We have shown previously that IgM from Ehrlich tumor (ET)-immunized mice, recognizing ET cell surface carbohydrates, protects control mice to a subsequent tumor challenge. The factors involved in such IgM-mediated protection were unknown, since it was independent of complement activation. Here, we have extended these in vivo studies by means of monoclonal IgM antibodies. Two of them (A10 and E1), strongly recognizing ET cells and with specificity to ET cell surface carbohydrates, were selected. The results show that A10 (but not E1 or unrelated IgM antibodies) is able to protect nonimmunized mice against ET growth. Protection by A10 was also seen by reducing 800-fold the initial dose; however, E1 was unsuccessful whatever the dose used. A10-mediated protection was observed in C3-defective mice (cobra venom factor treated) or in C5-deficient DBA/2, but not in silica-treated animals. Endotoxin removal did not affect the protection afforded by A10 while specific IgM depletion prevented any protective effect. In addition, the relationship between natural antibodies of IgM isotype recognizing ET cell surface carbohydrates and mouse strain resistance to this tumor is established. Similarly, this natural resistance seems to be complement independent but macrophage mediated. Therefore, these results indicate that some IgM molecules recognizing cell surface carbohydrates may participate in in vivo tumor suppression by a macrophage-dependent mechanism.

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This research was supported by Grants 86/647 and 89/052 from the Fondo de Investigaciones Sanitarias, Spain.

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