Location and expression kinetics of Tc24 in different life stages of Trypanosoma cruzi

Tc24-C4, a modified recombinant flagellar calcium-binding protein of Trypanosoma cruzi, is under development as a therapeutic subunit vaccine candidate to prevent or delay progression of chronic Chagasic cardiomyopathy. When combined with Toll-like receptor agonists, Tc24-C4 immunization reduces parasitemia, parasites in cardiac tissue, and cardiac fibrosis and inflammation in animal models. To support further research on the vaccine candidate and its mechanism of action, murine monoclonal antibodies (mAbs) against Tc24-C4 were generated. Here, we report new findings made with mAb Tc24-C4/884 that detects Tc24-WT and Tc24-C4, as well as native Tc24 in T. cruzi on ELISA, western blots, and different imaging techniques. Surprisingly, detection of Tc24 by Tc24-C/884 in fixed T. cruzi trypomastigotes required permeabilization of the parasite, revealing that Tc24 is not exposed on the surface of T. cruzi, making a direct role of antibodies in the induced protection after Tc24-C4 immunization less likely. We further observed that after immunostaining T. cruzi–infected cells with mAb Tc24-C4/884, the expression of Tc24 decreases significantly when T. cruzi trypomastigotes enter host cells and transform into amastigotes. However, Tc24 is then upregulated in association with parasite flagellar growth linked to re-transformation into the trypomastigote form, prior to host cellular escape. These observations are discussed in the context of potential mechanisms of vaccine immunity.
Author summary Chagas disease is a chronic infection with Trypanosoma cruzi (T. cruzi) that affects approximately 8 million people worldwide and may cause chronic heart inflammation. The vaccine candidate Tc24-C4 is a recombinant version of the Tc24 protein, which is a flagellar calcium-binding protein expressed by T. cruzi. While animal challenge studies have shown that targeting Tc24 is very promising, it is not fully understood how Tc24 is presented to the immune system. Here, we were able to localize Tc24 in flagellated T. cruzi parasites using a novel Tc24-specific monoclonal antibody. The results showed that Tc24 is not exposed on the outside of the parasite, which suggests that antibodies against Tc24 could not bind parasites during the infection. Then, by analyzing Tc24 expression in T. cruzi—infected host cells over time, we observed that Tc24 expression is reduced after the parasite enters the cells but is restored when parasites escape the host cell again. Our study provides more insights on the location and presence of Tc24 in T. cruzi during infection in the host, and we discuss our current understanding on the mechanisms of how the Tc24 vaccine may work.