Nano-scale treatment for a macro-scale disease: nanoparticle-delivered siRNA silences CCR2 and treats myocarditis

This editorial refers to ‘Silencing of CCR2 in myocarditis’†, by F. Leuschner et al., on page 1478.

Myocarditis is a significant cause of sudden death, acute heart failure, and chronic dilated cardiomyopathy.¹ The cellular and molecular pathogenesis of chronic dilated cardiomyopathy that sometimes follows acute myocarditis has been explored in animal models but is not well understood in human disease. Clinical myocarditis research has largely focused on modifying the mediators of acute myocarditis based on the theory that lowering acute inflammation may prevent long-term left ventricular remodelling. However, the primary mediators of fibrosis and chronic cardiomyopathy can differ from those involved in the acute immunological reaction that initially leads to cardiac inflammation.

Although T cells are important in the pathogenesis of acute myocarditis, CD11b⁺ monocytes/macrophages are the most common cardiac inflammatory cell in myocarditis patients and in viral and experimental autoimmune myocarditis (EAM) mouse models.^2–6 Strategies aimed at blocking CD11b⁺ cells and the profibrotic pathways they can mediate may impact the long-term cardiac damage from myocarditis. One attractive target is the receptor for monocyte chemotactic protein 1 (MCP-1), also named chemokine (C-C motif) ligand 2 (CCL2).⁷

Leuschner and colleagues collected heart tissue biopsied from patients with myocarditis and found that MCP-1 and its receptor CCR2 were elevated ∼5-fold in myocarditis patients compared with control samples from subjects who died due to trauma and had no history of cardiovascular disease.⁷ Previous work demonstrated that mice deficient in CCR2 developed significantly less acute EAM due to reduced recruitment of macrophages and, importantly in relation to this manuscript, a significant reduction in the profibrotic and pro-heart failure cytokine interleukin (IL)-1β.⁸ Based on those previous findings, the authors administered small interfering RNA (siRNA) for CCR2 (siCCR2) via lipidoid nanoparticles twice per week. Their EAM model administering troponin I with complete Freund's adjuvant in mice mimics the development of chronic dilated cardiomyopathy (DCM) after acute myocarditis in human disease. Anti-CCR2 treatment significantly reduced myocarditis compared with control nanoparticles, and, in particular, decreased CD11b⁺ immune cells within the heart. Left ventricular function improved, as assessed by mouse magnetic nanoparticle-enhanced magnetic resonance imaging (MRI) as late as day 60, the phase of disease associated with cardiac remodelling and DCM.

However, the timing of siCCR2 treatment would not be feasible clinically because myocarditis in patients presents at a later stage after inflammation is well underway. For this reason, the authors also administered siCCR2 during the peak of disease from day 14 to 28 and evaluated cardiac status on day 60. Importantly they found not only an improved ejection fraction with siCCR2 treatment, but also a 10-fold reduction in collagen and myocardial fibrosis. This reduction in cardiac remodelling is remarkable and a feat that has been difficult to achieve previously.

One reason for the success of the siCCR2 treatment is probably a reduction in the profibrotic cytokine IL-1β that occurs when CCR2 is blocked (Figure 1).⁸ IL-1β is known to be a key cytokine mediating myocardial fibrosis that is released from CD11b⁺ macrophages and mast cells.⁹ The next stage of translational research on anti-CCR2 therapy should determine whether the reduction in cardiac fibrosis following siCCR2 treatment is due to IL-1β or other mediators.

Figure 1.

Possible effects of siCCR2 (small interfering CCR2) on immune cell trafficking in experimental myocarditis. siCCR2 blocks bone marrow and splenic production of mononuclear leucocytes that become circulating CD11b⁺ monocytes. The decrease in CD11b⁺ monocytes leads to a decrease in myocarditis in the heart. siCCR2 may also block dendritic cell antigen presentation in the heart, leading to less T and B cell activation. Less antigen-specific immune activation in the draining lymph nodes may result in lower levels of anti-heart antibodies and antigen-specific effector T cells.

The MCP-1/CCR2 pathway has been recently considered for its therapeutic potential in other cardiovascular and autoimmune diseases. The pathogenic role of MCP-1 in atherosclerosis is well documented. MCP-1 is produced by many cell types in addition to macrophages, including cardiomyocytes, fibroblasts, and endothelial and smooth muscle cells, in response to oxidative stress, cytokines such as tumour necrosis factor (TNF) and IL-1β, metabolic factors, and shear stress.¹⁰ MCP-1 contributes to inflammation, cardiac hypertrophy, and heart failure in animal models of myocardial infarction.¹¹ Importantly, inhibition of CCR2 reduces left ventricular remodelling in an experimental myocardial infarction,¹² similar to the results found in this study of myocarditis. Blocking CCR2 reduces inflammation in animal models of rheumatoid arthritis, Crohn's disease, and type 2 diabetes.¹³ The ability of therapies targeted at CCR2 to decrease both inflammation and fibrosis is promising. It will be important to determine if therapies targeting CCR2 will be more effective at reducing myocardial infarction, stroke, or death due to heart failure than those targeting IL-1β.¹⁴

Several questions should be addressed in the pre-clinical development of anti-CCR2 therapies for myocarditis. The experiments of Leuschner and colleagues were conducted in A/J mice, which are highly susceptible to myocarditis due to complement C5 deficiency. The siCCR2 therapy for myocarditis should be demonstrated in other strains of mice such as BALB/c that are less susceptible, and in other animal models such as cardiac myosin-induced EAM and viral-induced models, due to the high number of clinical cases of myocarditis believed to result from common viral infections.

A clinical development programme should address other unanswered questions. Is anti-CCR2 treatment similarly efficacious in both sexes? CD11b expression is significantly greater in men and male mice with myocarditis than in females, and is directly related to the severity of acute myocarditis as well as the progression to DCM and chronic heart failure which is greater in men^4,5,9,15 Based on these observations, it may be that anti-CCR2 treatment will work best in men.¹⁶ Another important clinical question is whether anti-CCR2 treatment is effective if administered after the peak of disease, day 21 in EAM. It may be that in human DCM the profibrotic phase that follows the peak of inflammation may extend for months or years after the initial injury. Patients who fail to recover left ventricular function after acute myocarditis might have an extended time window to benefit from CCR2-dependent pathway inhibition. Today the reduction in chronic inflammation and fibrosis shown by Leuschner and colleagues makes the investigation of anti-CCR2 therapy via nanoparticle delivery an exciting horizon for future research.

Funding

This work was supported by National Institutes of Health (NIH) awards from the National Heart, Lung, and Blood Institute [grant R01 HL111938] to D.F., an American Heart Association Grant-in-Aid [12GRNT12050000] to D.F.

Conflicts of interest: none declared.