Modulation of murine cellular immune responses and cytokine production by sand fly saliva

Abstract

Sand flies (Diptera: Phlebotominae) are hematophagous arthropods and important vectors of Leishmania parasites. During blood feeding, both vector saliva and parasites are injected by an infected sand fly. The aim of this study was to characterize how sand fly saliva interferes with host immunity in order to clarify the mechanism underlying enhancing effect of sand fly saliva on Leishmania infection. Spleen cells from naive BALB/c mice were incubated in vitro with Phlebotomus papatasi, P. sergenti or Lutzomyia longipalpis salivary gland lysate (SGL). We studied the effect of saliva on cell proliferation and cytokine production in non‐stimulated and concanavalin A‐stimulated splenocytes. Both spontaneous and mitogen‐stimulated lymphocyte proliferation were significantly suppressed with SGLs of all three sand fly species and all SGLs doses tested (equivalent to 1/16, 1/4 and 1 gland per well). The proliferation index (a lysate‐treated/lysate‐untreated cells ratio) was significantly lower when cells were incubated with SGL and was ranging between 0.52 and 0.30 for non‐stimulated splenocytes and 0.69–0.32 for mitogen‐stimulated lymphocytes. This result indicates that saliva from different sand fly species is able to suppress host proliferative response even to the potent Leishmania‐unrelated mitogen. In parallel experiments, we analysed the effect of SGL (1 gland per well) on cytokine production. The levels of Th1 (IL‐2, IFN‐γ) and Th2 (IL‐4) cytokines were determined in cell culture supernatants by capture ELISA. Exposure to SGL alone did not affect levels of IL‐2 or IL‐4. The production of IFN‐γ was inhibited by P. papatasi SGL, whereas the SGLs of other two species were ineffective. In mitogen‐stimulated cells both Th1 cytokines were modulated; the level of IFN‐γ production was significantly suppressed and IL‐2 enhanced by SGL of all three sand fly species. The production of Th2 cytokine IL‐4 was inhibited only by L. longipalpis SGL; no changes in IL‐4 levels were noted after incubation with Phlebotomus saliva. This species‐specific immunomodulation is in agreement with our previous finding that the composition of sand fly saliva varies among species. Taken together, sand fly saliva modulate cell proliferation as well as cytokine production. These findings further increase our understanding of sand fly–host immune relationship.