Immunomodulatory effects of cholera toxin in mice

Immunomodulatory effects of cholera toxin (CT) were investigated in a murine model using various immunological parameters. C3H/HeN mice were injected with 2 micrograms of CT at various intervals (from 6 h to 21 days) before the immunological assays. Thymocytes were markedly decreased in their absolute number, and the phenotypes in such cells were clearly shifted from Thy1.2high+ PNAhigh+ to Thy1.2low+ PNAlow+ 2-4 days after the CT treatment. Spleen T cells were relatively increased, while surface IgM positive B cells were rather decreased. Natural killer activity and in vivo and in vitro cytotoxic T lymphocyte activity were markedly suppressed during the early stages after the CT treatment but recovered completely within 21 days. Mixed lymphocyte reaction was profoundly suppressed at least for the 1st week after the CT treatment. Furthermore, EL-4 tumor of C57BL/6 origin grew progressively and killed the recipient C3H mice when such tumor cells were inoculated 6 h after the CT treatment. On the contrary, a marked augmentation of direct (IgM) and indirect (IgG) plaque-forming cell responses to sheep red blood cells was seen after CT treatment. Delayed footpad reaction to SRBC was also augmented after CT treatment. As the mechanisms, both direct augmentation of CD4+ T cells and direct suppression of CD8+ T cells appeared to occur at a time due to the CT treatment. An indirect effect of CT through the release of the endogenous steroids was dismissed in the present study. Taken together, CT appears to have differential immunomodulatory effects on various immune effector cells through various mechanisms.