Enzyme histochemical comparison of biomphalaria glabrata amebocytes with human granuloma macrophages

In fresh water snails, amebocytes are the principal cells that react to parasitic infection. Ultrastructurally, amebocytes resemble mammalian macrophages. To clarify the relationship between amebocytes and macrophages, we compared the histochemical staining for seven enzymes in Biomphalaria glabrata snail amebocytes, both in the amebocyte-producing organ (APO) and in the encapsulation reaction formed around parasite sporocysts with the staining in macrophages from the lymph nodes of patients with sarcoid or tuberculosis. Snails were infected with Echinostoma paraensei and Schistosoma mansoni miracidia. APOs and ventricular tissue with encapsulated parasites were fixed and embedded in glycol methacrylate monomer. Hardened blocks were sectioned at 2 micron and stained for alkaline phosphatase, acid phosphatase, alpha-naphthyl acetate esterase (ANAE), ATPase, peroxidase, 5'nucleotidase, and chloroacetate esterase. The amebocyte-producing organ contained cells that were positive for acid phosphatase, ANAE, and ATPase. Amebocytes in the capsules formed around echinostome sporocysts showed stronger staining for the same three enzymes. Capsules did not form around schistosome sporocysts, but the connective tissue around them contained numerous amebocytes that were also positive for these three enzymes. The amebocyte enzyme histochemistry resembled that in human granuloma macrophages, but differed from that in neutrophils. The increased expression of enzymes in amebocytes involved in the encapsulation reaction as compared to those in the APO was reminiscent of the alterations observed when human monocytes convert to tissue macrophages. These studies support the hypothesis that the amebocyte is an "invertebrate macrophage."