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It is known that exposure to As+3 via drinking water causes a disruption of the immune system and significantly compromises the immune response to infection. The purpose of these studies was to assess the effects of As+3 on bone marrow progenitor cell colony formation and the humoral immune response to a T-dependent antigen response (TDAR) in vivo. In a 30 day drinking water study, mice were exposed to 19, 75, or 300 ppb As+3. There was a decrease in bone marrow cell recovery, but not spleen cell recovery at 300 ppb As+3. In the bone marrow, As+3 altered neither the expression of CD34+ and CD38+ cells, markers of early hematopoietic stem cells, nor CD45−/CD105+, markers of mesenchymal stem cells. Spleen cell surface marker CD45 expression on B cells (CD19+), T cells (CD3+), T helper cells (CD4+) and cytotoxic T cells (CD8+), natural killer (NK+), and macrophages (Mac 1+) were not altered by the 30 day in vivo As+3 exposure. Functional assays of CFU-B colony formation showed significant selective suppression (p<0.05) by 300 ppb As+3 exposure, whereas CFU-GM formation was not altered. The TDAR of the spleen cells was significantly suppressed at 75 and 300 ppb As+3. In vitro studies of the bone marrow revealed a selective suppression of CFU-B by 50 nM As+3 in the absence of apparent cytotoxicity. Monomethylarsonous acid (MMA+3) demonstrated a dose-dependent and selective suppression of CFU-B beginning at 5 nM (p<0.05). MMA+3 suppressed CFU-GM formation at 500 nM, a concentration that proved to be nonspecifically cytotoxic. As+5 did not suppress CFU-B and/or CFU-GM in vitro at concentrations up to 500 nM. Collectively, these results demonstrate that As+3 and likely its metabolite (MMA+3) target lymphoid progenitor cells in mouse bone marrow and mature B and T cell activity in the spleen.

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Our previously published data show that As(+3) in vivo and in vitro, at very low concentrations, inhibits lymphoid, but not myeloid stem cell development in mouse bone marrow. We also showed that the As(+3) metabolite, monomethylarsonous acid (MMA(+3)), was responsible for the observed pre-B cell toxicity caused by As(+3). Interleukin-7 (IL-7) is the primary growth factor responsible for pre-lymphoid development in mouse and human bone marrow, and Signal Transducer and Activator of Transcription 5 (STAT5) is a transcriptional factor in the IL-7 signaling pathway. We found that MMA(+3) inhibited STAT5 phosphorylation at a concentration as low as 50 nM in mouse bone marrow pre-B cells. Inhibition of STAT5 phosphorylation by As(+3) occurred only at a concentration of 500 nM. In the IL-7 dependent mouse pre-B 2E8 cell line, we also found selective inhibition of STAT5 phosphorylation by MMA(+3), and this inhibition was dependent on effects on JAK3 phosphorylation. IL-7 receptor expression on 2E8 cell surface was also suppressed by 50 nM MMA(+3) at 18 h. As further evidence for the inhibition of STAT5, we found that the induction of several genes required in B cell development, cyclin D1, E2A, EBF1, and PAX5, were selectively inhibited by MMA(+3). Since 2E8 cells lack the enzymes responsible for the conversion of As(+3) to MMA(+3) in vitro, the results of these studies suggest that As(+3) induced inhibition of pre-B cell formation in vivo is likely dependent on the formation of MMA(+3) which in turn inhibits IL-7 signaling at several steps in mouse pre-B cells.