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Early Block in Maturation Is Associated with Thymic Involution in Mammary Tumor-Bearing Mice¹

Becky Adkins^*, Vijaya Charyulu, Qi-Ling Sun^*, David Lobo^* and Diana M. Lopez^2,*

^* Department of Microbiology and Immunology, University of Miami Medical School, Miami, FL 33136; and Department of Medical Technology, Florida Atlantic University, Boca Raton, FL 33443

We previously reported that mice implanted with mammary tumors show a progressive thymic involution that parallels the growth of the tumor. The involution is associated with a severe depletion of CD4⁺8⁺ thymocytes. We have investigated three possible mechanisms leading to this thymic atrophy: 1) increased apoptosis, 2) decreased proliferation, and 3) disruption of normal thymic maturation. The levels of thymic apoptosis were determined by propidium iodide and annexin V staining. A statistically significant, but minor, increase in thymic apoptosis in tumor-bearing mice was detected with propidium iodide and annexin V staining. The levels of proliferation were assessed by in vivo labeling with 5'-bromo-2'-deoxyuridine (BrdU). The percentages 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⁺ cells increased, while the percentage of CD25⁺ cells decreased among CD4^-8^- thymocytes from tumor-bearing vs control animals. Together, these findings suggest that the thymic hypocellularity seen in mammary tumor bearers is 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.

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