Suppressive effect of Ixodes ricinus salivary gland extract on mechanisms of natural immunity in vitro.

Tick saliva has been shown to contain a variety of pharmacologically active molecules, including those with immunosuppressive activities. There is increasing evidence that the nonspecific suppression of host immunity by tick saliva is exploited by tick-borne pathogens, e.g. the saliva-activated transmission (SAT) of some tick-borne viruses. We have demonstrated the suppressive effect of the salivary gland extract (SGE) derived from partially fed (five days) Ixodes ricinus females on important mechanisms of innate immunity: natural killer (NK) cells, interferon and nitric oxide production. SGE reduced the interferon induction by polyinosinic-polycytidylic acid (poly I:C) or Escherichia coli lipopolysaccharide (LPS) in a culture of Balb/c mouse splenocytes by 94% and 62%, respectively. SGE suppressed the cytotoxicity of nonstimulated and in vivo poly I:C-stimulated mouse NK cells by up to 31% and 26%, respectively. The induction of NK activity in vitro by LPS but not by Concanavalin-A (Con-A) was also downregulated in the presence of SGE. The addition of SGE to cultures of mouse macrophages partially inhibited the production of nitric oxide, induced by LPS. These data suggest that the facilitating effect of SGE on the transmission of some tick-borne pathogens might be associated with the suppression of the host innate resistance mechanisms, represented by interferon, nitric oxide and NK cells.