Salmonella has been used with other therapeutic agents to enhance the efficacy of anti-cancer activities. It has been used in combination with chemotherapy, radiotherapy, immune checkpoint inhibitors, and immunomodulatory cytokines. The combined administration of
Salmonella with chemotherapy reduces toxicity compared with individual therapy with bacteria or chemotherapeutics. For this, a murine melanoma model was treated with VNP20009 and cyclophosphamide, which induced a significant decrease in microvessel density and serum VEGF levels compared with either treatment alone.
[24]. Similarly, the combination of anti-angiogenic agent HM-3 (a polypeptide inhibiting angiogenesis) and VNP20009 harboring expression plasmids for siRNA targeting
Sox2 demonstrated efficient treatment for lung cancer
[25]. Another well known ST A1-R strain was implemented in combination with the chemotherapeutic drugs temozolomide, doxorubicin, and anti-angiogenic agents, which significantly suppressed the growth of tumors in patient-derived orthotopic xenograft models
[26][27][28]. The co-administration of radiotherapy and BMCT produced prominent anti-tumor effects compared to either of the treatments alone. The combination of X-rays either with VNP20009 or ΔppGpp ST expressing cytolysin A (ClyA) or γ-radiation with
Salmonella BRD509 induced a significant suppression of the tumor or delayed tumor growth
[29][30][31]. In addition, the treatment with A1-R post-surgical excision of tumors significantly inhibited surgery-induced breast cancer metastasis
[32]. In combination therapy, the use of prodrug strategy along with
Salmonella-expressing, prodrug-activating enzymes such as HSV TK, carboxypeptidase G2 (CPG2), and cytosine deaminase have more promising tumor retardation capabilities compared to the use of the therapeutic strain alone
[11][33][34].
Being an intracellular pathogen, with vast survival in different organs of the host, attenuated
Salmonella has been widely used as a vaccine delivery system against various diseases
[35]. As mentioned earlier,
Salmonella is a multifaceted antagonist for cancer
[36][37], apart from that, using auxotrophic
Salmonella as a therapeutic and prophylactic vaccine delivery system is also an ideal strategy. Medina et al. have demonstrated the anti-cancer effect of auxotrophic ST (∆
aroA) as a vaccine delivery system that expresses β-gal as a model TAA against aggressive fibrosarcoma
[38]. In another study, SPI-2 and T3SS of
Salmonella were used to deliver survivin as a TAA into antigen-presenting cells, and the PsifB::sseJ promoter/effector combination was found to have an excellent anti-cancer immune response of CD8 infiltration in the tumor environment
[39]. Similarly, elevated effector-memory CTL responses against CT26 colon cancer and orthotopic delayed brain tumor glioblastoma in mice were found after immunization with survivin, which was fused to the SseF effector protein and kept under the regulation of SsrB, the key regulator of SPI2
[40]. Heat shock protein 70, as an immuno-chaperone fused with SopE of
Salmonella T3SS, has elicited a considerable CTL response against murine melanoma
[41]. A multi-antigen DNA vaccine encoding fusion antigenic domains of tyrosine hydroxylase, survivin, and PHOX2B, delivered by auxotrophic ST
(∆aroA,
∆guaAB), has been demonstrated to exhibit significant elicitation of the CTL response, INFγ production, and excellent suppression of neuroblastoma in a mouse model
[42]. In addition to the CTL responses, the elicitation of humoral response by a
Salmonella-based oral DNA vaccine with a MG7-Ag mimotope against gastric cancer was confirmed
[43]. An H
2O
2-inactivated
S. Typhimurium RE88
(∆aroA,
∆dam) has been established to induce anti-cancer immunity by using ovalbumin as a model antigen
[44].
Salmonella has also been studied for the expression of oncogenic virus antigens. A recombinant ST that produced Human Papillomavirus Type 16 (HPV16) L1 Virus-like Particles (VLPs) induced the anti-tumor immune response in prophylactic as well as therapeutic contexts
[45]. The same group has constructed a
Salmonella that has expressed major capsid protein L1 of HPV16 virus via plasmid, and has shown the induction of anti-HPV16 neutralizing and humoral immune responses
[46]. They have also demonstrated the intravaginal immunization of the HPV16-L1
Salmonella construct and its innate, adaptive, Th1, and Th2 mucosal immune responses
[47]. Thus,
Salmonella can be used to deliver oncogenic viral antigens for prophylactic vaccine development. In addition,
Salmonella infection triggers the formation of gap junctions in melanoma that are typically lacking in tumor cells. The transfer of tumor antigens to dendritic cells and the resultant induction of immune responses depend on these gap junctions
[48]. Moreover,
Salmonella has the ability to induce MHC class I and II immune responses by delivering cancer-related antigens via bacterial surface and translocating the antigen or its gene to the antigen-presenting cells, respectively.
Salmonella has the virtue of being used as a delivery vehicle for extrinsic cancer antigens, oncogenic viral antigens, and to display the intrinsic antigens of the active tumor to achieve anti-cancer immunity based on these aspects.