The Innate and Adaptive Immune System of the Common Bed Bug, Cimex lectularius: Current Knowledge and Research Opportunities

The common bed bug, Cimex lectularius (Hemiptera: Cimicidae), is a blood-feeding ectoparasite of vertebrates, primarily humans. In contrast to many other hematophagous arthropods, such as kissing bugs, mosquitoes, sandflies, and ticks that intermittently seek blood meals from vertebrate hosts, C. lectularius does not vector disease-causing pathogens and parasites to their human hosts. In this review, we summarize currently known immune responses by C. lectularius, and propose worthy research topics. Challenged by microbe ingestion or infection, C. lectularius mounts cellular immune responses such as phagocytosis of bacteria, as well as humoral responses such as secretions of antimicrobial peptides into the hemolymph. The functional immune system of the hemimetabolous C. lectularius resembles that of holometabolous insects but exhibits distinct deviations, including a sparser immune repertoire, the production of DNA nets by cells in response to pathogen invasions, and reproductive immune anticipation in the context of sexual reproduction (traumatic insemination). Many components of the C. lectularius immune system still await discovery, including the receptor molecules and immune pathways involved in antiparasitic and antiviral immune responses. Why C. lectularius does not vector pathogens to human hosts is hardly understood. Potential explanations include upregulated antimicrobial peptide activities that help eliminate invading pathogens.