T-cell responses to pathogenic free-living amoebae, Acanthamoeba sp.,
were analyzed in healthy Japanese individuals. Of 20 healthy subjects,
10 (50%) showed significant proliferative responses of peripheral blo
od mononuclear cells to the soluble amoebic antigens in vitro. The ant
igens used were not mitogenic, and no evidence of amoebic superantigen
s was available. We established human T-cell clones reactive to Acanth
nmoeba, all of which were CD3- and CD4-positive, CD8-negative, and TCR
-alpha beta-positive. We isolated two strains of Acanthnmoeba from two
patients, one from a patient with meningoencephalitis (CSF strain) an
d the other from a patient with keratitis (K strain). Of 13 clones, 11
were reactive to the K-strain as well as to the CSF-strain antigen un
der human leukocyte antigen (HLA)-DR restriction, whereas the other tw
o were specific for the K-strain antigen. All but one clone tested sho
wed TH1-equivalent functions because these cells produced interferon (
IFN)-gamma in response to the amoebic antigen but produced no detectab
le level of interleukin 4 (IL-4). These results suggest that immunocom
petent hosts might have acquired protective immunity mediated by Acant
hamoeba-specific T-cells during natural sensitization.