Hypoxia and the phenomenon of immune exclusion

Abstract

Over the last few years, cancer immunotherapy experienced tremendous developments and it is nowadays considered a promising strategy against many types of cancer. However, the exclusion of lymphocytes from the tumor nest is a common phenomenon that limits the efficiency of immunotherapy in solid tumors. Despite several mechanisms proposed during the years to explain the immune excluded phenotype, at present, there is no integrated understanding about the role played by different models of immune exclusion in human cancers. Hypoxia is a hallmark of most solid tumors and, being a multifaceted and complex condition, shapes in a unique way the tumor microenvironment, affecting gene transcription and chromatin remodeling. In this review, we speculate about an upstream role for hypoxia as a common biological determinant of immune exclusion in solid tumors. We also discuss the current state of ex vivo and in vivo imaging of hypoxic determinants in relation to T cell distribution that could mechanisms of immune exclusion and discover functional-morphological tumor features that could support clinical monitoring.

Keywords: Dynamic barriers; Functional barriers; Hypoxia; Imaging; Immune exclusion; Physical barriers; Tumor microenvironment.

Fig. 1

Mechanisms of HIF-1α protein stabilization in hypoxia and degradation in normoxia. a Under normal oxygen tension, HIF-α subunits are expressed, hydroxylated by a family of oxygen dependent prolyl hydroxylases (PHDs), recognised by the von-Hippel Lindau tumor suppressor (pVHL) which leads to HIF-α poly-ubiquitination and subsequent degradation by the 26S proteasome. b Under hypoxic conditions HIF-α is no longer hydroxylated but it dimerizes with the constitutively expressed HIF-β, enters the nucleus and binds to HREs to upregulate transcription of a group of hypoxic responsive genes. c Extensive modifications in chromatin structure, both HIF dependent and independent, also promote gene silencing

Fig. 2

Hypoxia-induced mechanical barriers. Mechanical barriers represent a category of determinants of immune exclusion where a lack of interaction between T cells and cancer cells is due to physical impediments. Hypoxia induced-mechanical barriers include neoangiogenesis (vascular accessibility), ECM remodeling (stromal fibrosis, collagen remodelling and crosslinking), EMT and cancer cell coating

Fig. 3

Hypoxia-induced functional barriers. Functional barriers represent a class of impediments in which T cells are in close proximity with cancer cells, but their activity is impaired. Hypoxia-driven functional barriers include metabolic barriers, secreted soluble factors, danger sensing pathways and/or cell-intrinsic signaling

Fig. 4

Scheme of ex vivo and in vivo techniques to detect hypoxia and immune cells. a Ex-vivo techniques could be useful to create topographic maps of hypoxic-induced physical and functional barriers in immune excluded tumor areas, providing mechanistic highlights into the organization of immune exclusion determinants. Bioinformatic and spatial transcriptomics could investigate hypoxic transcriptional profiles in correlation with ICR or TIS. Immunohistochemistry of exogenous compounds or HIF-induced proteins coupled to digital pathology of immune cells could provide functional and morphological insights. b In-vivo techniques to detect gradients of hypoxia and immune infiltration may be useful to monitor immunotherapy efficiency in hypoxic tumors (clinical setting). To detect hypoxia and immune cells in patients, non-invasive methodologies are preferred

Fig. 5

Spatial transcriptomics workflow including the downstream analysis. a Histological tumor sections are stained with hematoxylin and eosin-stained and imaged before permeabilization. b The sections are placed on glass slides containing RT-primers arrayed as spots that corresponded to tissues domains. The barcoded microarrays contain printed spots of RT-primers with unique barcode sequences. The RT-primers at each spot have a unique spatial ID barcode, which is sequenced along with the transcript to enable trace-back to a specific tissue domain. c Sequencing. d After sequencing the gene expression profiles and factor activity map are created

Fig. 6

Examples of hypoxia imaging in glioma. Postsurgical assessment of residual tumor showing a 18F-FMISO gradient, hypoxic areas are found in the surgical margin of the tumor. b 18F-FDG staining, marker for the tissue uptake of glucose and C) preoperative MRI showing the tumor mass in the right posterior parieto-occipital lobe

Fig. 7

Comparative study of BOLD and TOLD to assess tumor oxygenation. BOLD and TOLD-MRI measures of oxygen gradients overlaid on a high-resolution image of a small Dunning prostate R3327-AT1 tumor in conditions of hyperoxia, 100% O₂ (O₂), return to normal air (Air) and carbogen breathing, 95% O₂ and 5% CO₂ (CB). Changes in signal intensity is represented by %ΔSI. Response maps compare final images versus baseline. Each map shows substantial intratumoral heterogeneity and differences in the heatmap pattern within the same treatment, between BOLD and TOLD