Strongyloides stercoralis: role of antibody and complement in immunity to the third stage of larvae in BALB/cByJ mice.

Mice immunized against Strongyloides stercoralis L3 were shown to kill greater than 90% of challenge larvae contained within diffusion chambers. The objective of the present study was to identify the host components responsible for immunity. Serum from unprotected, control mice and protected, immune mice in doses of 25-500 microliters was transferred into naive mice at the same time and location as larval challenge. Transfer of as little as 50 microliters of immune serum was able to confer protective immunity. The serum-transferred immunity was ablated by excluding cells from the larval microenvironment or by depleting granulocytes through monoclonal antibody treatment in the recipient mice. Specific antibody isotypes were isolated using protein G and isotype-specific affinity columns. The resulting transfer experiments identified IgM as the isotype responsible for protective immunity to S. stercoralis L3. Antibody binding studies in vivo were performed and only IgM bound to the surface of infective L3 and host-derived L3 (L3+) in immune animals. Elevated levels of C3 were also found bound to the surface of L3/L3+ in immune mice. Cobra venom factor treatment of immunized mice to deplete complement completely eliminated C3 binding to the surface of L3/L3+ and ablated immunity. Therefore, IgM, complement, and granulocytes are necessary for immune elimination of S. stercoralis L3/L3+. Identification of antigens recognized by IgM may help select possible vaccine candidates.