Phagocytosis of antibody-sensitized Trypanosoma brucei in vitro by bovine peripheral blood monocytes.

The interaction between bovine peripheral blood monocytes and Trypanosoma brucei organisms in the presence of immune whole serum or specific purified IgM or IgG1 antibodies from infected cattle was investigated. Adherence of antibody-sensitized trypanosomes to freshly isolated monocytes from non-infected animals was not observed, confirming the absence of receptors for IgM and IgG1 on fresh monocytes. After in-vitro cultivation of the monocytes for at least 3 hr, adherence of IgM-sensitized trypanosomes was consistently demonstrable, while the adherence of IgG1-sensitized trypanosomes was not observed until the monocytes had been cultured for 7 days. The adherence of IgM-sensitized trypanosomes occurred in the absence of bovine complement and was Ca++-dependent, indicating that the interaction between sensitized trypanosomes and bovine monocytes was mediated through a specific combining site for monocytes in the Fc region of the IgM molecule rather than through a C3b receptor. In contrast to the monocytes from uninfected animals, freshly isolated peripheral blood monocytes from T. brucei- or T. congolense-infected cattle were capable of binding IgM- or IgG1-sensitized trypanosomes after only 30 min and 24 hr of in-vitro cultivation, respectively, suggesting that peripheral blood monocytes from infected animals were already in an "activated' state. Electronmicroscopic examination revealed that IgM- or IgG1-sensitized trypanosomes were rapidly engulfed and digested by the monocytes.