Comparison of Immune Responses between Human Body Louse, Pediculus humanus humanus, and Head Louse, P. h. capitis: Insights into Vector Competence Difference

Abstract

The human body louse, Pediculus humanus humanus, and the head louse, Pediculus humanus capitis, are hematophagous ectoparasites of humans. Although both body and head lice belong to a single species, only body lice are known to transmit several bacterial diseases, including trench fever, through its feces. This difference in vector competence is assumed to be due to their different immune reactions. Here, the differences in the immune response between body and head lice were investigated by measuring the proliferation rates of two model bacteria, a Gram-positive Staphylococcus aureus and a Gram-negative Escherichia coli, following dermal challenge. Body lice showed a significantly reduced immune response compared to head lice particularly to E. coli at the early stage of the immune challenge. The immune reactions in the alimentary tract were also compared between body and head lice following oral challenge of E. coli. Head lice suppressed the growth of E. coli effectively at the early stage of infection, resulting in gradual reduction of E. coli number in alimentary tract tissues. In contrast, the number of E. coli steadily increased in alimentary tract tissues of body lice. The proliferation profiles of Bartonella quintana, the causative agent of trench fever, inside louse body and its excretion patterns were also investigated in the two louse subspecies following oral challenge with B. quintana-infected blood meal. GFP-expressing B. quintana cells gradually proliferated until 9 days after infection and the number was significantly higher in body lice than in head lice. The numbers of B. quintana detected in feces from infected lice were almost the same and steadily decreased over time in both body and head lice. Nevertheless, the viability of B. quintana, as determined by fluorescence, was significantly higher in body louse feces, especially at 1 day post-infection and this tendency lasted until no B. quintana is excreted. These findings demonstrate that body lice allow more extensive proliferation of B. quintana inside the alimentary tract and excrete feces containing more viable B. quintana following ingestion of infected blood meal, which primarily attribute to their higher vector competence. As the first step to elucidate the molecular basis of reduced immune responses in body lice, immune-related genes were annotated from the genomes of body and head lice. A total of 93 immune-related genes were identified in both body and head lice, suggesting that both body and head lice have the same immune components. Many gene families in the humoral immune system were considerably reduced in number or absent in the body louse genome. In case of the genes related with pathogen recognition, only one type of membrane-binding peptidoglycan recognition protein (PGRP) was annotated whereas beta-glucan binding protein (BGBP) was not found. In addition, Imd and its adaptor protein FADD in the Imd pathway were not identified. In contrast, all components in the Toll, JAK/STAT (Janus kinase and Signal Transducer and Activator of Transcription) and JNK (c-Jun N-terminal kinase) pathways were preserved. Among the various kinds of antimicrobial peptides (AMP), only two types of defensin were annotated. The Louse PGRP, defensin 1 and defensin 2 showed 99.3, 99.1 and 98.2% similarity between body and head lice. The differences in the cellular and humoral immune response between body lice were investigated. Body lice exhibited a significantly lower phagocytotic activity against E. coli than head lice, whereas the phagocytosis against S. aureus differed only slightly between body and head lice. Transcriptional profiling of representative genes involved in the humoral immune response revealed that both body and head lice showed an increased immune response to S. aureus but little to E. coli following dermal challenge. Nevertheless, the basal transcription levels of major immune genes, such as PGRP, Thioester containing protein (TEP) 1, TEP2, Scavenger Receptor CI (SRCI), Dual oxidase (Duox) in whole body level were significantly lower in body versus head lice. The epithelial cell-specific immune responses in the alimentary tract tissues were compared between body and head lice following bacteria oral challenge. Interestingly, the basal transcription levels of PGRP and defensins, which are the sole components of recognition and effector in the humoral immune response, respectively, were lower in body lice than in head lice. Defensin 1 was up-regulated by B. quintana oral challenge in head lice but not in body lice, whereas no differenece was observed by E. coli oral challenge. Thus, such non-inducible immune genes, along with the lower basal transcription levels of PGRP and defensins in body lice, were primarily responsible for the reduced immune response of body lice to Gram-negative bacteria including B. quintana. 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 Duox. These results suggest that both the lower basal transcription level of immune-related genes, which is not inducible by B. quintana, in conjuction with the lower amount of ROS in the alimentary tract of body lice reduce their immune responses, thereby allowing invading B. quintana to proliferate and excreting more viable bacteria in feces, which result in their higher vector competence compared to head lice.