Combined effects in toxicology--a rapid systematic testing procedure: cadmium, mercury, and lead.

A testing procedure is described for the assessment of the toxicological response (e.g., acute toxicity or mutagenicity) of any combination and number of chemical, physical, and biological agents, with no more effort for a particular combination than for a single agent. The method provides a simple, sensitive, and quantitative index of synergism, antagonism, and additivity, and it has been demonstrated experimentally in rats by determining the acute lethality of combinations of cadmium, mercury, and lead salts. In a combination of two metal salts, the dose of one metal of the pair was fixed at or near the no-effect level while the dose of the second metal was increased until the entire dose-response curve was obtained. To evaluate interactions of the three metals, the previous pair of metals were kept fixed at their combined extrapolated LD1 level, and the third metal was increased. The statistical treatment of the data employed a computer program that did not involve probit transformations, but rather the approximate linear relationship between the fractional response and the logarithm of the dose. A particular combination could be synergistic, antagonistic, or additive, depending on the relative doses employed. Generally, a combination was synergistic when the most toxic member was present at or near its LD1 dose in the presence of the much less toxic member; the same combination was protective when the least toxic member was present at or near its LD1 dose. The results clarify apparently contradictory reports regarding the biological effects of metal combinations. The application of the testing procedure to combinations of mutagens is described, and an example is cited involving, for a particular bacterial mutagen, a combination of N-methyl-N'-nitro-N-nitrosoguanidine with ethylmethanesulfonate.

In vitro suppression of immunocompetent cells by lymphomas from aging mice.

We studied the effects of lymphomas in the spleens of aging (C57BL/6 X BALB/c)F1 (B6CF1) mice on the immunologic responsiveness of normal spleen cell cultures to sheep erythrocytes. The cultures were assayed for antibody-forming cells (AFC). Cultures of 20 times 10-6 normal spleen cells produced hundreds of AFC, whereas similar cultures of lymphomatous spleen cells produced less than ten AFC. Normal spleen cells were immunosuppressed in cultures with as few as 10-4 lymphomatous spleen cells. Our results demonstrated that lymphoma cells inhibited the response of normal spleen cells to antigenic stimulation in vitro. The age response for male mice gave a biphasic decline with a steeper second phase. This second decline of immune responsiveness started to appear as the incidence of lymphomas rose. This suggested that the immunosuppressive activity of lymphoma cells was causally related to the age-dependent decline of immune responsiveness in B6CF1 mice.