Growth inhibition of Babesia bovis in culture by secretions from bovine mononuclear phagocytes.

Bovine blood mononuclear phagocytes from babesia-free cattle were cultured in vitro. Cell monolayers were treated with culture-derived soluble Babesia bovis exoantigens, immune complexes, and bovine anti-B. bovis immune serum. Subsequently, the monolayers were washed free of the reagents and allowed to develop further in the presence of standard culture medium. Transfer of supernatant media from these cultures to those of B. bovis revealed the presence of growth-inhibiting factors. These factors were thermostable, nondialyzable, and were degraded by freeze-thawing, and their action was concentration dependent. Supernatants from antigen- and immune complex-treated monolayers demonstrated greater inhibitory effects than did supernatants from antibody-treated or untreated monolayers. Erythrocytes incubated with supernatant medium from antigen-treated monolayers did not support growth of B. bovis as well as did erythrocytes incubated with supernatants from untreated monocyte monolayers. This result suggests that the mechanism of action of soluble factors could be through some modification of the erythrocyte such as the blockage of active transport of essential nutrients.