Previous studies of Ehrlichia chaffeensis infection in the mouse have demonstrated that passive transfer of polyclonal Abs from resistant immunocompetent mice to susceptible SCID mice ameliorated infection and disease, even when Abs were administered during established infection. To identify particular Abs that could mediate bacterial clearance in vivo, E. chaffeensis-specific mAbs were generated and administered to infected SCID mice. Bacterial infection in the livers was significantly lowered after administration of either of two Abs of different isotypes (IgG2a and IgG3). Moreover, repeated administration of one Ab (Ec56.5; IgG2a) rescued mice from an otherwise lethal infection for at least 5 wk. Both protective Abs recognized the E. chaffeensis major outer membrane protein (OMP)-1g. Further studies revealed that both Abs recognized closely related epitopes within the amino terminus of the first hypervariable region of OMP-1g. Analyses of human sera showed that E. chaffeensis-infected patients also generated serological responses to OMP-1g hypervariable region 1, indicating that humans and mice recognize identical or closely related epitopes. These studies demonstrate that OMP-specific mAbs can mediate bacterial elimination in SCID mice, and indicate that Abs, in the absence of cell-mediated immunity, can play a significant role in host defense during infection by this obligate intracellular bacterium.

Naegleria fowleri is an etiological agent of primary amoebic meningoencephalitis in humans and laboratory animals. The determinative factors in host resistance of mice to N. fowleri infections have not been fully characterized. Male or female B6C3F1 mice stimulated by intraperitoneal administration of 10(6) amoebae of N. fowleri nN68 per mouse produced agglutinating activity and markedly elevated levels of serum and immunoglobulins M and G. Despite a marked humoral response, protective immunity was increased only marginally by active immunization. Host resistance was not impaired by prior treatment with 350 rads of 60Co radiation or 200 mg of cyclophosphamide per kg or by concurrent daily treatment with 30 mg of cyclophosphamide per kg for 14 days. Moreover, host resistance was not impaired by daily treatment with 4 mg of diethylstilbestrol per kg for 14 days, with challenge on day 2 of drug exposure or 24 h after the last drug treatment. Mice depleted of hemolytic complement by cobra venom factor were more susceptible to N. fowleri infection than were untreated mice.

The capability of serum samples from 423 human subjects to agglutinate rounded cells of Naegleria fowleri nN68 was assessed. Sera from the umbilical cords of seven infants failed to agglutinate N. fowleri cells. The median agglutination titer was 1:4 for sera from children through age 4 years, 1:8 for sera from juveniles 5 to 15 years of age, and 1:16 for sera from subjects 15 to 30 years old. The agglutination titers of sera from older adults decreased to a median of 1:8 for the 40to 60-year-old age group and to 1:4 for the 60-to 90-year-old subjects. Serum samples from young adults agglutinated rounded cells of both N. fowleri and N. gruberi. The agglutination activity for N. fowleri was removed by absorption with N. fowleri but not with N. gruberi. Conversely, agglutination activity for N. gruberi was removed by absorption with N. gruberi but not with N. fowleri. The agglutinating activity for N. fowleri was immunoglobulin M. Serum samples from children displayed markedly disparate capabilities to agglutinate N. fowleri and N. gruberi. Only rounded cells of N. fowleri or N. gruberi were reliably agglutinated by human serum samples. Live or paraformaldehyde-killed cells could be used in the assay, but live N. gruberi cells returned to the amoeboid form, and these agglutinated poorly.

The capability of 154 serum samples from pediatric outpatients and 101 samples from adults to agglutinate amoebae of Naegleriafowleri nN68 was assessed. Sera from all 19 infants tested had an agglutination titer of 1:4 or less; sera of toddlers had a median agglutination titer of 1:8 and those of adults, 1:16. Only 13 of 154 serum samples from children had titers of 1:32 or 1:64; 7 of these high titered sera were from the 38 asthmatic children. Selected sera were found to give comparable titers when assayed against eight other strains of N . fowleri. Agglutination activity was removed by absorption with N . fowleri but not by absorption with N . gruberi. The agglutinating activity was specific for the human pathogenic N . fowleri. The ubiquitous free-living N . gruberi was not the immunogen responsible for the agglutinating activity in human serum.