(A) VNA2-Tcd levels detected in pig serum by ELISA using 0.5 g/ml of TcdA or TcdB and serum diluted 1:10. cause of community-acquired diarrhea in previously low-risk populations, including children, healthy adults, and pregnant women (1,C7). Manifestations of CDI vary from asymptomatic P005091 colonization; mild or moderate diarrhea; a severe or fulminant illness with complications, including pseudomembranous colitis, toxic megacolon, and small bowel ileus; or even systemic inflammatory response syndrome, a multisystem organ failure that can P005091 be fatal (8). The emergence of antibiotic-resistant hypervirulent strains and the increase in disease relapse have complicated the treatment of CDI, leading to increases in hospital stay, morbidity, and mortality (1). is a Gram-positive, spore-forming anaerobic bacterium that produces two toxins, designated TcdA and TcdB (9), which are the major virulence P005091 factors of CDI (10). They are large exotoxins that bind to human colonocytes, causing inflammation, fluid accumulation, and mucosal injury manifested as pseudomembranous colitis (11). survives, persists, and produces the two exotoxins in the gut after prolonged treatment with broad-spectrum antibiotics reduces normal microflora (12). The extensive use of antibiotics for treatment of CDI has increased the emergence of resistant strains, leading to a dramatic increase in the incidence of disease relapse estimated at 20% to 35% (13). Consequently, there is an urgent need to develop novel, nonantibiotic therapies that prevent persistence and toxin production by and minimally impact normal gut microflora. Ideally, approaches that P005091 specifically target toxins instead of bacterial cells and eliminate the possibility of antimicrobial resistance are favored (14, 15). Several therapeutic approaches are currently under development, including antibiotics (8, 16, 17), probiotics (18,C23), fecal transplants (24,C26), toxin-binding resins or polymers (27), vaccines (16, 28,C30), and toxin-specific antibodies (Abs) (31,C38). Several but not all antitoxin antibodies improve CDI outcomes in animal models and clinical trials (32, 34, 35, 39,C42), but these conventional antibodies are costly and challenging to engineer. There is some evidence from the pig model (43) that antibodies against TcdB alone may be sufficient for treating CDI; however, there are conflicting data on the roles of the toxins in disease (44,C46). As an efficient alternative, we produced and tested heavy-chain-only VH domains (VHHs), generated by species, seeking VHHs that neutralize each of the two toxins. DNAs encoding these unconventional IgGs (IgG2 and IgG3) are easily cloned (47) and can be expressed at high levels in soluble form (48). The VHH protein products are generally more stable than conventional antibodies and frequently bind the active sites of targeted proteins (48,C50). We previously showed that bispecific VHH-based neutralizing agents (VNAs) are highly efficacious as antitoxins in animal models of exposures to botulinum neurotoxins (51), ricin (52), Shiga toxins (53), and anthrax (54), significantly outperforming their monomer VHH components. To achieve protection from CDI, a VNA was engineered and expressed in bacteria containing four VHHs, two (AH3, AA6) that neutralize TcdA and two copies of the 5D VHH (5D, 5D) that neutralizes TcdB (41). This VNA, called ABA, provided potent protection from CDI in a mouse model. While some reports have indicated that TcdA does not play a significant role in disease pathogenesis in the gnotobiotic pig model of CDI (43), other evidence has shown that TcdA and TcdB toxins contribute to fulminant disease in hamsters (55) and in some mouse models of CDI (56). Since VHH agents remain functional when linked into multimers, we have chosen to include VHHs that neutralize both Tcd toxins in our antitoxin agent, as this should be effective in all of the models of CDI. SLC4A1 In the current study, we chose to.