Chlamydial isolates of bovine origin were serotyped by a plaque reduction method. Of the two major serotypes observed, type 1 included isolates from bovine abortion and enteric infections, whereas type 2 isolates were associated with polyarthritis or encephalomyelitis. These two serotypes were identical to those with a similar disease distribution previously observed in isolates of ovine origin. The two groups did not cross-react and they were serologically unrelated to chlamydiae of avian origin. Thus, it appears that many chlamydial isolates causing intestinal infections or abortion in sheep or cattle are closely related antigenically, as are those producing polyarthritis, encephalomyelitis, and conjunctivitis, and that the two groups are distinct.

Eight chlamydial isolates of ovine origin were tested in a plaque reduction system using homologous and heterologous rooster antisera. The eight isolates could be separated into two separate immunotypes. Type 1 included isolates associated with ovine abortion and one agent recovered from the feces of an apparently normal sheep. Type 2 isolates were associated with polyarthritis and conjunctivitis. These two serotypes were not cross-reactive with several chlamydiae of avian origin. Further application of the plaque reduction test may provide a useful means of typing chlamydiae.

A role for both the cellular and humoral components of the immune response has been established for chlamydial infection. The significance of helper (L3T4) T cells was evaluated by using a Chlamydia trachomatis murine salpingitis model for upper genital tract chlamydial infection. Mouse oviducts were inoculated with C. trachomatis by using the mouse pneumonitis agent (MoPn) or control medium. Mice depleted of L3T4-bearing lymphocytes had significantly higher (P < 0.05) numbers of organisms recovered at day 7 postinoculation. The rate of hydrosalpinx formation was significantly higher in the mice depleted of L3T4-bearing lymphocytes (27 of 31 [87%]) than in the infected undepleted group (8 of 16 [50%]) (P < 0.01). The geometric mean antichlamydial immunoglobulin G titers at day 54 postinoculation were significantly higher in the L3T4depleted mice (mean titer, 2,030) than in the undepleted group (mean titer, 776; P < 0.05). The rate of fertility was lower in the L3T4-depleted group (2 of 31 [6%]) than in the infected, undepleted mice (2 of 16 [13%]), but this difference did not reach statistical significance. In conclusion, the greater persistence of organisms in the oviduct and higher rates of hydrosalpinx formation in mice depleted of L3T4-bearing cells suggests that these cells play a role in the clearing of organisms following infection and thus in reducing the degree of oviduct obstruction and damage.

Some members of the genus Chlamydia (Bedsonia or psittacosis-lymphogranuloma venereum-trachoma-inclusion conjunctivitis group of microorganisms) produce plaques in monolayers of mouse fibroblast (L-929) cells. Hyperimmune chicken antisera may be capable of specifically reducing plaque counts. When the test was applied to chlamydiae isolated from avian species, the results indicated that different isolates with a common source had similar antigenic reactivity. The plaque reduction test is a potentially useful method for serotyping chlamydiae. The difficult aspect of the method appears to be a readily reproducible means of producing neutralizing antiserum.