Gutting it Out: Developing Effective Immunotherapies for Patients With Colorectal Cancer

Abstract

Risk factors for colorectal cancer (CRC) include proinflammatory diets, sedentary habits, and obesity, in addition to genetic syndromes that predispose individuals to this disease. Current treatment relies on surgical excision and cytotoxic chemotherapies. There has been a renewed interest in immunotherapy as a treatment option for CRC given the success in melanoma and microsatellite instable (MSI) CRC. Immunotherapy with checkpoint inhibitors only plays a role in the 4%-6% of patients with MSIhigh tumors and even within this subpopulation, response rates can vary from 30% to 50%. Most patients with CRC do not respond to this modality of treatment, even though colorectal tumors are frequently infiltrated with T cells. Tumor cells limit apoptosis and survive following intensive chemotherapy leading to drug resistance and induction of autophagy. Pharmacological or molecular inhibition of autophagy improves the efficacy of cytotoxic chemotherapy in murine models. The microbiome clearly plays an etiologic role, in some or most colon tumors, realized by elegant findings in murine models and now investigated in human clinical trials. Recent results have suggested that cancer vaccines may be beneficial, perhaps best as preventive strategies. The search for therapies that can be combined with current approaches to increase their efficacy, and new knowledge of the biology of CRC are pivotal to improve the care of patients suffering from this disease. Here, we review the basic immunobiology of CRC, current "state-of-the-art" immunotherapies and define those areas with greatest therapeutic promise for the future.

Trial registration: ClinicalTrials.gov NCT01206530 NCT04082572 NCT03832569 NCT02437071.

Copyright © 2021 Wolters Kluwer Health, Inc. All rights reserved.

Figures

Figure 1.. Mouse models to study the tumorigenic roles of the human microbiome.

A, Isolated bacterium candidate (e.g., Enterotoxigenic Bacteroides fragilis) is administered by oral gavage to a mouse susceptible for colon tumorigenesis. Specific pathogen free (SPF) mice are colonized after being conditioned by antibiotic treatment (3-4 days treatment and 2 days of recovery) to enable exogenous bacterial colonization. If germ-free mice are used, no prior conditioning is required. To study colon tumorigenesis, Min mice (for colitogenic bacterium such as ETBF), IL-10−/− min mice (for non-colitogenic bacteria such as pks+ E.coli) or azoxymethan (AOM) treated wild type mice with or without dextran sulfate sodium (DSS; for colitogenic versus non colitogenic bacteria, respectively) were utilized. In the case of ETBF, a genetic background (Min mice with an APC allele deletion and loss of heterozygosity or LOH), an oncogenic virulence factor (Bacteroides fragilis toxin, BFT) and protumoral inflammation (STAT3 activation and IL-17 colitis) are all required for effective tumorigenesis. Microadenomas are visible as early as 1week following infection. Macroadenomas are visible as early as 4 weeks; Mice are sacrificed at 12 weeks for tumor counting (81, 187). Without ETBF colonization, Min mice develop tumors in the small intestine and rarely in the colon. ETBF colonized selectively the colon and induced distal colon tumors. B, Study of the carcinogenic properties of colorectal microbiota and colon biofilms. Mucosa of frozen colorectal cancer specimen or colonoscopy biopsies are homogenized and are diluted in suspension for oral administration in germ free mice. Distal colons are harvested at 1 week post colonization in Carnoy’s solution, to preserve the mucus layer. Top: Confocal FISH images demonstrate that bacteria invade the inner mucus layer and form biofilms only in mice inoculated with Bf+ tissues (right panels; left panels are SHAM control, PBS). Red = universal probe EUB338 targeting a conserved region in the bacterial 16S rRNA. Blue = DAPI. Bottom: matched Periodic acid–Schiff (PAS) stains showing the mucus layer in pink (Right, BF+ inocula; laft panel, SHAM or PBS inocula). Scale bar, 100um. The mucus layer is located between the two yellow dashed lines. Slides are oriented with epithelial cells at the bottom and lumen at the top. For tumor count, the colon is harvested from mice at 10-15 weeks post-colonization. The tissues are fixed in formalin and paraffin embedded and 6um tissue sections of colon rolls are stained with haemotoxylin and eosin (H&E). Scale bar, 2mm)

Figure 2.. Parameters involved in clinical outcome, dissemination to metastases and response to T cell checkpoint inhibitors in CRC.

We are listing here the parameters most associated to each of these three aspects. However, there is assuredly a great overlap for many of these parameters, which have been shown to correlate to all three aspects. Hence, a high cytotoxic T cell infiltration correlates with a good clinical outcome, a reduced risk of dissemination to metastases and a high response rate to checkpoint inhibitor immunotherapy. The parameters listed are for the most part, associated either to a High or Low Immunoscore tumor category. IM: invasive margin; MSS: microsatellite stability; MSI: microsatellite instability.