(a) Tick–pathogen interaction: conflict for both ticks and pathogens

Like other intracellular bacteria, A. phagocytophilum have evolved mechanisms to subvert host response to facilitate infection, multiplication, and transmission [5]. These molecular mechanisms for infection of tick cells include but are not limited to remodeling of the cytoskeleton, inhibition of cell apoptosis, manipulation of the immune response, and control of host cell epigenetics [5]. For example, in tick salivary glands, A. phagocytophilum inhibits the intrinsic apoptosis pathway through porin down-regulation to facilitate bacterial infection, while tick cells respond through FAS down-regulation, resulting in the activation of the extrinsic apoptosis pathway to limit A. phagocytophilum infection and promote tick survival (Table 1) [9]. Alterations in the tick gut microbiome associated with feeding, development, and infection could modulate immune response in ticks [10]. Microbiota and A. phagocytophilum-induced activation of dual oxidase results in production of reactive oxygen species (ROS) to control bacteria and activate immune responses as well as epithelial regeneration and repair to protect ticks from infection (Table 1) [10]. However, ROS-mediated damage to gut epithelial cells results in activation of the Janus kinase/signal transducers and activators of transcription (JAK/STAT) pathway, which in turn inhibits apoptosis that facilitates infection of tick salivary glands [9,10].

(b) Tick–pathogen interaction: benefits for both ticks and pathogens

At the tick–pathogen interface, pathogens induce several mechanisms to increase tick survival and favor pathogen infection and transmission. These mechanisms include the induction of an antifreeze glycoprotein (AFGP) and heat shock proteins (HSP) (Table 1) [11,12]. The AFGP increases tick survival at cold temperatures [11], while the HSP response helps to increase tick survival by protecting from stress and preventing desiccation at high temperatures after enhancing questing speed in order to increase chances to attach to a host [12]. In addition, because pathogen infection occurs during blood feeding, ticks have developed a protective response to limit pathogen infection, which also contributes to their survival [12–14]. A. phagocytophilum subvert tick RNA interference by mechanisms other than reducing tudor staphylococcal nuclease (Tudor-SN) levels to preserve tick life cycle because of the role of this protein during tick feeding [14]. In contrast, subolesin, which is involved in tick innate immune response to limit pathogen infection [15], is not manipulated by A. phagocytophilum infection because it affects tick feeding and reproduction and infection with tick-borne bacteria (Fig 2) [15].

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Fig 2. Ixodes scapularis tick–A. phagocytophilum coevolution.

The pathogen inhibits apoptosis by reducing porin levels to increase infection but without affecting tick feeding and reproduction, as illustrated after gene knockdown to maintain tick vector capacity. However, the pathogens do not manipulate subolesin levels because, as shown after gene knockdown, it can affect infection and tick performance. These results illustrate coevolutionary mechanisms by which pathogens manipulate tick protective responses to facilitate infection while preserving tick feeding and vector capacity to guarantee survival of both pathogens and ticks.

https://doi.org/10.1371/journal.ppat.1005488.g002

On the pathogen side, A. phagocytophilum could promote protein misfolding in the endoplasmic reticulum (ER) to counteract the tick cell response to infection, but tick cells respond by activating protein targeting and degradation to prevent ER stress and cell apoptosis, a mechanism that facilitates pathogen infection (Table 1) [13]. Additionally, A. phagocytophilum may benefit from the tick cell ability to limit rickettsial infection through phosphoenolpyruvate carboxykinase (PEPCK) inhibition, leading to decreased glucose metabolism and the availability of essential metabolites for bacterial growth, which also results in the inhibition of cell apoptosis that increases infection of tick cells [13].

(c) Host–tick and host–pathogen interactions: conflict for hosts, ticks, and pathogens

Tick infestations produce a feeding lesion and inhibit host cell responses such as immunity by down-regulation of lectin, complement activation, and other mechanisms to impair the activity of natural killer cells, neutrophils, eosinophils, basophils, and T lymphocytes (Table 1) [7,16]. In turn, hosts respond to tick infestation by activating different mechanisms, including coagulation and platelet aggregation, that affect tick feeding and blood digestion (Table 1) [7,16]. Pathogens manipulate host biological processes to facilitate infection, multiplication, and transmission [4–7]. For example, A. phagocytophilum infection results in inflammatory histopathologic lesions and neutropenia (Table 1) [17]. However, host response to A. phagocytophilum infection produces pathogen-specific IgG against bacterial major surface proteins and CD4-dependent inflammatory responses of activated macrophages and neutrophils to control pathogen infection (Table 1) [18].

(d) Host–tick and host–pathogen interactions: benefits for ticks and pathogens

The mechanisms described above, by which tick saliva modulates host immunity and suppresses inflammatory responses, deviate the host immune response to facilitate tick feeding and pathogen transmission (Table 1) [4,6,7,16].

The mechanisms by which A. phagocytophilum subvert host response to facilitate infection, multiplication, and transmission appear to be common to tick vectors and vertebrate hosts, suggesting an evolutionary adaptation to a diverse number of vector and host species [5]. Therefore, these coevolutionary mechanisms are also reflected at the host–pathogen interface, where neutrophils infected with A. phagocytophilum show up-regulation of proinflammatory genes and increased levels of interleukin 8 (IL-8), IL-8 receptor (CXCR1), and other chemokines [19,20]. These protective responses result in the recruitment of neutrophils and increased granulocytic phagocytosis, which in turn facilitate A. phagocytophilum dissemination (Table 1) [20].

(e) Host–tick and host–pathogen interactions: benefits for the host

A remaining question is what is the benefit for the host from both ticks and pathogens? One hypothesis is that hosts may benefit from the capacity of ticks to manipulate their immune response [16]. For example, in humans, tick feeding may result in increased antibody levels to the carbohydrate α-gal (Gal α 1-3Gal β 1-[3]4GlcNAc-R) [21,22]. Although the tick-induced response to α-gal may result in anaphylactic reactions to red meat, tick bite, and cetuximab treatment, it could also increase protection to pathogen infection [21–23]. Likely, most tick-borne pathogens contain α-gal-modified proteins on their surface. Therefore, increased antibody levels to α-gal could contribute to reducing pathogen infection and multiplication (Table 1) [21–23].

It is difficult to consider that pathogen infection may have a benefit for the host. However, reservoir hosts and not accidental hosts that do not play a role during tick and pathogen life cycle can control pathogen infection to guarantee survival and facilitate pathogen transmission. These facts suggest the existence of mechanisms that evolved to produce a beneficial effect after pathogen infection. Pathogen interference may constitute one of these mechanisms in which pathogen infection, as shown for tick microbiota [10], may interfere with and reduce the susceptibility to infection with other more lethal pathogens [24]. Another possible mechanism is the epigenetic modification produced by A. phagocytophilum that affects the chromatin structure and transcriptional program of host cells [25]. Bacterial-induced epigenetic deregulations may affect host cell function, resulting in pathogen persistence but also promoting host defense to infection (Table 1) [26]. Additionally, the induction of IL-10 in response to A. phagocytophilum infection results in the control of histopathologic lesions induced by host-derived interferon gamma (IFN-γ) (Table 1) [17,18].