In its most severe form, anaphylaxis is characterized by bronchospasm, upper respiratory angioedema, and/or hypotension. Massive vasodilation, fluid extravasation, and repressed myocardial function can result in death (1
). Here, we demonstrate that the SphK1-S1PR2 axis regulates anaphylaxis-induced hypotension, the elimination of histamine from the circulation, and the duration of anaphylactic shock.
It is generally agreed that mast cells play a central role in the onset of many anaphylactic episodes by recognizing the presence of an allergen through allergen-specific IgE antibodies bound to the cell surface receptor, FcεRI. Engagement of FcεRI causes the release of potent allergic mediators, like histamine, from mast cell granules. The activation of mast cells is also intimately linked with the induction of SphK1 and SphK2 activity in mast cells (13
) and with the production and secretion of S1P (13
). Studies in embryonic liver–derived mast cells or in cells from the bone marrow of Sphk1–/–
neonates (prior to detecting any influence by varied levels of circulating S1P) showed that SphK2 is the main isoenzyme involved in the production of S1P and found that this isoenzyme is important for mast cell responses, whereas SphK1 did not appear to play a major role in the response of these cells (15
). In contrast, differences in plasma S1P levels of adult Sphk1–/–
(low amount) versus Sphk2–/–
(high amount) mice, relative to WT mice, were demonstrated to change the in vivo responsiveness of mast cells, with Sphk1–/–
mice being more resistant to anaphylaxis than Sphk2–/–
). Here, we found that the amount of histamine released from mast cells upon anaphylactic challenge was associated with the loss of body temperature within the first 10 minutes of anaphylaxis (Figure A). However, after 15 minutes, the recovery from anaphylaxis (as measured by increasing body temperature) was rapid for Sphk2–/–
mice and delayed in Sphk1–/–
mice, relative to WT littermates (Figure A). The recovery was mast cell–independent (Figure B) but was associated with the amount of circulating histamine and with the relative contribution of the SphK-S1PR gene products in clearing plasma histamine (Figure ) during the hypotension induced by anaphylaxis.
Histamine is responsible for some of the major symptoms associated with systemic anaphylaxis, as shown in a murine model of histamine deficiency upon genetic deletion of the histidine decarboxylase (HDC
) gene (21
mice showed no significant alterations in body temperature or in respiration during Ag-mediated anaphylaxis, but a histamine challenge led to a marked loss of body temperature, demonstrating the role of histamine in regulating body temperature. In agreement, we found that histamine decreased body temperature, decreased blood pressure (Figure ), and caused changes in the heart rate and respiration (Supplemental Figure 7). Moreover, a histamine challenge of Sphk1–/–
mice (which exhibited the most severe anaphylactic response) resulted in the death of only 1%–2% of the mice.This is consistent with the frequency of death reported by the American College of Allergy, Asthma and Immunology Epidemiology of Anaphylaxis Working Group (39
), emphasizing the physiological relevance of the model used herein.
Under normal conditions, secreted histamine is rapidly bound to its receptors or can be inactivated by oxidative deamination or methylation (36
). Nonetheless, a considerable amount of histamine is still excreted unmodified (35
), and in humans, cases of histamine intolerance due to ingestion of spoiled fish (40
) or other amine-rich foods (41
), endogenous histamine overproduction, or an imbalance between the accumulation of histamine and its elimination are known to result in elevated plasma histamine levels and severe allergic reactions (41
) or recurrent anaphylaxis (42
). This suggests that histamine exceeds its physiological usefulness during exposure to large amounts of it, and this overwhelms the inactivation mechanisms, causing unwanted effects on blood pressure, vascular tone, and permeability. In addition, there may be differences in the responses of individuals to histamine, based in part on the physiological effects mediated by the 4 known histamine receptors (H1–4
), which differ in tissue distribution and affinity for histamine (43
). Polymorphisms discovered in histamine receptors may also account for some of the physiological differences (45
). It is well known that histamine increases endothelial permeability (47
). The recent finding that histamine also increases SphK1 expression and activity in human arterial endothelial cells (29
) provides evidence of an additional mode by which histamine might modulate the vasculature. Here, we found that histamine increased the levels of S1P in an SphK1-dependent manner (Figure A), and this is important for blood pressure recovery during anaphylaxis. In agreement, alterations of S1P levels by overexpression of SphK1 in resistance arteries increased the resting tone of these arteries (49
), which are thought to regulate blood pressure and tissue perfusion, and mediated transmural pressure-induced reactive oxygen species formation and myogenic reactivity (50
). Degradation of SphK1-produced S1P by overexpression of a S1P phosphatase (S1P phosphohydrolase 1) results in the negative regulation of resting and myogenic tone in resistance arteries, indicating a role for S1P (derived from SphK1 activity) in the contractility of these arteries (51
) and thus in the regulation of blood pressure. Our data provide what we believe to be new insights into an additional mechanism that may contribute to the differences in sensitivity of individuals to histamine or in cases of histamine intolerance.
The receptors for S1P, particularly S1PR1, S1PR2, and S1PR3 are abundant and widely distributed in the vascular system, in which they have important roles in the contractility and permeability of the vasculature (18
). In endothelial cells, S1PR1 preserves vascular integrity and regulates vascular permeability (12
); S1PR3 appears to function in myocardial contractility and regulation of bradycardia and hypertension induced by S1P (32
); and S1PR2 increases vascular permeability and regulates renal, mesenteric, and local blood flow in various organs (20
). Here, we show that in a model of non-lethal anaphylaxis, S1PR2 played an important role, counteracting the vasodilatory actions of histamine, while the other receptors for S1P seemed to be dispensable (Figures –). This is consistent with prior studies (51
) that suggest a role for S1PR2 in the vasoconstriction induced by S1P in resistance and coronary arteries. Additional studies in S1pr2–/–
mice showed a decreased contractile responsiveness and vascular tone in vivo, suggesting a role for S1PR2 in the maintenance of vascular hemodynamics (54
). We did not observe differences in the blood pressure or body temperature of S1pr2–/–
versus WT mice in the absence of a challenge, but a marked difference was observed during the hypotension induced by anaphylaxis (Figures and ). This demonstrates that the S1PR2 receptor is particularly important in response to acute vascular vasodilatory influences. The marked hypotension of these mice during anaphylaxis correlates with their inability to properly clear histamine, suggesting that the defective renal function may derive from a compromised GFR, although a direct function of this receptor on the kidney cannot be excluded.
Proposed model for the dual actions of S1P in the onset of and recovery from anaphylaxis.
However, histamine- or platelet-activating factor–induced anaphylaxis was reported to be fatal in mice lacking S1P (Sphk1–/–Sphk2–/–
), and this lethality was attributed to a lack of signaling through the S1PR1 receptor, since administration of a selective agonist of S1PR1 (AUY954) to these mice without S1P increased survival. The S1PR1 receptor would function to regulate vascular permeability during anaphylaxis, preventing the extensive vascular fluid extravasation that can cause lethality. We found that injection of histamine in the footpad of the S1PR1 inducible knockdown mice (S1pr1loxp/loxp-Mx
mice), which had a reduced presence of S1pr1
in tissues (Supplemental Figure 3) and in the endothelium of the aorta (Supplemental Figure 8E), induced a temporal enhancement in local swelling (Supplemental Figure 8C), confirming the involvement of this receptor in vascular permeability. In contrast, S1PR1 deficiency did not alter the rate of recovery from histamine-induced anaphylaxis (Figure ), although a trend for elevated hematocrit and accumulation of Evans Blue in their lungs (albeit not significant) was seen after IgE/Ag challenge (Supplemental Figure 8, A and B). Furthermore, inhibition of S1PR1 receptors by VPC23019 (57
) (Supplemental Figure 8, F and G) did not result in lethality or the worsening of anaphylaxis in Sphk1–/–Sphk2+/–
mice, which showed a significantly enhanced vascular permeability (Supplemental Figure 4), while it modestly increased the intensity of the shock in the Sphk2–/–
mice, which did not show significant vascular leakage during anaphylaxis as determined by hematocrit changes (Supplemental Figure 4). Thus, altogether our data suggest a modest involvement of S1PR1 and vascular permeability in this non-lethal model of histamine-induced anaphylaxis (see model in Figure ). However, we recognize the possibility that some S1PR1 may remain in the S1pr1loxp/loxp-Mx
mice and may be sufficient to regulate microvascular permeability and prevent worsening of the disease. Similarly, poor recovery from anaphylaxis in Sphk1–/–
mice may well be due to slightly higher plasma exudation in these mice (Supplemental Figure 4), which can also contribute to the hypotension. Regardless, our findings demonstrate that the activation of S1PR2 and its role in blood pressure regulation during anaphylaxis is dominant over the role of S1PR1 in controlling vascular permeability (see model in Figure ) (under conditions in which S1P is present in the circulation), because S1pr2–/–
mice suffer a severe anaphylaxis (despite an apparent reduction in plasma exudation relative to WT mice; Supplemental Figure 4) that cannot be reversed by S1P injection. Further support is also provided by the finding that antagonism of S1PR1 did not worsen anaphylaxis in S1pr2–/–
mice, and S1pr2–/–/S1pr1loxp/loxp-Mx
double-deficient mice did not exhibit lethality or a more severe anaphylactic response when compared with S1pr2–/–
mice (Supplemental Figure 8, H and I). The data shown herein and the findings by Camerer et al. (12
) in the mice without S1P, using a more severe model of anaphylaxis, are not inconsistent, since cooperation between S1PR1 and S1PR2 has been shown to regulate microvascular permeability (20
). Moreover, our findings and those of Camerer et al. demonstrate the importance of S1P in recovery from anaphylactic shock, whether in the circulation (12
) or in the local environment of the vasculature.
Our findings argue for a feedback regulatory mechanism that is important in recovery from anaphylaxis; it involves the histamine-dependent stimulation of SphK1 in both the hematopoietic and nonhematopoietic compartments, the production of S1P, and the activation of S1PR2, which controls vascular tone and facilitates recovery of blood pressure and clearance of histamine (see model in Figure ). In fact, administration of S1P after the initiation of shock improved the recovery from anaphylaxis in WT mice (Figure C) to a similar extent as adrenaline (data not shown). While adrenaline is the recommended first-line treatment in anaphylaxis (22
), the administration of adrenaline carries risks, particularly in the elderly population, that include cardiac arrhythmias, myocardial infarction, and hypertensive intracerebral bleeds (5
). Therefore, adrenaline overdosing in non–life-threatening allergic reactions is a serious risk. Since the S1PR2 receptor does not mediate bradycardia, tachycardia, or influence myocardial function in vivo and its effects on blood pressure and flow rate appear to be mostly vascular (54
), specific receptor agonists may be a safer and useful alternative for anaphylaxis treatment in the high-risk population. This would need to be balanced with the possibility of increased lung vascular permeability by engagement of S1PR2 receptors (55
). Vascular permeability in the lung could be counteracted by administration of S1P or engagement of S1PR1 receptors in the lung (58
). In light of our findings and the recent report from Camerer et al. (12
), a combination treatment with specific agonists to S1PR1 and S1PR2 receptors could be particularly effective for the reversal of vascular leakage and hypotension, respectively, during anaphylaxis.