Tick vaccines became commercially available in the early 1990's for the control of cattle tick infestations (Willadsen et al., 1995; Canales et al., 1997; de la Fuente et al., 1998, 2007b). TickGARD (in Australia) and Gavac (in Latin American countries) are both derived from R. microplus midgut membrane-bound recombinant protein BM86. The protective action of BM86-based vaccines in cattle is due to the positive correlation between antigen-specific antibodies and reduction of ticks infestations and fertility (Rodríguez et al., 1995; de la Fuente et al., 1998; Merino et al., 2011a). The mechanism by which BM86 immunization affects ticks involves antibody-antigen interaction that interferes with the still unknown BM86 biological function thus reducing the number, weight and reproductive capacity of engorging female ticks (de la Fuente et al., 1998, 1999). As a result, the prevalence of some tick-borne pathogens can indirectly be affected (de la Fuente et al., 2007b). Vaccine trials with BM86 resulted in a reduction in the incidence of babesiosis, as well as reduced tick infestations in vaccinated cattle herds, and these results were corroborated in extensive field trials (de la Fuente et al., 1998, 2007a; Rodríguez Valle et al., 2004). However, because A. marginale is also mechanically transmitted by blood-contaminated mouth parts of biting insects and fomites, BM86 antigen vaccination controlled the transmission of A. marginale only in regions where ticks are the main vectors (de la Fuente et al., 1998).

Despite the effectiveness of these commercial BM86-based vaccines for the control of cattle tick infestations, they show strain-to strain variation in efficacy and are effective against Rhipicephalus tick species mainly (de la Fuente and Kocan, 2003; Willadsen, 2006; de la Fuente et al., 2007a,b; Odongo et al., 2007) hence the need to develop improved vaccine formulations (Guerrero et al., 2012).

BA86 is a recombinant R. annulatus BM86 ortholog protein with over 90% similarity to BM86 (Canales et al., 2008). Experimental trials in cattle proved the efficacy of recombinant BA86 for the control of R. annulatus and R. microplus infestations, showing that the efficacy of both BM86 and BA86 is higher against R. annulatus. These results suggested that physiological differences between R. microplus and R. annulatus and those encoded in the sequence of BM86 orthologs may be responsible for the differences in susceptibility of tick species to BM86 vaccines (Canales et al., 2009a; Jeyabal et al., 2010).

A BM86 ortholog of Hyalomma anatolicum anatolicum, HAA86, was cloned and expressed by Azhahianambi et al. (2009). Jeyabal et al. (2010) reported that vaccination of cattle with the recombinant HAA86 antigen did not only protect against homologous tick challenge but also reduced tick transmission of Theileria annulata, thus protecting the animals against lethal exposure.

The R. microplus BM95 glycoprotein is a BM86 homologue that protects cattle against infestations by South American cattle tick strains not protected by BM86 vaccination (Canales et al., 2009b). Studies with BM95 have shown it protects against a broader range of tick strain infestations suggesting BM95 could be a more universal antigen against infestations by R. microplus strains from different geographical areas (García-García et al., 2000; de la Fuente and Kocan, 2003).

The number of new upcoming promising targets that can affect both tick infestations and pathogen transmission is rising. Nijhof et al. (2010) have recently identified a novel protein from metastriate ticks with structural similarities to BM86, named ATAQ after a part of its signature peptide. Although its function is unknown, ATAQ is expressed in both midguts and Malpighian tubules, while BM86 is expressed only in midguts. The vaccine efficacy of recombinant ATAQ proteins against tick infestations has not been evaluated but it may constitute a good vaccine candidate with an increased cross-protective effect against heterologous ticks compared to BM86-based vaccines because ATAQ proteins are more conserved.

Arthropod vectors induce immunosuppression in the host during feeding and secrete pathogen transmission-enhancing factors that counteract host rejection responses. For example, the Lyme disease agent B. burgdorferi exploits tick salivary proteins (B-cell inhibitory protein BIP and Salp15 from I. ricinus and I. scapularis, respectively) to facilitate transmission to the mammalian host (Anguita et al., 2002, 2003; Hannier et al., 2004; Ramamoorthi et al., 2005). During feeding, tick salivary glands secrete a large variety of pharmacologically active molecules with immunosuppresive properties that facilitate pathogen transmission and are potential candidates for anti-tick vaccines that limit infestations and interfere with tick-borne pathogen transmission (Valenzuela, 2002; Ribeiro and Francischetti, 2003; Nuttall et al., 2006; Titus et al., 2006; Nuttall, 2009).

64TRP is a 15 kDa protein that resembles mammalian host skin proteins, identified in expression libraries as a putative tick cement protein involved in the attachment and feeding of R. appendiculatus (Trimnell et al., 2002; Havlíková et al., 2009). The protein derives from the cement cone that secures the tick's mouthparts in the host skin and, as a broad-spectrum vaccine antigen, is effective against adult and immature stages of several tick species, including I. ricinus (Trimnell et al., 2005). Recombinant forms of R. appendiculatus 64TRP induce potent humoral and delayed type hypersensitivity responses (Trimnell et al., 2002). In hamster, guinea pig and rabbit models this cement antigen acts as a dual-action vaccine by targeting the tick-feeding site (impairing attachment and feeding) and cross-reacting with “concealed” midgut antigens, resulting in death of engorged ticks (Trimnell et al., 2002, 2005; Havlíková et al., 2009). Histological and immunocytological studies have indicated that the key mode of action of 64TRP immunisation is the local cutaneous delayed type hypersensitivity response induced at the skin site of tick feeding (Labuda et al., 2006). Recent experiments have illustrated how vaccination with this antigen also affects tick vector capacity. Labuda et al. (2006) reported that vaccination of mice with 64TRPP antigen prevented transmission of tick-borne encephalitis virus (TBEV) by I. ricinus thus having a protective effect on pathogen transmission.

Salp15 is another secreted salivary protein with host immunosuppressive properties, inhibiting CD4⁺ T-cell activation (Anguita et al., 2002), complement activity (Schuijt et al., 2008), and dendritic cell function (Hovius et al., 2008a). OspC is an outer surface protein produced by B. burgdorferi. When ticks take a blood meal, the spirochetes initiate its synthesis in the midguts of infected ticks. Salp15 physically binds to OspC on B. burgdorferi spirochetes surface during exit from the salivary glands, facilitating the survival of spirochetes, pathogen transmission and host infection (Ramamoorthi et al., 2005; Dai et al., 2009). Salp15-OspC interaction potentially conceals OspC from the host immune response protecting the spirochete (Ramamoorthi et al., 2005). Mice immunized with recombinant Salp15 and challenged with B. burgdorferi infected nymphs were significantly protected from infection (Dai et al., 2009). Antibodies directed against Salp15 may separate Salp15 away from OspC leaving it exposed to the immune responses, or, hypothetically, Salp15 antibodies could bind to Salp15-coated spirochetes and release the spirochetes more effectively to phagocytes (Dai et al., 2009). Immunization of murine hosts with a combination of Salp15 and OspA provide better protection from B. burgdorferi infection than either alone (Dai et al., 2009). Salp15 homologs have been identified in I. ricinus ticks, they also bind B. garinii and B. afzelii OspC to facilitate spirochete transmission (Hovius et al., 2008b).

Salp25D is expressed by I. scapularis salivary glands and midguts (Das et al., 2001) and has homology to peroxiredoxins antioxidants (Barr and Gedamu, 2003). Immunization of mice with rSalp25D reduces Borrelia acquisition by I. scapularis (Narasimhan et al., 2007) demonstrating it plays a critical role during tick feeding in the mammalian host, protecting the bacteria from reactive oxygen produced by neutrophils and facilitating Borrelia acquisition by ticks. Therefore it could be used to vaccinate reservoir hosts to interrupt the spirochete life cycle and reduce its prevalence in ticks in Lyme disease endemic areas. Interestingly, Salp25D does not influence transmission from the tick to the mammalian host (Narasimhan et al., 2007).

The tick histamine release factor (tHRF) from I. scapularis was characterized by Dai et al. (2010). tHRF is secreted in tick saliva, upregulated in B. burgdorferi-infected ticks and it appears to have a role in tick engorgement and efficient B. burgdorferi transmission (Dai et al., 2010). Silencing tHRF by RNAi significantly impaired tick feeding and decreased B. burgdorferi infection levels in mice. Actively immunized mice with recombinant tHRF, or passively transferring tHRF antiserum, also markedly reduced the efficiency of tick feeding and B. burgdorferi infection in mice. Blocking tHRF might offer a viable strategy to develop vaccines that block tick feeding and therefore transmission of tick-borne pathogens.

The I. scapularis salivary protein TSLPI (Tick Salivary Lectin Pathway Inhibitor) identified by Schuijt et al. (2011a) protects B. burgdorferi from direct killing by the host complement system. Silencing TSLPI mRNA significantly reduces Borrelia loads in nymphs and also impairs transmission to mice. TSLPI plays a significant role in both transmission and acquisition of Borrelia (Schuijt et al., 2011a) but immunization against rTSLPI does not completely block bacterial transmission from the tick to the host, suggesting the need for a combination of tick proteins in future tick antigen-based vaccines to prevent Lyme disease (Schuijt et al., 2011b).

Ferritins are iron-storage proteins that play a pivotal role in the homeostasis of iron during tick feeding. A common heavy chain type ferritin 2 (Kopacek et al., 2003), without functional orthologs in vertebrates, has been recently characterized as a gut-specific protein secreted into the tick hemolymph, where it acts as an iron transporter (Hajdusek et al., 2009). Ferritin 2 (RmFER2) knockdown by RNAi and vaccination with the recombinant protein resulted in reduction of feeding, oviposition and fertility in I. ricinus, R. microplus and R. annulatus (Hajdusek et al., 2009, 2010) thus highlighting its potential use as a future dual action tick and tick-borne diseases protective antigen candidate.

TROSPA is a tick receptor for B. burgdorferi OspA that has been identified in the tick midgut (Pal et al., 2004; Antunes et al., 2012). Tick-borne pathogens can adapt from the vector to the mammalian host by differential gene expression. For example, outer surface proteins OspA and OspB are produced when Lyme disease spirochetes enter and reside in ticks (Pal and Fikrig, 2003) but they are downregulated during transmission to the host. Other genes that facilitate transmission from ticks and colonization of the host such as bba52 and OspC are upregulated. TROSPA expression is upregulated during B. burgdorferi infection and downregulated during tick engorgement. The receptor's physiological function is unknown but binding of OspA to TROSPA is essential for B. burgdorferi to colonize the tick gut, thus supporting bacterial infection in the vector (Pal et al., 2004). B. burgdorferi infection enhances expression of specific tick genes such as TROSPA and salp15 that can be targeted to prevent the transmission of Borrelia spirochetes and other tick-borne microbes (Hovius et al., 2007). Blocking TROSPA with TROSPA antisera or via RNA interference (RNAi) reduces B. burgdorferi adherence to the gut of I. scapularis, and as a result reduces bacterial colonization of the vector and, potentially, pathogen transmission to the host (Pal et al., 2004). Bacterial OspA has been used as a Lyme disease vaccine that blocks pathogen transmission as anti-OspA antibodies destroy the spirochetes in the tick gut before transmission to the host occurs (Pal et al., 2000). Studies by Tsao et al. (2001) suggested that vaccination of mice with OspA could reduce transmission of the bacteria to the tick vector regardless of whether the reservoir host was previously infected or not. A combination of OspA with TROSPA antigens may enhance vaccine protective efficacy against Lyme disease.

Serpins (serine protease inhibitors) are a large family of structurally related proteins found in a wide variety of organisms, including hematophagous arthropods. They are known to regulate many important functions such as blood coagulation, food digestion, inflammatory and immune responses (Mulenga et al., 2001) and therefore are attractive target antigens for tick vaccine development. Combining different serpins to vaccinate cattle results in a reduction of engorgement rates and increased mortality of Haemaphysalis and Rhipicephalus ticks (Imamura et al., 2005, 2006). Furthermore, immunization of cattle with a cocktail vaccine containing recombinant R. appendiculatus serpins RAS-3, RAS-4, and a 36 kDa immune-dominant protein RIM36, reduces tick infestations and also has an effect on the tick mortality rate of Theileria parva-infected ticks by increasing it from 10.8 to 48.5% in the vaccinated group (Imamura et al., 2008). Infection of cattle with T. parva could not be prevented by the vaccine although the presence of the pathogen in peripheral blood was delayed by a couple of days indicating the vaccine also had an effect on pathogen transmission to the mammalian host.

Tick Subolesin (SUB), the ortholog of insect and vertebrate akirins (AKR) (de la Fuente et al., 2006a; Goto et al., 2008; Canales et al., 2009c; Galindo et al., 2009; Macqueen and Johnston, 2009; Mangold et al., 2009), was discovered as a tick protective antigen in I. scapularis (Almazán et al., 2003). Most vertebrates have two closely related AKR homologues, AKR1 and AKR2 (Goto et al., 2008). Only one SUB/AKR gene has been identified in insects and ticks, which is evolutionary and functionally related to mammalian AKR2 (de la Fuente et al., 2006a; Goto et al., 2008; Galindo et al., 2009; Macqueen and Johnston, 2009). SUB has a role in tick immunity and other molecular pathways and has been shown to protect against tick infestations and infection by vector-borne pathogen such as A. phagocytophilum, A. marginale, B. bigemina, and B. burgdorferi (de la Fuente et al., 2006b; Merino et al., 2011b; Bensaci et al., 2012). RNAi experiments have demonstrated that SUB knockdown affects the expression of genes involved in multiple cellular pathways (de la Fuente et al., 2006c, 2008c). It also has an effect on pathogen infection by reducing tick innate immunity that results in higher infection levels but also indirectly by affecting tick tissue structure and function and the expression of genes required for pathogen infection, therefore interfering with pathogen infection and multiplication (Zivkovic et al., 2010; de la Fuente et al., 2011) (Figure 2). Vaccines containing conserved SUB/AKR protective epitopes have been shown to protect against tick, mosquito and sand fly infestations, thus suggesting the possibility of developing universal vaccines for the control of arthropod vector infestations (Moreno-Cid et al., 2013). However, the effects of SUB/AKR vaccines on vector-borne viruses showed no effect on tick-borne encephalitis virus infection and transmission (Havlíková et al., 2013).

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.