflammatory mediators induce vasodilation of local vessels which are leaky and promote edema and facilitate immune cell infiltration. While inflammation controls infection, it can also cause tissue damage, as the effector function of the innate immune response is nonspecific.

Major histocompatibility complex (MHC) – Highly polymorphic molecule expressed on the cell surface that presents self and foreign peptides for T-cell recognition through the TCR.

Opsonization – The coating of cell surfaces with molecules that facilitate phagocytosis. Phagocytosis – The process of pathogen engulfment through recognition of surface receptors.

Polymorphonuclear leukocytes (PMN) – Cells of the innate immune response characterized by a multilobed nucleus including neutrophils and eosinophils.

Signal transduction – The process by which signals received by the cell are converted into gene expression within the nucleus.

The human body and the microbes (bacteria, viruses, and parasites) that reside within it share the same simple mandate: to survive and to reproduce. The ground rules of the host are clear. The host–microbe relationship cannot cause damage that compromises the health of the host. When health is compromised, the host immune system eliminates or controls the growth of these unruly microbes, now termed pathogens. Immune responses in the eye are critical for protection from pathogens. For example, immunesuppressed HIVþ patients are more susceptible to ocular

infection. However, ocular immune responses also pose a threat as inflammation can damage the delicate microanatomy of the eye and compromise vision. Examples of blinding inflammation include viral, bacterial, and parasitic keratitis, retinitis, bacterial endophthalmitis, and autoimmune uveitis, which are reviewed elsewhere in this encyclopedia. As an evolutionary adaptation to preserve vision, immune responses in the eye are normally tightly regulated to control pathogens while minimizing inflammation.

Innate and Adaptive Immunity

The two major arms of the immune system, innate and adaptive immunity, can be distinguished by the unique molecular structures they employ to recognize molecules expressed by pathogens or expressed by a host cell in response to a pathogen. The innate immune response utilizes a limited set of pathogen recognition receptors (PRRs) to detect pathogen-associated molecular patterns (PAMPs), which are conserved pathogen products. There are PRRs that recognize PAMPs expressed by all major classes of microbes for example: toll-like receptor (TLR) 4 recognizes lippopolysaccharide (LPS) of Gram-negative bacteria; TLR3 recognizes double-stranded RNA expressed by viruses; and TLR11 recognizes proteins expressed by parasites. PRRs can also recognize molecules associated with pathogen-induced cell death. For example, RAGE (receptor of advance glycation end products), TLR 2, 4, and 9 recognize nonoxidized high mobility group box 1 (HMGB1), which is released by normal cells upon necrotic but not apoptotic cell death. PRRs are expressed on epithelial cells at sites of microbial entry and on cells of the innate immune response which include: macrophages, dendritic cells (DC), and polymorphonuclear leukocytes (PMN).

Pathogen recognition by the adaptive immune response is not predetermined by a limited set of receptors. Rather, a random process of gene segment rearrangement by recombinase activation genes (Rag-1 and Rag-2) generates over a billion different receptors tailored to recognize essentially any molecule. Cells of the adaptive immune response include B cells, which express immunoglobulin (Ig) molecules and T cells that express T-cell receptors (TCRs). Individual B or T cells express receptors with a single specificity creating a repertoire of many B and T cells each with different specificities. B cells mediate humoral immunity through production of secreted Ig molecules (antibodies) which directly recognize

unique shapes, and conformations of pathogen-associated molecules including proteins, carbohydrates, lipids, nucleic acids, and simple chemical groups. Cellular immunity is mediated by T cells through TCR recognition of pathogen-associated proteins processed into peptides and presented on the cell surface by the major histocompatibility complex (MHC). These differences in antigen recognition specialize humoral immunity for defense against extracellular pathogens and cell-mediated immunity for defense against intracellular pathogens. The focus of this article is a general overview of T-cell- mediated immunity introducing how T-cell responses are regulated in the eye and conditions where ocular T-cell responses cause immunopathology.

Antigen Trafficking, Processing, and Presentation to T cells

The lymphatic system drains extracellular fluids through a network of collection channels and vessels that ultimately empty into the bloodstream through the thoracic duct. Interspersed between lymphatic vessels are lymph nodes that are filled with immune cells. This strategic placement of an immune organ within a drainage pathway allows the immune system to survey for foreign antigens contained within extracellular fluids from different regions of the body. For example, the eye is drained to cervical lymph nodes within the neck whereas the arms drain to axillary and brachial lymph nodes. Internal organs also have lymphatic drainage: lungs drain to mediastinal lymph nodes and intestine drains to mesenteric lymph nodes.

T cells, which have not encountered their antigen, are found in the lymph nodes, spleen, and blood but not in nonlymphoid tissues, such as the eye. Therefore, these naive T cells are not the first responders to an ocular pathogen. Rather, innate immunity is the first line of defense. Recognition of PAMPs through PRRs induces rapid expression of inflammatory cytokines (IL (interleu- kin)-1, IL-6, and tumor necrosis factor (TNF)) that increase the production of chemokines, such as IL-8 and acute phase proteins (e.g., C-reactive protein). Chemokines promote the migration of immune cells to the eye while acute phase proteins opsonize microbial surfaces where they activate complement and promote phagocytosis by macrophages and PMN. To control the pathogen, infiltrating PMN and macrophages produce reactive oxygen species, including superoxide which can be transformed into H2O2 by superoxide dismutase. H2O2 and chloride are substrates for myeloperoxidase which generates antimicrobial hypochlorous acid, the active ingredient of bleach. PMN and macrophages also release granules containing antimicrobial molecules and generate nitric oxide (NO) that reacts with superoxide to form the potent oxidant peroxynitrite. In addition to controlling the pathogen, DCs and macrophages which have internalized pathogens

and/or pathogen products migrate to the lymph nodes through lymphatic vessels to activate pathogen-specific T cells.

Adaptive immune responses depend on innate immune responses, as T cells only recognize cell-associated antigens. Hence, antigens that flow freely into the lymph node or that are carried by DCs and macrophages from the pathogen site have to be processed and presented by antigen-presenting cells (APCs) for recognition by T cells. As a general rule, pathogen-expressed proteins from the extracellular milieu are digested by phagocytic APC and presented through MHC class II molecules for recognition by CD4þ Tcells, whereas intracellular pathogen-expressed proteins are processed and presented through MHC class I molecules for recognition by CD8þ T cells (Figure 1). However, certain types of APC, for example CD8aþ CD11cþ DCs from mice, cross present extracellular proteins through MHC class I molecules.

T-cell activation

Very few T cells within the entire T-cell repertoire are specific for any one particular antigen. Therefore, to most effectively respond to a pathogen, these antigen-specific T cells undergo cell division to expand their numbers. This process of clonal expansion takes time, which explains why adaptive immune responses lag behind innate immune responses.

The activation of T cells to undergo proliferation results from several signals delivered from the APC. These signals initiate and sustain signal transduction cascades culminating in the transcription of genes necessary for T-cell proliferation, most importantly IL-2 (Figure 2). Transducing signals from the APC to T cells occurs by phosphorylation and dephosphorylation of intracellular substrates by kinases and phosphatases, respectively. Signal one is delivered by TCR engagement of MHC þ peptide complexes, which results in a conformational change in the TCR that makes immunoreceptor tyrosine-based activation motifs (ITAMs) accessible on chains of the CD3 complex which is comprised of five distinct molecules (g, d, e, z, and Z). The CD3 complex is associated with the TCR and is necessary for signal transduction upon TCR engagement, as the TCR has a short cytoplasmic domain incapable of signaling. Upon

TCR antigen recognition, CD3 ITAMS are phosphoylated by p56lck, and p59fyn kinases, then bound by ZAP-70 ((CD3)

zeta chain-associated protein of 70 kDa) kinase, which activates phospholipase Cg1 (PLCg1). PLCg1 is an enzyme which catabolizes membrane-bound phosphatidlyinositol-1, 4-bisphosphate (PIP2) into inositol-1,4,5-triphosphate (IP3) and diacylglcerol (DAG). IP3 induces an intracellular calcium increase which then activates the serine phosphatase calcineurin to dephosphorylate nuclear factor of activated T cells (NFAT). Dephosphorlated NFAT leaves the cytosol to enter the nucleus. DAG activates protein kinase C (PKC) initiating another cascade leading to degradation of the

inhibitor of NFkB (ikB) and subsequent translocation of NFkB from the cytosol to the nucleus. DAG also activates the Ras pathway, which through mitogen-activated protein kinase (MAPK) leads to formation and activation of the activator protein-1 (AP-1) molecule. NFAT, NFkB, and AP-1 are all transcription factors that cooperate to induce transcription of IL-2 and other T-cell activation genes.

Signal one alone, however, is not sufficient to induce complete T-cell activation.

Signal two is delivered by invariant molecules expressed on the cell surface of APC that are upregulated after PRR engagement of PAMPs. The best characterized of these co-stimulatory molecules are CD80 (B7-1) and CD86 (B7-2), which bind to CD28 on T cells. CD28

engagement further activates the Ras/MAPK pathway to promote IL-2 production by making the IL-2 promoter accessible to transcription factors by chromatin remodeling, and by stabilizing IL-2 mRNA. Immediately, T-cell proliferation ensues.

The benefit of generating T cells with TCR capable of recognizing a tremendously diverse array of antigens carries the consequence that the same process generates TCR that recognize self molecules. While the most dangerous self-reactive T cells are deleted during T-cell development in the thymus by a process referred to as negative selection, many T cells with cross reactivity to both foreign and selfantigens escape negative selection. The requirement for two signals to induce T-cell activation may have developed to ensure that these cross-reactive T cells are activated only under appropriate circumstances; for example, when foreign antigen is encountered. During normal physiological conditions, APC that present self-antigens will not be activated by PAMPs to upregulate co-stimulatory molecules. Hence, signal one but not signal two will be delivered. Signaling through the TCR alone leads to long-lived T-cell unresponsiveness referred to as anergy, which is characterized by inhibited expression of IL-2. Therefore, anergy induction protects the body from the generation of autoimmunity while preserving the most extensive repertoire of TCR.

The current paradigm for anergy induction suggests that signal one induces NFAT activation with only partial AP-1 activation causing a signaling imbalance that leads to the expression of anergy-inducing genes, such as diacylglycerol kinase alpha (DGKa) (Figure 2(a)). DGKa phosphorylates DAG promoting its degradation and further decreasing activation of the Ras/MAPK and PKC pathways. Ultimately, the IL-2 promoter becomes inaccessible to transcription factors due to chromatin remodeling, and as a result, the cell becomes anergic. Anergy induction also involves T-cell expression of inhibitory co-stimulatory molecules, such as cytotoxic T-lymphocyte antigen 4 (CTLA-4), which antagonize activating co-stimulatory molecules.

T cell differentiation and effector function

Upon activation, T cells receive further instructions in the form of cytokines that promote differentiation that is best suited to respond to the offending pathogen. For example, CD4þ T helper (Th) responses are directed toward at least four effector lineages: Th1, Th2, Th17, and adaptive T regulatory cells (Treg) by exposure to particular cytokines IL-12/ interferong (IFNg), IL-4, transforming growth factor-b (TGF-b)/IL-6, or TGF-b, respectively (Figure 3). CD8þ T cells differentiate into cytolytic T lymphocytes (CTLs) by exposure to IL-12 and express granzymes and perforin. CD8þ T cells also produce cytokines and similar to Th cells are directed toward four effector lineages: Tc1, Tc2, Tc17, and Treg by exposure

to IL-12/IFNg, IL-4, TGF-b/IL-6, or TGF-b. PAMPs directly or indirectly induce the expression of these polarizing cytokines. For example, bacterial LPS and viral RNA induce DC expression of IL-12. The cellular source of IL-4 and TGF-b have not been defined. IL-6 is commonly expressed in inflammatory environments. These critical cytokines induce or repress the expression of key transcription factors. Th1 cells and CTL express T-bet while suppressing Gata-3 which is critical for Th2 development. Th17 cells do not express T-bet or Gata-3, but express retinoic acid-related orphan receptorgt (RORgt) which is fundamental for their development. Forkhead box P3 (FoxP3) is a signature transcription factor for Tregs. Each transcription factor promotes specific expression of subset-specific genes: Th1/Tc1 cells express IFNg, Th2/Tc2 cells produce IL-4, 5, and 13, Th17/Tc17 cells produce IL-17, and Treg cells produce TGF-b.

The primary effector function of Th cells is to provide T cell help through co-stimulatory molecule expression and cytokine production, which orchestrates the immune response by inducing: T and B cell proliferation, Ig heavy chain class switching in B cells, and activation of innate cells. T follicular helper cells (Tfh) are another recently defined Th effector lineage characterized by expression of the chemokine receptor (CXCR5) and localization to B cell follicles within lymphoid tissues. Tfh cells promote B cell proliferation by production of IL-21. Activated B cells initially express IgM heavy chain molecules but upon engagement of Th cells through TCR and CD40L interactions, B cells change their Ig heavy chain isotype in response to Th expressed cytokines. For example, Th1-expressed IFNg or Th2 expressed IL-4 promotes switching to IgG or IgE isotypes, respectively. Antibodies opsonize pathogen surfaces to: neutralize toxins, inhibit pathogen invasion by blocking cell surface receptors critical for binding host cells, and activate complement. In addition, antibody heavy chain isotypes are bound by Fc receptors expressed by innate cells to mediate antibody-dependent cellular cytotoxicity (ADCC). Antibody engagement of Fc receptors expressed by neutrophils (FcgRI), macrophages (FcgRI), NK cells (FcgRIII), and eosinophils (FceR1), induces the release of lytic granules leading to targeted cell lysis. IgG molecules mediate ADCC through macrophages, neutrophils, and NK cells, whereas IgE molecules mediate ADCC through eosinophils.

Each Th lineage displays very different effector functions with clear protective and pathological consequences. Th1 cells mediate type IV delayed-type hypersensitivity responses (DTH) through production of IFNg, which activates macrophages to increase their phagocytic activity and induce expression of TNF, NO, and reactive oxygen species. Hence, Th1 infiltration of the pathogen site amplifies inflammation initiated by PAMPs by further activation of macrophages. Th2 cells produce IL-4 and

IL-21 CD4+

Tfh

TGFβ CD4+

Treg

(a)

? ?

CD4+

Th

β TGF

FoxP3

CD4+ IFNγ

Th1

CD4+ IL-4, IL-5, IL-13

Th2

CD4+ IL-17

Th17

β TGF

CD8+

CTLp

CD8+ IL-4

Tc2

IL-5, which control extracellular helminthic infections by activating eosinophils. The recently described Th17 subset produces IL-17 and has been implicated in the response against a wide variety of pathogens requiring a strong inflammatory response predominated by neutrophils. The nonspecific effector mechanisms elicited by macrophages, neutrophils, and eosinophils are associated with significant tissue damage. In contrast, Treg cells inhibit immune responses by expression of CTLA-4 and/or by the production of immunosuppressive cytokines such as TGF-b and IL-10, which inhibits T-cell proliferation and decreases co-stimulatory molecule and cytokine expression of APC. Tregs also suppress cytokine production and lytic activity of T cells, at sites of inflammation.

CD4þ T helper cells are also critical for the generation of long-lived memory CD8þ T cells that respond quickly and efficiently upon secondary exposure to antigen. CD8þ T cells that expand in numbers to a pathogen in

the absence of CD4þ T cells rapidly undergo activationinduced cell death (apoptosis) through TRAIL/TRAIL receptor interactions upon secondary exposure to antigen. In addition, memory CD8þ T cells generated in the presence of CD4þ Th cells are not maintained if transferred to mice deficient in CD4þ T cells, indicating that Th cells are somehow necessary to maintain long-lived memory CD8þ T cells in vivo.

The primary effector function of CD8þ T cells is control of intracellular pathogens. CD8þ T cells differentiate into lytic effectors (CTL) which are characterized by expression of perforin and granzymes. CTL target infected cells by their expression of pathogen peptides presented on the cell surface by MHC class I molecules. Perforin molecules form pores in cell membranes which facilitate transfer of granzymes into the cytoplasm that activate caspases to induce apoptosis. Interestingly, granzyme B can also inhibit herpes simplex virus-1 (HSV-1) replication within latently infected neurons without

inducing apoptosis by caspase-mediated cleavage of viral proteins. Whether the neuron or the HSV-1 infection prevents apoptosis under this circumstance remains to be determined. CD8þ CTL also produce IFNg, which inactivates viral proteins and activates innate cells as previously mentioned.

Differentiation of T-cell effectors is coincident with the differentiation of long-lived memory T cells. Memory T cells are divided into central (Tc) and effector (Te) memory subpopulations based on their location and unique expression of cell surface molecules. Tc memory cells localize to lymph nodes and spleen through expression of the chemokine receptor7 (CCR7) and CD62L, whereas Te memory cells are present in nonlymphoid tissues and do not express these molecules. Several cell surface molecules have been useful in distinguishing naı¨ve T cells from memory and effector T-cell populations. For example, naı¨ve T cells express low levels of CD44 and killer cell lectin-like receptor G1 (KLRG1) and high levels of the IL-7 receptor (CD127). In contrast, both effector and memory T-cell populations express high levels of CD44. The retention or reexpression of CD127 along with low expression of KLRG1 distinguishes memory T cells from CD127 low KLRG1 hi T cell effectors.

The differentiation of effector and memory T cells from a single naı¨ve T-cell precursor is an area of active research and several models are being tested. Lineage determination could involve dedifferentiation of effector cells. Another model suggests that the strength of signal delivered by APC to T cells may favor a particular lineage. For example, T effector generation may be favored when inflammation is high, whereas T memory generation may be favored when inflammation is low. Inverse regulation of T-bet and eomesodermin transcription factors by IL-12 expressed during inflammation may be critical for lineage determination. Alternatively, lineage commitment may be determined upon the first cell division by asymmetric distribution of critical lineage determining molecules to daughter cells.

T-cell immune responses in the eye

The eye was characterized as a site of immune privilege by the Nobel laureate Peter Medawar in 1948 based on the observation that foreign tissue transplanted in the anterior chamber persisted indefinitely whereas the same tissue transplanted in the skin was rapidly rejected by the host immune response. Based on the absence of demonstrable lymphatic drainage of the interior of the eye, Medawar concluded that ocular antigens were sequestered and the immune system was ignorant of their presence. However, subsequent investigations found that Medawar was incorrect. A reevaluation of ocular immune privilege almost 30 years later by Kaplan and Streilein indicated that soluble and cell-associated antigens injected into the anterior

chamber escaped from the eye and induced systemic T-cell nonresponsiveness. Upon secondary exposure to the same antigen under immunogenic conditions, such as antigen administration with adjuvant, the magnitude of Th1 or Th2 type cell-mediated immune responses was markedly reduced in comparison to mice in which the first exposure to antigen was through a nonprivileged route (skin or conjunctiva of the eye). This phenomenon, termed anterior chamber-associated immune deviation (ACAID), is mediated by CD4þ Tregs, which inhibit the induction of T-cell effector responses and CD8þ Tregs, which inhibit the expression of T-cell effector responses specific for ocular antigens. ACAID may contribute to the tremendous success of corneal transplantation that does not routinely require donor/recipient MHC matching or systemic immunosuppression because mice that accept corneal allografts demonstrate reduced DTH responses to donor antigens.

Ocular immune privilege is also maintained by soluble molecules contained within the aqueous humor and by cellassociated molecules expressed on tissues lining the anterior chamber, which are immunosuppressive (Table 1). These molecules directly inhibit T-cell effector function, convert T effectors into Treg, or inhibit the activation of

Table 1 Immune suppressive factors that maintain ocular immune privilege

innate cells. These barriers to T-cell responses are thought to represent a necessary compromise to limit tissue damage during pathogen control. The inhibition of the adaptive immune response would suggest that the eye would be more susceptible to infection. However, this is not the case. Rather, the eye is more prone to immunopathology when T-cell responses break immune privilege to control an ocular pathogen.

Several ocular infections induce T-cell responses that promote immunopathology. Viral (HSV-1) and bacterial (Pseudomonas aeruginosa) infections of the cornea induce CD4þ Th1 cells that infiltrate the cornea and orchestrate neutrophil infiltration leading to keratitis. Parasitic Onchocerca volvulus infections induce Th2 responses that contribute but are not alone sufficient to induce keratitis. Interestingly, endosymbiotic Wolbachia bacteria activate TLRs to induce infiltration of the cornea by neutrophils, which are the primary mediators of inflammation. Th17 and Th1 cells have been shown to contribute to immunopathology in experimental rodent models of uveitis. CD4þ Th cells are also critical in the rejection of corneal allografts through induction of a classic DTH response in the cornea and mediate lacrimal keratoconjunctivitis in mice exposed to dessicating stress.

The factors that break immune privilege are not completely understood. However, changes in immunoregulatory elements due to inflammation must be involved. Interestingly, in animal studies, the type but not intensity of ocular inflammation has been shown to be critical in breaking immune privilege. For example, the induction of

ACAID is preserved in mice with LPS-induced uveitis but not in mice with inflammatory pigmentary glaucoma, although neutrophil inflammation is more pronounced in uveitic mice. Differences in the duration of inflammation may distinguish whether privilege is broken, as LPSinduced uveitis resolves within days whereas glaucoma persists for months. Identification of the regulatory factors affected by pathogens that break immune privilege will be critical for our understanding of how T-cell responses to ocular antigens are modulated to promote the elimination of ocular pathogens while producing minimal tissue damage.

See also: Immunobiology of Acanthamoeba Keratitis; Immunopathogenesis of Onchocerciasis (River Blindness); Immunopathogenesis of Pseudomonas Keratitis; Pathogenesis of Fungal Keratitis.

Further Reading

Abbas, A. K. and Lichtman, A. H. (2009). Basic Immunology: Functions and Disorders of the Immune System, 3rd edn. Philadelphia, PA: Saunders/Elsevier.

Forrester, J. V., Dick, A. D., McMenamin, P. G., and Lee, W. R. (2008).

The Eye: Basic Sciences in Practice, 3rd edn. Philadelphia, PA: Saunders/Elsevier.

Niederkorn, J. Y. and Kaplan, H. J. (2007). Immune Response and the Eye, Chemical Immunology and Allergy, vol. 92, 2nd, rev. edn. Basel: Karger.

Paul, W. E. (2008). Fundamental Immunology, 6th edn. Philadelphia, PA: Wolters Kulwer/Lippincot Williams and Wilkins.

Glossary

Endocytosis – The process by which cells absorb molecules, such as proteins, from outside the cell by engulfing them with their cell membrane to form an endosome.

Endophthalmitis – An infection of the posterior of the eye.

Opsonization – The process by which a pathogen or infected cell is marked for destruction by a phagocyte.

Phagocytosis – The engulfment of solid particles, such as bacteria, by the cell membrane to form an internal phagosome.

Introduction

Innate immunity comprises a large number of molecules and cells that recognize and respond rapidly to pathogens, providing immediate defense against infection. However, innate immunity also carries with it the potential of highly destructive inflammation that presents an important dilemma for the eye. Inflammation is necessary for successfully eradicating pathogens. An ideal response would eliminate the microorganisms before they are able to directly damage any ocular tissues. The innate immunity would be limited and produce little or no damage to the surrounding normal tissues. However, some types of ocular infections trigger inflammation that is either (1) insufficient to clear the microorganisms, resulting in direct destruction of ocular tissue by the pathogen, or (2) excessive inflammation that clears the microorganisms, but destroys a significant amount of normal tissue. Either of these two scenarios is undesirable and can lead to significant loss of vision. Therefore, a delicate balance must be achieved between the amount of inflammation required for pathogen clearance and the amount of nonspecific tissue damage.

The innate immune system of the eye is similar to other mucosal surfaces. The first tier is passive consisting of several anatomic, physical, and chemical barriers that work together to prevent infection without inducing inflammation. The second tier is active consisting of cellular and secretory components that together cause acute inflammation aimed at eradicating the pathogen. The delicate

tissues of the eye that make up the visual axis (cornea, lens, and retina) have a very low tolerance for inflammation, as a very small amount of damage can produce a significant loss of vision. The two-tiered system helps to prevent unnecessary inflammation and the active mechanisms of innate immunity are only turned on once the passive barriers have been breached. Both the passive and active arms of ocular innate immunity are the focus of this article.

Passive Innate Defense System

Anatomic and Physical Barriers

Several anatomic and physical barriers protect the anterior and posterior of the eye from invading pathogens (Figure 1). The active arm of innate immunity is only triggered when pathogens breach these barriers. The cornea is exposed to the external environment, making the anterior segment highly vulnerable to potential pathogen invasion. Therefore, the anterior segment possesses a multilayer barrier system that includes: eyelids and eyelashes, tear film, and the corneal epithelium. By contrast, the posterior segment is not exposed to the external environment, and is therefore less vulnerable to infection. The critical barriers of the posterior segment include

(1)the retinal pigment epithelium (RPE), which lies between the blood-rich choroid and the neural retina, and

(2)the posterior lens capsule that forms the barrier between the anterior and posterior segments. Each component of the passive innate defense system is described briefly below.

Eyelids and eyelashes

The outermost barrier of the ocular surface consists of the eyelids and eyelashes. The eyelashes protect the ocular surface from dust and foreign debris. The regular blinking action of the eyelids moves the tears across the ocular surface, washing away potentially colonizing or infecting organisms.

Tear film

Tears form the second barrier, lubricating and protecting the ocular surface. Tears also posses a potent defense system that limits the growth, colonization, and survival of microorganisms. The tear film consists of three layers: the outermost lipid layer, an aqueous layer, and the inner mucus layer (Figure 2). The lipid layer lubricates the eyelid and slows evaporation of the aqueous tear film layer. The aqueous layer forms the major component of the tear film and contains numerous antimicrobial

Bowman’s

membrane Stroma

Descemet’s membrane

Endothelium

Eye lid and eye lashes

Corneal epithelium

Posterior lens capsule

Figure 1 Anatomic and physical barriers of the eye. The eyelid, eye lashes, tear film, and corneal epithelium serve as barriers of the anterior segment of the eye. The posterior lens capsule and RPE serve as barriers of the posterior segment of the eye.

proteins including: lysozyme, lactoferrin, defensins, secretory IgA (sIgA), and complement. Many of these antimicrobial proteins are constitutively expressed and provide early, broad-spectrum protection against invading pathogens and also prevent the overgrowth of commensal bacteria. The innermost mucus layer of the tear film is made up of secreted and membrane-bound mucins that protect the epithelium from debris, pathogens, and desiccation. Mucins are high-molecular weight glycoproteins characterized by extensive O-glycosylation. Membrane-bound mucins expressed by the ocular surface epithelia include MUC1, MUC4, and MUC16. Secreted mucins are also found in the mucus layer and include MUC2, MUC5AC, and MUC19. The membrane-bound mucins anchor the ocular tear film to the corneal epithelium and are thought to act as a physical barrier against pathogen penetrance. Secreted mucins bind to pathogens in the tear film, facilitating their clearance from the ocular surface. Under normal conditions, mucin production and secretion by goblet cells and corneal epithelial cells are constitutive. However, mucin production can also be induced via Tolllike receptors (TLRs) expressed on the surface of corneal epithelial cells. Moreover, inflammatory cytokines, such as IL-1b, IL-6, and TNFa, have also been shown to induce mucin production and secretion. Together, these data reveal that constitutively expressed mucins make up

a critical component of the passive defense system, while at the same time, upregulation of mucin production and secretion can also be a product of the active arm of innate immunity in the eye.

Corneal epithelium

The final barrier of the ocular surface consists of nonkeratinized stratified epithelial cells bound together by tight junctions. The corneal epithelium acts as a physical barrier to invasion of microorganisms due to the presence of epithelial intercellular tight junctions and the rapid renewal of epithelial cells with frequent shedding of the superficial layers of potentially infected epithelium. As mentioned in the previous section, the epithelium also expresses membrane-bound mucins that inhibit bacterial binding to the epithelial surface and produce several of the antimicrobial factors that are present in the ocular tear film.

Posterior lens capsule

The posterior lens capsule forms a physical barrier between the anterior and posterior segments of the eye after extracapsular cataract surgery and prevents the spread of microorganisms from the anterior chamber into the posterior chamber in the postsurgical eye. The best example of this is the fact that an intact posterior lens capsule is critical in preventing endophthalmitis following cataract surgery. Contamination of the aqueous humor can occur during cataract surgery. However, the pathogens are quickly cleared and endophthalmitis does not develop. By contrast, when the posterior capsule is breached, the rate of endophthalmitis increases significantly. This supports the finding that the anterior segment is much more efficient at clearing bacteria as compared to the posterior segment. Studies suggest the difference in ability to clear pathogens in the anterior versus posterior of the eye may be linked to expression of antimicrobial peptides. One major difference is that the AH is continuously secreted and drained, whereas the vitreous humor is not. The vitreous also offers greater opportunity for microbes to bind its fibrils. However, the exact molecular mechanisms involved remain unclear.

Retinal pigment epithelium

The RPE consists of a single layer of cells joined by tight junctions that lie between the photoreceptors of the neural retina and the blood-rich choroid. The RPE serves multiple functions aimed at protecting and maintaining the health of the neural retina. RPE cells (1) phagocytose shed disks from the photoreceptor outer segments and recycle their components; (2) transport nutrients from the choroid to the retina; (3) absorb light; (4) provide adhesive properties for the retina; and (5) serve as a rich source of cytokines, chemokines, and growth factors. More recently, RPE have also been linked to immunity and have been