Suggestive evidence that the highly metastatic variant ESb of the T-cell lymphoma Eb is derived from spontaneous fusion with a host macrophage

Two lines of evidence are reported which suggest that the highly metastatic variant ESb of the T-cell lymphoma Eb is derived from spontaneous fusion with a host macrophage. Firstly, ESb cells are shown to express the macrophage differentiation antigen Mac-1 which was not found on Eb cells or on any other tumor cells tested except the macrophage tumor line Pu5. Secondly, the progression from low to high metastatic capacity could be reproduced in vitro following hybridization of thioguanine-resistant Eb cells (EbTGR) with syngeneic bone-marrow-derived macrophages. Two HAT medium-selected hybrid tumor lines (Eb-F1 and Eb-F2) could be established. They were found to express cell surface markers of both parental lines: T lymphoid differentiation antigens from T-lymphoma and macrophage antigens (Mac-1, class II MHC antigens) from the normal cell fusion partner. The antigens were identified on the hybrids and subclones thereof by means of monoclonal antibodies and 3 different detection assays: cytofluorography, complement-dependent cytotoxicity and immunoprecipitation followed by gel electrophoresis. Animals inoculated s.c. with the parental line EbTGR developed local tumors but not metastases and survived for more than 40 days. In contrast, animals inoculated similarly with Eb-F1 or Eb-F2 cells quickly developed metastases in visceral organs and died as early as 10-14 days following inoculation. In many but not all respects, the in vitro-derived T-lymphoma-macrophage hybrids resembled the spontaneous in vivo-derived variant ESb. These findings, together with the presence of Mac-1 antigen on ESb cells, suggest (1) that ESb variant cells may be derived from spontaneous fusion with a host cell, most likely a macrophage and (2) that somatic cell fusion may be an important mechanism of genetic rearrangements leading to metastatic variants. The new highly metastatic tumor lines which were developed under well-defined in vitro conditions, and their subclones, may become very useful tools for studying the contribution of specific genetic traits and of membrane-related structures to various steps of the metastatic process.