Arguments to Acknowledge the Bystander Killing effect as a Tool to Modify the Tumor Microenvironment by the Action of Antibody Drug Conjugates

The bystander killing effect: The term „bystander effect“ has already been mentioned in 1996, mediated by a cytokine cascade released by tumor infiltrating cells mainly monocytes. The term „ bystander killing effect“ described the action of ADC`s on tumors with antigen heterogeneity. The concept of intratumor pharmacokinetics could be visualized in an antigen heterogeneous model by phosphor-integrated dots imaging analysis [6].

Single cell analysis of human breast cancer (CC), colorectal cancer (CRC), and gastric cancer (GC): Single cell and transcriptomics analysis of human breast cancers were documented analysing more than 100 000 cells by RNA sequencing and including the expression and clustering of 133 clinically and targetable receptor or ligand immunomodulation markers accross all cell types grouped by clinical breast cancer subtypes (TNBC, HER2+, and ER+) [7]. Genome and transcriptome signatures of 1063 colorectal cancers yielded mutations in WNT, EGFR and TGFßpathway genes, the mitrochondtrial CYB gene and 3 regulatory elements along with 21 copy-number variations, and the COSMIC SBS44 signature correlated with survival [8]. A single-cell gastric analysis of pre-cancerous and cancer lesions showed that intestinal and diffuse type cancer were characterized be different cell populations and described cancer associated fibroblast harbouring pro-stemness properties and malignant cells with high expression level of epithelial-myofibroblast transition which was correlated with poor clinical prognosis [9].

Effects of tumor associated macrophages (TAM) in clinical situations: An overview on the present known TAM landscape is given in a review descrinbing the CD expression of macrophages with favourable and unfavourable prognosis and of macrophage subpopulations with inhibition or with cooperation in anti-cancer therapy [10]. In breast cancer high numbers of CD163+ tumor associated macrophages predicted poor prognosis in HER2+ patients [11]. In colorectal cancer CXCL8 induced M2 macrophage polarization and inhibited CD8+ T cell infiltration to generate an immunosuppressive microenvironment [12]. Patients with peritoneal metastasis of gastric cancer and high M2 status in CD68+CD163+ cells had worse overall survival than those with low expression [13].

Effects of endothelials cells in clinical situations: A summary describes the interaction between endothelial cells and targeted therapy in the tumor microenvironment [14]. A single-cell analysis of various cancer types revealed differences in endothelial cells between tumors and normal tissues claiming that tip-like endothelial cells are the main differential endothelial cell subcluster promoting angiogenesis and inhibiting anti-tumor responses. TIP-like endothelial cells may be found in CRC, renal cell cancer, gastric cancer, and lung cancer [15]. In CRC endothelial cells exhibited enhanced angiogenesis. An increase in the density and proportion of tip cells correlated with CRC occurence, progression, and poor prognosis. Effective PD-1 blockade reduced tip cells disrupting the VEGFA-KDR-ESM1 positive feedback loop [16].

Effects of fibroblasts in clinical situations: A cancer-associated fibroblast-derived gene signature discriminated distinct prognoses by integrated single-cell and bulk-seq analyses in breast cancer [17]. In CRC stromal fibroblasts shaped the myeloid phenotype in normal colon and CRC and induced CD163 and CCL2 expression in macrophages [18]. In gastric cancer cancer-associated fibroblasts (CAF) affected progression via the CXCL12- CXCR4 axis [19].

Role of epithelial-mesenchymal transition (EMT) in tumor growth: Aberrant reactivation of EMT was associated with malignant properties of tumor cells during cancer progression and metastasis. A review summarized the contribution of EMT to tumor progression [20].

Contribution of extracellular vesicles (EV) and RNA binding proteins as bystanders in tumor growth: An interaction between the TME and cancer cells is involved in CRC metastasis. EV`s have been shown to play a role in cancer-TME interaction and in cancer progression. The RNA LINC00543 was overexpressed in CRC tissues, correlates with advanced TNM stage and poorer prognosis of CRC patients. Overexpression promoted metastasis by enhancing and remodeling EMT [21].

Macrophage conversion / reprogramming: Considering the unfavourable condition of the M2 polarization status there have been attempts to alter the macrophage polarization towards a M1 status. A summary on the present landscape of macrophage–reprogramming cancer immunotherapies has been reported [22].

ADC targeted constituents of the TME: Considering the BKE in conversion tumor cells may be viewed as bystander cells to TME cells elucidating the importance of TME cells in the action of ADC `s. In 2018 a tumor stroma targeting ADC of an anti TEM8 antibody and the payload MMAE was shown to trigger localized anticancer drug release resulting in the idea to apply stroma- targeting ADC`s as an alternative to cancer-targeting in cases of heterogeneous target expression [23]. The immunosuppressive contribution of TME cells has been shown in CAR-T-cell therapy [24] and the elimination of tumor associated neutrophils by ADC`s in preclinical models of cervical cancer [25].