Disease-relevant targets of infection by obligate intracellular parasites are beyond the reach of conventional antibodies. The neutralizing targets of intracellular pathogens in host cells are largely unknown.
Ohio State University researchers created nanobodies intended to target a protein that makes E. chaffeensis bacteria particularly infectious. Ehrlichia translocated factor-1 (Etf-1), a type IV secretion system (T4SS) effector, is a primary virulence factor. In this study, the Ohio State University researchers developed Etf-1–specific nanobodies (Nbs). A series of experiments in cell cultures and mice showed that one specific nanobody they created in the lab could inhibit infection by blocking three ways the protein enables the bacteria to hijack immune cells. 24 distinct anti–Etf-1 Nbs, NbD7 blocked mitochondrial localization of Etf-1–GFP in cotransfected cells. NbD7 and control Nb (NbD3) bound to different regions of Etf-1. Size-exclusion chromatography showed that the NbD7 and Etf-1 complex was more stable than the NbD3 and Etf-1 complex. Intracellular expression of NbD7 inhibited three activities of Etf-1 and E. chaffeensis: up-regulation of mitochondrial manganese superoxide dismutase, reduction of intracellular reactive oxygen species, and inhibition of cellular apoptosis. To safely and effectively deliver Nbs into the host cell cytoplasm, NbD7 was conjugated to cyclized cell-permeable peptide 12 (CPP12-NbD7). CPP12-NbD7 effectively entered mammalian cells and abrogated the blockade of cellular apoptosis caused by E. chaffeensis and inhibited infection by E. chaffeensis in cell culture and in a severe combined-immunodeficiency mouse model.
The study provided support for the feasibility of nanobody-based ehrlichiosis treatment, but much more research is needed before treatment would be available for humans. There is a certain urgency to coming up with an alternative to the antibiotic doxycycline, the only treatment available. The broad-spectrum antibiotic is unsafe for pregnant women and children, and it can cause severe side effects.
Zhang, Wenqing, et al. "An intracellular nanobody targeting T4SS effector inhibits Ehrlichia infection." Proceedings of the National Academy of Sciences 118.18 (2021).