A portion of a
cDNA encoding a 35-kDa
antigen from Toxoplasma gondii was cloned into the CKS expression vector and expressed in Escherichia coli. By using the
enzyme-linked
immunosorbent assay (ELISA), the
recombinant protein (rP35
antigen) was examined for reactivity with
immunoglobulin G (
IgG)
antibodies in the sera of pregnant women. Of these women, 41 had a toxoplasma serologic profile suggestive of recently acquired T. gondii
infection (Sabin-Feldman
dye test [DT] titers from 1:256 to 1:32,000, positive
IgM ELISA titers from 2.3 to 9.7, positive
IgA ELISA from 1 to >28, and acute patterns in the differential agglutination [AC/HS] test) (group I), and 50 women had a toxoplasma serologic profile suggestive of
infection acquired in the distant past (low DT titers from 1:16 to 1:512, negative
IgM ELISA titers from 0 to 0.8, and chronic patterns in the AC/HS test) (group II). The classification of acute or chronic profile was based on the individual's clinical history as well as the combination of the results of the toxoplasma serological profile. An additional group (group III) was composed of sera from 50 women who were seronegative for T. gondii
antibodies in the DT. The results revealed that whereas 85.3% of women in group I had
IgG antibodies that reacted with the rP35
antigen, only 8% of women in group II had
IgG antibodies that reacted with the same
antigen. In immunoblots, the rP35
antigen was recognized by
IgG antibodies in a pool of sera from individuals with a toxoplasma serologic profile compatible with acute
infection but not in a pool of sera from individuals with a serologic profile characteristic of a
chronic infection. These results reveal that
IgG antibodies against the P35
antigen are produced during the acute stage of the
infection but are uncommon in the latent or chronic phase of the
infection. Thus, the rP35
antigen may be a useful serologic marker to differentiate between recently acquired
infection and that acquired in the more distant past.