A Nod to disease vectors: mitigation of pathogen sensing by arthropod saliva

Dynamic gene expression in salivary glands of the cat flea during Rickettsia felis infection

The cat flea, Ctenocephalides felis, is an arthropod vector capable of transmitting several human pathogens including Rickettsia species. Earlier studies identified Rickettsia felis in the salivary glands of the cat flea and transmission of rickettsiae during arthropod feeding. The saliva of hematophagous insects contains multiple biomolecules with anticlotting, vasodilatory and immunomodulatory activities. Notably, the exact role of salivary factors in the molecular interaction between flea-borne rickettsiae and their insect host is still largely unknown. To determine if R. felis modulates gene expression in the cat flea salivary glands, cat fleas were infected with R. felis and transcription patterns of selected salivary gland-derived factors, including antimicrobial peptides and flea-specific antigens, were assessed. Salivary glands were microdissected from infected and control cat fleas at different time points after exposure and total RNA was extracted and subjected to reverse-transcriptase quantitative PCR for gene expression analysis. During the experimental 10-day feeding period, a dynamic change in gene expression of immunity-related transcripts and salivary antigens between the two experimental groups was detected. The data indicated that defensin-2 (Cf-726), glycine-rich antimicrobial peptide (Cf-83), salivary antigens (Cf-169 and Cf-65) and deorphanized peptide (Cf-75) are flea-derived factors responsive to rickettsial infection.

The Tick Protein Sialostatin L2 Binds to Annexin A2 and Inhibits NLRC4-Mediated Inflammasome Activation

Tick saliva contains a number of effector molecules that inhibit host immunity and facilitate pathogen transmission. How tick proteins regulate immune signaling, however, is incompletely understood. Here, we describe that loop 2 of sialostatin L2, an anti-inflammatory tick protein, binds to annexin A2 and impairs the formation of the NLRC4 inflammasome during infection with the rickettsial agent Anaplasma phagocytophilum Macrophages deficient in annexin A2 secreted significantly smaller amounts of interleukin-1β (IL-1β) and IL-18 and had a defect in NLRC4 inflammasome oligomerization and caspase-1 activation. Accordingly, Annexin a2-deficient mice were more susceptible to A. phagocytophilum infection and showed splenomegaly, thrombocytopenia, and monocytopenia. Providing translational support to our findings, better binding of annexin A2 to sialostatin L2 in sera from 21 out of 23 infected patients than in sera from control individuals was also demonstrated. Overall, we establish a unique mode of inflammasome evasion by a pathogen, centered on a blood-feeding arthropod.

The tick salivary protein sialostatin L2 inhibits caspase-1-mediated inflammation during Anaplasma phagocytophilum infection

Saliva from arthropod vectors facilitates blood feeding by altering host inflammation. Whether arthropod saliva counters inflammasome signaling, a protein scaffold that regulates the activity of caspase-1 and cleavage of interleukin-1β (IL-1β) and IL-18 into mature molecules, remains elusive. In this study, we provide evidence that a tick salivary protein, sialostatin L2, inhibits inflammasome formation during pathogen infection. We show that sialostatin L2 targets caspase-1 activity during host stimulation with the rickettsial agent Anaplasma phagocytophilum. A. phagocytophilum causes macrophage activation and hemophagocytic syndrome features. The effect of sialostatin L2 in macrophages was not due to direct caspase-1 enzymatic inhibition, and it did not rely on nuclear factor κB or cathepsin L signaling. Reactive oxygen species from NADPH oxidase and the Loop2 domain of sialostatin L2 were important for the regulatory process. Altogether, our data expand the knowledge of immunoregulatory pathways of tick salivary proteins and unveil an important finding in inflammasome biology.