title: The local tumour immune response following systemic Salmonella enterica serovar Typhimurium infection
Creators: name: Johnson, Sile Ann
Publisher: name: University of Glasgow
Year: 2018

description: In recent years, there is renewed interest in the potential of bacteria as an alternative cancer therapeutic strategy. Salmonella enterica serovar Typhimurium is arguably the most well studied strain of bacteria for cancer therapy, examined in both pre-clinical and clinical settings. Many of the studies which have demonstrated a role for S. Typhimurium in tumour growth inhibition or regression have focused on increasing the tumour-specific localisation of the bacteria, or enhancing the efficacy of the treatment modality. However, the exact mechanisms underlying S. Typhimurium-mediated tumour growth inhibition are incompletely elucidated, particularly with respect to the myeloidderived immune cells, such as monocytes and macrophages. The current study intended to address the dearth of information in the literature pertaining to the overall tumour-local immune response to systemically administered S. Typhimurium. This was achieved through the development of an in vivo tumour model and the optimisation of the S. Typhimurium administration protocol to maximise therapeutic effect. This allowed for the investigation of changes in multiple immune cell types in the tumour, both in number and functional phenotype, following infection. It was found that following systemic SL7207 infection, there was an increase in the secretion of pro-inflammatory mediators in the tumour milieu. This was accompanied by the activation of both neutrophils and monocytes, and possibly increased migration of tumour-associated dendritic cells. Interestingly, we found evidence to suggest that resident tumour-associated macrophages (TAMs) do not participate in mediating the pro-inflammatory tumour microenvironment following infection, which is suggested in some published reports. We were also interested in the types of T cell responses stimulated in the tumour following infection. This investigation revealed increases in the frequency of tumour-associated T helper (TH)1, but also TH17 cells following infection. There was also a concomitant decrease in the frequency of the tumour-promoting, T regulatory (Treg), cells in the tumour mass. To our knowledge, this is the first report to suggest a role for either TH17 or Tregs in playing a role in bacterial-mediated cancer therapy. Given the phenotypic changes in the tumour-associated monocytes following infection, we chose to assess the contribution of this cell population to S. Typhimurium-mediated tumour growth inhibition. This was attempted though the employment of transgenic mice lacking circulating monocytes and clodronate liposome-mediated depletion of monocytes/macrophages. Both of these approaches were proficient in depleting tumour monocytes in the uninfected state, with TAMs also affected by clodronate liposome treatment. However, we found that neither of these approaches was sufficient to mediate the depletion of tumour-recruited monocytes following systemic S. Typhimurium infection. Interestingly, clodronate liposome treatment abrogated the S. Typhimurium-induced tumour growth inhibitory effects anyway. Upon further investigation, it was observed that the spleens of clodronate liposome-treated mice that were systemically infected with S. Typhimurium did not experience splenomegaly like their control PBS liposome-treated counterparts. As the spleen is a source of systemic inflammatory mediators following infection and splenic monocytes contribute to the tumour monocyte/macrophage population, the current hypothesis is that the splenic monocytes mediate tumour-growth inhibition in S. Typhimurium infected mice. This concept antagonises the prevailing ideology in the literature that tumour-local immune cells are the effectors of bacterial-mediated tumour growth arrest. This study also sought to enhance the tumour arrest effects of S. Typhimurium through transformation of the bacteria with a eukaryotic expression vector encoding tumour inhibitory genes, destined for transfer to the tumour cells. However, through this investigation, it was discovered that the bacteria transformed with such a plasmid exhibited an aberrant morphology and phenotype, which we subsequently discovered was due to a phage origin of replication encoded in the plasmid. The data generated in this thesis provides valuable information pertaining to the general immune response in the tumour following systemically administered S. Typhimurium. Furthermore, we propose a role for monocytes, possibly of splenic origin, in mediating the effects of S. Typhimurium-induced tumour growth inhibition. Finally, we identified a feature of eukaryotic expression plasmid, a phage origin of replication, which is not compatible with S. Typhimurium and should be avoided for bactofection, and other bacteriological, studies.