Neutrophil-mediated killing of Dipetalonema viteae microfilariae: simultaneous presence of IgE, IgG antibodies and complement is required.

Neutrophils from the peripheral washings of normal rats in the presence of sera obtained from rats immune to circulating microfilariae adhered to and killed the microfilariae of Dipetalonema viteae in vitro within 16-24 hr. No significant adherence or cytotoxicity was mediated by sera collected from animals with a high microfilaraemia or from normal rats. Ultrastructural studies show that neutrophils, which are bigger than microfilariae, can easily internalize the small larvae resulting in the disintegration of the parasite. Immunoadsorption and inhibition experiments showed that the adherence-promoting activity resides both in IgG and IgE classes of antibody. However, the mere participation of these two antibodies is not sufficient to effect neutrophil adherence towards microfilariae, the presence of complement is also required. Samples of fresh immune rat serum (fIRS) depleted in alternative pathway components of complement by treatment with zymosan A failed to mediate cell adherence to the parasite. fIRS inactivated for the classical pathway of complement by the chelating agent EGTA partially retains its activity in mediating cytotoxicity to microfilariae. The striking antigenic specificity of D. viteae antibodies was shown by their ability to mediate cytotoxicity only to D. viteae but not towards Brugia malayi microfilariae.