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A small percentage of healthy donors identified in the Western population carry antibodies in their peripheral blood which convey cytotoxic activity against certain human melanoma and neuroblastoma cell lines. We measured the cytotoxic activity of sera and plasmas from healthy donors on the human neuroblastoma cell line Kelly and various melanoma cell lines. Antibodies of IgM isotype, presumably belonging to the class of naturally occurring antibodies, exerted cytotoxic activity in a complement-dependent fashion. Apart from complement-dependent tumor cell lysis, we observed C3 opsonization in all tumor cell lines upon treatment with cytotoxic plasmas. Cell lines tested primarily expressed membrane complement regulatory proteins (mCRP) CD46, CD55 and CD59 to various extents. Blocking of mCRPs by monoclonal antibodies enhanced cell lysis and opsonization, though some melanoma cells remained resistant to complement attack. Epitopes recognized by cytotoxic antibodies were represented by gangliosides such as GD2 and GD3, as evidenced by cellular sialidase pretreatment and enhanced expression of distinct gangliosides. It remains to be clarified why only a small fraction of healthy persons carry these antitumor cytotoxic antibodies.

The alpha 5 beta 1 integrin (CD49e/CD29), a heterodimeric membrane protein, is the "classical" fibronectin receptor on many cell types. During B-cell ontogeny, expression of the alpha 5-subunit is developmentally regulated. The alpha 5 beta 1 is decisive for migration on fibronectin substrate and very likely cooperates with other adhesion molecules in transvascular trafficking. To test whether alpha 5 beta 1 influences local growth vs. disseminative spread of neoplastic B-cells in vivo, human B-cell lines mimicking different maturational stages were transferred s.c. into severe combined immunodeficiency (SCID) mice and examined for alpha 5 beta 1 expression and for adherence on fibronectin substrate in vitro and ex vivo. All cell lines were locally tumorigenic. Dissemination was observed in all animals carrying Nalm-6 tumors, in one animal with a BL 60 and in 2 mice carrying a Raji tumor. By contrast, Daudi, BJAB and U266 tumors did not disseminate. As evidenced by immunohistochemistry and flow cytometry, all lines and their tumors were to various extents beta 1-positive but showed considerable differences in alpha 5 expression. The functional surface expression of alpha 5 beta 1 correlated with fibronectin adherence of the lines. Daudi expressed alpha 5 beta 1 in a non-functional configuration which was rendered functional only upon applying high concentrations of Mg++ and Mn++. B-cell lines functionally expressing alpha 5 beta 1 at high or moderate levels disseminated in SCID mice while alpha 5-negative lines and Daudi did not. These results support the conclusion drawn from an earlier in situ analysis of human B-cell lymphomas/leukemias that the alpha 5 beta 1 integrin contributes to the disseminative phenotype of malignant B cells.

Two monoclonal antibodies, HD 66 and CRIS-4, by which the new CD 76 B-cell-associated cluster was defined, bound to several gangliosides (sialic acid containing glycolipids) of different polarity. One of the gangliosides recognized by HD 66 could be identified as NeuAc alpha 2-6Gal beta 1-4GlcNAc beta 1-3Gal beta 1-4Glc-beta 1-1'Cer. This antigen was enzymatically synthesized. Sialidase treatment of the ganglioside antigens abolished binding of HD 66 and CRIS-4.