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Acanthamoeba castellanii  is a pathogenic and opportunistic free-living amoeba that causes  Acanthamoeba  keratitis (AK) and granulomatous amebic encephalitis (GAE) in immunocompromised individuals. The biological and pathogenic characterizations behind this opportunistic protozoan is not fully understood. This study aimed to determine the biological functions of heat shock protein (HSP)-20 of  A. castellanii  ( Ac -HSP20) involved in the maintenance of life cycle and the infectivity of  A. castellanii . Immunoscreening  A. castellanii  cDNA library with  A. castellanii  infected rabbit sera identified three positive clones, one of them was a putative heat shock protein ( Ac -HSP20). The recombinant 23 kDa  Ac -HSP20 protein (r Ac -HSP20) was successfully expressed in  Escherichia coli  BL21 (DE3) and purified using metal affinity chromatography. The rabbits immunized with r Ac -HSP20 produced high titer antibody (1:25,600). Immunolocalization with the antibody identified the expression of native  Ac -HSP20 on the surface of both  A. castellanii  trophozoites and cysts. Further, Western blot with antibody identified that the expression of native Ac-HSP20 was 7.5 times higher in cysts than in trophozoites. Blocking  Ac -HSP20 on the membrane of trophozoites with specific antibody or silencing  Ac-hsp20  gene transcription by siRNA inhibited their transformation into cysts at the early stage but returned to normal at the late stage by stimulating the transcription of  Ac-hsp20 . Incubation of trophozoites with anti- Ac -HSP20 IgG increased macrophage-involved phagocytosis to the protozoa and inhibited trophozoite infectivity on the cornea of rabbits compared with that without antibody. Our study provides that  Ac -HSP20 is a surface antigen involved in the encystation and infectivity of  A. castellanii  and thus an important target for vaccine and drug development.

Ac-HSP20 Is Associated With the Infectivity and Encystation of Acanthamoeba castellanii