Molecules such as viral PAMPs (e.g. nucleic acids) serve as potent danger signals, triggering the activation of innate immune receptors in neighboring cells, thereby amplifying the antiviral response. For example, endocytosed or phagocytosed debris containing viral RNA is detected by Toll-like receptors-3, 7, or 8 (TLR-3, TLR7, or TLR-8) within antigen-presenting cells like plasmacytoid dendritic cells (pDCs) and macrophages [
29]. The recognition of double-stranded viral RNA in the endosomes is mediated through TLR3 (
Figure 2B). Upon encountering dsRNA, TLR3 activates a signaling cascade leading to the activation of transcription factors IRF3 and IRF7 [
44]. Consequently, the expression of pivotal immune mediators such as interferons (IFNα and IFNβ), as well as various chemokines (CXCL9, CXCL10, CXCL11), and co-stimulatory molecules like clusters of differentiation 40, 80 or 86 (CD40, CD80, CD86) is induced, orchestrating a robust antiviral response. Single-stranded RNA derived from oncolytic viruses is sensed by endosomal TLR7 and/or 8 (
Figure 2B-C) [
12]. Activation of these receptors triggers downstream signaling pathways, culminating in the activation of the transcription factor NFΚB1. This activation prompts the expression of pro-inflammatory cytokines (TNF, IL1B, IL6, IL12A) and chemokines (CXCL8, CCL5, CCL3L3, CCL4L1), fostering an inflammatory milieu conducive to innate immune responses. This orchestrated response enhances the immune surveillance against viral infections and contributes to the shaping of an immune response in the tumor milieu. For example, Newcastle disease virus infection activates various signaling mechanisms, including TLR-signaling in tumors to regulate immune response and tumor-susceptibility markers [
45]. Oncolytic parvovirus exploits TLR-signaling to induce human immune responses, enhancing dendritic cell maturation and stimulating NFκB-dependent activation of the adaptive immune system, thereby priming immune responses against tumors [
46]. It has also been shown that immune responses induced by viral infection also synergize to enhance oncolysis. For example, IL-24 enhances apoptosis induced by influenza A virus via TLR3 and caspase-8 pathways, sensitizing cancer cells to TLR-mediated apoptosis [
47]. Similarly, measles virus-based therapies induce oncolysis by activating plasmacytoid dendritic cells, resulting in the production of interferon-alpha and cross-presentation of tumor antigens, thereby facilitating antigen-specific immune responses against tumors [
48]. Alternatively, some variants of parvoviruses escape TLR/RIG-I mediated immune sensing by tumor cells, resulting in lower IFN production and subsequent immune responses [
49].