Infiltration of CD4+ T cells into cornea during development of Onchocerca volvulus-induced experimental sclerosing keratitis in mice

Gamma interferon and interleukin-1 receptor 1 regulate neutrophil recruitment to the corneal stroma in a murine model of Onchocerca volvulus keratitis

Toll-like receptor 2 (TLR2) is an essential mediator of corneal inflammation induced by the filarial nematode Onchocerca volvulus, which harbors endosymbiotic Wolbachia bacteria. TLR2 is also required for dendritic cell activation, gamma interferon (IFN-gamma) production, and neutrophil recruitment to the cornea. To examine the role of IFN-gamma in O. volvulus keratitis, C57BL/6 and IFN-gamma(-/-) mice were immunized subcutaneously, and a soluble antigen extract from O. volvulus adult worms (OvAg) was injected into the corneal stroma of each animal. We found that, in the absence of IFN-gamma, neutrophil recruitment to the cornea was significantly impaired, whereas there was no effect on eosinophil infiltration. Since the cornea contains resident macrophages and fibroblasts and our previous studies showed that CXC chemokines mediate neutrophil recruitment, we examined the role of recombinant IFN-gamma (rIFN-gamma) on each cell type. We found no effect of rIFN-gamma on CXC chemokine production by macrophages or corneal fibroblasts, either alone or with filarial extracts; in contrast, rIFN-gamma was found to enhance OvAg-induced tumor necrosis factor alpha (TNF-alpha), interleukin-6 (IL-6), IL-1alpha, and IL-1beta in macrophages. Furthermore, we found that rTNF-alpha, rIL-1alpha, or rIL-1beta induced CXC chemokine production by corneal fibroblasts but not by macrophages. To determine the relative contributions of endogenous cytokines, we injected OvAg into the corneal stroma of C57BL/6, IL-1 receptor 1(-/-) (IL-1R1(-/-)), and TNF-alphaR1/2(-/-) mice and examined neutrophil recruitment. We found that neutrophil infiltration was impaired in IL-1R1(-/-) mice but not in TNF-alphaR1/2(-/-) mice. IFN-gamma therefore appears to regulate neutrophil recruitment to the corneal stroma by enhancing TLR2 expression and OvAg-induced IL-1alpha and IL-1beta production by macrophages in the cornea, which then induce IL-1R1-dependent production of CXC chemokine by resident corneal fibroblasts.

Onchocerca volvulus keratitis (river blindness) is exacerbated in BALB/c IL-4 gene knockout mice

To determine the outcome of Onchocerca volvulus keratitis in IL-4(-/-) BALB/c mice, animals were immunized subcutaneously and injected into the corneal stroma with soluble O. volvulus antigens. IL-4(-/-) BALB/c mice had a deviated cellular response, with decreased serum IgE and IgG1 and elevated IgG2a compared with control BALB/c mice. In marked contrast to control BALB/c, C57BL/6, and IL-4(-/-) C57BL/6 mice, IL-4(-/-) BALB/c mice developed severe corneal opacification and neovascularization that was associated with a pronounced neutrophil infiltrate to the corneal stroma. STAT-6(-/-) BALB/c mice had the same phenotype as IL-4(-/-) BALB/c mice, and complement depletion had no effect on the severity of O. volvulus keratitis in these mice. These findings indicate that on a BALB/c background, IL-4 has a critical role in regulating neutrophil recruitment to the cornea and development of O. volvulus keratitis.

Immune mechanisms in Onchocerca volvulus-mediated corneal disease (river blindness)

Infection with the parasitic nematode Onchocerca volvulus can lead to severe visual impairment and blindness. In an effort to characterize the molecular basis for the inflammatory response in the cornea, we have developed a murine model for O. volvulus-mediated keratitis in which parasite antigens are injected into the corneal stroma of sensitized mice. This model reproduces the two main clinical features of human disease, corneal opacification and neovascularization. Histological analysis of corneas from these mice reveals a biphasic recruitment of neutrophils and eosinophils to the central cornea, along with a small, but persistent number of CD3+ cells. In this review, we present evidence that production of antigen-specific T cell and antibody responses are essential for development of O. volvulus keratitis, and we propose a sequence of molecular and cellular events that lead to migration of inflammatory cells to the cornea and to loss of corneal clarity.

CD4(+) depletion selectively inhibits eosinophil recruitment to the cornea and abrogates Onchocerca volvulus keratitis (River blindness)

Previous studies demonstrated that in the murine model of Onchocerca volvulus keratitis, neutrophils and eosinophils are recruited into the cornea in a biphasic manner in response to intrastromal injection. To determine if CD4(+) T cells regulate migration of neutrophils and eosinophils into the cornea, CD4(+) cells were depleted using monoclonal antibody GK1.5 before intrastromal injection of parasite antigens. Depletion of CD4(+) cells abrogated corneal opacification at later but not early stages of disease. Consistent with this observation, CD4 depletion significantly impaired recruitment of eosinophils to the cornea but had no effect on neutrophils. These data indicate that CD4(+) T cells mediate sustained O. volvulus keratitis by regulating eosinophil recruitment to the cornea.

Twenty-five-year panorama of corneal immunology: emerging concepts in the immunopathogenesis of microbial keratitis, peripheral ulcerative keratitis, and corneal transplant rejection

PURPOSE

To describe the most recent advances in our understanding of the cellular and molecular mechanisms involved in the immunopathogenesis of corneal immunoinflammatory disorders including microbial keratitis, peripheral ulcerative keratitis. and allograft rejection.

METHODS

Review of the published peer-reviewed literature that has contributed significantly to our modern understanding of corneal immunology. In addition, the authors have summarized the information in conceptual diagrams that highlight the critical cellular and molecular pathways that lead to corneal immune responses in the two most thoroughly studied corneal immune disorders, herpes simplex keratitis (HSK) and transplant rejection.

RESULTS

In spite of the wide array of molecular and cellular factors that mediate corneal immunity, critical mechanistic facets are shared by the various corneal immunoinflammatory disorders. These include activation and migration of local antigen-presenting cells (APCs), including Langerhans cells (LCs), upregulation in pleiotropic proinflammatory cytokines such as interleukin-1 (IL-1) and tumor necrosis factor-alfa (TNF-alpha) that can mediate a wide array of immune functions in addition to up-regulating protease expression. and chemokines that play a critical role on the one hand in attracting nonantigen-specific inflammatory cells such as neutrophils and on the other in attracting CD4+ T helper type 1 (Th1) cells that mediate most of the destruction in the cornea.

CONCLUSIONS

In the last 25 years, we have seen our field develop from a descriptive stage into a new phase where the fundamental processes that mediate and effect corneal immunity are being accurately deciphered. It is anticipated that this new knowledge will allow development of specific molecular and genetic therapeutic strategies that could target critical steps in the immunopathogenesis of disease without the untoward side-effects of nonspecific generalized immune suppression that still remains the standard of care today.

An Essential Role for Antibody in Neutrophil and Eosinophil Recruitment to the Cornea: B Cell-Deficient (μMT) Mice Fail to Develop Th2-Dependent, Helminth-Mediated Keratitis

Invasion of the corneal stroma by neutrophils and eosinophils and subsequent degranulation disrupts corneal clarity and can result in permanent loss of vision. In the current study, we used a model of helminth-induced inflammation to demonstrate a novel role for Ab in mediating recruitment of these inflammatory cells to the central cornea. C57BL/6 and B cell-deficient (μMT) mice were immunized s.c. and injected intrastromally with Ags from the parasitic helminth Onchocerca volvulus (which causes river blindness). C57BL/6 mice developed pronounced corneal opacification, which was associated with an Ag-specific IL-5 response and peripheral eosinophilia, temporal recruitment of neutrophils and eosinophils from the limbal vessels to the peripheral cornea and subsequent migration to the central cornea. In contrast, the corneas of μMT mice failed to develop keratitis after intrastromal injection of parasite Ags unless Ags were injected with immune sera. Eosinophils were recruited from the limbal vessels to the peripheral cornea in μMT mice, but failed to migrate to the central cornea, whereas neutrophil recruitment was impaired at both stages. With the exception of IL-5, T cell responses and peripheral eosinophils were not significantly different between C57BL/6 and μMT mice. Taken together, these findings not only demonstrate that Ab is required for the development of keratitis, but also show that recruitment of neutrophils to the cornea is Ab-dependent, whereas eosinophil migration is only partially dependent upon Ab interactions.

Pathogenesis of onchocercal keratitis (River blindness)

Onchocerciasis is a major cause of blindness. Although the World Health Organization has been successful in reducing onchocerciasis as a public health problem in parts of West Africa, there remain an estimated 17 million people infected with Onchocerca volvulus, the parasite that causes this disease. Ocular pathology can be manifested in any part of the eye, although disease manifestations are frequently characterized as either posterior or anterior eye disease. This review focuses on onchocerca-mediated keratitis that results from an inflammatory response in the anterior portion of the eye and summarizes what is currently known about human disease. This review also describes studies with experimental models that have been established to determine the immunological mechanisms underlying interstitial keratitis. The pathogenesis of keratitis is thought to be due to the host inflammatory response to degenerating parasites in the eye; therefore, the primary clinical symptoms of onchocercal keratitis (corneal opacification and neovascularization) are induced after injection of soluble O. volvulus antigens into the corneal stroma. Experimental approaches have demonstrated an essential role for sensitized T helper cells and shown that cytokines can regulate the severity of keratitis by controlling recruitment of inflammatory cells into the cornea. Chemokines are also important in inflammatory cell recruitment to the cornea, and their role in onchocerciasis is being examined. Further understanding of the molecular basis of the development of onchocercal keratitis may lead to novel approaches to immunologically based intervention.

Identification of an epitope of a recombinant Onchocerca volvulus protein that induces corneal pathology

Ocular onchocerciasis results from immune recognition of parasite proteins released into the eye by degenerating microfilariae. Previous studies have shown that pathology similar to human ocular onchocerciasis can be induced in sensitized mice by intracorneal injection with Onchocerca volvulus antigens. In the current study, we used this murine model to map the segments of O. volvulus protein disulfide isomerase (OvPDI) associated with the development of corneal pathology. Subclones of OvPDI were constructed encompassing one or more predicted T cell epitopes. Keratitis was induced in BALB/c mice after subcutaneous immunizations with OvPDI, followed by intracorneal challenge of OvPDI constructs. Truncated OvPDI proteins containing amino acids 450-481 of OvPDI were found to induce keratitis, whereas constructs that did not include this region did not induce corneal pathology. Consistent with this observation, two peptides derived from the 450-481 region stimulated T cell proliferation to a greater degree than control carrier protein. DNA sequence analysis of cDNAs encoding OvPDI from blinding and non-blinding strains of O. volvulus indicated no differences in the primary amino acid sequence of the 450-481 domain. Immunization of animals with OvPDI induced antibodies recognizing a 55 kDa host protein, identical to the predicted molecular weight of the mouse PDI homologue. Together, these data implicate specific antigenic epitopes of OvPDI in the development of O. volvulus mediated corneal pathology.

Pleomorphism of stromal eosinophils in murine experimental onchocercal keratitis

Onchocercal keratitis (river blindness) is one of the leading worldwide causes of blindness. Light microscopic analysis of human specimens and corneal tissue from experimental models has implicated the eosinophil as an important cell in the inflammatory response. Our previous studies in experimental murine onchocercal keratitis have demonstrated that the inflammatory infiltrate is composed primarily of eosinophils displaying ring shaped or bilobed nuclei. However, a number of cells were not characterizable by light microscopy, presumably due to mechanical distortion. To more fully characterize the inflammatory cell infiltrate, we examined corneal specimens by transmission electron microscopy. In addition to typical eosinophils with bilobed and ring shaped nuclei, this approach revealed cells with variable nuclear morphology and cell shape which contained the dense cored granules characteristic of eosinophils. Hence, the degree of pleomorphism of eosinophils is broader than appreciated and underscores the importance of this cell in experimental murine onchocercal keratitis.

In vivo molecular analysis of cytokines in a murine model of ocular onchocerciasis. I. Up-regulation of IL-4 and IL-5 mRNAs and not IL-2 and IFN gamma mRNAs in the cornea due to experimental interstitial keratitis

Sclerosing keratitis is the major cause of blindness due to onchocerciasis which results from chronic infection with the filarial parasite Onchocerca volvulus. Using a murine model of onchocercal sclerosing keratitis, we have demonstrated previously that predominantly (> 85%) CD3 + /CD4+ T-cells as well as the IL-2 receptor bearing cells infiltrate into the cornea in vivo during development and progress of the disease. The identification of CD4+ subsets TH1 and TH2 based on the cytokine secretion patterns of murine T-lymphocytes has been useful for understanding the immune basis of resistance and pathogenesis in murine models of several parasitic diseases. The present investigation was carried out to demonstrate whether the local immune response at the corneal lesion due to onchocercal interstitial keratitis correlated with such distinct patterns of cytokine production. For that purpose, mRNA was extracted separately from corneas obtained from the diseased eyes and the normal eyes of A/J mice with onchocercal interstitial keratitis, reverse transcribed and amplified by the polymerase chain reaction with four different cytokine specific primers. In corneas obtained from the eyes affected with onchocercal interstitial keratitis, mRNAs coding for IL-4 and IL-5 were up-regulated compared to the normal eyes having no lesions from the same animals. However, the levels of mRNAs for IL-2 and IFN gamma were found to be the same in the diseased and normal eyes. Taken together, these data suggest that IL-4 and IL-5 producing TH2-lymphocytes are active at the corneal lesion due to onchocercal interstitial keratitis.

Experimental onchocercal keratitis

In individuals with onchocerciasis, severe visual impairment (river blindness) occurs as a result of corneal inflammation induced by antigens released from dead and dying Onchocerca volvulus microfilariae. To characterize the underlying immune response, animal models have been developed that partially reproduce many of the clinical features of human onchocercal keratitis, Eric Pearlman here discusses how these studies have identified systemic and local immune responses associated with keratitis, including T helper-cell subset and cytokine responses.

Interleukin 4 and T helper type 2 cells are required for development of experimental onchocercal keratitis (river blindness)

Inflammation of the corneal stroma (stromal keratitis) is a serious complication of infection with the nematode parasite Onchocerca volvulus. Because stromal keratitis is believed to be immunologically mediated in humans, we used a murine model to examine the role of T cells and T helper cell cytokines in the immunopathogenesis of these eye lesions. BALB/c mice immunized subcutaneously and injected intrastromally with soluble O. volvulus antigens (OvAg) developed pronounced corneal opacification and neovascularization. The corneal stroma was edematous and contained numerous eosinophils and mononuclear cells. Stromal keratitis in immunized mice was determined to be T cell dependent based on the following observations: (a) T cell-deficient nude mice immunized and injected intrastromally with OvAg fail to develop corneal pathology; and (b) adoptive transfer of spleen cells from OvAg-immunized BALB/c mice to naive nude mice before intrastromal injection of OvAg results in development of keratitis. OvAg-stimulated lymph node and spleen cell cytokine production was dependent on CD4 cells and included interleukin (IL)-4 and IL-5, but not interferon gamma, indicating a predominant T helper type 2 cell-like response. Inflamed corneas from immunized BALB/c mice and from reconstituted nude mice had greatly elevated CD4 and IL-4 gene expression compared with interferon gamma. Mice in which the IL-4 gene was disrupted failed to develop corneal disease, demonstrating that IL-4 is essential in the immunopathogenesis of O. volvulus-mediated stromal keratitis.