Arsenite Alters Lysosome-Mediated Degradation and the Autophagy Process Leading to Immunosuppression in Human B-Lymphoblastoid Cell Lines

Abstract

The immune system is a target of arsenic toxicity. Epidemiological data have shown that arsenic exposure is associated with characteristics of immunosuppression. Human B-lymphoblastoid cell lines (LCL) were used as an in vitro model of immune cell targeting by arsenic to investigate the mechanism of arsenic-induced cytotoxicity, which could provided insight into the mechanism underlying arsenic-induced immunotoxicity leading to the immunosuppression observed in humans. In LCL arsenite-induced cytotoxicity was not associated with apoptosis, but associated with hallmarks of autophagy, a cell stress-responsive process that facilitates the removal of cellular components through lysosome-mediated degradation. At environmentally relevant concentrations, arsenite-induced toxicity resulted in a decrease in cell proliferation that was correlated with hallmarks of autophagy including expansion of acidic vesicles, global induction of lysosomal gene expression, increased flux of the autophagosome marker LC3-II, and increased enzymatic activity of the lysosomal hydrolase cathepsin D. Investigation of the upstream cellular damage leading to the induction of autophagy revealed that arsenite induces proteotoxic damage leading to an accumulation of protein aggregates that may be targeted to the lysosome for degradation. In addition, global gene expression data showed an enrichment of ER stress responsive genes after arsenite exposure. Further evaluation of global gene expression data indicated that the global induction of lysosomal genes occurs before the activation of ER stress genes, suggesting that the induction of autophagy may occur before the generation of ER stress. To investigate the effect of arsenite-induced proteotoxicity and autophagy on normal immune function, the ability of LCL to process and present exogenous antigens onto MHC class II molecules was evaluated. Arsenite decreased antigen presentation of the exogenous antigen HSA. This decrease was associated with decreased lysosomal degradation of the model substrate DQ-Ova, suggesting that arsenite is disrupting lysosome-mediated degradation. In addition, arsenite exposure was associated with an increase in MHC class II protein aggregates, which could render them unavailable to bind peptide fragments. Through the identification that arsenite induces proteotoxicity and autophagy in LCL, it provides novel insight into the mechanisms of arsenic-induced immunotoxicity that could lead to a better understanding of the mechanisms underlying arsenic-induced immunosuppression observed in humans.