Modulating the immune response to colorectal cancer in mice using a chitosan gel vaccine

Abstract

Vaccines modulate the host’s anti-tumour immune response and represent an area of emerging immunotherapy research for the treatment of cancer, including colorectal cancer (CRC). Murine subcutaneous (SC) injections of tumour cell lines are often used to test cancer vaccines for the treatment of CRC. However, the baseline immune responses to cancers are often not quantified in mouse models. In the cancer models presented here, the immune cells: dendritic cells, macrophages, T cells, (CD4+ and CD8+), and B cells were identified via flow cytometry at the tumour site (local immune response) and in the spleen (systemic immune response). T cell phenotypes were identified in the inguinal lymph nodes and spleen, which included: antigen-experienced memory T cells (CD44+ CD122+), T regulatory cells (CD25+ CD127lo), proliferating cells (Ki67+), and the expression of cytokines (IFN-γ, IL-2, and IL-17). To quantify the baseline immune response to cancer, CT26 colon carcinoma cells were subcutaneously injected into mice. Control mice received B16-OVA melanoma cells or saline. SC CT26 tumours had a lower frequency of B cells, T cells, and CD8+ T cells than B16-OVA tumours. Spleens from mice that received either tumour were dominated by F4/80+ macrophages, CD3+ T cells and CD4+ T cells. The spleens of mice that received B16-OVA tumours had more IFN-γ+ CD4+ and CD8+ T cells and a higher frequency of CD44+ CD122+ T cells than mice that received CT26 tumours. These results illustrate select intrinsic differences between CT26 and B16-OVA subcutaneous tumours and showcase the incongruities of using the two tumour models interchangeably. These data will enable researchers using these mouse tumour models to have an understanding of immune cell populations present before therapeutic intervention, better enabling therapeutics to be tested in the proper murine cancer model. Cancer vaccines are a current area of immunotherapy which are progressing through clinical trials. Unfortunately, many CRC vaccines are not able to alter the immune response towards a protective phenotype. To determine if the immune response to CRC could be modulated with a cancer vaccine, mice were vaccinated with chitosan gel gel alone, gel and AH1 peptide (the specific endogenous antigen of CT26 tumours), or PBS control. Mice were challenged subcutaneously with CT26 cells. Vaccination with chitosan gel reduced the growth rate, endpoint volume, and endpoint weight of SC CT26 tumours. This protection correlated with an increase in splenic B cell frequency and a decrease in lymph node T cell frequency. No correlations with T cell phenotype were found. There were no differences in cell subset or T cell frequency between gel alone and gel + AH1 peptide. In a SSM3 breast cancer model, the immune response to two inflammatory treatments was analysed for alterations in the immune response to tumour. Treatment with anti-PD-1 antibodies + aspirin increased the frequency of splenic dendritic cells and intra-tumoural B cells and decreased the frequency of CD3+ and CD4+ T cells in the tumour. Treatment with Letrozole + Celecoxib reduced the frequency of F4/80+ macrophages in the SSM3 tumours. We have also shown that CRC can also be modelled by a microsurgical intracaecal injection of CT26 cells. The surgery was optimised to be performed in sterile conditions, using inhalant anaesthesia. These data show that the immune response to CRC can be modulated towards a protective phenotype, in a mouse model, which could be tested in human patients and potentially translate into a human therapy.