Aberrant secondary antibody responses to sheep erythrocytes in rabbits with experimental syphilis.

Rabbits infected with Treponema pallidum have strikingly depressed in vivo immunoglobulin G responses to sheep erythrocytes. To gain further insight into the nature of this suppression, the immune responses of splenic and peripheral blood lymphocytes from infected rabbits to sheep erythrocytes were studied in vitro. Spleen cells from rabbits that had been sensitized with sheep erythrocytes during active syphilis had greatly decreased immunoglobulin M and G responses after in vitro incubation with sheep erythrocytes, when compared to the results obtained with cells from sensitized uninfected animals. Suppressor cells could be demonstrated in peripheral blood lymphocytes of control rabbits 6 months after sensitization with sheep erythrocytes; these cells could be removed by nylon wool filtration. When primary sensitization with sheep erythrocytes was carried out during active syphilis, these suppressor cells were not detectable in peripheral blood lymphocytes 6 to 9 months later. These findings provide further evidence that induction of immune responses may be abnormal early in treponemal infection and may help to explain the failure of the host to produce antibodies which eradicate the organism during the first 2 to 3 months of infection.