Naegleria fowleri, the causative agent of primary amebic meningoencephalitis, is resistant to complement lysis. The presence of a complement regulatory protein on the surface of N. fowleri was investigated. Southern blot and Northern blot analyses demonstrated hybridization of a radiolabeled cDNA probe for CD59 to genomic DNA and RNA, respectively, from pathogenic N. fowleri. An 18-kDa immunoreactive protein was detected on the membrane of N. fowleri by Western immunoblot and immunofluorescence analyses with monoclonal antibodies for human CD59. Complement component C9 immunoprecipitated with the N. fowleri "CD59-like" protein from amebae incubated with normal human serum. In contrast, a gene or protein similar to CD59 was not detected in nonpathogenic, complement-sensitive N. gruberi amebae. Collectively, our studies suggest that a protein reactive with antibodies to human CD59 is present on the surface of N. fowleri amebae and may play a role in resistance to lysis by cytolytic proteins.Naegleria fowleri, an ameboflagellate found in freshwater lakes and ponds, is the causative agent of primary amebic meningoencephalitis, a rapidly fatal disease of the central nervous system (1, 7). The determinants of virulence for this ameba are unknown, but resistance to complement lysis appears to play a major role in its pathogenicity (13,30). Both pathogenic and nonpathogenic Naegleria species activate the alternative pathway of complement (16). However, pathogenic N. fowleri amebae are resistant to the lytic effect of complement (36). Immunoelectrophoretic studies have established that complement components C3 and C5 are converted to C3b and C5b, respectively, following incubation of complementresistant N. fowleri in normal human serum (NHS) (37). These results suggest that complement regulation by N. fowleri occurs at the stage of membrane attack complex (MAC) formation.CD59 is an 18-to 20-kDa glycosyl-phosphatidylinositol/inositol-anchored glycoprotein found on the surface of a variety of cell types which functions to inhibit complete formation of the MAC of complement. CD59 inhibition of complement lysis occurs by binding complement components C8 and C9, ultimately preventing C9 insertion into and polymerization in the cell membrane (9). The purpose of the present study was to determine whether complement-regulatory protein CD59, which is present on mammalian cells, could be detected on pathogenic N. fowleri amebae.Molecular and immunology-based assays were used to test the hypothesis. Southern blot analysis was performed to determine whether N. fowleri possesses the CD59 gene. Northern blot analysis was utilized to identify the presence of CD59 transcripts expressed by complement-resistant N. fowleri amebae. With monoclonal antibodies to human CD59, an immunoreactive protein was detected in the membrane fraction of N. fowleri by Western immunoblot analysis. Immunoprecipitation studies were used to establish whether the ameba "CD59-like" protein was able to associate with human complement component C9. In addition, an...