| Environmental pollutants, including polycyclic aromatic hydrocarbons (PAHs) and dioxins, are profoundly immunosuppressive. The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that mediates the biological effects of these toxins. In animal models, B lymphocyte growth and function appear to be targeted by these toxins. Therefore, we reasoned that the AhR must be expressed during B lymphocyte maturation and postulated that the AhR normally functions to regulate cell growth. The following goals were set to test these predictions and to extend studies to primary human B lymphocytes: (1) evaluate AhR expression and activity in resting and activated B lymphocytes, (2) determine the effects of environmental AhR ligands on B lymphocyte growth and differentiation, and (3) determine the physiological role of the AhR, specifically as it relates to mature B lymphocyte growth.; Unmethylated cytosine guanine dinucleotides and CD40 ligand were employed to model physiologically-relevant stimuli during innate and adaptive immune responses in primary human B lymphocytes, respectively. Activated B lymphocytes dramatically up-regulated AhR expression. Furthermore, nuclear AhR localization and expression of the AhR-regulated CYP1A1 enzyme gene, in the absence of exogenous ligands, indicated constitutive AhR activation. Benzo[ a]pyrene (B[a]P), a prototypic AhR ligand, significantly suppressed activated B lymphocyte proliferation, whereas a non-metabolizable ligand (2,3,7,8-tetrachlorodibenzo-p-dioxin) did not. Using a CYP1A1specific inhibitor, it was shown that metabolism of the parent compound is essential for growth suppression. B[a]P-treatment significantly suppressed plasma cell differentiation, further demonstrating that human B lymphocytes, presumably undergoing effector functions, are sensitive targets of PAH-mediated immunosuppression. Interestingly, when cell proliferation was blocked, plasma cell differentiation and the effects of B[a]P were unaffected, indicating that PAHs suppress differentiation by a mechanism independent of proliferation. Finally, to provide insights into a physiologic role for the AhR, AhR function and expression were blocked with a selective repressor and small interfering RNAs, respectively. Both approaches significantly inhibited proliferation, confirming that the AhR positively regulates B lymphocyte growth. Together, these data define for the first time the physiological role of the AhR in lymphocytes as well as provide a greater appreciation and mechanistic understanding of the suppressive effects of these environmental toxins on the human immune system. |
