Acanthamoeba-specific human T-cell clones isolated from healthy individuals

Understanding Acanthamoeba Keratitis: An In-Depth Review of a Sight-Threatening Eye Infection

Acanthamoeba keratitis (AK) is a rare but potentially sight-threatening corneal infection caused by the Acanthamoeba parasite. This microorganism is found ubiquitously in the environment, often in freshwater, soil, and other sources of moisture. Despite its low incidence, AK presents significant challenges due to delayed diagnosis and the complex nature of therapeutic management. Early recognition is crucial to prevent severe ocular complications, including corneal ulceration and vision loss. Diagnostic modalities and treatment strategies may vary greatly depending on the clinical manifestation and the available tools. With the growing reported cases of Acanthamoeba keratitis, it is essential for the ophthalmic community to thoroughly understand this condition for its effective management and improved outcomes. This review provides a comprehensive overview of AK, encompassing its epidemiology, risk factors, pathophysiology, clinical manifestations, diagnosis, and treatment.

Three encephalitis-causing amoebae and their distinct interactions with the host

Naegleria fowleri, Balamuthia mandrillaris, and Acanthamoeba spp. can cause devastating brain infections in humans which almost always result in death. The symptoms of the three infections overlap, but brain inflammation and the course of the disease differ, depending on the amoeba that is responsible. Understanding the differences between these amoebae can result in the development of strategies to prevent and treat these infections. Recently, numerous scientific advancements have been made in the understanding of pathogenicity mechanisms in general, and the basic biology, epidemiology, and the human immune response towards these amoebae in particular. In this review, we combine this knowledge and aim to identify which factors can explain the differences between the lethal brain infections caused by N. fowleri, B. mandrillaris, and Acanthamoeba spp.

The biology of Acanthamoeba keratitis

Acanthamoeba keratitis (AK) is a rare protozoal infection of the cornea. At least eight species of Acanthamoeba are known to cause this sight-threatening disease of the ocular surface. Acanthamoeba spp. exist in a wide array of niches ranging from thermal springs to under ice and every conceivable habitat in between. Contact lens wear is the leading risk factor for AK and is practiced by over 30 million individuals in the United States, yet the incidence of AK is less than 33 cases per one million contact lens wearers. Serological studies have reported that 90%-100% of individuals with no history of AK possess antibodies specific for Acanthamoeba antigens indicating that exposure to this organism is commonplace, yet disease is remarkably rare. Animal studies have shed light on the pathobiology and immunobiology of AK and indicate that a constellation of factors including the ocular surface microbiome and the microbiome of Acanthamoeba itself contribute to the pathogenesis of AK. Interesting, secretory antibodies produced by the adaptive immune response can prevent the initiation of corneal infection, but once Acanthamoeba trophozoites breach the corneal epithelium the adaptive immune system is helpless in altering the course of AK. It has been almost 50 years since AK was first described, yet many questions remain unanswered about this curious and enigmatic disease of the ocular surface.

SJL mice infected with Acanthamoeba castellanii develop central nervous system autoimmunity through the generation of cross-reactive T cells for myelin antigens

We recently reported that Acanthamoeba castellanii (ACA), an opportunistic pathogen of the central nervous system (CNS) possesses mimicry epitopes for proteolipid protein (PLP) 139–151 and myelin basic protein 89–101, and that the epitopes induce experimental autoimmune encephalomyelitis (EAE) in SJL mice reminiscent of the diseases induced with their corresponding cognate peptides. We now demonstrate that mice infected with ACA also show the generation of cross-reactive T cells, predominantly for PLP 139–151, as evaluated by T cell proliferation and IA^s/dextramer staining. We verified that PLP 139–151-sensitized lymphocytes generated in infected mice contained a high proportion of T helper 1 cytokine-producing cells, and they can transfer disease to naïve animals. Likewise, the animals first primed with suboptimal dose of PLP 139–151 and later infected with ACA, developed EAE, suggesting that ACA infection can trigger CNS autoimmunity in the presence of preexisting repertoire of autoreactive T cells. Taken together, the data provide novel insights into the pathogenesis of Acanthamoeba infections, and the potential role of infectious agents with mimicry epitopes to self-antigens in the pathogenesis of CNS diseases such as multiple sclerosis.

Acanthamoeba: biology and increasing importance in human health

Acanthamoeba is an opportunistic protozoan that is widely distributed in the environment and is well recognized to produce serious human infections, including a blinding keratitis and a fatal encephalitis. This review presents our current understanding of the burden of Acanthamoeba infections on human health, their pathogenesis and pathophysiology, and molecular mechanisms associated with the disease, as well as virulence traits of Acanthamoeba that may be targets for therapeutic interventions and/or the development of preventative measures.

Inability to make a premortem diagnosis of Acanthamoeba species infection in a patient with fatal granulomatous amebic encephalitis

Granulomatous amebic encephalitis (GAE), an infection of immunocompromised hosts, is almost uniformly fatal. A case of GAE in a patient who failed to mount a serologic response to Acanthamoeba polyphaga is presented. Although Acanthamoeba polyphaga that is sensitive to multiple antimicrobials grew from brain tissue, an inability to make a premortem diagnosis precluded therapy.

Acanthamoeba spp. as agents of disease in humans

Acanthamoeba spp. are free-living amebae that inhabit a variety of air, soil, and water environments. However, these amebae can also act as opportunistic as well as nonopportunistic pathogens. They are the causative agents of granulomatous amebic encephalitis and amebic keratitis and have been associated with cutaneous lesions and sinusitis. Immuno compromised individuals, including AIDS patients, are particularly susceptible to infections with Acanthamoeba. The immune defense mechanisms that operate against Acanthamoeba have not been well characterized, but it has been proposed that both innate and acquired immunity play a role. The ameba's life cycle includes an active feeding trophozoite stage and a dormant cyst stage. Trophozoites feed on bacteria, yeast, and algae. However, both trophozoites and cysts can retain viable bacteria and may serve as reservoirs for bacteria with human pathogenic potential. Diagnosis of infection includes direct microscopy of wet mounts of cerebrospinal fluid or stained smears of cerebrospinal fluid sediment, light or electron microscopy of tissues, in vitro cultivation of Acanthamoeba, and histological assessment of frozen or paraffin-embedded sections of brain or cutaneous lesion biopsy material. Immunocytochemistry, chemifluorescent dye staining, PCR, and analysis of DNA sequence variation also have been employed for laboratory diagnosis. Treatment of Acanthamoeba infections has met with mixed results. However, chlorhexidine gluconate, alone or in combination with propamidene isethionate, is effective in some patients. Furthermore, effective treatment is complicated since patients may present with underlying disease and Acanthamoeba infection may not be recognized. Since an increase in the number of cases of Acanthamoeba infections has occurred worldwide, these protozoa have become increasingly important as agents of human disease.

Association of antigen specificity and migratory capacity of memory T cells in rheumatoid arthritis

Among the T cell pool of multiple specificities in the rheumatoid synovial tissues (ST) we have previously shown a lack of proliferative response of T cells to Acanthamoeba polyphaga [1]. In contrast, peripheral blood (PB) derived T cells proliferate to the antigen. The aim of the present study was to establish whether there is a preferential migration of some T cell specificities to the joint in rheumatoid arthritis (RA) patients dependent on the chemokine system, and to identify which chemokine receptors are involved in the migratory process. For this purpose, PB-derived T cell lines and clones from RA patients specific for A. polyphaga, herpes simplex virus (HSV) and Campylobacter jejuni were developed. Their migratory capacities towards ST-derived chemokine supernatants were analysed. Expression of CCR1, CCR2, CCR5, CCR6, CCR7, CXCR3 and CXCR4 were analysed by FACS, and attracting chemokines were identified by blocking studies. We found that the migratory capacities of T cells specific for C. jejuni and HSV were markedly higher against synovial chemokines than T cells specific for A. polyphaga. CCR5 and CXCR3 were expressed by all high-migrating T cell lines and clones. CCR2 was expressed at higher levels on the high-migrating T cell lines compared with the low-migrating A. polyphaga lines. Neutralization of RANTES (regulated upon activation normal T cell expressed and secreted) in the ST cell-derived supernatant reduced T cell migration of all T cell lines and clones by 60-90%, while neutralization of MCP-1 reduced the migratory capacity of CCR2-expressing T cells by 45-80%. In conclusion, the ability of T cells to migrate towards chemokines produced by ST cells is associated with the T cell specificity. Blocking of single chemokines substantially reduced the migratory capacity of memory T cells to ST cell-derived supernatant indicating unique roles for each chemokine receptor in the process of T cell migration.

Tear IgA and serum IgG antibodies against Acanthamoeba in patients with Acanthamoeba keratitis

PURPOSE

Exposure to Acanthamoebaspecies appears to be ubiquitous, as 50% to 100% of healthy human subjects display anti-Acanthamoebaantibodies. However, the presence of specific anti-Acanthamoebaantibodies in the serum and tears of patients has not been investigated. The prevalence of serum immunoglobulin G (IgG) and tear IgA against three species of Acanthamoebawas assessed in healthy subjects and patients with Acanthamoebakeratitis.

METHODS

The level of specific serum IgG and tear IgA against A. castellanii, A. astronyxis, and A. culbertsoniin the sera of 23 patients and 25 healthy subjects was tested by enzyme-linked immunosorbent assays. Total serum IgM, IgG, and IgA concentrations were measured by nephelometry. Acanthamoebakeratitis was diagnosed clinically and confirmed by in vivo confocal microscopy. In some patients, corneal biopsies were also performed and trophozoites were cultured on lawns of Escherichia colion non-nutrient agar.

RESULTS

All healthy subjects and patients with Acanthamoebakeratitis had detectable serum IgG antibodies against all Acanthamoebaantigens. However, patients with Acanthamoebakeratitis had significantly higher anti-AcanthamoebaIgG antibody titers than healthy subjects. In contrast, Acanthamoeba-specific tear IgA was significantly lower in patients with Acanthamoebakeratitis in comparison with healthy subjects. Total serum immunoglobulins did not differ significantly between healthy subjects and patients with Acanthamoebakeratitis.

CONCLUSIONS

The results suggest that a low level of anti-AcanthamoebaIgA antibody in the tears appears to be associated with Acanthamoebakeratitis.

Immunological inter-strain crossreactivity correlated to 18S rDNA sequence types in Acanthamoeba spp

Various species of the genus Acanthamoeba have been described as potential pathogens; however, differentiation of acanthamoebae remains problematic. The genus has been divided into 12 18S rDNA sequence types, most keratitis causing strains exhibiting sequence type T4. We recently isolated a keratitis causing Acanthamoeba strain showing sequence type T6, but being morphologically identical to a T4 strain. The aim of our study was to find out, whether the 18S rDNA sequence based identification correlates to immunological differentiation. The protein and antigen profiles of the T6 isolate and three reference Acanthamoeba strains were investigated using two sera from Acanthamoeba keratitis patients and one serum from an asymptomatic individual. It was shown, that the T6 strain produces a distinctly different immunological pattern, while patterns within T4 were identical. Affinity purified antibodies were used to further explore immunological cross-reactivity between sequence types. Altogether, the results of our study support the Acanthamoeba 18S rDNA sequence type classification in the investigated strains.

Discrimination between clinically relevant and nonrelevant Acanthamoeba strains isolated from contact lens- wearing keratitis patients in Austria

Eighteen cases of Acanthamoeba-associated keratitis among contact lens wearers seen at the Department of Ophthalmology, Karl-Franzens-University, Graz, Austria, between 1996 and 1999 are reviewed. The amoebae were proven to be the causative agents in three patients. The aim of our study was to discriminate between clinically relevant and nonrelevant isolates and to assess the relatedness of the isolates to published strains. Altogether, 20 strains of free-living amoebae, including 15 Acanthamoeba strains, 3 Vahlkampfia strains, and 2 Hartmannella strains, were isolated from clinical specimens. The virulent Acanthamoeba strains were identified as A. polyphaga and two strains of A. hatchetti. To our knowledge this is the first determination of keratitis-causing Acanthamoeba strains in Austria. Clinically relevant isolates differed markedly from nonrelevant isolates with respect to their physiological properties. 18S ribosomal DNA sequence types were determined for the three physiologically most-divergent strains including one of the keratitis-causing strains. This highly virulent strain exhibited sequence type T6, a sequence type not previously associated with keratitis. Sequence data indicate that Acanthamoeba strains causing keratitis as well as nonpathogenic strains of Acanthamoeba in Austria are most closely related to published strains from other parts of the world. Moreover, the results of our study support the assumption that pathogenicity in Acanthamoeba is a distinct capability of certain strains and not dependent on appropriate conditions for the establishment of an infection.

The pathogenesis of Acanthamoeba keratitis

Free-living amoebae of the genus Acanthamoeba produce a progressive, blinding infection of the corneal surface. The pathogenesis of Acanthamoeba keratitis involves parasite-mediated cytolysis and phagocytosis of corneal epithelial cells and induction of programmed cell death. Acanthamoeba spp. elaborate a variety of proteases which may facilitate cytolysis of the corneal epithelium, invasion of the extracellular matrix, and dissolution of the corneal stromal matrix.