Early block in maturation is associated with thymic involution in mammary tumor-bearing mice

We previously reported that mice implanted with mammary tumors show a progr essive thymic involution that parallels the growth of the tumor, The involu tion is associated with a severe depletion of CD4(+)8(+) thymocytes. We hav e investigated three possible mechanisms leading to this thymic atrophy: 1) increased apoptosis, 2) decreased proliferation, and 3) disruption of norm al thymic maturation. The levels of thymic apoptosis were determined by pro pidium iodide and annexin V staining. A statistically significant, but mino r, increase in thymic apoptosis in tumor-bearing mice was detected with pro pidium iodide and annexin V staining. The levels of proliferation were asse ssed by in vivo labeling with 5'-bromo-2'-deoxyuridine (BrdU), The percenta ges of total thymocytes labeled 1 day following BrdU injection were similar in control and tumor-bearing mice. Moreover, the percentages of CD4(-)8(-) thymocytes that incorporated BrdU during a short term pulse (5 h) of BrdU were similar. Lastly, thymic maturation was evaluated by examining CD44 and CD25 expression among CD4(-)8(-) thymocytes, The percentage of CD44(+) cel ls increased, while the percentage of CD25(+) cells decreased among CD4(-)8 (-) thymocytes from tumor-bearing vs control animals. Together, these findi ngs suggest that the thymic hypocellularity seen in mammary tumor bearers i s not due to a decreased level of proliferation, but, rather, to an arrest at an early stage of thymic differentiation along with a moderate increase in apoptosis.