Immunopathology of ocular onchocerciasis. 2. Anti-retinal autoantibodies in serum and ocular fluids

Presumed Onchocerciasis Chorioretinitis Spilling over into North America, Europe and Middle East

BACKGROUND

Newer generation ophthalmologists practicing in the developed world are not very familiar with some tropical ocular diseases due to the absence of reports in the ophthalmic literature over the past thirty years. Because of world globalization or due to influx of immigrants from sub-Saharan Africa, exotic retinal diseases are being encountered more often in ophthalmology clinics.

METHODS

A multicenter case series of chorioretinitis or optic neuritis with obscure etiology that used serial multimodal imaging.

RESULTS

Four cases qualified with the diagnosis of presumed ocular onchocerciasis based on their residence near fast rivers in endemic areas, multimodal imaging, long term follow-up showing progressive disease and negative workup for other diseases. Characteristic findings include peripapillary choroiditis with optic neuritis or atrophy, subretinal tracts of the microfilaria, progressive RPE atrophy around heavily pigmented multifocal chorioretinal lesions of varying shapes, subretinal white or crystalline dots, and response to ivermectin. Typical skin findings are often absent in such patients with chorioretinitis rendering the diagnosis more challenging.

CONCLUSIONS

Familiarity with the myriad ocular findings of onchocerciasis, and a high-degree of suspicion in subjects residing in endemic areas can help in the correct diagnosis and implementation of appropriate therapy. Onchocercal chorioretinitis is a slow, insidious, progressive, and prolonged polymorphous disease.

Preliminary assessment of Onchocerca-induced visual impairment using clinical fundus camera in Gashaka local government area of Taraba state, north eastern Nigeria

Introduction

Onchocerciasis is the world's second leading cause of infectious blindness and remains a major problem in parts of Africa. In light of the efforts targeted towards improving ongoing elimination program, this study assessed onchocerca-induced visual impairments in Gashaka local government areas (LGA) in Taraba State, north-eastern Nigeria.

Methods

In 2019, we recruited 158 consenting visually impaired persons across three communities in Garbabi ward of Gashaka LGA. To avoid confusion with co-endemic trachoma, the integrity of the tarsal conjunctiva, eyelashes were assessed using direct light. The anterior segment of the eye was also examined using a torchlight with oblique illumination. However, the posterior segment of the eye was assessed using a fundus camera. Two photographic images for the left and right eye of each participant were captured using the clinical fundus camera. The photographic eye images that were too dark were discarded, and only clear images were analyzed by two ophthalmologists. An ocular manifestation report was recorded for each participant following consensus between the ophthalmologists.

Results

Of the 316 photographic eye images, almost half 146 (46.2%) from 73 participants were just too destroyed for light to penetrate and was not included in the analysis. Only 170 from 85 participants were clear and examinable. A total of 33 (39%) participants had chorioretinitis suggestive of onchocerciasis, including 22(25.9%) with chorio-retinal atrophy, 7(8.2%) and 4(4.7%) had chorioretinal atrophy in combination with early cataract and signs of trachoma respectively. In addition, 3(3.5%) of the participant had eye images which showed lens opacities, 1(1.2%) showed signs of keratoconus and 1(1.2%) showed a scared and pigmented cornea, possibly due to onchocerciasis. Furthermore, 28 (32.9%) had some ill-defined changes and 19 (22.4%) showed poorly defined chorio-retinal atrophy.

Conclusion

In a bid to sustain MDA gains towards elimination of onchocerciasis, this work highlights the need for continuous assessment of onchocerciasis induced visual impairment, strengthening of ivermectin delivery and optimizing compliance and patient care among affected populations. These would require resource acquisition and local capacity building. Our preliminary findings call for further operational research on ocular morbidity as well as future stakeholders' consultations in this important and understudied area.

The Pathogenesis of Nodding Syndrome

Nodding syndrome is a rare, enigmatic form of pediatric epilepsy that has occurred in an epidemic fashion beginning in the early 2000s in geographically distinct regions of Africa. Despite extensive investigation, the etiology of nodding syndrome remains unclear, although much progress has been made in understanding the pathogenesis of the disease, as well as in treatment and prevention. Nodding syndrome is recognized as a defined disease entity, but it is likely one manifestation along a continuum of Onchocerca volvulus-associated neurological complications. This review examines the epidemiology of nodding syndrome and its association with environmental factors. It provides a critical analysis of the data that support or contradict the leading hypotheses of the etiologies underlying the pathogenesis of the syndrome. It also highlights the important progress made in treating and preventing this devastating neurological disease and prioritizes important areas for future research.

Diagnostic features of the autoimmune retinopathies

The term autoimmune retinopathy encompasses a spectrum of rare autoimmune diseases that affect retinal function, often but not exclusively at the level of the photoreceptor. They typically present with painless visual loss, which may be accompanied by normal fundus examination. Some are progressive, often rapidly. They present a diagnostic challenge because there are no standardised clinical or laboratory based diagnostic criteria. Included within the spectrum are cancer-associated retinopathy, melanoma-associated retinopathy and presumed non-paraneoplastic autoimmune retinopathy. Differentiation from other retinopathies can be challenging, with overlap in symptoms, signs, and investigation findings, and an absence of pathognomonic features. However, technological developments in ophthalmic imaging and serological investigation over the past decade are adding novel dimensions to the investigation and classification of patients with these rare diseases. This review addresses the clinical, imaging, and serological features of the autoimmune retinopathies, and discusses the relative strengths and limitations of candidate diagnostic features.

Genetic response to an environmental pathogenic agent: HLA-DQ and onchocerciasis in northwestern Ecuador

The aim of this study is to explore human leukocyte antigen (HLA)-DQ variability in two populations (Cayapas Amerindians and Afro-Ecuadorians) who live near one another along the Cayapa River and who are exposed to the same environmental stresses, such as infection by Onchocerca volvulus. HLA-DQA1 and HLA-DQB1 of 149 unrelated individuals (74 Cayapas and 75 Afro-Ecuadorians) have been analyzed. HLA high-resolution molecular typing was performed by sequence-based typing, sequence-specific oligonucleotides hybridization and sequence-specific primer (SSP) amplification. The comparison between affected (cases) and unaffected people (controls) in both populations shows the key role of several HLA-DQA1 alleles in susceptibility and protection against onchocerciasis. In both populations, there is strong evidence related to the protective role of DQA1*0401 against onchocerciasis. Alleles HLA-DQA1*0102 and *0103 seem to represent risk factors in Afro-Ecuadorians, while HLA-DQA1*0301 is only a suggestive susceptibility allele in Cayapas. These findings represent new positive/negative associations with onchocerciasis in South America, whereas previous findings pertained only to African populations.

Do helminths cause epilepsy?

Both helminthiases and epilepsy occur globally, and are particularly prevalent in developing regions of the world. Studies have suggested an association between epilepsy and helminth infection, but a causal relationship is not established in many helminths, except perhaps with neurocysticercosis. We review the available literature on the global burden of helminths, and the epidemiological evidence linking helminths to epilepsy. We discuss possible routes that helminths affect the central nervous system (CNS) of humans and the immunological response to helminth infection in the CNS, looking at possible mechanisms of epileptogenesis. Finally, we discuss the current gaps in knowledge about the interaction between helminths and epilepsy.

The need for standardization of antiretinal antibody detection and measurement

PURPOSE

To review the current literature on the detection and measurement of antiretinal antibodies.

DESIGN

Collaborative essay.

METHODS

Literature review and interpretation.

RESULTS

There is strong evidence to suggest a role for antiretinal antibodies, particularly those targeting recoverin and alpha-enolase, in the pathogenesis of autoimmune retinopathy (AIR). Additionally, numerous other autoantibodies have been described as putative mediators of retinal degeneration and more remain to be discovered. However, assay methods described in the literature by many laboratories for the detection of circulating antiretinal antibodies have been varied and diverse, making it difficult to interpret and compare their results.

CONCLUSIONS

There is currently little standardization of laboratory methods used to detect and monitor antiretinal antibodies. To measure and monitor levels of circulating antiretinal antibodies optimally in patients with AIR, development of standardized assays with stringent internal controls is required. A multicenter collaborative and validation effort is encouraged to reach a consensus on this issue.

Autoantibodies to p75/LEDGF, a cell survival factor, found in patients with atypical retinal degeneration

We have identified a group of patients with an atypical retinal degeneration having anti-retinal autoantibodies in their sera. This select population is characterized by a progressively severe loss of vision associated with a decrease in photoreceptor function, abnormal pigmentation of the retinal pigment epithelium and a negative family history of retinal dystrophy. Immunohistochemical analysis on mouse retinal tissues with sera from this group of patients demonstrated high titers of anti-retinal antibodies (320 to 1,280). Anti-retinal reactivity at these levels was not detected in sera from normal individuals, or from patients with uveitis or known genetic retinal degenerations. One antigen that was identified from a retina cDNA library with sera from a patient with atypical retinal degeneration was lens epithelium-derived growth factor (LEDGF). Western blot analysis revealed that sera from all three patients demonstrated reactivity to p75/LEDGF, a survival factor that protects cells from oxidative, thermal and UV damage. In conclusion, we have found a novel group of patients with a retinal degeneration of non-paraneoplastic, non-familial origin demonstrating immunoreactivity to an autoantigen, p75/LEDGF, heretofore not associated with this disease. Finally, identification of specific anti-retinal antibodies may have applications in the diagnosis and management of retinal degeneration.

The pathogenesis of chorioretinal disease in onchocerciasis

The pathogenesis of onchocercal chorioretinopathy is poorly understood. In this article, Philip Cooper, Ronald Guderian, Roberto Proaño and David Taylor discuss the important clinical, histological and epidemiological features of the resulting lesions that cause blindness, and review the numerous mechanisins that have been put forward to explain its pathogenesis. The pathogenesis of anterior segment disease, particulary sclerosing keratitis, has been reviewed in depth previously(1) and will not be discussed here.

The involvement of autoimmunity against retinal antigens in determining disease severity in toxoplasmosis

PURPOSE

Ocular lesions are frequent in various individuals infected with Toxoplasma gondii. Disease intensity in ocular toxoplasmosis varies greatly between patients. Autoimmunity has been suggested as a possible component to retinal destruction.

METHODS

Immunologic parameters in the response to retina antigens were evaluated in infected persons with and without ocular lesions and in non-infected controls. Subjects were divided into groups on the basis of titers of serum antibodies to T. gondii, presence and severity of ocular lesions, and clinical history.

RESULTS

Peripheral blood mononuclear cells from patients with mild disease responded to one or more retinal antigens with a significantly higher frequency than patients without disease or with severe disease. Interestingly, the cytokines produced by the proliferating mononuclear cells did not follow any specific patterns, except for the fact that IL-4 and IL-5 were seldom detected.

CONCLUSIONS

Our results suggest that although the presence of an immune response towards autoantigens is not protective against the development of ocular lesions by the T. gondii, it may protect against the development of severe disease.

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.

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.

Molecular mimicry between host and pathogen: examples from parasites and implication

The studies summarized in this paper suggest that parasites may trigger activation of autoimmune mechanisms. The association between parasites and autoimmunity could by manifested by the development of pathogenic anti-parasitic antibodies and cytotoxic T cells that attack and damage self tissues as a result of molecular mimicry between host and parasites. On the other hand, the homology between self and parasitic antigens may enable parasites to protect themselves from the immune system and to induce a state of immunosuppression. Although classic autoimmune diseases have not been shown to be more common amongst patients with chronic parasitic infections than in the general population, it is clear that autoimmune activity does occur in patients with chronic parasitic infections. It is possible that infection with parasites and other microbial agents may be followed by the activation of the immune system and, in genetically predisposed individuals, by loss of functional tolerance to self, activation of autoreactive cell that leads to progression to an overt autoimmune disease.

Human antiretinal antibodies in toxoplasma retinochoroiditis

BACKGROUND/AIMS

Toxoplasma retinochoroiditis (TR) is an important cause of blindness and visual morbidity, affecting young adults. It has been postulated that some of the retinal damage observed in TR is due to antiretinal autoimmune mechanisms.

METHODS

Humoral antiretinal autoimmunity in TR was investigated by indirect immunofluorescence (IIF) on normal human cadaveric retina and by a human retinal S-antigen ELISA. 36 patients with TR were separated on clinical grounds into those with first recurrence of disease (n = 18) or those with multiple recurrences (n = 18). Patients were also segregated into those with active (n = 28) or quiescent disease (n = 8). Serum from 16 normal controls (six with positive toxoplasma serology and 10 without) with no evidence of eye disease and 12 patients with idiopathic retinal vasculitis (IRV) were also tested.

RESULTS

Sera from 34 of the 36 patients (94%) with TR demonstrated photoreceptor layer reactivity by IIF contrasting with six of 16 normal controls (p = < 0.001) and three of 12 IRV patients (p = < 0.001). Titres of antiphotoreceptor antibody were also higher among TR patients than controls. Sera from 27 of the 36 TR patients, 10 of 16 normals, and nine of 12 retinal vasculitis patients possessed anti-human retinal S-antigen antibodies at a titre of 1:400 or more as assessed by ELISA (p = > 0.05). Antiretinal autoantibody as detected by IIF did not run in parallel with S-antigen reactivity.

CONCLUSIONS

The data indicate that the extent of antiretinal reactivity within TR is not accounted for by anti-S-antigen antibodies alone. This remarkably high prevalence of antiphotoreceptor antibody in TR as opposed to that found in either healthy or disease controls suggest that these antibodies may be co-pathogenic in toxoplasma retinochoroiditis.

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.

Molecular and immunological characterization of hr44, a human ocular component immunologically cross-reactive with antigen Ov39 of Onchocerca volvulus

Structural similarities between host self-antigens and infectious organisms may be involved in the expression of autoimmune reactivity and development of autoimmune disease. The unique eye pathology associated with Onchocerca volvulus infection, particularly the development of posterior segment lesions, may be promoted by such autoreactive responses. Ov39 is a parasite-derived antigen that has been shown previously to be antigenically cross-reactive with a 44,000-M(r) host ocular component. A clone, designated hr44, was isolated from a cDNA library of human retina by immunoscreen using serum to Ov39. A monoclonal antibody raised to Ov39 also reacted with hr44 and gave evidence for a shared conformational epitope. The primary structure analysis showed that identities between the antigens are limited and confined to small peptides. The cross-reactivity between the antigens appears to involve T cells, since Ov39-specific T cells can be stimulated by hr44, a neural-specific antigen. Based on secondary structure prediction, hr44 has the typical features of a membrane-associated type I antigen with an amino-terminal extracellular domain. mAbs and antisera localized the antigen in the optic nerve, neural retina, retinal pigment epithelium, as well as the epithelial layers of ciliary body and iris.

Cross-reactive antigens in the pathogenesis of onchocerciasis

The ocular disease associated with infection with Onchocerca volvulus is unique in that there is a wealth of epidemiological evidence to support the casual nature of the association but there is little known about the pathogenic mechanisms involved. We have identified a 44,000 M(r) component of ocular tissues that shows immunological cross-reactivity with an O. volvulus antigen. This immunological cross-reactivity between parasite and a component of host tissues may underlie the development of ocular disease in onchocerciasis. Preliminary experiments indicate that it is possible to initiate ocular disease in susceptible rats using the recombinant parasite antigen. This should allow the development of a laboratory model of ocular onchocerciasis and further our understanding of the mechanisms by which an infective organism can produce an auto-immune-like disease in the host.

Effects of repeated doses of ivermectin on ocular onchocerciasis: community-based trial in Sierra Leone

Ivermectin seems to be a safe and effective treatment for onchocerciasis when given in a single dose, but less is known about the effects of repeated doses. Also, there seem to be differences in its effectiveness in anterior and posterior segment ocular disease. The ocular effects of ivermectin were studied in 586 villagers who were taking part in a double-blind, placebo-controlled, randomised trial in Sierra Leone. Only those who had received four doses, with 6-month intervals, of ivermectin or placebo were eligible. The 296 ivermectin-treated subjects and the 272 who received placebo were comparable with respect to age, sex, Onchocerca infection, blindness, and visual impairment before treatment. After treatment, the ivermectin group had less anterior segment disease than the placebo group, with significantly lower prevalences of microfilariae in the anterior chamber and cornea, and punctate keratitis (all p less than 0.001), and iritis (p less than 0.05). There was no significant difference in the prevalence of sclerosing keratitis, optic atrophy, or chorioretinitis between the groups. Visual acuities tended to be better in the ivermectin group, but the difference was not significant. There was a small but significant (p less than 0.01) excess of vascular sheathing in the ivermectin group. These differences persisted when subjects who were blind or visually impaired at baseline were excluded from analysis. The long-term effects of ivermectin, particularly on posterior segment disease, need further evaluation. In the mean time, the mass distribution of ivermectin should be promoted for all communities with hyperendemic onchocerciasis at risk of anterior segment disease.

Immunological crossreactivity between a cloned antigen of Onchocerca volvulus and a component of the retinal pigment epithelium

Onchocerciasis (river blindness) is a major blinding disease in Africa, Central America, and South America. Loss of vision can be due to corneal change, optic atrophy, or chorioretinal disease. It has been suggested that autoimmunological reactions resulting from crossreactivity between parasite antigens and components of eye tissues contribute to development of ocular pathology. Using sera collected from onchocerciasis patients as a screening reagent, a cDNA clone (Ov39) has been isolated from a lambda gt11 expression library of Onchocerca volvulus. This antigen exhibits immunological crossreactivity with a component of retinal pigment epithelium cells (RPE). Antiserum raised against this recombinant peptide immunoprecipitates a 22,000 Mr antigen of adult O. volvulus and recognizes a 44,000 Mr component of bovine RPE by Western blotting. A 44,000 Mr antigen of cultured human RPE metabolically labeled with 35S-methionine can be immunoprecipitated with the same antiserum. An antigen of the same size is recognized by a rabbit antiserum raised against whole O. volvulus extract. Immunocytochemical studies on cryostat sections of the bovine eye using the antirecombinant sera localizes this antigen to the RPE.

Antiretinal antibodies in serum of patients with age-related macular degeneration

Antibodies, immunoreactive with normal human retinal proteins, were detected by Western immunoblot analysis in the sera of 30 patients with age-related macular degeneration (AMD). Sera from 14 of these patients demonstrated positive binding predominantly to a doublet protein of molecular weight between 58 and 62 kD. The sera from the remaining 16 patients and from 12 control subjects reacted either weakly or not at all with the doublet protein. No correlation was found with any specific type of AMD. The serum antibodies also immunocrossreacted with the same proteins from isolated photoreceptor outer segments; this was confirmed by indirect immunofluorescence on intact retinas. The crossreactivity of the serum antibodies with a protein of Mr 58 to 62 kD, the lower band present in the bovine purified neurofilament-68 kD preparation, suggests strongly that this protein may be a component of the neuronal cytoskeleton. However, it is not clear whether these autoantibodies play a direct role in the etiology of AMD or represent a nonspecific response to retinal damage.

Experimental autoimmune anterior uveitis (EAAU), a new form of experimental uveitis. I. Induction by a detergent-insoluble, intrinsic protein fraction of the retinal pigment epithelium

The uveitogenicity of several protein fractions of the bovine retinal pigment epithelium (RPE) was studied in Lewis rats, and a major pathogenic fraction was selected. Fresh RPE cells were carefully isolated and purified in order to minimize the presence of rod outer segments (ROS). The buffer-insoluble part of the cells was extracted by Triton X-100. Most uveitogenicity was found in the Triton-insoluble pigment and cytoskeleton-containing fraction of RPE (RPE-TI). The S-antigen and opsin contents of RPE-TI were too low to induce an inflammatory response, while transducin, IRBP and cGMP-phosphodiesterase were absent. Hence, a hitherto unknown uveitogenic RPE protein, called PEP-X, evoked the pathogenic response. A typical dose-dependent experimental autoimmune anterior uveitis (EAAU) developed when the rats were immunized with RPE-TI. Initially, mononuclear cells infiltrated the anterior segment. In subsequent severe stages polymorphonuclear cells predominated in the anterior chamber. EAAU differed in particular from the known forms of EAU induced by photoreceptor proteins in that the inflammation remained exclusively anterior and the photoreceptor cells and the pineal gland were not affected. In immunized rats the immune responses to ROS proteins were very low. In contrast, there were consistently high cellular and humoral immune responses to RPE-TI. As in experimental autoimmune (uveo)retinitis (EAU), the development of EAAU could be inhibited by cyclosporin treatment indicating T-cell-dependency. A combination of histopathological, immunological and biochemical results indicates that PEP-X is an intrinsic RPE protein that is highly pathogenic. In view of its characteristics, EAAU may be a valuable model for human acute anterior uveitis, the most prevalent form of uveitis.

Ocular manifestations of onchocerciasis in a rain forest area of west Africa

The epidemiology and natural history of onchocerciasis and its ocular complications in rain forest areas are poorly understood. The present study was conducted on a rubber plantation in a hyperendemic area in the rain forest of Liberia, West Africa, where 800 persons were examined. The prevalence of infection was 84% overall 29% had intraocular microfilariae, and 2.4% were blind in one or both eyes. Onchocerciasis was the cause of all binocular blindness and one-third of all visual impairment. Over half of the visual impairment caused by onchocerciasis was due to posterior segment diseases. Chorioretinal changes were present in 75% of people, and included intraretinal pigment clumping in 52% and retinal pigment epithelium atrophy in 32%. Atrophy of the retinal pigment epithelium was associated with increasing age and severity of infection. Intraretinal pigment was strongly associated with anterior uveitis. There was a strong correlation between uveitis and the inflammatory chorioretinal sequelae: retinitis, intraretinal pigment, subretinal fibrosis, and optic neuropathy. These findings indicate that considerable visual impairment associated with rain forest onchocerciasis is common and is due largely to chorioretinal disease.

Analysis of aqueous humour in ocular onchocerciasis

The development of an onchocercal chorioretinopathy from the first detectable signs to a full blown oncho fundus is not fully understood. We investigated the intraocular humoral immune response against Onchocerca volvulus, human S-antigen, IRBP and crude retinal extract (using an ELISA) by examining paired aqueous humour and serum samples obtained from onchocerciasis patients (without [n = 10] and with ocular symptoms [n = 8]) and endemic controls [n = 14] from Sierra Leone (West Africa). A local intraocular anti-retinal IgG antibody production could not be demonstrated in onchocerciasis patients, whether they had ocular symptoms or not. A significantly higher level of O. volvulus antibodies and IgG was measured in the aqueous of onchocerciasis patients with ocular involvement, as compared to patients without ocular symptoms (Mann-Whitney ranksum test; p less than 0.001 and p less than 0.02 respectively). Since interleukin-6 (IL-6) plays an essential role in the differentiation of B cells into immunoglobulin producing plasma cells, we therefore measured this cytokine in paired aqueous and serum samples. Elevated IL-6 levels were found in the aqueous of two out of eight onchocerciasis patients tested. In view of these findings it seems improbable that retinal autoimmunity is a major factor in the pathogenesis of onchocercal chorioretinopathy. The high intraocular levels of antibodies against the parasite suggest a direct involvement of the parasite in the pathogenesis of onchocercal chorioretinopathy.

Parasitic infection and autoimmunity

The studies summarized in this paper indicate that parasitic infections can serve as a trigger factor of autoimmune reactivity by several mechanisms. The relationship between parasites and autoimmunity could be manifested by the presence of autoantibodies or T-cells with autoreactivity. In spite of the evidence that has accumulated, the specific association between infection and autoimmunity is still obscure. The reasons for tissue damage in parasitic diseases are controversial. Some believe it is the result of pathogenic autoantibodies or autoreactive T-cells. Others argue against the causative role of autoimmunity in the formation of tissue lesions. The parasite itself could be the cause of tissue destruction, thus releasing high amounts of self antigens which might stimulate the autoreactivity. There is now little doubt that some degree parasite/host cross-reactivity occurs, and definition of cross-reacting antigens and epitopes is now taking place. It seems likely that a combination of events could result in cross-reactivity including: parasites themselves have cross reactive molecules and altered self antigens by adsorbing of parasite material to surrounding host cells. The mechanisms involved in parasites autoimmunity are complex and numerous, requiring a rigorous experimental approach to rationalize each step and determine its clinical importance. The developed methods in immunochemistry, monoclonal antibodies and hybridoma technology, and recombinant DNA research not only facilitate this kind of approach but also allow optimism for a successful outcome.

Serum antibody response to human and bovine IRBP in uveitis

Interphotoreceptor retinoid binding protein (IRBP) is a 136,000 molecular weight photoreceptor cell protein capable of inducing an experimental autoimmune uveitis (EAU) in susceptible animal strains. The occurrence of serum antibodies against human (Hu) or bovine (Bo) IRBP was investigated in patients with uveitis and healthy controls. A sensitive ELISA detected anti-IRBP in approximately 50% of patients and controls, without apparent differences in the mean level, titre or avidity and irrespective of the origin of the antigen. Although the correlation (p less than 0.001) between anti-HuIRBP and anti-BoIRBP levels in uveitis sera suggested the presence of crossreacting antibodies, these sera also contained antibodies specific for either the human or the bovine antigen. The only difference between patients and controls was the greater ability of antibodies in uveitis sera (p less than 0.05) to recognize a synthetic peptide of HuIRBP, which induces severe EAU in rats. We conclude that autoantibodies to IRBP occur naturally in man and are not increased in patients with uveitis.

Autoimmune responsiveness to retinal IRBP, S-antigen and opsin in proliferative vitreoretinopathy

Proliferative vitreoretinopathy (PVR) was induced in rabbits by intravitreal injection of homologous fibroblasts. During the 8 weeks after injection the immune responsiveness to three purified retinal autoantigens was studied. From 2 weeks after injection, animals that developed serious forms of PVR exhibited definite mitotic responses of their lymphocytes to stimulation by the retinal antigens. These responses could consistently be demonstrated for S-antigen and interphotoreceptor retinoid-binding protein (IRBP) during the subsequent period of examination. Marked responses were also noted to opsin, however, their occurrence was more variable. In mild forms of PVR or in controls the responses were weak or absent. This showed that the elevated cellular reactivities were induced by the development of PVR and not by some other experimental factor. Humoral immune responses to the three antigens were absent (as assayed by ELISA). The control groups did not exhibit any elevated immune responsiveness. There appears to be accumulating evidence that inflammation may play a role in the development of PVR. The present results indicate that cellular autoimmune responses to photoreceptor antigens are a secondary phenomenon in PVR, nevertheless, they may be an important factor in the subsequent development of severe PVR. This autosensitization may consequently be taken into consideration in the treatment of complicated human PVR.