Host Defense Mechanisms

A virus causes infection by invading host cells, multiplying new virions, and exiting the host cell to attack others. As part of their survival strategies, hosts have evolved effective mechanisms to defend against viral invaders by employing multifaceted immune responses. Virulence and pathogenesis are the consequences of the complex interactions between the infecting virus and host immunity. Vertebrates deal with viral infections by two types of immune responses, innate/nonspecific and adaptive/specific responses. The innate immune response is a rapid response to prevent the spread of viruses during the early phase of the invasion. The innate immune response includes synthesis of interferons to inhibit virus replication and the induction of natural killer (NK) cells to lyse virus infected cells. The adaptive immune response has two components, the humoral and cell-mediated responses. The humoral response attacks viruses when they are present in the host's circulation by B-lymphocyte-produced antibodies (immunoglobulins). The cell-mediated response destroys virus-infected cells by T-lymphocyte-produced cytokines once viruses have resided inside of the host cells. The adaptive immune response can also result in the production of ''memory cells'' which endow the immune system with the ability to respond much more rapidly and effectively to a subsequent infection of the same virus, which provides long-term protection against a given virus. In insects, NK cells, antibodies, cytotoxic T cells, and memory cells are all lacking and the entire immune system is innate. In general, insects utilize three lines of defense to combat infections: physical and chemical barriers, humoral immune responses, and cellular immune responses. In insect cellular immune responses, hemocytes confer cellular immunity to insects and hemocytic response is mediated by phygocytosis, nodule formation, and encapsulation of microbes. The insect humoral response is characterized by the activation of the phenoloxidase cascade and biosynthesis of antimicrobial peptides. The hemocytic and phenoloxidase responses are rapid and present the first line of defense behind the physical and chemical barriers, while the synthesis of antimicrobial peptides is much slower and begins to appear some hours after the actual infection has been recognized. Together, these responses constitute an effective defense system to protect insects from challenges by numerous invaders (Schmid-Hempel, 2005).

While the humoral and cellular immune responses to bacterial and fungal infections have been characterized and documented in honey bees, relatively little is known concerning how honey bees recognize and fight viral infections. However, we believe that honey bees do possess effective defense mechanisms that protect them from virus infections.

The commonly observed phenomenon that viruses persist in apparently healthy colonies as latent infections is a good indication that honey bees have the innate ability to resist the multiplication of virus infections.

Recent work has indicated that RNA interference (RNAi) is a natural, conserved mechanism of antiviral immunity in plants, vertebrates, and insects (Keene et al., 2004; Li et al., 2002; Voinnet, 2001). RNAi is an RNA-dependent gene silencing process triggered by a long double-stranded RNA (dsRNA). When dsRNA is introduced into a cell, a specific RNaseIII endonuclease, Dicer, binds and cleaves dsRNA to produce double-stranded fragments of 20-25 base pairs with 2-nt 3' overhangs, called small interfering RNAs (siRNAs). The siRNAs are integrated into the RNA-induced silencing complex (RISC) to activate the RISC. Activated RISC bind to homologous mRNA and cause sequence-specific degradation of the target mRNA. Positive-stranded RNA viruses appear to be potentially vulnerable to RNAi because the viruses replicate their genomes through complementary strands resulting in dsRNA replication intermediates that are attractive targets for siRNAs. Since the genomes of most honey bee viruses are positive-stranded RNA molecules, we would expect RNAi to also be an important defense mechanism against viruses in honey bees.

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