The following abstract presents results of a study conducted by a contract laboratory for the National Toxicology Program. The findings have not been peer reviewed and were not evaluated in accordance with the levels of evidence criteria established by NTP in March 2009. The findings and conclusions for this study should not be construed to represent the views of the NTP or the U.S. Government.
Sodium metasilicate was selected for evaluation as an immunomodulatory agent because it is used extensively to manufacture numerous industrial and consumer products, and sodium silicate has been reported to elicit contact uritcaria in humans. SMS is most commonly used to make silica gel; however, it is also used as an additive in detergents, adhesives, fireproofing mixtures, and paint strippers. Occupational exposure to these compounds occurs primarily by inhalation or skin exposure.
The objective of this study was to determine the potential for SMS to elicit an allergic response when applied dermally to female BALB/c mice. Concentrations used in the contact hypersensitivity assays were determined by irritancy testing. The minimal irritating concentration was found to be 6.0% and the maximal non-irritating concentration to be 4.0%. Measurement of the contact hypersensitivity response was accomplished using the Mouse Ear Swelling Test and the local lymph node assay. SMS treatment levels for the MEST were 0.4%, 2%, and 4% for sensitization phase, and 6.0% for challenge phase. In the LLNA, mice were sensitized to 2%, 4%, and 6% SMS. 1-fluoro-2,4-dinitrobenzene (2,4- dinitrofluorobenzene) was used as the positive control at a concentration of 0.15% for the irritancy test and LLNA, and 0.20% for the MEST. An evaluation of lymph node subpopulations, cytokine mRNA expression, and serum IgE levels was also conducted.
Dermal exposure to SMS (2-6%) did not induce cell proliferation in the draining lymph nodes, as measured by the LLNA. However, a delayed-type hypersensitivity response was observed when mice were sensitized on the back with 4% SMS, then challenged on the ear with 6% SMS. The positive control, DNFB, induced cell proliferation in the DLN, and elicited a DTH response. Lymph node subpopulations were also altered by treatment with SMS. While an increasing trend in the absolute number of CD3+, CD4+, CD8+, and B220+lg+ cells was observed across treatment groups, only the B220+lg+ lymph node cells were shown to increase when the data were presented as a percentage of the total lymph node cell count. This response was observed at concentrations as low as 4% SMS. An evaluation of the cytokine mRNA revealed an increase in the expression of IFN-γ, TNF-β, and MIF mRNAs in mice treated with SMS. No change in total serum IgE levels was detected.
The studies were conducted at the Virginia Commonwealth University/Medical College of Virginia's Immunotoxicology Laboratory.
In conclusion, sodium metasilicate is observed to be an irritant at concentrations of 6% or greater. However, based on the results of the LLNA, it does not appear to be a contact sensitizing compound. Furthermore, SMS appears to have minimal potential to be a respiratory chemical allergen.