Effects of prolonged low-level cadmium exposure on the tadpole immune system.

Uninjected (Group I) and sheep erythrocyte (SRBC)-injected (Group II) Rana tadpoles were exposed to varying sublethal concentrations of cadmium (Cd) for 6 weeks. In order to assess possible effects on the tadpole immune system we determined pre-B, B mu, and plasma cell (PC mu) frequencies in liver and mesonephros of Group I larvae, and hemagglutinating antibody (HA) titers of Group II animals. Group I and Group II control animals were cultivated in water with no added Cd (0 ppm), while treatments were set at 0.1, 0.2, 0.4 and 0.8 ppm Cd. We found that (a) Cd appeared to stimulate a significant increase in the frequency of B mu cells in animals treated at 0.4 and 0.8 ppm, (b) certain treated Group II larvae contained significantly increased amounts of HA in their serum, while their serum protein concentrations were not significantly different, and (c) there was a dose-related increase in tissue Cd levels in treated Group II larvae. Our observations suggest that chronic low-level exposure to Cd may (a) result in a slight increases in the frequency of B mu cells in unimmunized animals, (b) increase immune responsiveness of immunized larvae, and (c) increase liver and mesonephros accumulations of Cd. Preliminary studies indicated that cytosolic Cd is associated with a protein which appears to be similar to mammalian metallothionein.