EFFECTS OF 2,3,7,8-TETRACHLORODIBENZO-p-DIOXIN (TCDD) ON THE IN VITRO ANTIBODY RESPONSE: Differential effects on the B lymphocyte depending on the state of in vivo activation and the modulation by serum-derived growth factors

Previous reports have indicated a dichotomy in the actions of TCDD on humoral immunity, both in vitro and in vitro, in which enhancements and suppression have been identified. The latter effect has been correlated with induction of liver P4501A1 enzyme activity, a response which is regulated by the Ah-gene locus. Additionally, the primary alteration in suppression of antibody responses is in the differentiation of the B cell. Therefore, the current investigation was undertaken to determine the relationship between these dual actions of TCDD on humoral immunity as related to its direct actions on B lymphocyte function. Specific emphasis was placed on determining the potential role of the Ah-gene locus in the modulation of the B cell and in vitro antibody responses. Initial observations determined that: 1) the degree of suppression of in vitro antibody responses in B6C3F1 mouse (an Ah-high responder strain) splenocytes is dependent on both the lot, concentration, and type (i.e., fetal bovine versus newborn bovine) of serum used in culture; and 2) there is a similarity in the actions of TCDD and Staphylococcus aureus Cowan strain I (SAC); a polyclonal B cell activator. These observations prompted the study of the direct effects of TCDD on B cells in different stages of in vivo activation and in the presence of different serum environments for which SAC has been reported to have opposing actions. TCDD was found to increase background levels of proliferation and differentiation in resting B cells (Go); thereby suggesting that resting B cells are a primary target in enhancement of immune responses by TCDD. However, no effect was seen on either response when the cells were stimulated with lipopolysaccharide (LPS). In cycling B cells (G1), TCDD caused suppression of both background and LPS-stimulated proliferation and differentiation and demonstrated a serum dependency that paralleled its actions in whole lymphocyte antibody responses; thereby demonstrating that it is the cycling B cell that is the primary target in suppression of antibody responses by TCDD and that both proliferation and differentiation are affected. The modulatory role of serum was also determined in primary hepatocyte cultures using P4501Al enzyme induction as the endpoint. Sera which supported either no effect or enhancements in both whole lymphocyte and purified B cell responses did not support induction of P4501A1 activity above BSA, a protein control. Conversely, sera which supported a consistent and dose-related suppression of Mug antibody responses were found to enhance the induction of P4501A1 activity. More importantly, normal mouse serum was found to allow for the full expression of the Ah-dependent phenomena in vitro, where primary hepatocytes and splenocytes from Ah-high responder (B6C3F1) and Ah-low responder (DBA/2) mouse strains were affected by TCDD in a manner that parallels the effects seen in the two strains following in vivo exposures. The latter results using mouse sera are consistent with a role by the Ah-gene locus in the direct effects of TCDD on whole lymphocyte and purified B cell antibody responses. However, the results with other, more traditional sources of sera, indicate that the ultimate expression of the TCDD- induced responses can be modulated by serum factors, which are at present unidentified. Furthermore, the results of this investigation indicate that the susceptibility of the cell, as related to its stage in the cell cycle, can contribute to the complex effects that are seen in the alteration of antibody responses following TCDD exposure