Angiotensin-converting enzyme inhibitor treatment early after myocardial infarction attenuates acute cardiac and neuroinflammation without effect on chronic neuroinflammation

Myocardial infarction (MI) remains a major contributor to morbidity, mortality, and healthcare costs worldwide [1]. Despite improved acute survival due to rapid reperfusion, surviving patients are at risk of progression to heart failure [2]. Conventional long-standing drug therapy often ameliorates the chronic remodeling of the left ventricle to spare contractile function through interference with the neurohumoral system. Yet, the early inflammatory response to MI is now well recognized as a therapeutic target [3]. This physiologic response in the first hours to days after MI is a critical time period to determine adequate healing [3].

Specifically, inflammation after MI involves the dynamic migration of leukocytes, particularly macrophages, to the damaged area [4]. Early infiltrating cells including granulocytes and pro-inflammatory monocytes and macrophages mediate removal of cellular debris [5, 6]. Later, the predominant cell populations express reparative markers, which contribute to matrix remodeling and the formation of a stable collagen-rich scar [5]. In addition, during systemic inflammation post-MI, the leukocyte content is dominated by infiltrating leukocytes mobilized from hematopoietic organs, with a supplementary involvement of resident macrophages [7]. The severity of acute inflammation is inversely proportional to late functional outcome, but optimal healing requires a balance of inflammatory leukocytes to allow clearance of damaged cells and mediate repair and stable scar formation [8]. The goal of therapeutic strategies in the early post-MI phase is therefore to modulate inflammation in order to achieve optimal healing.

But cardiovascular disease is also associated with higher risk of cognitive impairment, dementia, and Alzheimer’s disease [9, 10]. Shared risk factors may contribute to this epidemiologic overlap, but we recently identified a contribution of heart–brain inflammatory crosstalk by demonstrating a direct neuroinflammatory response early after cardiac ischemia and in chronic heart failure [11, 12]. This biphasic neuroinflammation may contribute to subsequent neurodegeneration and cognitive decline as seen in Alzheimer’s disease [13, 14]. However, the precise mechanisms underlying this connection are not fully understood. Moreover, the reciprocal effects of anti-inflammatory therapies after MI on neuroinflammation are poorly characterized.

It has been shown that moderate doses of angiotensin-converting enzyme (ACE) inhibitors exhibit anti-inflammatory action besides their effects on neurohumoral activation [8] by interfering with excessive leukocyte mobilization from the spleen to the heart, and thereby contributing to improved contractile function after MI. In our prior work, continuous ACE inhibitor therapy lowered the acute and chronic cardiac and brain TSPO PET signals and spared cardiac function [12], but it remains unclear whether attenuated neuroinflammation derives from acute anti-inflammatory response to the therapy or chronic anti-remodeling action.

Accordingly, we aimed to evaluate the contribution of early enalapril therapy and delayed enalapril therapy on target cardiac inflammation and function, and on distant neuroinflammation. Serial non-invasive whole-body PET imaging of TSPO was employed to evaluate cardiac and neuroinflammation after coronary artery occlusion in mice under various treatment regimens, and was validated by autoradiography and histological tissue workup.

Using whole-body TSPO-targeted molecular imaging, we previously demonstrated that myocardial infarction imparts concomitant cardiac and neuroinflammation early after the insult, with recurrent neuroinflammation in chronic heart failure. In this prior study, continuous treatment with angiotensin-converting enzyme inhibitor enalapril lowered the acute inflammation in the heart and brain, as well as chronic neuroinflammation in parallel with improved function [12]. Current clinical practice suggests that chronic therapy with ACE inhibitors attenuates adverse ventricular remodeling, but the impact of such treatment beyond the heart remains equivocal. In the present study, we demonstrate that the impact on neuroinflammation of ACE inhibitor therapy after myocardial infarction requires acute application and is directly related to the severity of inflammatory activity in the heart. Late application of therapy, omitting the acute anti-inflammatory effects, does not reduce chronic neuroinflammation, providing insights into the mechanistic interaction between acute cardiac inflammation, chronic heart function, and brain health. Serial whole-body ¹⁸F-GE180 PET in mice after MI identifies cardiac and neuroinflammation and TSPO as a marker for cardiac remodeling.

ACE inhibitors are a standard heart failure medication, targeting a range of physiologic processes to mediate adverse remodeling and improve cardiac function by reducing pre- and afterload, and dampening neurohumoral activation upstream of the myocardium to improve ejection fraction [16, 17]. More recently, ACE inhibitor therapy in mice was established to abolish the release of spleen-derived monocytes into the circulation, resulting in lower inflammatory cell infiltration to the infarct territory after coronary artery occlusion [8]. Reduced CD11b cell content was paralleled by a reduction of pro-inflammatory markers including Ly6C, TNF-α, and CD68 [8]. Indeed, angiotensin II has pro-inflammatory activity [18], and its suppression via ACE inhibitors or angiotensin receptor blockers lowers the production of IL-6 [19]. Continuous treatment with moderate-dose enalapril from 2 days prior to 8 weeks after coronary ligation evoked a decline in TSPO PET image–derived inflammation in the infarct territory, and confirmed improved chronic contractile function [8, 9]. However, the contributions of the acute anti-inflammatory benefit of enalapril on late ventricular function relative to chronic benefits on remodeling are not definitive. In the present study, both early and delayed ACE inhibitor therapies improved ejection fraction and attenuated ventricle dilatation at 8 weeks, suggesting an independent benefit of early anti-inflammatory therapy for late function. Indeed, acute ACE inhibitor application lowered inflammatory TSPO PET signal proportional to CD68 cell infiltration into the infarct territory. Notably, the improved cardiac function at 8 weeks was associated with lower chronic TSPO signal from the remote territory, which, due to the localization of the target to mitochondria, may provide an indirect indication of mitochondrial function and metabolism.

In chronic heart failure, the TSPO PET signal is elevated in the remote myocardium, confirming previous results [12], but the cellular basis of this signal is difficult to define. Here, immunostaining identified punctate intracellular TSPO expression in cardiomyocytes, reflected by semi-quantitative increase in the staining area in the remote myocardium. ACE inhibitor therapy is known to reduce oxidative stress [20], which may normalize oxidative metabolism and mitochondrial function in surviving cardiomyocytes. As such, TSPO PET imaging in chronic heart failure appears to provide a surrogate measurement of mitochondrial density which may be helpful for assessing ventricular remodeling and response to heart failure therapies. The precise relationship between TSPO expression and mitochondrial stress requires further evaluation, e.g., applying direct anti-oxidant therapy. Increased TSPO expression has been reported in the failing mouse heart after transverse aortic constriction, proportional to impaired mitochondrial energetics [21]. Angiotensin II has been suggested to regulate TSPO expression during stress, including in the heart [22]. The response to enalapril may be partially explained by this interaction, though we cannot exclude that the overall improvement in cardiac function also influences the TSPO signal independent of direct regulation.

The mechanisms underlying the response to ACE inhibitor therapy are largely related to cardiac function. With early treatment, modulated inflammation reduces adverse remodeling, leading to improved contractile function at 8 weeks. Conversely, delayed therapy impacts sympathetic neuronal activation and late remodeling, culminating in improved contractile function. The end result is lower stress on the myocardium, which may reduce mitochondrial dysfunction and explain the decline in TSPO signal. While limited to enalapril therapy in the present study, other cardiac interventions may have comparable ancillary benefits.

Inter-organ communication, the heart–brain axis, and systems biology are increasingly recognized as important contributors to the copresentation of disease in the aging population [11, 23]. We have previously established concomitant neuroinflammation after acute myocardial infarction and in chronic heart failure which was attenuated by continuous enalapril therapy [12]. Notably, the present study shows no reduction of TSPO PET signal or microglial content in the whole brain late after myocardial infarction with either early or delayed enalapril therapy. Nonetheless, chronic brain TSPO signal correlates with contractile function, with rising neuroinflammation corresponding to greater decline in ejection fraction. This observation suggests that despite persistent neuroinflammation with early or delayed enalapril therapy alone, neuroinflammation is inherently tied to the progression of heart failure. Worse contractile function translates to recurrent neuroinflammation, independent of inflammatory cell content in the myocardium. This microglial activation may occur in response to systemic cues such as impaired blood flow [24], compensatory neurohormonal activation [25], or systemically elevated inflammatory cytokines [26]. Further treatment studies should evaluate the contributions of such pathogenetic processes.

The chronic cardiac functional benefits of ACE inhibitor therapy appear to derive from both acute anti-inflammatory and chronic anti-remodeling benefits, as evidenced by improved ejection fraction with either early or delayed therapy. Neither early nor delayed ACE inhibitor therapy alone alleviated neuroinflammation, in contrast to our previous observation that continuous enalapril treatment lowered both the acute and chronic TSPO PET signals [12]. This suggests that therapeutic strategies may differ between the target and distant organs. Accordingly, higher dose or continuous enalapril administration may be necessary to optimize anti-remodeling efficacy. Alternatively, this discrepancy may reflect the limitations of gated perfusion SPECT for assessment of ventricle volumes in mice, which may be more accurately measured by dedicated anatomic modalities [15]. It should be noted that the enalapril dose used in the present study is potentially high relative to clinical doses. A dose–response study could provide additional translational insights.

The presence of neuroinflammation in chronic heart failure may derive from various contributing factors, including reduction of cerebral blood flow [27], heightened immune response [28], or whole-body oxidative stress [29]. The inverse relationship of chronic neuroinflammation to ejection fraction suggests that contractile dysfunction contributes to late microglial activation, but the precise mechanism warrants further investigation.

The early TSPO brain signal predicts late brain signal, suggesting that chronic neuroinflammation is influenced by acute microglial activation. Such an observation is consistent with the concept of central immune priming following a primary insult, as is observed in stroke or sepsis [30, 31]. In the pathogenesis of Alzheimer’s disease, microglial activation foreshadows subsequent neurodegeneration, prefacing a secondary wave of inflammatory activation in response to accumulating amyloid-β [32]. A recent study demonstrated that a single exposure to systemic lipopolysaccharide evoked acute neuroinflammation and exacerbated amyloid-β plaque development in a slow-developing Alzheimer’s disease mouse model [31]. Accordingly, the acute inflammatory activation following myocardial infarction may bear grave consequences for distant organs, including the brain, which may benefit from acute anti-inflammatory therapy. Previous studies identified elevated pro-inflammatory cytokines in the brain in mice with chronic heart failure which was associated with increased microglial activation and impaired cognitive performance [26]. Patients in chronic heart failure also exhibit higher levels of pro-inflammatory cytokines including TNF-α, IL-6, IL-1β, and C-reactive protein [33]. Inflammatory cytokines are known to cross the blood–brain barrier [34]. Accordingly, therapies targeted to upstream inflammatory mediators such as interleukin-1β, as shown in the Canakinumab CANTOS trial, may impart systems biology benefits following acute ischemic injury and in progressive heart failure [35, 36]. Interestingly, antibody-mediated suppression of IL-1β reduces the cerebral infarct size in a mouse stroke model [37], suggesting potential therapeutic opportunities with targeted agents. Consistent with this concept, the early anti-inflammatory enalapril therapy influenced neuroinflammation more definitively.

Some limitations of the present study should be considered. First, it should be noted that early and late ACE inhibitor therapies were not directly compared with continuous treatment, but with a historical dataset [12]. The duration between these experiments was minimal, and the same animal models and conditions were used to minimize fluctuation between groups. Second, the cognitive response to MI and enalapril therapy was not directly assessed. Nonetheless, previous evidence suggests impaired prefrontal memory in chronic heart failure mice [26], and the acute and chronic neuroinflammations are consistent with mouse models of cognitive impairment [38, 39]. Dedicated evaluations of cognitive outcomes after myocardial infarction are warranted. Third, the cellular basis of the TSPO PET signal is not completely defined. However, our immunostaining experiments demonstrate colocalization of TSPO with peripheral macrophages and microglia but not astrocytes in the acute phase, supporting specificity for inflammatory cells. This is consistent with experience in cell culture uptake assays [40]. Immunostaining of heart failure remote myocardium also suggests localization to mitochondria in cardiomyocytes, which aligns with the regulation of TSPO in heart failure [21]. Fourth, improvement of ejection fraction after enalapril therapy did not directly correspond to reduction in ventricular volumes. This may be related to the perfusion SPECT-derived calculation of ventricular geometry, which is subject to spillover from the ventricle wall to the lumen, or to the broader range of ventricle size typical of permanent left coronary ligation which limits statistical power [15]. Nonetheless, the modestly higher ejection fraction is consistent with prior reports using similar treatment [8, 12]. Finally, as TSPO PET primarily reflects inflammation, we have focused on these related mechanisms as influencing chronic ejection fraction and recurrent neuroinflammation. Other factors, such as brain perfusion or sympathetic neuronal activation [24, 25], may play a role in this observation, but require in-depth dedicated experiments.

In conclusion, whole-body TSPO PET imaging provides quantitative non-invasive indication of peripheral macrophages and central microglial activation after acute myocardial infarction, and altered mitochondrial function in chronic heart failure. Considering the previous evidence of reduced TSPO signal under continuous ACE inhibition, the functional benefits of ACE inhibitor therapy may capitalize on the combination of both acute anti-inflammatory effect and chronic attenuation of adverse remodeling, and may be required to alleviate chronic neuroinflammation, though further study is warranted.