An antivector vaccine protects against a lethal vector-borne pathogen

Abstract

Vaccines that target blood-feeding disease vectors, such as mosquitoes and ticks, have the potential to protect against the many diseases caused by vector-borne pathogens. We tested the ability of an anti-tick vaccine derived from a tick cement protein (64TRP) of Rhipicephalus appendiculatus to protect mice against tick-borne encephalitis virus (TBEV) transmitted by infected Ixodes ricinus ticks. The vaccine has a "dual action" in immunized animals: when infested with ticks, the inflammatory and immune responses first disrupt the skin feeding site, resulting in impaired blood feeding, and then specific anti-64TRP antibodies cross-react with midgut antigenic epitopes, causing rupture of the tick midgut and death of engorged ticks. Three parameters were measured: "transmission," number of uninfected nymphal ticks that became infected when cofeeding with an infected adult female tick; "support," number of mice supporting virus transmission from the infected tick to cofeeding uninfected nymphs; and "survival," number of mice that survived infection by tick bite and subsequent challenge by intraperitoneal inoculation of a lethal dose of TBEV. We show that one dose of the 64TRP vaccine protects mice against lethal challenge by infected ticks; control animals developed a fatal viral encephalitis. The protective effect of the 64TRP vaccine was comparable to that of a single dose of a commercial TBEV vaccine, while the transmission-blocking effect of 64TRP was better than that of the antiviral vaccine in reducing the number of animals supporting virus transmission. By contrast, the commercial antitick vaccine (TickGARD) that targets only the tick's midgut showed transmission-blocking activity but was not protective. The 64TRP vaccine demonstrates the potential to control vector-borne disease by interfering with pathogen transmission, apparently by mediating a local cutaneous inflammatory immune response at the tick-feeding site.

Figure 1. Effect of Immunization on Mice Infested with Virus-Infected and -Uninfected Ticks

Comparison of immunization with either 64TRP antigens, a commercial TBEV vaccine, or the commercial anti-tick vaccine (TickGARD) on (A) transmission = % uninfected nymphal ticks that became infected; (B) support = % mice supporting cofeeding virus transmission between an infected adult female tick and uninfected nymphs; and (C) survival = % mice that survived an infected tick bite (only animals surviving subsequent i.p. inoculation with 1,000 PFU TBEV were included in the analyses).

Figure 2. Skin Histological Response in Immunized Mice Infested with Virus-Infected and -Uninfected Ticks

Histological profiles of skin sections taken at d 4 of TBEV-infected I. ricinus tick challenge on Balb/c mice immunized with either TRP2 (A, D), TRP5 (E), or GST (C, F), or unimmunized (B). Stained with hematoxylin and eosin (A–D) or “Hema Gurr” Rapid stain BDH (E, F) [19]. (A) (magnification 20×) TRP2-immunized animals, (B) (magnification 20×) control unimmunized, and (C) (magnification 20×) GST-immunized animals. Ep, epidermis; De, dermis; CC, cement cone of I. ricinus. (D–E) (magnification 63×) denote skin sections from TRP2 and TRP5 immunized mice, respectively, showing: perivascular cuffing = yellow circle; degranulating mast cells = green arrow; numerous lymphocytes = blue arrow; frequent macrophages = yellow arrow; and some eosinophils = white arrow; dermal dendrocytes = black arrow; neutrophils = gray arrow; and basophils = light blue arrow. (F) (magnification 63×) skin sections from GST control immunized mice showing lymphocytes, macrophages, and dermal dendrocytes.

Figure 3. Skin Immunocytochemical Profile of Immunized Mice Infested with Virus-Infected and -Uninfected Ticks

Immunocytochemical profiles of skin sections taken at d 4 of TBEV-infected I. ricinus tick challenge on Balb/c mice immunized with either GST (A, B) or TRP5 (C, D), or unimmunized (E, F), using rat anti-mouse CD4⁺ antiserum (B, D, and F) and rat anti-mouse CD8⁺ (A, C, and E) antiserum, with a negative control sample (G, PBS plus normal rabbit serum). (C) TRP5-immunized mice: red arrowheads = numerous CD8⁺ T cells; red circles = CD8⁺ T cells occluding the dermal blood vessels; and (D) yellow arrows = CD4⁺ T cells. (E) and (F) unimmunized mice, few CD8⁺ T cells = red arrows and CD4⁺ T cells = yellow arrows/yellow circle, respectively. (B) Control GST-immunized mice yellow arrows = few CD4⁺ T cells. (G) PBS-negative control skin sample = no T cells. Magnification 20×.