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Benzo[a]pyrene-induced immunotoxicity: comparison to DNA adduct formation in vivo, in cultured splenocytes, and in microsomal systems.

Benzo[a]pyrene (BaP)/DNA adduct formation appears to be involved in carcinogenesis, but the relationship between adduct formation and BaP-induced immunotoxicity is unknown. We compared DNA adduct formation (32P-postlabeling analysis) to suppression of polyclonal immune responses (3H-TdR incorporation and IgM secretion) and decreases in cell viability in B6C3F1 female mouse splenic leukocytes (SPL). BaP administration (200 mg/kg, ip) resulted in suppression of polyclonal responses and substantial DNA adduct formation in mouse SPL. SPL adduct levels were similar to those in liver, lung, kidney, and stomach. In vitro exposure of SPL to BaP without rat liver activation enzymes (S9) caused decreases in SPL viability and immune responses that were dependent on dose and exposure period. However, DNA adduct formation in SPL was very low between 1 and 200 microM BaP. S9 enhanced the toxicity of BaP for SPL cultures. Adduct formation was rapid and dose related in +S9 incubates. The low level of BaP activation by SPL was confirmed in microsomal incubations in which splenic microsomes exhibited much lower aryl hydrocarbon hydroxylase (AAH) activity and ability to form DNA-adducting metabolites than did microsomes from liver or lung. Results indicate that immunosuppression produced by BaP in these systems was due to cytotoxic effects. It appears that these effects were caused by two separate mechanisms, one dependent on and one independent of DNA adduct formation. Since SPL had high levels of DNA adducts after ip injection of BaP, reactive metabolites of BaP may be involved in the immunotoxicity seen in vivo.