Antitumor effects of a Toxoplasma mutant lacking lactate dehydrogenases.

"Bug as drug" is a concept recognized over a century ago and has gained significant research attention recently for fighting diseases such as immune disorders and others. Bacteria and viruses are constantly studied for this purpose, but the use of parasitic organisms is still rare. Recently, we found that Toxoplasma gondii mutants lacking two lactate dehydrogenases (ME49 Δldh1-Δldh2) were avirulent in mice but able to stimulate high levels of Th1 immunity. This outcome prompted us to determine whether Δldh mutants also displayed antitumor activities. Using a mouse melanoma model, we showed that intratumoral administration of Δldh1-Δldh2 repressed the growth of established tumors and helped to inhibit lethal tumor development in the mice. The sera of parasite-treated mice had high levels of TNF-α and INF-γ, which likely contributed to the tumor-repressing activity. We also found that chronic Toxoplasma infection, which is common in animals and humans, also led to antitumor activity. In addition, pre-existing chronic infections did not affect the antitumor efficiency of the Δldh1-Δldh2 mutant. Together, these results suggest that the attenuated T. gondii mutant Δldh1-Δldh2 has the potential to be a good antitumor therapy and provide new insights into the development of novel tumor therapeutics.