Regulation of preB cell apoptosis by aryl hydrocarbon receptor transcription factor-expressing stromal adherent cells

Polycyclic aromatic hydrocarbons (PAH) are environmental chemicals that med iate immunosuppression. In long-term bone marrow B-cell lymphopoiesis model s, PAH induce apoptosis in immature (preB) lymphocytes. Since the biologic function of PAH is often mediated by the aryl hydrocarbon receptor/transcri ption factor (AhR), the role of the AhR or AhR-regulated genes was assessed in preB cell apoptosis, Specifically, a bone marrow-derived preB cell line (BU-11) was cultured on monolayers of the AhR(+) bone marrow-derived strom al cell line BMS2, hepatoma sublines that express various levels of AhR act ivity (Hepa-1c1c7 and variants), AhR(+) thymic epithelial cells, and primar y bone marrow stromal cells from wildtype or AhR(-/-) mice. Cultures were t reated with one of two prototypic PAH, 7,12-dimethyl-benz[a]anthracene (DMB A) or benz[a]pyrene (B[a]P), and the percentage of cells undergoing apoptos is measured. The data demonstrated that: 1) bone marrow- and hepatic-derive d stromal/adherent cells support preB cell growth and regulate apoptosis in duced by DMBA or B[a]P; 2) B[a]P is more effective than DMBA when preB cell s are maintained on Hepa-1c1c7 monolayers than when maintained on BMS2 mono layers; 3) DMBA is more effective than B[a]P when preB cells are cultured o n BMS2 monolayers; 4) alpha-naphthoflavone, an AhR antagonist and cytochrom e P-450 inhibitor, blocks preB cell apoptosis in both BU-11/Hepa-1c1c7 and BU-11/BMS2 cultures; 5) although preB cells grow well in Hepa-1c1c7 or BMS2 supernatants, addition of PAH in the absence of hepatic- or bone marrow-de rived adherent cells does not result in preB cell apoptosis; 6) preB cell a poptosis is dependent on AhR activity in adherent hepatic- or bone marrow-d erived stromal cells; and 7) apoptosis is induced by DMBA when preB cells a re maintained on primary bone marrow stromal cell monolayers from wildtype but not from AhR(-/-) mice. Collectively, the data indicated that AhR-regul ated activities in the hematopoietic microenvironment influence the suscept ibility of immature lymphocytes to low-dose PAH exposure.