Radiation-induced cystitis, a known side effect of radiation exposure to the urinary bladder, can significantly impact patient outcomes [2, 25, 26]. The lack of evidence-based research and therapeutics available in treating this disease remains a significant issue that needs to be addressed. Current pre-clinical research has demonstrated a potential role for macrophages [6] and mast cells [7,8,9,10,11] in the pathophysiology of radiation-induced toxicities; however, a clear link has not been established for the involvement of these immune cells, and other myeloid-derived immune cells, in the prognosis of radiation-induced cystitis.

This systematic review revealed that the effect of radiation on myeloid-derived immune cells in radiation-induced cystitis is poorly defined. In other literature, a role for the urinary bladder mast cell [3, 5] in the pathophysiology of radiation-induced cystitis has been suggested. In the healthy urinary bladder, mast cells are critical regulators of immunity [27,28,29] distributed throughout all layers of the urinary bladder wall [30]. In the context of radiation-induced toxicities, mast cells have been associated with the development of radiation-induced fibrosis around various tissues of the body [7,8,9,10,11]. Specifically, mast cells are believed to release various chemokines, such as TGF-β, which cause fibrosis by promoting fibroblast recruitment and proliferation [12]. Mast cells are also believed to disrupt vasculature by increasing the permeability of blood vessels [16] as well as playing a direct role in chemical cascades that result in radiation-induced inflammation [9].

In the reviewed studies, several investigated mast cells in the irradiated urinary bladder [8, 19, 20] [22, 23]. Two studies indicated no statistical difference in the infiltration of mast cells between irradiated and non-irradiated groups [8, 19]. These studies suggest that mast cells are resistant to radiation exposure in the urinary bladder, consistent with reports on mast cells exhibiting resistance to radiation-induced cytotoxicity [31, 32]. Contrastingly, two studies indicated a decrease [20, 23] and one study indicated an increase in mast cell density following irradiation [22]. Mast cells were reported to decrease in the urothelium [20] and lamina propria [23], which may suggest that mast cells are more susceptible to radiation-induced toxicities in the most apical layers of the urinary bladder wall following irradiation at 14-day, and 7- and 15-day timepoints, respectively. After 28 days, Chen, Chen [22] reported that mast cells infiltrated the bladder, suggesting that this increase in mast cells may be indicative of inflammatory response. Despite this, further research is needed to validate these findings.

In addition to mast cells, macrophages have also been implicated in the development of radiation-induced toxicities through similar mechanisms of inflammation and fibrosis [33]. Macrophages were investigated in three of the included studies [19, 21, 24], in which all three studies concluded no statistical difference in macrophage prevalence following irradiation. Despite no statistical difference between non-irradiated and irradiated groups, Podmolíková, Mukanyangezi [19] and Lombardo, Obradovic [24] reported a marginal increase in CD206+ macrophages and increased CD86+ macrophage-associated markers in the urinary bladder. Ito, Yamamoto [21], however, reported a reduction in CD68þ macrophages, suggesting that CD68þ macrophages may be susceptible to radiation.

Though previous research has suggested a role for urinary bladder mast cells and macrophages in the pathophysiology of radiation-induced cystitis, this systematic review has determined that these cells do not have a clear prognostic value in the development of this disease. Firstly, only four studies assessed inflammation following irradiation of the urinary bladder [8, 20,21,22], two of which identified inflammation in the irradiated urinary bladder [8, 20]. Incongruencies in techniques used to assess cystitis, as well as a lack of assessment in several studies, make it difficult to draw meaningful conclusions about the involvement of mast cells or macrophages in relation to radiation-induced cystitis, a disease characterized by inflammation. Thus, it is recommended that a guideline is developed for the assessment of radiation-induced cystitis in pre-clinical models to allow for more meaningful and accurate comparisons in the assessment of radiation-induced cystitis, with particular regard to its triphasic development [3, 4].

Of particular interest was the lack of any literature on the impact of myeloablative radiotherapies, or the associated HSCT on myeloid lineage immune cells. Despite its curative potential, HSCT and its conditioning regiments have been linked to radiation-induced cystitis. The development of radiation-induced cystitis following HSCT is associated with increased disease morbidity, prolonged hospitalisations [2, 25] and significant increases in non-relapse-related mortality, according to Galli, Sorà [26]. The prevalence of radiation-induced cystitis, associated with HSCT conditioning regimens, has a reported prevalence ranging between 10% and 20% in paediatric transplant recipients [34] and 17% in adult transplant recipients [35], with the median incidence of cystitis reported to occur within the first 30 days of transplantation [26]. Due to its prevalence and apparent risk to immunocompromised patients, future research on HSCT and myeloablative radiotherapies is highly recommended.

Only three of the included studies in this systematic review assessed the prevalence of other immune cells in the urinary bladder [19, 20, 24], and found that there were no statistically significant differences in assessed lymphocytes [19, 20]. However, Lombardo, Obradovic [24] reported no change to the number of CD4+ and CD8+ T-cells in response to irradiation compared to controls (NSD). To this end, it is well known that myeloablative radiotherapy can induce immunosuppression [36]. As all included studies utilized EBRT to evaluate myeloid-lineage immune cells in the irradiated urinary bladder, it is unclear as to whether these findings are applicable to myeloablative radiotherapy. Dually, it is also recommended that future research is done to validate a model of radiation-induced cystitis, in consideration of different species, strains, dosimetry, and modes of radiation. Due to scarcity in the assessment of other myeloid-lineage immune cells, namely monocytes, neutrophils, eosinophils and basophils, future studies should also aim to assess the function, prevalence, and distribution of a variety of immune cells in the irradiated bladder, focusing on how these characteristics may contribute to disease pathology or how they may differ in the context of myeloablative radiotherapy.

Limitations of this systematic review include the small number of identified studies and the inability to make valuable comparisons due to the different species, strains, dosimetry, and techniques used. Additionally, it is important to recognise that only four of the included studies sought to investigate cystitis resulting from radiation exposure [8, 19, 20, 22]. The remaining three studies [21, 23, 24] had discussed inflammation resulting from radiation exposure, but did not explicitly identify cystitis. These studies were still included in this systematic review, however, as they still met the inclusion criteria.