Systemic sclerosis (SSc) is a devastating multisystem autoimmune connective tissue disease with lung involvement as the primary cause of death [
1]. Interstitial lung disease (ILD) occurs early in the disease course and affects 40–70% of patients. Diagnostic tools such as pulmonary function tests (PFTs) or high-resolution computed tomography (HRCT) often only detect irreversibly compromised lung function and structure [
2]. Consequently, there is a need for the diagnosis of early, potentially reversible disease stages. This need could be met by nuclear medicine applications such as single photon emission computed tomography (SPECT/CT) and positron emission tomography (PET). These highly sensitive and specific methodologies allow the real-life visualization of pathophysiological processes and have become valuable diagnostic tools in oncology [
3].
In SSc-ILD, pulmonary damage at its early stages is characterized by apoptosis of epithelial cells (EPC) (up to 80%) [
4] and inflammatory cells caused by, for example, cigarette smoke, infections, environmental exposures or micro-aspiration and/or locally increased oxidative and endoplasmatic reticulum stress [
5,
6]. Notably, in experimental animal models of ILD, EPC damage [
7] or the delivery of apoptotic cells induce lung fibrosis [
8,
9], whereas blocking of apoptotic pathways prevents or attenuates the development [
10‐
12]. Dying cells release cellular contents such as adenosine triphosphate (ATP), uric acid or high-mobility group protein B1 (HMGB-1), some of which being recognized as danger-associated pathogens (DAMPs) [
13,
14]. In experimental ILD, signaling via danger receptors, including, for example, toll like receptors (TLRs), initiates innate immune responses, thereby promoting inflammation and fibrosis mainly through the NFkB/inflammasome and IL-1 pathways [
15]. In physiologic conditions, the DAMP-mediated influx of inflammatory cells leads to the clearance of apoptotic debris and the resolution of inflammation. In ILD, probably due to a genetic predisposition, exaggerated DAMP signaling occurs with a sustained pro-inflammatory response, part of which is attributed to inefficient phagocytosis of apoptotic cell debris (= efferocytosis) [
16,
17]. Among the phagocytosing cells, alternatively activated macrophages, which are predominant in ILD [
18,
19], are a major source of transforming growth factor (TGF)β [
20]. TGFβ induces apoptosis of EPCs, thereby further enhancing the loss of functional epithelium [
21,
22]. Furthermore, TGFβ mediates the differentiation of fibroblasts into myofibroblasts rendering them resistant to apoptosis [
23]. This results in massively increased and perpetuated secretion of extracellular matrix proteins. Although less well-investigated, it has been suggested that adaptive immune responses might also be involved in pathogenesis in ILD. Potential mechanisms include cross-presentation of cellular DAMPs from apoptotic epithelial or inflammatory cells by, for example, dendritic cells, which could drive the activity of cytotoxic T cells and thereby increase lung damage [
24,
25]. In addition, patient-derived data and data derived from experimental ILD suggest a potential pathogenic involvement of B cells [
25‐
27] and a propensity towards a T helper 2 (Th2) response [
28]. Overall, in ILD, there is a vicious cycle of dysregulated pro-apoptotic and anti-apoptotic mechanisms involving different cell types, which identifies apoptosis as an important initiator and driver of lung fibrosis.
One of the first signals of cells undergoing apoptosis is the rapid redistribution of phosphatidylserine (PS) onto the cell surface, where annexin V binds with high affinity. PS constitutes 10–15% of the phospholipids of the inner leaflet of the plasma membrane [
29]. Upon the onset of apoptosis, closely following activation of caspase 3, translocation of PS onto the cell surface results in a 100–1000-fold increase of annexin V binding sites per cell [
30]. Notably, annexin V may also identify necrotic cells, since the disruption of the membrane of necrotic cells may allow binding of annexin V to PS at the inner leaflet [
31]. In human pilot studies, technetium-99 m (
99mTc)-labeled annexin V has been used to detect apoptosis and necrosis in the context of acute myocardial infarction [
32] and cardiac allograft rejection [
33]. Recent data from animal studies using models of (infectious) endocarditis [
34], atherosclerosis [
35], myocarditis [
36] and rheumatoid arthritis [
37] suggest a potential use for the detection of early inflammatory disease stages.