The effects of prolonged administration of 5-bromodeoxyuridine on cells of the immune system.

We have determined the in vivo effect of 5-bromodeoxyuridine (BrdU) administered to mice in the drinking water for various lengths of time on the performance of T and B lymphocytes in a number of experimental protocols. Young mice continuously exposed to BrdU fail to gain weight, and the lymphocytes recovered after a prolonged period of exposure are fewer in number than in control mice. The recovery of normal levels of T and B lymphocytes after irradiation is severely impaired. Ag-specific cells responding to Ag in an adoptive transfer model fail to expand as much in the presence of BrdU as in the absence, and the Ag-specific effectors produced in the presence of BrdU are less able to secrete cytokines upon restimulation in vitro. Polarized populations of Tc1 and Tc2 effectors generated in vitro proliferate less in the presence of BrdU, and the resulting effectors make less cytokines per cell upon restimulation. Thus, the incorporation of BrdU into T or B lymphocytes can, under some circumstances, seriously impair the performance of the labeled cells, and these findings raise a note of caution in the interpretation of studies that make use of long-term exposure to BrdU.