Protective immune responses and the virulence of Actinobacillus pleuropneumoniae (APP) have been attributed, in part, to toxins (Apx) produced by the bacterium. A mutant of the serovar 7 strain HS93 (HS93Tox-), lacking the genes encoding the structural toxin ApxA and the post-translational activating protein ApxC, but retaining the genes required for secretion ApxB and ApxD, was isolated and shown to be attenuated in a mouse model. A plasmid vector system was developed and used to express the ApxA gene from within the HS93Tox- strain. The resulting strain, HS93Tox-/pIG-T1K, expresses the Apx structural protein in a non-activated form. HS93Tox-/pIG-T1K was shown to be attenuated in a mouse model and to be capable of inducing Apx-specific antibodies, which were boosted on re-inoculation. Live vaccination of mice with HS93Tox-/pIG-T1K offered protection against homologous wild-type serovar 7 challenge, and also heterologous challenge with a serovar 1 strain. This is in contrast to vaccination with the HS93Tox- strain, which failed to protect mice against a heterologous challenge.