The Effect of Molecular Weight and Gel Preparation on Humoral Adjuvancy of Silicone Oils and Silicone Gels

Abstract

Silicone gels from commercial breast implants have been shown previously by our laboratory to be potent humoral adjuvants, while the low molecular size 20 centistoke (cs) silicone oil (M.W. 1900) possesses no measurable adjuvant properties. It became necessary to shear the silicone gel during our previous experiments in order to facilitate injection through a syringe and needle; it is conceivable that shearing may reduce the molecular weight of the silicone gel used. This investigation was undertaken to determine whether humoral adjuvancy of silicone oils is dependent on molecular weight and whether the method of shearing the silicone gel affects its adjuvancy. Four Dow Corning 360 Medical silicone oils (100 cs, M.W. ± 5,000; 350 cs, M.W. ± 10,000; 1000 cs, M.W. ± 16,500 and 12,500 cs, M.W. ± 60,000) and Dow Corning octamethylcyclotetrasiloxane (D4, M.W. 296) were tested for their humoral adjuvancy by immunizing 64 Sprague Dawley rats with 50 μg of bovine serum albumin (BSA) mixed with each oil. The rats were periodically bled and the sera were analyzed for anti-BSA antibodies by ELISA. In a separate experiment, three silicone gel preparations with reproducible characteristics were prepared by using a tissue homogenizer and varying the applied shear force. Each of these preparations was tested for its humoral adjuvancy as previously described for silicone oils. Rats immunized with BSA mixed with the highest molecular size silicone oil tested (M.W. ± 60,000) showed a significant increase in anti-BSA antibodies as compared to the lower molecular size oils. The three silicone gel preparations showed no difference in their adjuvancy effect. Thus, the humoral adjuvancy of silicone oil appears to be dependent on molecular weight. Differential shearing of the silicone gel does not alter its humoral adjuvancy.