A toxoid prepared from cholera toxin by iodination

A cholera toxoid was prepared by iodinating purified cholera toxin having an activity of 25 Limit of blueing (Lb) doses/1 microgram with 0.8 mumol of iodine monochloride per mg toxin, and the residual lesion capacity was tested in mice. The blueing dose (BD) test was strongly positive for the native toxin, and completely abolished in the iodinated toxoid when tested at up to 25 times on Lb dose. The dermal microscopic lesions with intradermal doses of 1 microgram virulent toxin presented intense leucocyte infiltration, proteinaceous edema and active hyperemia, whereas none of these effects was observed with the same amount of toxoid. To determine antigenicity, two groups of mice received toxin or toxoid, 8.5 micrograms adsorbed to aluminum hydroxide, followed by a booster of 17 micrograms in saline 21 days later. Measurement of antibodies by ELISA at day 28 indicated that the toxoid was 2.5 times more antigenic than the toxin. These data show that iodination converts cholera toxin to an effective toxoid.