*1.2. The role of the Inflammatory Response in Generating the Cystic Compartment in ACP*

ACPs often have large cystic components that contribute to the adverse clinical outcomes associated with the disease (Figure 3). Their large size and at times rapid growth can injure or exert mass effect on critical adjacent structures, such as the pituitary, the hypothalamus, the optic apparatus and third ventricle, which may necessitate urgent surgical intervention to preserve function and prevent morbidity and mortality. As a result, a better understanding of the pathogenesis of ACP cysts and the development of better treatments to limit their growth is clearly desirable. Numerous studies have analyzed the content of these cysts and the results of these studies have demonstrated a significant inflammatory content within them. A summary of some of these papers is presented in Table 1.

**Figure 3.** Example of an adamantinomatous craniopharyngioma with a massive cystic component.



Some of the first work examining the role of inflammation in ACP pathogenesis was carried out by Mori et al. who demonstrated highly elevated levels of IL-6 in the cyst fluid of 15 pediatric ACPs and posited that IL-6 plays an important role in the inflammatory reaction associated with ACPs [37]. Another study that demonstrated the role played by the inflammatory response in the generation of the ACP cyst was that by Pettorini et al. [36]. Using high-performance liquid chromatography and mass spectrometry to analyze cyst fluid from 6 patients, they found high levels of alpha defensins 1–3, proteins that are present in neutrophils and are involved in the inflammatory-mediated response. Furthermore, their group demonstrated that these levels were significantly reduced after treatment with intracystic interferon alpha (IFN-alpha). They posited that the detection of these proteins suggested that the innate immune response was playing a critical role in cyst generation and that a possible mechanism of action of IFN-alpha in treating the cyst was via an immune-modulatory effect [36]. A later study by the same group performed more extensive proteomic analysis on nineteen patient samples [15]. In this study, they used reverse phase liquid chromatography in conjunction with high resolution ESI-I TQ-Orbitrap mass spectrometry to analyze ACP cyst fluid from nineteen children. In addition to again revealing elevated levels of alpha-defensins (that again were reduced after treatment with IFN-alpha), they also demonstrated elevated levels of several other proteins of inflammation. Specifically, these included alpha2-HS-glycoprotein, alpha1-antichymotrypsin and apolipoproteins.

In another study, Donson et al. [31] used cytometric bead analysis to measure the concentration of 24 cytokines and 11 chemokines in cyst fluid from five pediatric ACPs and five pediatric pilocytic astrocytomas (PAs). Their analysis demonstrated that six cytokines were present at statistically significant increased levels in ACPs versus PAs. These cytokines included IL-6, IL-10, CXCL8 (IL-8), and CXCL1 (GRO). Of these, levels of IL-6 demonstrated the greatest difference between ACPs and PAs. Apps et al. also demonstrated similar findings when analyzing the protein content of ACP cyst fluid [10]. They analyzed the content of cyst fluid from 6 patients with ACP using multiplex ELISA (Enzyme-linked immunsorbent assay). They found that the cyst fluid contained several proteins associated with inflammation, such as apolipoproteins, complement system proteins and immunoglobulins. In addition, their analysis revealed the presence of cytokines such as IL-1B, IL-6, IL-8, IL-10, IL-18 as well as TNF (Tumor necrosis factor) and Interferon gamma.

Further evidence for the role of inflammation in the genesis of the cystic component in ACPs is provided by the efficacy of treatment with IFN-alpha. IFN-alpha has been used with varying degrees of success in the treatment of multiple cancers [38]. The mechanism of action in the treatment of neoplasms is complex and multifaceted but likely involves the stimulation of an anti-cancer immune response [38]. The use of intracystic IFN-alpha in the treatment of cystic ACP has been established for several years and numerous studies have demonstrated its safety and efficacy [35,39,40]. The treatment involves the surgical placement of a catheter within the cyst with the position of the catheter confirmed radiologically prior to the administration of the drug. This method of treatment has been shown to delay disease progression and can allow the clinical team to delay a more definitive treatment via surgical resection and radiotherapy [35]. Such a delay is often desirable, as it by may allow a child's developing brain to mature further prior to undergoing inherently risky surgery and radiation therapy. The mechanism of action of intracystic IFN-alpha in treating ACP has not been confirmed but, as in other cancers, is likely to involve an immunomodulatory effect. Indeed, the previously mentioned proteomic analyses would seem to lend significant weight to this argument [36].
