Abstract
A high infiltrate of CD3+ and CD8+ T cells is associated with positive patient prognosis in colorectal cancer (CRC), highlighting the importance of T cells in the anti-tumour immune response. T cell migration into the tumour microenvironment (TME) is dependent on expression of chemokine receptors and adhesion molecules. Once within the TME, chronic exposure to tumour antigens can promote upregulation of inhibitory receptors (IRs). T cells within the TME of CRC tumours have higher expression of IRs compared to surrounding tissue. Signalling via IRs can induce an “exhausted” T cell state, which is associated with expression of IRs and diminished cytokine production. The use of immune checkpoint blockade (ICB) can be used to reinvigorate exhausted T cells by targeting IRs and preventing subsequent signalling. ICBs have been effective in inducing strong anti-tumour immunity and better patient prognosis in melanoma, however, use of ICB has had limited success in treating CRC. Greater understanding of T cell migration and function in CRC is vital to improving these treatments and patient prognosis. The aim of this research was to investigate how T cells migrate into the TME via expression of chemokine receptors and adhesion molecules, and to understand how the TME affects T cell function. Chemokine receptor expression and adhesion molecules on T cells enriched in CRC tumours, non-tumour bowel (NTB) and blood were assessed by mass cytometry. Analysis of these data revealed that T cell populations enriched in CRC tumours expressed CXCR4, CCR4, CXCR3, and CCR5, a distinct profile from the blood and NTB. T cell populations enriched in CRC patient blood expressed CD11b, an adhesion molecule commonly associated with macrophages, granulocytes, and natural killer (NK) cells. I developed an in vitro TranswellTM migration model to identify the importance of CXCR4, CXCR3, CCR4, CCR5, and CD11b in T cell migration into the TME. Blocking CD11b inhibited T cell migration towards CXCL12 in vitro, which suggested CD11b expression on T cells may be involved in migratory functions. Next, I investigated the relationship between IR expression and cytokine production on T cells in CRC tumours and patient blood using mass cytometry. IR+ T cell populations enriched in CRC tumours had both an exhausted phenotype with diminished cytokine production, and an activated T cell phenotype that produced high levels of effector cytokines. In addition, PD-1 expression alone was not a determinant of T cell exhaustion. Signalling via PD-1 did not promote an exhausted phenotype in all T cell populations. These data suggest that IR+ T cells can be both activated and exhausted in CRC tumours and signalling via one IR is not sufficient to induce diminished T cell function. Identification of the chemokine receptors involved in T cell migration into CRC tumours could provide the basis for novel targets for immunotherapies. Targeting the chemokine/chemokine receptor axis for immunotherapy could enhance T cell infiltration into tumours and therefore enhanced anti-tumour immunity. In addition, these data investigating relationships between IR expression and T cell function may help to explain why successful treatment with ICBs is limited in CRC patients.