The presence of abundant macrophages is characteristic morphological hallmark for the diagnosis of Burkitt´s lymphoma. They are considered to be the removers of cellular debris which occurs due to the rapid proliferation of such lymphomas. However, although macrophages represent the main non-malignant population within such lymphomas and therefore can be considered a substantial part of the stroma, little is known about how these cells are being recruited and how they influence malignant growth. Since malignant B–cells show high plasticity and have capacity to differentiate into various phenotypes we analysed the origin of macrophages in a murine model of Burkitt´s lymphoma. In this transgenic model a human c-myc gene is overexpressed in a B-cell specific fashion.
When long term cultured lymphoma cells from transgenic animals were transferred into wild type animals expressing the congenic marker CD45.1 lymphomas grew out within 14 days post transfer. Lymphomas were minced into single cell suspension and macrophages were sorted by flow cytometry based on the expression of CD11b and F4/80. Approximately 6% of the cells expressed F4/80 and CD11b as macrophage markers and displayed an M2 phenotype. Surprisingly approximately 50% of these macrophages also expressed CD45.2 and therefore were not of host origin but derived from the transferred lymphoma cells. These cells were positive for the c-myc transgene, but displayed reduced expression of c-myc as shown by RT PCR in comparsion to the lymphoma cell line in culture. Furthermore, lymphoma derived macrophages displayed reduced PAX5 expression and were negative for CD19 surface expression. When lymphoma cells were retrovirally transduced to express bcl2 to block apoptosis, and were treated with chemotherapy in vivo, we observed a massive increase in macrophage numbers in remaining tissues suggesting transdifferenziation as an alternative exit strategy in case of cellular stress.
These results demonstrate that BL cells are able to differentiate into a macrophage phenotype to form their own supporting stroma. In addition, differentiation after DNA damage such as chemotherapy is increased suggesting a new exit strategy when classic apoptotic pathways are blocked.
Mackensen: Eli Lilly: Consultancy.
Asterisk with author names denotes non-ASH members.