A mutated cholera toxin without the ADP-ribosyltransferase activity induces cytokine production and inhibits apoptosis of splenocytes in mice possibly via toll-like receptor-4 signaling

Native cholera toxin (CT) and its mutated form (CT-2*) without ADP-ribosyltransferase activity differ in their immunomodulatory effects on host cells, and the mechanisms of these differences are poorly understood. In this study, we demonstrated that CT-2* induced higher levels of cytokine production and down-regulated ex-vivo apoptosis of splenocytes from C57BL/6 mice. After exposure of the splenocytes ex-vivo to CT or CT-2* (2 μg/ml) for 48 h, CT-2* stimulated expression of the toll-like receptor (TLR-4) gene was much higher and the cells produced increased levels of interleukin (IL)-12, interferon (IFN)-γ, and tumor necrosis factor (TNF)-α, compared to splenocytes of mice exposed to native CT. We confirmed these findings by observing that CT-2*, induced much lower levels of IL-12, IFN-γ, and TNF-α in a TLR-4 knockout macrophage cell line derived from C57BL/6 mice. In addition, while CT is known to stimulate apoptosis in splenocytes, we observed that CT-2* significantly down-regulated apoptosis (4.2%), compared to splenocytes exposed to CT (18.7%) or PBS (negative control, 8.5%). On the contrary, we noted both native CT and CT-2* to exhibit similar levels of apoptosis in TLR-4^-/- cell line. Overall, the evidence supports the conclusion that CT-2* modulated cytokine production and apoptosis in splenocytes of mice possibly through the TLR-4 signaling pathway.