The present invention provides an outer membrane vesicle (OMV) from a Gram-negative bacterium, comprising at least one heterologous protein that is free in the lumen of the OMV, wherein the OMV is capable of eliciting an immune response to the heterologous protein when administered to a mammal. The invention also provides methods for preparing the OMVs of the invention, pharmaceutical compositions comprising the OMVs of the invention, especially immunogenic compositions and vaccines, and methods of generating an antibody immune response in a mammal using OMVs.
