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Plasticizer di(2-ethylhexyl) phthalate (DEHP) and its active metabolite MEHP have important immunotoxic effects in mammalian species, including inhibition of cell proliferation, inflammation inhibition, lowering of the antibody response, and apoptosis. Virtually nothing is known about the potential detrimental effects of DEHP/MEHP on the teleost immune system, although phthalates are a likely threat to fish health. Here we investigated whether short-term in vitro DEHP exposure would affect B lineage cells in the rainbow trout, using cultured immune tissues. Cell culture conditions, evidence of cellular incorporation of DEHP, and possible effects of DEHP on immune genes were first established using the mouse pre-B cell line PD31 and data confirmed a dose-dependent cellular uptake of DEHP using liquid chromatography-coupled ion trap mass spectrometry. Effects of in vitro DEHP exposure on trout B cell proliferation were tested by flow cytometry. Significant, dose-dependent inhibition was evident in both anterior and posterior kidney cultures after 24 h exposure to ≥4 μM DEHP. DEHP-induced cell death was not significant for the range of DEHP tested. Further, the abundance of IgM-secreting plasmablasts and plasma cells was significantly reduced after in vitro exposure of ≥16 μM DEHP for 2 or 7 days. Finally, in vitro DEHP exposure significantly lowered the levels of secreted HCmu transcripts in a dose-dependent manner. B lineage cells from posterior kidney were more sensitive to effects of in vitro DEHP exposure than those from anterior kidney. Together, the data support a model where DEHP modifies the normal B cell activation pathways in rainbow trout, promoting B cell differentiation while suppressing plasmablast expansion, resulting in fewer IgM-secreting plasma cells. Insufficient production of protective antibody make fish more susceptible to infection, and increases their risk for disease and mortality in polluted waters. [ABSTRACT FROM AUTHOR]