Date on Master's Thesis/Doctoral Dissertation

5-2024

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Physiology and Biophysics

Degree Program

Physiology and Biophysics, PhD

Committee Chair

Galandiuk, Susan

Committee Member

Joshua, Irving G

Committee Member

Metz, Cynthia J

Committee Member

Bhatnagar, Aruni

Committee Member

Schuschke, Dale

Author's Keywords

Colorectal cancer; macrophage polarization; immunotherapy; immune checkpoint protein expression

Abstract

The majority of patients with colorectal cancer do not respond to treatment with immunotherapy. Immunotherapy requires the expression of cell surface immune checkpoint proteins (e.g. PDL1) to exert its effect. Most (>85%) of colorectal cancers do not express these proteins and this contributes in part to the poor prognosis and survival in patients with advanced disease. The patients in which immunotherapy is a potential treatment option, are a genetic subtype known as mismatch repair deficient, and have demonstrated an excellent response to anti-PD-1/PDL1 immunotherapy. A key feature of mismatch repair deficient (MMRd) cancers is their immune cell rich tumor microenvironment. Macrophages and T cells are the most abundant immune cells of the tumor microenvironment in colon cancer and T cells are known to contribute to cancer PDL1 expression via IFNg expression. The role macrophages play in the expression of PDL1 in cancer is not well established. This dissertation investigated the role of macrophage polarization, and its mechanisms of inducing immune checkpoint protein expression in colorectal cancer. These studies led to the following results: Mismatch repair deficient (MMRd/microsatellite high [MSI-H]) colorectal cancers have a higher proportion of M1-like (pro-inflammatory) macrophages in their tumor microenvironment. Tumors with higher proportion of M1 macrophages have increased PDL1 expression, in both mismatch repair proficient and deficient colorectal cancers. The co-culture of colon cancer cell lines with M1 macrophages significantly increased cancer PDL1 expression, even in those with proficient mismatch repair systems. M1 macrophages have a higher cell surface PDL1 expression compared to the anti-inflammatory M2 phenotype. THP-1 derived M1 polarized macrophages increased expression of JAK/STAT pathway genes in colon cancer along with PDL1 expression, despite not producing IFNg. CXCL9 and CXCL10 gene expression was upregulated in MMRd/MSI-H tumor samples; their expression correlated strongly with M1 macrophage infiltration of the tumor micro-environment and CXCL9 and CXCL10 gene expression was significantly upregulated in our M1 polarized macrophages. Increased M1 macrophage fraction and higher CXCL9 and CXCL10 gene expression were associated with improved overall CRC patient survival, including those with MMR proficient CRC. These results suggest that M1 macrophages in the tumor microenvironment can induce PDL1 expression in colon cancer providing a potential treatment target for mismatch repair proficient CRC, which at present do not respond to immunotherapy. While IFNg continues to play an important role in PDL1 expression, PDL1 can be induced via the JAK/STAT pathway by alternative mechanisms such as the CXCL9/CXCL10/CXCR3 axis.

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