Abnormal increase in multinuclear macrophages in primary cultures of BUF/Mna rat thymomas

The in vitro characteristics of spontaneous thymomas from BUF/Mna rats, a strain with a high incidence of these tumors, were studied. In primary cultures, the adherent cells consisted of mononuclear macrophages, mono- and multi-nuclear epithelial cells and some fibroblastic cells on day 3. The macrophages rapidly increased in number with the formation of large multinuclear cells by day 9. A modest increase in the number and nuclearity of macrophages was also noted in adherent cultures of normal thymuses from 5-week-old BUF/Mna rats. On the other hand, in cultures of thymic cells from 1-year-old or 5-week-old ACI/NMs rats, a normal control rat strain, macrophages did not increase in number and only rarely formed multinuclear cells in adherent cell cultures. These results suggest that abnormal proliferation signal(s) to thymic macrophages and/or their progenitor cells accompanies and may be involved in the development of thymomas in BUF/Mna rats.