EFFECTS ON IMMUNE RESPONSE 103Figure 5.5. Comparison of effects of small versus large amounts of whole-body irradiation
administered to rabbits at various times before or after primary immunization
with SRBC. Adapted from Taliaferro and Taliaferro.5
amounts of antigen that would otherwise be subimmunogenic.12 In nonirra-
diated mice, such “low-dose tolerance” is apparently due to the induction of
suppressor T cells. Figure 5.7 shows the effect of whole body exposure to
small amounts (15 rad) on the response of mice to inactivated (mitomycin-
treated) tumor cells. The mice were irradiated or sham-irradiated immedi
ately prior to injection with the indicated numbers of mitomycin-treated
tumor cells. A control group was given phosphate-buffered saline. After 21
days, all mice received 10 4 viable tumor cells. As seen in the figure, sham-
irradiated mice injected with small numbers of mitomycin-treated Sarcoma
I (Sal) cells exhibited larger tumors than the saline-injected controls (solid
line) when subsequently challenged with untreated tumor cells. Exposure to
low-dose irradiation not only abolished this partial tolerance to the tumor
cells, but actually rendered the irradiated mice partially immune. The aug
mented antitumor activity of these (low-dose) irradiated spleen cells can
also be shown in a cell transfer system (Figure 5.8).23
The above experiments have involved rabbits and mice irradiated in vivo.
A similar phenomenon can be demonstrated in vitro. The latter approach
has the added advantage of permitting the dissection of the various compo
nents of the immune response by subtracting or adding specific cell types.
Figure 5.9 shows the response of murine spleen cells exposed to several
doses of radiation in vitro to sheep red blood cells (SRBCs) as antigen.16 The