Substance P (SP) is an undecapeptide present in the CNS and the peripheral nervous system. SP released from the peripheral nerves exerts its biological and immunological activity via high-affinity neurokinin 1 receptor (NK1R). SP is also produced by immune cells and acts as an autocrine or paracrine fashion to regulate the function of immune cells. In addition to its proinflammatory role, SP and its metabolites in combination with insulin-like growth factor-1 are shown to promote the corneal epithelial wound healing. Recently, we showed an altered ocular surface homeostasis in unmanipulated NK1R−/− mice, suggesting the role of SP-NK1R signaling in ocular surface homeostasis under steady-state. This review summarizes the immunobiology of SP and its effect on immune cells and immunity to microbial infection. In addition, the effect of SP in inflammation, wound healing, and corneal epithelial homeostasis in the eye is discussed.

Substance P (SP) is an 11-aa-long neuropeptide, which is produced by neuronal and nonneuronal cells, including the immune cells. SP is known to exert its biological activity through G protein–coupled neurokinin receptors (NKRs) named neurokinin 1 receptor (NK1R), NK2R, and NK3R. Among the three, NK1R has the highest affinity for SP. SP-NK1R interaction is widely reported to regulate immune cells’ function and the immunity to microbial infection. In addition, SP is also reported to promote ocular inflammation, wound healing, and tissue homeostasis. The focus of this review is 3-fold: 1) to provide an overview of the biology of SP; 2) to describe the effect of SP on immune cells and the immunity to microbial infection; and 3) to describe the role of SP in ocular inflammation, wound healing, and tissue homeostasis.

In 1931, v. Euler and Gaddum (1) discovered that the powdered extracts of the equine (horse) brain and intestine had hypotensive and spasmogenic activity. Because the activity of this factor resided in the water-soluble powdered extracts of the brain and intestinal tissue, it was initially named as “preparation P” and then later “Substance P.” In the early 1970s, Chang et al. (2) determined the amino acid composition of SP. SP is comprised of 11 aa (Arg-Pro-Lys-Pro-Gln-Gln-Phe-Phe-Gly-Leu-Met-NH2) and has a net positive charge under physiological pH (2). SP is a member of the tachykinin (TAC) family of neuropeptides and is encoded by the TAC1 gene (3). In humans, the major mammalian TAC genes are TAC1, TAC3, and TAC4. The human TAC1 gene consists of seven exons and encodes for SP, neurokinin A (NKA), neuropeptide K, and neuropeptide-γ (4, 5). TAC1 encodes for preprotachykinin A (PPTA) mRNA, whereas TAC3 and TAC4 encode for preprotachykinin B and preprotachykinin C mRNA, respectively (6, 7). PPTA mRNA undergoes alternative splicing to give rise to αPPTA, βPPTA, γPPTA, and δPPTA mRNA transcripts (Fig. 1A). Out of the four isoforms of PPTA mRNA, αPPTA and δPPTA give rise to SP peptide only, whereas βPPTA yields to SP and NKA peptide, and the γPPTA transcript encodes for SP, NKA, and neuropeptide-γ (8).

FIGURE 1.

(A) Schematic of human TAC1 gene mRNA splice variants. The neuropeptide products are indicated by colored boxes on mRNA structures, and the number denotes the coding exon for each TAC derived from the various isoforms of PPTA. (B) Schematic representation of SP production by a neuronal cell and an immune cell type. The outcome of SP binding to a target cell expressing high-affinity SP receptor, NK1R, is depicted as NK1R-associated signaling events along with an internalization of the SP-NK1R complex, and the recycling of free NK1R to the cell surface after degradation of SP in the endosomal complex. AC, adenylate cyclase; DAG, diacylglycerol; ECE-1, endothelin-converting enzyme-1; GRK, G-protein coupled receptor kinase; IP3, inositol triphosphate; MEKS, MAPK; PKC, protein kinase C; PLC, phospholipase C.

FIGURE 1.

(A) Schematic of human TAC1 gene mRNA splice variants. The neuropeptide products are indicated by colored boxes on mRNA structures, and the number denotes the coding exon for each TAC derived from the various isoforms of PPTA. (B) Schematic representation of SP production by a neuronal cell and an immune cell type. The outcome of SP binding to a target cell expressing high-affinity SP receptor, NK1R, is depicted as NK1R-associated signaling events along with an internalization of the SP-NK1R complex, and the recycling of free NK1R to the cell surface after degradation of SP in the endosomal complex. AC, adenylate cyclase; DAG, diacylglycerol; ECE-1, endothelin-converting enzyme-1; GRK, G-protein coupled receptor kinase; IP3, inositol triphosphate; MEKS, MAPK; PKC, protein kinase C; PLC, phospholipase C.

Close modal

SP is present in a large amount in the CNS and the peripheral nervous system (9). In neurons, SP is expressed in the soma. Once synthesized, SP is transported in large dense-core vesicles (LDCVs) and the peptide is released through the exocytosis of LDCVs either at axonal terminals or at the neuronal soma (Fig. 1B) (10). Once exocytosed, SP binds to membrane-bound SP receptor expressed either on the same cell or on the neighboring cells. Unbound SP can be degraded by a cell surface metalloendopeptidase named neprilysin, and thereby suggests a shorter half-life of SP in tissues (11, 12). However, SP can prolong its half-life by interacting with high m.w. factors such as fibronectin (13). In support, SP is reported to be more stable in blood plasma (14). Thus, SP may form a complex with other molecules to prolong its half-life in tissue or in the blood.

Biological activity of SP is mediated through NKRs. NKRs belong to the class I (rhodopsin-like) family of G protein–coupled receptors (15, 16). There are three different types of NKRs: NK1R, NK2R, and NK3R. The affinity of these NKRs in binding to SP is in the order of NK1R > NK2R > NK3R (17). In humans, two naturally occurring isoforms of NK1R are found in neuronal and immune cell types (1821). They are named as full-length NK1R or NK1R and truncated NK1R (NK1R-T). NK1R is made up of 407 aa residues and has one C-terminal intracellular domain, whereas NK1R-T is 311 aa long, but lacks the 90-aa-long C-terminal intracellular domain (22).

NK1R signaling involves phosphorylation of the C-terminal domain of NK1R by G protein–coupled receptor kinases, resulting in the interaction of NK1R with β-arrestin adapter proteins. β-Arrestin plays a central role in desensitization of cells to SP signaling (8, 23). The desensitization process involves β-arrestin–associated internalization of SP-NK1R complex in an endosome (Fig. 1B). The endosomal acidification dissociates SP from the NK1R complex followed by degradation of SP by endothelin-converting enzyme-1, a membrane metalloendopeptidase (8). The degradation of SP frees NK1R to recycle back to the cell surface (Fig. 1B). Recycling of NK1R is also regulated by the concentration of SP. After stimulation with a low concentration of SP (<1 nM), NK1R is minimally phosphorylated and gets internalized into endosomes, but rapidly dissociates from β-arrestin to recycle back to the cell surface (24). However, a high concentration of SP (>10 nM) causes extensive NK1R phosphorylation, its internalization into endosomes, and prolonged association with β-arrestin (23). After prolonged stimulation with SP, the NK1R gets ubiquitinated and traffics to lysosomes, where it gets degraded (Fig. 1B) (25).

SP binding to NK1R also activates phospholipase C, which causes the formation of inositol triphosphate and diacylglycerol. Inositol triphosphate increases the level of cytosolic Ca2+, whereas diacylglycerol activates protein kinase C. NK1R signaling can also activate adenylyl cyclase, which causes the generation of cAMP, and the latter activates protein kinase A (Fig. 1B). In NK1R-transfected rat kidney epithelial cells, these signaling events cause an activation of ERK1/2, a member of MAPKs (26). The nuclear translocation of ERK1/2 is necessary for the proliferation and antiapoptotic effect of SP (26). NK1R signaling may also cause an activation of p38 MAPK and NF-κB transcription factor (27, 28). In macrophages, SP-induced NF-κB activation requires ERK1/2 and p38 MAPK activation, which results in the expression of MIP-2 and MCP-1 chemokine (29).

In contrast with NK1R, the signaling property of NK1R-T is less well studied. NK1R-T does not interact with β-arrestin, resulting in the impairment of SP-induced desensitization and endocytosis of the SP-NK1R complex (26). The NK1R-T is found on human monocytes, macrophages, colonic epithelial cells of the colitis patients, and in the peripheral tissues, including heart, lung, prostate, and spleen (19, 30). Furthermore, NK1R-T has 10-fold lower affinity to SP than full-length NK1R (20, 31). The intracellular signaling pathways for NK1R and NK1R-T are different; for example, SP stimulation of NK1R-T did not activate NF-κB and showed reduced mRNA expression of IL-8 (32, 33). NK1R-T stimulation in HEK 293 cells causes ERK1/2 phosphorylation, but the effects are seen after 15 min of incubation with SP (20, 32). In contrast, full-length NK1R stimulation results in a rapid activation of ERK (within 1–2 min). NK1R-T stimulation increases CCL5-induced intracellular calcium level in monocytes/macrophages (19). These results demonstrate a cross-talk between NK1R-T and CCR5, while interacting with their respective ligands. However, more detailed studies are needed to address the factors involved in upregulating NK1RT in chronic inflamed tissues, and whether an increased level of NK1RT has beneficial or detrimental effects in resolving the inflammation and promoting the tissue repair.

Nerves containing SP are reported to innervate primary (thymus and bone marrow) and secondary lymphoid organs (spleen, lymph nodes, tonsils, and lymphoid aggregates in the gut) (3436). This suggests that SP may act as a mediator of cross talk between the nervous and immune systems. In addition to their production from neurons, SP and its receptor NK1R are well documented to be expressed in different immune cell types. The expression of SP and its effect on immune cells are summarized in Table I. Brief descriptions of the role of SP in regulating the function of different immune cell types follow.

Table I.
Expression and effects of SP in immune cell types
Immune Cell TypesSP ExpressionNK1R-Mediated EffectSP or NK1R Agonist Signaling on Immune CellsReferences
DCs Shown in human and mouse Shown in human and mouse Potentiate immunostimulatory function and survival of DCs, IL-12 production and generation of type I immunity. (7579
Monocytes/macrophages Shown in human and mouse Shown in human and mouse NK1R-T stimulation in monocytes augments CCR5 signaling and promotes M-tropic HIV infection. NK1R signaling enhances IL-12 production in macrophages. (6770
Eosinophils Shown in human and mouse Data not shown SP effects are NK2R mediated. Inhibit spontaneous apoptosis of cells in culture, exerts chemotactic effect and causes the release of O2(51, 52
Neutrophils Shown in human and mouse Shown in human and mouse Causes superoxide production, enhances phagocytosis, increases expression of chemokine and chemokine receptor. (55, 57, 58, 62
Mast cells Data not shown Shown in human and mouse Causes mast cell degranulation, TNF-α and vascular endothelial growth factor secretion. (40, 41, 45, 46
NK cells Shown in human and mouse Shown in human and mouse Increases IFN-γ production, either inhibits or enhances NK cell cytotoxicity under defined set of condition. (81, 8386
T lymphocytes Shown in human and mouse Shown in human and mouse Enhances IL-2 and IFN-γ production, promotes T cell proliferation, generation of memory Th17 from non-committed memory CD4 T cells. (89, 90, 92, 94
Immune Cell TypesSP ExpressionNK1R-Mediated EffectSP or NK1R Agonist Signaling on Immune CellsReferences
DCs Shown in human and mouse Shown in human and mouse Potentiate immunostimulatory function and survival of DCs, IL-12 production and generation of type I immunity. (7579
Monocytes/macrophages Shown in human and mouse Shown in human and mouse NK1R-T stimulation in monocytes augments CCR5 signaling and promotes M-tropic HIV infection. NK1R signaling enhances IL-12 production in macrophages. (6770
Eosinophils Shown in human and mouse Data not shown SP effects are NK2R mediated. Inhibit spontaneous apoptosis of cells in culture, exerts chemotactic effect and causes the release of O2(51, 52
Neutrophils Shown in human and mouse Shown in human and mouse Causes superoxide production, enhances phagocytosis, increases expression of chemokine and chemokine receptor. (55, 57, 58, 62
Mast cells Data not shown Shown in human and mouse Causes mast cell degranulation, TNF-α and vascular endothelial growth factor secretion. (40, 41, 45, 46
NK cells Shown in human and mouse Shown in human and mouse Increases IFN-γ production, either inhibits or enhances NK cell cytotoxicity under defined set of condition. (81, 8386
T lymphocytes Shown in human and mouse Shown in human and mouse Enhances IL-2 and IFN-γ production, promotes T cell proliferation, generation of memory Th17 from non-committed memory CD4 T cells. (89, 90, 92, 94

Mast cells.

Mast cells are shown in close proximity to SP-positive nerves in many tissues, including lung, intestine, dura matter, diaphragm, and skin, suggesting the involvement of mast cells in neurogenic inflammation (3739). Earlier studies showed that SP can induce the histamine release by mast cell degranulation, and this SP-mediated activity was considered to be the outcome of direct G protein–mediated activation rather than an NKR-mediated process (40, 41). However, later studies in both rat and human cultured mast cells showed the expression of the high-affinity SP receptor NK1R (42, 43), whereas the human intestinal mast cells (mucosal type) do not respond to SP, because they do not express any of the three NKRs under the homeostatic condition (44). Together, these studies suggest that SP exerts its function in an NK1R-independent and -dependent fashion on mast cells. In addition, studies show a bidirectional signal between mast cells and SP-expressing nerves. SP released from sensory neurons acts on mast cells to produce TNF-α and vascular endothelial growth factor (45, 46). In contrast, mast cells can release the tryptase enzyme, which activates protease-activated receptor-2 on neurons and mediates the release of SP to induce neurogenic inflammation (47).

Eosinophils.

SP is shown to present in human eosinophils (48). Similarly, immunostaining of SP is noted in eosinophils derived from liver granulomas of mice infected with murine schistosomiasis (4850). Eosinophil activation by SP causes their degranulation and release of O2 (51). This effect of SP is mediated via its N terminus and was thought to be a receptor-independent event. However, more recently, eosinophils isolated from atopic dermatitis patients were shown to express the higher levels of NK2R, but not NK1R, at mRNA and protein levels (52). SP stimulation of eosinophils showed an increased influx of Ca2+ and inhibited their spontaneous apoptosis in cultures (52). SP was also shown to have a chemotactic effect on eosinophils derived from atopic dermatitis patients (52), suggesting that eosinophils may propagate the itch cycle in atopic conditions via SP.

Neutrophils.

Neutrophils expressing SP are found in granulomas and inflamed bronchoalveolar lavage (53, 54). SP stimulation of human neutrophils causes superoxide production (55). This biological activity of SP is NK1R dependent and involves an increased level of intracellular Ca2+ (56). SP also enhances the phagocytic activity of neutrophils and regulates the influx of neutrophils in inflamed tissues (57). The influx of neutrophils in inflamed tissue is NK1R mediated. A reduced influx of neutrophils and MPO activity was reported in the pancreas and lung tissue of NK1R-deficient mice in an acute pancreatitis mouse model (58). Similarly, IL-1β–induced neutrophil accumulation in a murine air-pouch model was impaired in NK1R−/− mice (59). SP administration in the skin has been shown to cause the neutrophil accumulation in the skin tissue of mice, and the effect is dependent on skin-resident mast cells (60, 61). However, in vitro SP stimulation of primary mouse neutrophils, purified from the peripheral blood by density gradient centrifugation, showed an increased expression of CCL3, CXCL2 chemokines and CCR1, CXCR2 chemokine receptors (62). These effects were NK1R dependent. Together, these studies suggest a direct and indirect effect of SP in regulating neutrophil function and influx in inflamed tissues.

Monocytes/macrophages.

Monocytes/macrophages in both human and rodents are reported to express SP and its receptor NK1R (6365). In human mononuclear phagocytes, NK1R antagonist treatment downregulates SP mRNA expression (66). While determining which form of NK1R is expressed, primary human monocytes and the undifferentiated human monocyte cell line THP-1 were reported to express only NK1R-T (19, 67). The NK1R-T–expressing THP-1 cells, when stimulated with SP, did not trigger Ca2+ response, but SP stimulation augmented CCL5-induced intracellular levels of Ca2+ in undifferentiated THP-1 cells, as well as in primary human monocytes (67). These studies suggest a critical interaction between NK1R-T and CCR5 in augmenting CCL5-induced effects in monocytes. In fact, the NK1R antagonists have been shown to inhibit HIV infection of macrophages, suggesting the importance of SP via NK1R-T in enhancing M-tropic HIV infection (68, 69). In mice, SP induces p35 and p40 mRNA in cultured macrophages via NK1R, and LPS stimulation further augments the secretion of bioactive IL-12p70 (70). Moreover, IL-12 induces SP (PPTA) expression in splenic macrophages, suggesting that IL-12 and SP regulate each other’s expression in murine macrophages (71). In addition to IL-12, IL-23 has also been reported to regulate SP levels in mouse macrophages, which is subject to inhibition by TGF-β cytokine (72). SP exerts its action on mouse macrophages through high-affinity NK1R, and it causes NF-κB activation without increasing the intracellular Ca2+ levels (29, 73). NK1R expression in mouse peritoneal macrophages is upregulated by Th1 (IFN-γ) and Th2 (IL-4) cytokines (74). Together, these reports provide convincing evidence for the expression and function of SP in both human and rodent monocytes/macrophages.

Dendritic cells.

Dendritic cells (DCs) play an important role for the generation of an adaptive T cell response to non-self-antigens. SP and its receptors NK1R and NK2R are present in both mouse and human DCs (75, 76). In mice, bone marrow–derived DCs (BMDCs) were shown to express the γ-transcript of the PPTA gene and produce SP at protein levels as determined by immunostaining (75). BMDCs expressing SP potentiate allogeneic T cell proliferation through NK1R (75). In addition, BMDCs stimulated with NK1R agonist produce IL-12, which when injected in vivo induces type I immunity (77). Similarly, in vivo administration of NK1R agonist has been shown to potentiate the immunostimulatory functions of skin-resident Langerhans cells to induce an Ag-specific type I immunity in a mouse model (78). NK1R signaling of BMDCs has also been shown to promote the survival of DCs (79), suggesting a possible in vivo role of SP in maintaining tissue-resident DC populations under the homeostatic condition, especially in tissues highly innervated with sensory nerves such as the skin and cornea.

NK cells.

NK cells are granular lymphocytes, which are primarily involved in controlling viral load in inflamed tissues by targeting virus-infected cells. Human NK cells are reported to express functional NK1R, and a dose-dependent effect of SP on NK cell migration was seen in vitro with or without IL-2 stimulation (80). SP-NK1R interaction has also been shown to promote IFN-γ production from murine NK cells in a bacterial infection model (81). In contrast, the role of SP in inhibiting the cytotoxicity of NK cells is well described in studies related with HIV-seropositive subjects. In patients with HIV infection, higher amounts of SP are seen in human plasma, and it correlates with lower number and depressed function of NK cells (82). The ex vivo experiments carried out on PBMCs isolated from HIV-seropositive subjects showed that SP antagonist CP-96345 increases the cytolytic activity of NK cells (83). A detailed study further showed that human NK cells express both full-length NK1R and NK1R-T, and a preincubation with SP inhibits the NK cell’s contact-dependent cytotoxicity. The inhibitory effect of SP was evident in the NK cell line (YTS), as well as in NK cells purified from the human blood of healthy volunteers (84). In this study, preincubation with SP did not affect NF-κB activation, but inhibited the prolonged increase in Ca2+ level in NK cells after its interaction with the target cell. SP preincubation also inhibited activation receptor–induced phosphorylation of ERK in NK cells. However, in the absence of preincubation and the use of different concentrations, SP has been shown to stimulate the cytotoxicity of NK cells (85, 86). Thus, under a defined set of conditions, SP can inhibit the cytotoxicity of NK cells.

T lymphocytes.

Resting T cells do not express SP and its receptor, but activated human T lymphocytes are shown to express the PPTA gene and produce SP neuropeptide (87). Similarly, in rodents, activated T cells express NK1R, and SP released from activated T cells may act in an autocrine fashion to regulate T cell proliferation (75). It is reported that IL-12 augments, whereas IL-10 blocks, the SP production from murine T cells (72). IL-12 also induces SP receptor NK1R in murine T cells (88). SP has also been shown to enhance IL-2 expression in activated human T cells and promote T cell proliferation (89, 90). In human T lymphocytes, SP is reported to enhance the expression of MIP-1β, suggesting that SP action on T cells may promote the chemotaxis of CCR5 (MIP-1β receptor)-expressing immune cells (91). Recently, in the presence of monocytes, SP-NK1R interaction has been shown to promote the generation of human memory Th17 cells from non-Th17 committed memory CD4 T cells (92). In murine schistosomiasis parasitic infection, granuloma eosinophils produce SP, and CD4 T cells isolated from the granulomas are shown to express NK1R, suggesting that SP-NK1R interaction in granulomas may involve CD4 T cells (93). In fact, SP is reported to enhance schistosome egg Ag-induced IFN-γ production in NK1R-expressing T cells from schistosome-infected mice (94). Together, these studies show the functional role of SP on activated, but not naive, T cells.

Multiple studies have documented the role of SP in viral, bacterial, and parasitic infection (9597). The known roles of SP in regulating the immunity to viral, bacterial, and parasitic infections are summarized in the following subsections.

SP and its receptor in viral infections.

Among viral infections, the role of SP is well reported in murine γ herpes virus 68 (HV-68) infection. HV-68 is a γ-2 herpes virus that infects the epithelial cells and establishes the latency in immune cell types such as B cells, macrophages, and DCs (98, 99). HV-68 infection of C57BL/6 (B6) mice is shown to cause an increased mRNA expression of PPTA and SP receptor in the spleen and mesenteric lymph node of infected mice (100). In the same study, NK-1R–deficient mice exhibited reduced CTL response and an increased level of latent virus. Similarly, mice genetically deficient for the PPTA gene, after HV-68 infection, were shown to have an ∼100-fold higher viral load in the lungs in comparison with control B6 mice (101). This was associated with an increased lung pathology and an increased level of latent virus in the spleens of PPTA-deficient mice. Together, these studies suggest that, in the absence of SP or its receptor, the host immunity to HV-68 infection is significantly compromised.

The role of SP has also been investigated in detail in HIV infection. An increased level of SP is reported in the blood plasma of HIV-infected men and women (82, 102). In addition, HIV infection of cultured macrophages causes an increased expression of SP (103). In contrast, in vitro addition of SP has been shown to enhance the replication of HIV in blood-isolated mononuclear phagocytes (104, 105). This effect of SP is mediated via NK1R-T. The latter enhances HIV entry into immune cells via CCR5, because the use of non-peptide SP antagonist (CP-96,345) inhibits HIV infection of macrophages and downregulates CCR5 (68). In comparison with CP-96,345, NK1R antagonist aprepitant is more potent ex vivo to suppress the HIV infection of myeloid-derived macrophages (69). Aprepitant treatment has also been shown to reduce viral load in SIV-infected macaques and reduce the proinflammatory cytokines in HIV-positive individuals, suggesting that the NK1R antagonist might be of use as an adjunct therapy in HIV infection (106).

SP is also reported to play an important role during infection with paramyxoviruses. Sendai virus is an ssRNA virus of the Paramyxoviridae family. Sendai virus infection of the Guinea pig increases the expression of SP in the cell body of the afferent neurons in nodose ganglia (107). The inducible expression of SP suggests its involvement in virus-induced airway inflammation. In fact, NK1R antagonist treatment of Guinea pigs, infected with Sendai virus, was shown to limit virus-induced bronchoconstriction (108). Similarly, in a rat model of respiratory syncytial virus (RSV) infection, an increased expression of SP receptor was noted after lung infection with RSV (109). Challenge of RSV-infected rats with capsaicin resulted in airway inflammation, and NK1R antagonist treatment abrogated the effect of capsaicin (110).

Similar to HIV, the spread of measles virus (MV) from one cell to another is partly dependent on SP receptor NK1R. The fusion protein of MV shares the C-terminal sequence of SP, and pharmacological inhibition of the NK1R is shown to reduce the spreading of MV (111, 112). Neurkinin-1 receptor signaling is also reported to regulate innate immune defense to genital herpes virus infection (113). NK1R-deficient mice exhibited an enhanced level of HSV-2 in the genital tract, which was associated with an impaired NK cell cytotoxicity in the vaginal tissue. This study suggested that lack of NK1R signaling compromises innate immunity to HSV-2 infection. NK1R signaling is also reported to regulate corneal HSV-1 infection–induced inflammation, as described later in the Role of SP in ocular inflammation section.

SP and its receptor in bacterial infections.

The best studied role of SP in augmenting the immunity to bacterial infection was shown in a mouse model of Salmonella infection. The successful clearance of Salmonella infection requires IL-12–induced production of IFN-γ, and the latter enhances macrophage and DC activation to eliminate the Salmonella infection (114). Oral infection with Salmonella causes a rapid and dramatic upregulation of SP and NK1R mRNA in mucosal tissues (115, 116). Mice pretreated with spantide II, NK1R antagonist, before oral challenge with Salmonella became highly susceptible to bacterial infection, because spantide II treatment reduced IL-12p40 mRNA expression and impeded the bacterial clearance (116). In a murine model of pneumococcal meningitis, therapeutic targeting of NK1R was shown to limit neuroinflammation (117). Another proinflammatory role of SP was described in two clinically relevant bacterial CNS pathogens, Neisseria meningitides and Borrelia burgdorferi. After exposure to either of these two pathogens, SP was shown to augment proinflammatory cytokine production in the isolated cultures of CNS-resident microglia and astrocyte cell types (118). Moreover, NK1R-deficient mice demonstrated a decreased level of inflammatory cytokines after CNS infection of these pathogens (118).

SP and its receptor in parasitic infections.

The role of SP in parasitic infection is also well documented. It is reported that SP is involved in regulating the size of granuloma formation postinfection with larval cysts of the cestode taenia solium. Mice deficient in SP precursor or NK1R showed much smaller granulomas and a lesser level of IL-6, TNF-α, and IL-1β protein in comparison with wild-type infected mice (119). Mice can get infected with Schistosoma mansoni, which colonize the human intestine, and thus mice are considered a good model to study schistosomiasis. Schistosome eggs in liver induced chronic granulomatous inflammation, and SP expression in granulomas was shown to control the levels of inflammatory cytokines (120). NK1R knockout mice in response to schistosome infection develop smaller granulomas and show reduced levels of IFN-γ and IgG2a (120, 121). SP involvement has also been shown in a murine model of Trypanosoma brucei infection, because the infected mice treated with SP antagonist RP67,580 developed reduced neuroinflammation (122). However, Trypanosoma infection of NK1R knockout mice showed more severe neuroinflammation than wild-type mice (123). Interestingly, treatment with NK2 and NK3 antagonist reduced neuroinflammation in the knockout mice, suggesting that in the absence of NK1R signaling SP can still exert its function through weaker affinity receptors NK2R and NK3R. In fact, NK1R has been shown to act as a negative feedback for basal and activity-induced release of SP from neurons, and an increased level of SP is noted in the skin of NK1R−/− mice (124).

Multiple reports have shown the role of SP in ocular inflammation (125). Human samples obtained from the patients with pterygium showed the expression of SP and NK1R in pterygium fibroblasts. Moreover, the cell culture studies show that SP via NK1R induces the migration of pterygium fibroblast and microvascular endothelial cells, suggesting that SP may contribute to the pathogenesis of Pterygia through its profibrogenic and angiogenic action (126). In allergic conjunctivitis patients, a higher level of SP was found in the tear fluid (127). The treatment of conjunctivitis in patients with ocular allergy and in an animal model is reported to reduce the SP level in tears (128130). Subconjunctival injections of SP are also shown to cause conjunctivitis and increase the permeability of conjunctival blood vessels (129). Recently, SP-NK1R interaction is reported to cause the loss of anterior chamber–associated immune deviation after retinal laser burns in a rodent model (131). The study showed that retinal laser burns in one eye resulted in an increase in NK1R expression in the retina of both eyes, and the local use of NK1R antagonists prevented the bilateral loss of anterior chamber–associated immune deviation. SP has also been shown to cause the rejection of second corneal allograft transplant (132). Using a mouse model of penetrating keratoplasty, it was shown that severing the corneal nerves during surgery released SP in both eyes, which disabled CD4+CD25+ regulatory T cells (Tregs) that were required for allograft survival (132). Although the underlying mechanism by which SP abolished the immunosuppressive activity of Tregs is not clear, the study showed the in vivo ability of SP to regulate Treg function in an animal model.

SP has also been shown to stimulate the corneal neovascularization through NK1R (133). The possible underlying mechanism could be the direct action of SP on vascular endothelial cells, because functional NK1R is known to express on HUVECs, and the growth-promoting effects of SP on vascular endothelial cells are reported in serum-free culture condition (134, 135). SP may also indirectly promote hemangiogenesis in tissues by recruiting granulocytes with angiogenic potential from the blood circulation (136). Under these conditions, SP can perform its chemotactic activity or cause mast cell degranulation to recruit the granulocytes in inflamed tissue (137). In addition to the development of new blood vessels, SP via NK1R is known to promote the vasodilation of existing blood vessels, resulting in plasma extravasation (138, 139). Recently, SP has been shown to regulate lymphangiogenesis in an animal model of diet-induced obesity (140). The study showed that NK1R antagonist treatment reduced abnormal lymphangiogenesis in visceral adipose tissue of allergen-sensitized obese mice. SP is also known to effectively modulate the contractility of lymphatic vessels via its direct effect on lymphatic muscle cells (141). The direct effect of SP is mediated via NK1R and NK3R molecules, which are expressed on lymphatic muscle cells and are involved in the activation of p38MAPK and ERK1/2 molecules (142). The development of hemangiogenesis and lymphangiogenesis in avascular cornea is likely to affect the visual acuity by causing edema and the influx of immune cells in corneal tissue (143). Recently, in two different models of corneal angiogenesis (alkali burn and sutures), topical application of NK1R antagonist Lanepitant is shown to effectively reduce both hemangiogenesis and lymphangiogenesis in the inflamed cornea (144). Moreover, treatment with the NK1R antagonist Fosaprepitant is shown to inhibit corneal (hem and lymph) angiogenesis after this has been established in an animal model.

SP has also been reported to regulate virus- and bacteria-induced inflammation in the cornea. In a mouse model of corneal HSV-1 infection, SP has been shown to regulate the severity of herpes stromal keratitis (HSK) induced in response to corneal HSV-1 infection (145). In this study, intense SP staining was noted in the posterior stroma of the eyes with severe HSK, whereas the eyes with mild HSK showed SP staining on the apical surface of the corneal epithelium. The intense SP staining noted in the corneal stroma could be the outcome of SP expression by stromal keratocytes/fibroblasts, diffusion of SP from aqueous humor, and SP secreted from the sympathetic nerves. A recent study has shown hyperinnervation of the corneal stroma of HSK developing in eyes with sympathetic nerves that are originating from superior cervical ganglion (146). The cultured explants of superior cervical ganglion are reported to increase the level of SP in response to stimulation with IL-1β cytokine (147). In HSV-1–infected corneas, NK1R expression was seen in both CD45 and CD45+ cells, suggesting that SP is likely exerting its action on both immune and nonimmune cells. Moreover, subconjunctival injection of NK1R antagonist spantide I, during the clinical phase of HSK, was shown to reduce the corneal opacity and angiogenesis (145). In addition to HSK, SP is also reported to regulate the severity of bacterial keratitis induced in response to corneal Pseudomonas aeruginosa infection in resistant BALB/c and susceptible B6 mouse model (81, 148). In these studies, SP was shown to adversely affect the disease by elevating the levels of proinflammatory cytokines and the growth factors in infected corneas (149). The use of SP antagonist spantide I was shown to reduce type I and enhanced type II cytokine in the infected cornea of B6 mice, leading to an improved disease outcome (150). SP was also reported to delay the apoptosis of polymorphonuclear cells, and blocking SP interaction with NK1R promotes an earlier apoptosis and improves disease outcome in susceptible B6 mice (151). Together, these studies show the involvement of SP in promoting infection-induced corneal inflammation.

In addition to its role in microbial inflammation, SP is reported to play an important role in corneal wound healing. SP is present in corneal nerves, normal tears, and is also expressed in the corneal epithelium and stromal keratocytes (152156). Although normal cornea has resident immune cell populations such as macrophages and DCs, they are not reported to express SP in the corneal tissue. Among SP receptors, NK1R is expressed in corneal epithelial and keratocytes (155, 156). SP via NK1R increases the half-life of IL-8 transcripts in human corneal epithelial cells, resulting in the enhancement of IL-8 synthesis (154). Similarly, the primary human keratocytes expressing the full-length form of NK1R upon SP stimulation secrete increased levels of chemotactic IL-8 protein, which contributes significantly to SP-enhanced keratocyte migration (155). Together, these events suggest that SP-NK1R interaction promotes corneal wound healing. In fact, SP is reported to promote diabetic corneal epithelial wound healing via NK1R (157). In an alkali-burn model of mouse and rabbit eyes, SP was shown to mobilize CD29+ stromal cells from the bone marrow into the circulation, and subsequently to the injured tissue to accelerate the process of wound healing (158). The process of corneal epithelial wound healing involves migration of corneal epithelial cells over the denuded surface followed by their proliferation to increase the normal thickness of the epithelium (159). Unlike in a diabetic mouse model, topical application of SP treatment alone is not effective to promote injury induced corneal epithelial wound healing in a rabbit model (160). However, SP is known to act synergistically with insulin-like growth factor-1 (IGF-1) to promote corneal epithelial cell migration and their attachment to extracellular matrix proteins in ex vivo cultures (161). The combined effect of SP/IGF-1 was concentration dependent of each factor and was mediated by NK-1R, but not by NK2R and NK3R (162). The signaling events associated with the combined treatment involve protein kinase C activation, p38 MAPK activation, and upregulation of α5 integrin (163165). The latter is expressed on actively migrating corneal epithelial cells. In rabbits, the administration of eye drops containing both SP and IGF-1 was shown to promote corneal epithelial wound healing (166). SP/IGF-1 treatment was also reported to improve the barrier function and enhance epithelial wound healing in animal models of neurotrophic keratopathy, a persistent corneal epithelial defect condition that results from insults to the trigeminal nerve (167, 168).

In addition to full-length SP, the 4-aa sequence (FGLM-amide) derived from the C-terminal end of SP along with IGF-1 effectively stimulates the corneal epithelial cell migration in a rabbit model of corneal wound healing (169). SP-derived peptide and IGF-1 have also been shown to promote corneal epithelial wound healing in diabetic rats (170). Similarly, a tetrapeptide (SSSR) derived from the C-domain of IGF-1 acts synergistically with FGLM-amide to promote corneal epithelial wound healing (171). Shortening of SP and IGF-1 retained their synergistic effect but prevented their undesirable side effects, such as SP-induced miosis (172) and IGF-1–induced angiogenesis (173). These encouraging observations resulted in the clinical trials to treat persistent corneal epithelial defects with topical application of SP or SP-derived peptide along with IGF-1 molecule. Eye drops containing FGLM-amide and IGF-1 were shown to treat a patient with neurotrophic keratopathy (174). The treatment resulted in reduced corneal opacity and the closure of the corneal epithelial defect. Similarly, eye drops containing FGLM-amide and SSSR peptides were shown to induce a rapid resurfacing of the corneal epithelium in individuals with neurotrophic keratopathy (175). Together, these studies show the beneficial effect of SP in association with IGF-1 to induce corneal epithelial wound healing under experimental and clinical conditions.

The cornea is highly innervated with sensory nerves that produce neuropeptides such as SP and calcitonin gene-related peptide (cGRP) (176). The neuronal soma of sensory nerves innervating the corneal tissue is localized in trigeminal ganglia. Electrical stimulation of the trigeminal ganglia is reported to cause an increase of tear secretion, which is dependent on the release of SP from the sensory nerve endings (177). This study suggested the role of SP in reflex tear production. Recently, we showed that mice genetically deficient for functional NK1R had a reduced level of basal tears in comparison with wild type B6 mice as measured with phenol red thread test and developed dry eye disease–associated clinical features (178). SP and its metabolites are reported to present in normal human tears without inducing any overt inflammation, suggesting SP may have a physiological role in maintaining the corneal epithelium homeostasis (153, 179). In fact, SP has recently been shown to inhibit hyperosmotic stress-induced apoptosis of the corneal epithelial cells in ex vivo cultures (180). SP-NK1R interaction is also reported to regulate the expression of E-cadherin and ZO-1 tight junction proteins in the corneal epithelial cells, and thereby demonstrate its protective role in preserving the corneal epithelial barrier function (181183). Our results showed an excessive exfoliation of the apical corneal epithelial cells in unmanipulated NK1R−/− mice, possibly because of the dysregulation in the expression or localization of adhesion/tight junction proteins in corneal epithelial cells (178). Together, these studies suggest an important physiological role of SP-NK1R interaction in maintenance of the corneal epithelium homeostasis under the steady-state condition.

The existing literature strongly supports the role of SP in promoting an inflammatory condition. The current experimental strategy to control SP-induced inflammatory events is heavily dependent upon blocking its high-affinity receptor NK1R via peptide or nonpeptide NK1R antagonists. However, this strategy does not take into account the interaction of available SP in inflamed tissue with lesser affinity SP receptor NK2R/NK3R and its outcome on ongoing tissue inflammation. Future experimental and clinical studies should look into strategies to further increase the efficacy of the treatments targeting SP-induced inflammation. At the same time, one should not overlook the beneficial effects of SP, especially on wound healing or in homeostasis of the corneal epithelium.

This work was supported by National Institutes of Health/National Eye Institute Grant EY022417 (to S.S.), Research to Prevent Blindness (to the Department of Ophthalmology), and National Institutes of Health/National Eye Institute Core Grant for Vision Research EY004068 (to Dr. Linda D. Hazlett, Wayne State University School of Medicine).

Abbreviations used in this article:

B6

C57BL/6

BMDC

bone marrow–derived DC

DC

dendritic cell

HSK

herpes stromal keratitis

HV-68

herpes virus 68

IGF-1

insulin-like growth factor-1

LDCV

large dense-core vesicle

NKA

neurokinin A

NKR

neurokinin receptor

NK1R

neurokinin 1 receptor

NK1R-T

truncated NK1R

PPTA

preprotachykinin A

RSV

respiratory syncytial virus

SP

substance P

TAC

tachykinin.

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The authors have no financial conflicts of interest.