Botulinum neurotoxin (BoNT) serotypes A, B and E cause the vast majority of human botulism cases and pose the greatest bioterrorism threats. We previously identified multiple camelid single-domain antibodies (VHHs) that each neutralize BoNT/A, BoNT/B or BoNT/E. We also demonstrated that heterodimers of linked toxin-neutralizing VHHs are much more potent than VHH monomer pools in preventing BoNT intoxication. In this study, we expressed two different heterohexamer proteins (VNA1-ABE and VNA2-ABE) of ~100 kDa secreted from mammalian host cells, each containing the same six linked anti-BoNT VHH components ordered in two different combinations. Each heterohexamer contained two VHHs that neutralize BoNT/A, BoNT/B or BoNT/E. Both heterohexameric antitoxins displayed similar strong binding properties for the three targeted BoNT serotypes by ELISA. One ug of each heterohexameric antitoxin fully protected groups of mice co-administered with 100 LD50 of BoNT/A, BoNT/B or BoNT/E, or a pool containing 100 LD50 of each of the three toxins. The results demonstrate that long chains of at least six different linked VHHs can be expressed such that all component VHHs in the multimer retain their target binding activities. These findings make more feasible the development of a BoNT antitoxin product consisting of a small pool of proteins that, in combination, neutralize all known BoNT serotypes and subtypes.

Enteric microbial pathogens, including Escherichia coli, Shigella and Cryptosporidium species, take a particularly heavy toll in low-income countries and are highly associated with infant mortality. We describe here a means to display anti-infective agents on the surface of a probiotic bacterium. Because of their stability and versatility, VHHs, the variable domains of camelid heavy-chain-only antibodies, have potential as components of novel agents to treat or prevent enteric infectious disease. We isolated and characterized VHHs targeting several enteropathogenic Escherichia.coli (EPEC) virulence factors: flagellin (Fla), which is required for bacterial motility and promotes colonization; both intimin and the translocated intimin receptor (Tir), which together play key roles in attachment to enterocytes; and E. coli secreted protein A (EspA), an essential component of the type III secretion system (T3SS) that is required for virulence. Several VHHs that recognize Fla, intimin, or Tir blocked function in vitro. The probiotic strain E. coli Nissle 1917 (EcN) produces on the bacterial surface curli fibers, which are the major proteinaceous component of E. coli biofilms. A subset of Fla-, intimin-, or Tir-binding VHHs, as well as VHHs that recognize either a T3SS of another important bacterial pathogen (Shigella flexneri), a soluble bacterial toxin (Shiga toxin or Clostridioides difficile toxin TcdA), or a major surface antigen of an important eucaryotic pathogen (Cryptosporidium parvum) were fused to CsgA, the major curli fiber subunit. Scanning electron micrographs indicated CsgA-VHH fusions were assembled into curli fibers on the EcN surface, and Congo Red binding indicated that these recombinant curli fibers were produced at high levels. Ectopic production of these VHHs conferred on EcN the cognate binding activity and, in the case of anti-Shiga toxin, was neutralizing. Taken together, these results demonstrate the potential of the curli-based pathogen sequestration strategy described herein and contribute to the development of novel VHH-based gut therapeutics.