# Protective efficacy of the recombinantly expressed C-terminal domain of Mannheimia haemolytica leukotoxin in mice and goats

## TL;DR

Researchers tested a part of a toxin from a bacteria that causes respiratory disease in cattle and found it can protect mice and goats, suggesting it could be used in a vaccine.

## Contribution

The C-terminal domain of LktA is shown to be a stable and highly protective subunit vaccine candidate against Mannheimia haemolytica.

## Key findings

- cLktA elicited significantly higher antibody levels in mice and goats compared to a control group.
- A single dose of cLktA conferred 80% and 100% survival in mice and goats after bacterial challenge.
- cLktA was found to be a stable and effective immunogen suitable for subunit vaccine development.

## Abstract

Bovine respiratory disease (BRD) complex is a leading cause of economic losses in the beef and dairy cattle industries. Mannheimia haemolytica is recognised as the primary pathogen associated with this disease. While antibiotics and vaccines are widely used against it, antimicrobial resistance and limited vaccine efficacy remain obstacles. Mannheimia haemolytica leukotoxin A (LktA) has been identified as a promising candidate for subunit vaccine development against BRD. However, the low expression and biological instability of the full-length LktA complicate its production. This study evaluated the immunogenic potential of the truncated LktA protein for subunit vaccine development.

Truncated proteins of LktA N-terminal (nLktA) and C-terminal (cLktA) were expressed in E. coli which were small enough for stable expression yet large enough to function as effective immunogens. The immunogenicity of the recombinant truncated LktA proteins was evaluated in mouse and goat models against a phosphate-buffered saline (PBS) negative-control group. Recombinant cLktA was emulsified with oil adjuvant and used to immunise mice and goats.

The cLktA group had significantly higher antibody levels at four weeks post-immunisation (wpi) than the PBS group. In goats, cLktA elicited high antibody responses up to six wpi. A single administration of cLktA conferred 80% and 100% survival against a M. haemolytica challenge.

These findings show the C-terminal region of Mannheimia haemolytica LktA to be a highly immunogenic and protective antigen and suggest its potential as a candidate for subunit vaccine development.