Background
The neuropeptide substance P (SP) is produced at high levels within the central nervous system (CNS) and its selective receptor, the neurokinin-1 receptor (NK-1R), is expressed by resident cells such as neurons, microglia, and astrocytes, and by immune cells that can infiltrate the CNS including macrophages and lymphocytes (as reviewed in [
1,
2]). In addition to its functions as a neurotransmitter in the perception of pain and its essential role in gut motility, this tachykinin is now recognized to exacerbate inflammation at peripheral sites including the skin, lung, and gastrointestinal and urogenital tracts. Indeed, this neuropeptide appears to contribute to disease pathology for some infectious agents. For example, SP increases the bronchoconstriction and damaging cardiac inflammation following infection with respiratory syncytial virus and encephalomyocarditis virus, respectively [
3,
4]. Likewise, SP contributes to the severity of inflammation associated with
Trypanosoma brucei brucei infection, and inflammation and granuloma size in a mouse model of
Taenia solium cysticercosis [
5‐
7].
Recently, a number of studies have identified a similar role for SP and NK-1R interactions in neuroinflammation (as discussed in [
1,
2]), and our data suggest that SP exacerbates damaging inflammation elicited within the CNS in response to disparate bacterial pathogens. We determined that the absence of SP/NK-1R interactions in SP receptor-deficient mice or prophylactic pharmacological NK-1R inhibition in wild type animals significantly reduces bacteria-induced neuroinflammation and resultant CNS damage [
8,
9]. NK-1R null mice and mice treated with an NK-1R antagonist showed reduced inflammatory and maintained immunosuppressive, cytokine production, as well as decreased astrogliosis, cellularity, and demyelination following intracerebral administration of the Gram-negative bacterial pathogens
Neisseria meningiditis and
Borrelia burgdorferi, or the Gram-positive bacterium
Streptococcus pneumoniae [
8,
9]. These rodent studies therefore indicate that SP/NK-1R interactions are essential for the progression of damaging inflammation following bacterial CNS infection.
In the present study, we have assessed the role played by endogenous SP/NK-1R interactions in damaging CNS inflammation in an established nonhuman primate (NHP) model of Lyme neuroborreliosis using the specific NK-1R antagonist, aprepitant [
10]. We have previously demonstrated that this NHP model faithfully reproduces the key features of neuroborreliosis including the development of pleocytosis, as well as the classical lesions associated with leptomeningitis of the brain and spinal cord and radiculitis observed in human patients with
B. burgdorferi-associated CNS infection [
11]. We demonstrate that inhibition of SP/NK-1R interactions limits inflammatory nervous system immune responses associated with intrathecal
B. burgdorferi administration in rhesus macaques. This ability, and the availability of centrally acting NK-1R inhibitors that are approved for clinical use, raises the intriguing possibility that targeting SP/NK-1R interactions could be useful as an adjunctive therapy for the treatment of bacterial CNS infections.
Discussion
Bacterial infections of the CNS constitute a group of highly damaging and often life-threatening diseases. What makes the etiology of these diseases so perplexing is that severe CNS inflammation can be initiated by bacterial species that are generally regarded to be of low virulence [
17]. While such responses may be protective, inflammation elicited by infectious agents often results in progressive CNS damage. Indeed, we have recently demonstrated that inflammation plays a key role in pathogenesis in a NHP model of acute Lyme neuroborreliosis [
12]. A hallmark of developing inflammation is the synergistic interaction between cells and their products that can amplify the response. It is now widely accepted that SP, the most abundant tachykinin in the CNS, can exacerbate the inflammatory responses of both leukocytes and resident glial cells via the high affinity NK-1R (as reviewed in [
1,
2]). Importantly, we have demonstrated that SP can augment proinflammatory mediator production by murine glia in response to
B. burgdorferi [
9]. Consistent with this finding, we have shown that endogenous SP/NK-1R interactions are required for maximal proinflammatory cytokine expression in vivo following direct CNS administration of this spirochete in mice [
9]. More recently, we have shown that an NK-1R antagonist can attenuate the neuronal and glial production of inflammatory mediators including CCL2 and IL-6 in rhesus macaque frontal cortex explants and isolated DRG cells following
B. burgdorferi challenge [
18].
In the present study, we have confirmed that levels of gene expression of select inflammatory cytokines and chemokines, including CCL2, CXCL13, IL-17A, and IL-6, are increased in the DRG and spinal cord tissue samples and the CSF from rhesus macaques at 2 to 4 weeks following intrathecal
B. burgdorferi administration. An increase in IL-17A gene expression following infection is particularly interesting since cell signaling mediated by this cytokine plays a key role in regulating the expression of other inflammatory mediators, such as IL-6, via a mechanism that involves NF-κB-mediated transcription [
19]. Consistent with our previous studies using frontal cortex explants and isolated DRG cells [
12], treatment with the NK-1R antagonist, aprepitant, was able to significantly attenuate the transcription of inflammatory mediators in our in vivo NHP model of Lyme neuroborreliosis. Taken together, these data support the contention that endogenous SP/NK-1R interactions play a significant role in the initiation and/or progression of neuroinflammation associated with
B. burgdorferi infection of the CNS.
Interestingly, our studies also demonstrate that NK-1R expression is increased in the NHP brain cortex at 2 weeks following infection and that this effect can be abolished by treatment with aprepitant. While the mechanisms underlying the ability of
B. burgdorferi to increase NK-1R expression are not clear, this finding is in agreement with previous studies demonstrating the ability of bacteria and/or their products to upregulate NK-1R expression by leukocytes [
20,
21]. However, the ability of aprepitant to prevent increases in NK-1R expression suggests that
B. burgdorferi-induced effects occur secondary to a response that is, at least in part, dependent upon SP/NK-1R interactions. This would be consistent with the documented ability of inflammatory mediators to increase NK-1R expression by leukocytes [
20,
21] and glial cells [
22].
Finally, our immunohistochemistry analysis of NHP frontal cortex tissue demonstrates that the number and/or activation level of astrocytes as determined by GFAP expression is decreased in NHPs at 2 and 4 weeks following
B. burgdorferi administration. While the mechanisms underlying this effect are unclear, decreases in astrocyte number/activation may result from increased apoptotic death of this population following infection or could occur as a result of the compensatory production of suppressive mediators, such as IL-10 and IL-19, that have been shown to be produced in a delayed manner by
B. burgdorferi- challenged microglia and/or astrocytes [
14,
23]. Interestingly, treatment with aprepitant prevented
B. burgdorferi-induced decreases in GFAP expression indicating that this effect, either directly or indirectly, is mediated by endogenous SP/NK-1R interactions.
Non-peptide NK-1 receptor antagonists are known to exert central effects [
24]. Of the latest generation of NK-1R antagonists, aprepitant has been shown to cross the blood-brain barrier after oral administration using human positron emission tomography to demonstrate its ability to occupy NK-1R within the brain in an oral dose and plasma concentration-dependent manner [
25]. The ability of NK-1R antagonists to cross the blood-brain barrier means that these agents have the potential for use in the treatment of a wide range of CNS disorders [
26‐
29]. Inhibitors of this tachykinin receptor have been the subject of extensive study for the clinical treatment of depression and anxiety, and aprepitant and its pro-drug fosaprepitant are currently employed clinically as post-chemotherapy anti-emetic agents [
10]. While we did not formally investigate the safety of aprepitant in this study, no adverse events were recorded for any of the animals receiving this drug that could be attributed to the treatment at any point in the study period. This agrees with other studies in which aprepitant was used (performed both in humans and in mice), where no safety issues were detected [
30,
31].
Given a potential role for SP/NK-1R interactions in damaging inflammatory responses within the CNS following infection, there has been considerable interest in targeting this receptor to limit neuroinflammation and neurological sequelae associated with infectious agents. For example, an NK-1R antagonist has been shown to prevent seizure activity in a rodent model of helminth brain infection [
32], while our own studies have shown that pharmacological targeting of NK-1R with the antagonist L703,606 can not only prevent the development of damaging inflammation due to streptococcal CNS infection when administered prophylactically but can also reverse infection-associated gliosis and demyelination when delivered therapeutically without increasing CNS bacterial burden [
8]. Furthermore, aprepitant has been tested as an adjunctive therapy for the treatment of HIV-associated neurocognitive disorder in human clinical trials and has been shown to reduce in vivo serum levels of several pro-inflammatory cytokines and chemokines, and lower CD4-positive T cell expression of sCD163 and PD-1 [
33,
34]. As such, while further studies are needed to define the specific mechanisms underlying the ability of SP to augment CNS inflammation and its role in pathogen clearance, the available data raise the intriguing possibility that currently approved NK-1R antagonists, such as aprepitant, could be repurposed for use as a co-therapy to limit the neuroinflammatory damage associated with infectious agents. Clearly, further investigation of the prophylactic and therapeutic benefits of NK-1R antagonists in such conditions is warranted.
Acknowledgements
Not applicable.