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Tumour cell binding by a human monoclonal IgM antibody from the spleen of a non‐tumour‐associated patient is due to somatic mutations in the VH gene

Recently we described the occurrence of B cells producing polyspecific natural IgM with anti‐tumour specificity in the spleen of non‐tumour‐bearing individuals as well as in fetal organisms. Immunoprecipitation and 2‐D electrophoresis showed the binding of such antibodies to a 55‐kD (pI 60) membrane surface glycoprotein. In vitro cultivation of human cancer cell lines in the presence of the purified IgM antibodies resulted in growth inhibition and complement‐mediated cell lysis. Furthermore, the antibodies were shown to be able to induce MHC class I molecule expression on tumour cells. Because of this, a role for naturally occurring antibodies with anti‐tumour specificity in preventing neoplasias had been suggested. We have constructed and expressed in Escherichia coli single‐chain fragments (scFv: VH‐linker‐VL) derived from a polyspecific human monoclonal IgM autoantibody produced by a human × mouse heterohybridoma which was obtained from the spleen of an autoimmune patient. The mutated complementarity determining region (CDR) gene segments were replaced by the equivalent germ‐line sequences and the CDR3 region was swapped for that from another polyspecific human natural antibody with no binding to tumours. Using these four scFv constructs for binding analyses and in vitro cultivation experiments we found: (i) scFv containing the mutated VH region of the original antibody were able to bind to tumour cells, to induce MHC class I molecule expression, and to inhibit tumour growth in a way similar to what had been described for the complete antibody; (ii) replacement of the mutated by the germ‐line VH gene independently of the CDR3 to which it had been recombined, resulted in failure to bind to tumour cells. Nevertheless, other antigens (ssDNA, tetanus toxin) were still recognized, although with lower affinity. We discuss the significance of the replacement mutations in the VH gene CDRs. selected probably by B cell contact to an (auto)antigen, for generating a tumour binding capacity, not encoded by the germ‐line gene.