The action of macrophages and immune serum on growth of Babesia microti in short-term cultures.

The effect on growth of B. microti of macrophages from normal or B. microti recovered animals was tested in short-term cultures. Normal macrophages did not or only slightly inhibited parasite growth. When immune serum was added to the normal macrophage containing cultures, inhibition greater than that produced by immune serum alone was observed. Macrophages from immune animals had an inhibitory effect on parasite growth even in the absence of immune serum. Inhibition of B. microti growth was greater in cultures containing immune serum and immune macrophages than in cultures containing either immune serum or immune macrophages alone. Supernatants from B. microti cultures which contained immune macrophages or macrophages which had been stimulated by addition of immune serum inhibited parasite growth while cell free supernatant from cultures which contained no macrophages or normal macrophages did not. At 72 and 96 hr of culture erythrophagocytosis by macrophages was more prominent in immune serum containing B. microti cultures than in B. microti cultures not containing immune serum. More macrophages were retained in the immune serum containing cultures than in cultures not containing immune serum. Alternate culture-hamster passage yielded a B. microti strain which grew better in hamster erythrocyte short-term cultures than did the original strain.