Investigation on the humoral immune factors putatively associated with different vector competence between human body and head lice

Human body and head lice have been obligatory human ectoparasites for thousands of years. Although both body and head lice belong to a single species, Pediculus humanus, only body lice are known to be a vector of several bacterial diseases. The higher vector competence of body lice is assumed to be due to their weaker immune response than that of head lice. In this study, we compared the epithelial cell-specific immune responses in the alimentary tract tissues between body and head lice. The number of Escherichia coli, as a Gram-negative model bacterium, steadily decreased in alimentary tract tissue of head lice following oral challenge whereas E. coli proliferated exponentially in body lice. Similarly, head lice suppressed effectively the growth of Bartonella quintana, the causative agent of trench fever, whereas body lice allowed its proliferation in the alimentary tract. Interestingly, the basal transcription levels of peptidoglycan recognition protein and defensins, which are the sole components of recognition and effector in the humoral immune response, respectively, were significantly lower in body lice than in head lice. In addition, the level of cytotoxic reactive oxygen species (ROS) generated by epithelial cells, especially the hydroxyl radical and superoxide, was significantly lower in body lice than in head lice regardless of blood feeding although there was no difference in the transcription level of dual oxidase. These results suggest that both the lower basal transcription level of immune-related genes and lower amount of ROS in the alimentary tract tissues of body lice reduce the immune responses, thereby allowing invading bacterial pathogens to proliferate and resulting in their higher vector competence compared to head lice.