[Protective immunity against Naegleria meningoencephalitis in mice]

Pathogenic free-living amoebae in Korea

Acanthamoeba and Naegleria are widely distributed in fresh water, soil and dust throughout the world, and cause meningoencephalitis or keratoconjunctivitis in humans and other mammals. Korean isolates, namely, Naegleria sp. YM-1 and Acanthamoeba sp. YM-2, YM-3, YM-4, YM-5, YM-6 and YM-7, were collected from sewage, water puddles, a storage reservoir, the gills of a fresh water fish, and by corneal washing. These isolates were categorized into three groups based on the mortalities of infected mice namely, highly virulent (YM-4), moderately virulent (YM-2, YM-5 and YM-7) and nonpathogenic (YM-3). In addition, a new species of Acanthamoeba was isolated from a freshwater fish in Korea and tentatively named Korean isolate YM-4. The morphologic characters of its cysts were similar to those of A. culbertsoni and A. royreba, which were previously designated as Acanthamoeba group III. Based on experimentally infected mouse mortality, Acanthamoeba YM-4 was highly virulent. The isoenzymes profile of Acanthamoeba YM-4 was similar to that of A. royreba. Moreover, an anti-Acanthamoeba YM-4 monoclonal antibody reacted only with Acanthamoeba YM-4, and not with A. culbertsoni. Random amplified polymorphic DNA marker analysis and RFLP analysis of mitochondrial DNA and of a 18S small subunit ribosomal RNA, placed Acanthamoeba YM-4 in a separate cluster based on phylogenic distances. Thus Acanthamoeba YM-4 was identified as a new species, and assigned Acanthamoeba sohi. Up to the year 2002 in Korea, two clinical cases were found to be infected with Acanthamoeba spp. These patients died of meningoencephalitis. In addition, one case of Acanthamoeba pneumonia with an immunodeficient status was reported and Acanthamoeba was detected in several cases of chronic relapsing corneal ulcer, chronic conjunctivitis, and keratitis.

Pathogenic free-living amoebae

Studies on pathogenic free-living amoebae performed in Korea were briefly reviewed. One strain of Naegleria fowleri was isolated from the sewage, and 3 strains of Acanthamoeba culbertsoni from a reservoir and the gill of fish. They were identified by morphological characteristics. Three strains among the 4 were experimentally proved pathogenic to cause primary amoebic meningoencephalitis in mice. The virulence of N. fowleri depended upon various factors such as strain, weight and sex of mice, and number of the inoculated amoebae. A. culbertsoni was found to retain cytolytic activities which were related to the pathogenicity. Also these amoebae were demonstrated to harbour the activities of acid phosphatase, peroxidase and ATPase. There is one case record of systemic Acanthamoeba infection in Korea, proved at autopsy. Cellular or humoral responses to these amoebae have been studied in vitro or in vivo. Immunization could reduce the mortality of experimentally infected mice.

[Cell-mediated immunity in experimental amoebic meningoencephalitis]

Cell-mediated and humoral immune reactions in mice infected with pathogenic Acanthamoeba culbertsoni were observed according to the period of time after amoebic infection by intranasal inoculation. The degrees of blastogenesis of spleen cells induced by mitogens, which were measured using radioactive [3H]-thymidine, were compared between infected and non-infected control groups. The mitogens used in this blastogenesis experiment were concanavalin A (Con A) and lipopolysaccharide(LPS). On the other hand, enzyme-linked immunosorbent assay(ELISA) was employed for the detection of humoral antibodies against A. culbertsoni. The levels of blastogenesis of splenocytes and serum titres in the experimental group showed increasing tendency a week after inoculation of A. culbertsoni, although there was no difference between the experimental and control groups in other periods of the experimental time.

[Passive immunity by splenocyte transfer against amebic meningoencephalitis in mice]

The role of passive cell-mediated transfer of immunity against primary amoebic meningoencephalitis(PAME) in mice was studied. Naegleria fowleri, ITMAP 359, were cultured in CGVS medium. The ICR mice used were six week-old males of average weight of 15 g. Immunization was done by three intraperitoneal injections of l x 10(6) N. fowleri trophozoites at the interval of one week. Splenocytes were obtained from normal and immune mice spleens, and 1 x 10(7) cells were administered intraperitoneally into mice 3 days before challenge infection. Mice were infected intranasally with 7 x 10(4) N. fowleri trophozoites in a 3 microliter suspension under secobarbiturate anesthesia. Transplants of normal or immune splenocytes seem to alter the pattern of the PAME development. The splenocytes transferred from immune mice reduced the mortality rate in the N. fowleri infected mice, as compared with the mice transferred with the same number of normal splenocytes or without splenocyte. The blastogenic response of the splenocytes to both lipopolysaccharide and concanavalin A was elevated on day 7 after infection the mice transinoculated with immune splenocytes. The serum antibody titers in the mice transferred with immune splenocytes were increased gradually from day 7 up to day 20 after infections by mean of ELISA. It is suggested that the transfer of splenocytes from immunized mice conferred immunity against N. fowleri infection.