Antigenic differences among osteogenic sarcoma tumor cells taken from different locations in human tumors

Sections were taken from the center, midzone, and margin of four human osteogenic sarcomas and one fibrosarcoma. Single-cell suspensions of tumors were examined in an indirect immunofluorescence assay with autologous or homologous anti-osteogenic sarcoma antisera as the intermediate reactant and fluorescein-labeled anti-human IgG as the final reactant. Cells were stained under conditions in which the fluorescence intensity was directly proportional to the density of the tumor-associated antigen on these cells. The density of tumor-associated antigen on cells from the center of the five tumor masses was low; cells from the midzone had intermediate levels of tumor antigen density, and cells at the margin had the highest levels. Similar preparations stained with polyspecific anti-HLA antisera did not demonstrate such a gradient. Since osteogenic sarcomas grow outward from the center, with the outer margin populated by the youngest cells, these results suggest that the oldest cells in the tumor bear the least tumor antigen, and the youngest tumor cells have the most. This is not compatible with theories which postulate that the immune system modulates the growth of a tumor so that only the least antigenic cells are allowed to grow. Alternative mechanisms are discussed.