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Review
. 2016 Mar 10;164(6):1233-1247.
doi: 10.1016/j.cell.2016.01.049.

The Basis of Oncoimmunology

A Karolina Palucka  1 , Lisa M Coussens  2
Affiliations
Review

The Basis of Oncoimmunology

A Karolina Palucka et al. Cell. .

Abstract

Cancer heterogeneity, a hallmark enabling clonal survival and therapy resistance, is shaped by active immune responses. Antigen-specific T cells can control cancer, as revealed clinically by immunotherapeutics such as adoptive T-cell transfer and checkpoint blockade. The host immune system is thus a powerful tool that, if better harnessed, could significantly enhance the efficacy of cytotoxic therapy and improve outcomes for cancer sufferers. To realize this vision, however, a number of research frontiers must be tackled. These include developing strategies for neutralizing tumor-promoting inflammation, broadening T-cell repertoires (via vaccination), and elucidating the mechanisms by which immune cells organize tumor microenvironments to regulate T-cell activity. Such efforts will pave the way for identifying new targets for combination therapies that overcome resistance to current treatments and promote long-term cancer control.

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Figures

Figure 1
Figure 1. The makings of tumor immunity
The communication between cancer and the immune system is a dynamic process, reminiscent of a balance. When immunity to cancer is ‘up’ and the suppressive processes are ‘down’, cancer is under control. However, a strong anti-tumor immune response will trigger largely physiological processes designed to dampen effector T cells to prevent tissue damage and maintain tissue homeostasis. Given that the immunity might have evolved mainly to maintain self, to establish coexistence with environment and to occasionally protect self from external threats, the suppression prevails. Multiple pathways of suppression are at play in tumor microenvironments including cells such as TH2-polarized macrophages, immature and suppressive monocytes, regulatory B cells and regulatory T cells, as well as molecules such as check points that control T cell differentiation (for example CTLA-4 and IDO) and effector function (such as PD-1). Pharmacological blockade of these inhibitory pathways can tip the balance towards anti-cancer effector T cells. The latter ones can be primed or boosted by antigen presenting cells (DCs) and/or by co-stimulatory signals (for example CD137 ligands). Recent studies demonstrate that thymus-independent neo-antigens generated in adult life by somatic mutation or post-translational regulation (for example phosphorylation) might be more immunogenic (or perhaps linked with less suppression) than shared tumor antigens. Neo-antigens can occur as random results of somatic mutation, as well as a by-product of anticancer treatments, e.g., chemotherapy (CTX) or radiation therapy (RT), or by targeting epigenetic control mechanisms or drugs intervening with DNA repair pathways. They can be presented to T cells in exogenous vaccines, as well as endogenously via DCs that captured dying neoplastic cells. When T cells specific to defined antigens kill neoplastic cells, such process can enable generation of responses to other antigens, so called epitope spreading. A critical factor in the balance between immunogenicity and suppression is inflammation (which in turn is impacted by the microbiome); indeed, the type of inflammation (tumor destructing TH1 or tumor promoting TH2 and TH17) should become a part of TNM grading, along with pathology, microbiome phenotype, and immune infiltrate assessment.
Figure 2
Figure 2. Immune-mediated landscape
The yin and yang implications of tumor-immune system communications form the basis for disease pathophysiology, and at the same time, targets for therapeutic intervention. The disease landscape emerging from these multi-factorial interactions is orchestrated by the three compartments, i.e., the cancer, the immune system and the host. The outputs are numerous and dramatically opposite, as well as both local and systemic, and include: immunity that might control cancer; chronic inflammation which can be linked with tissue remodeling processes and metabolic changes that support neoplastic cell survival and primary tumor development; angiogenesis and lymphangiogenesis that can also support metastatic dissemination; as well as systemic consequences for the host including cachexia. Clearly, therapy going forward will require a well-timed and orchestrated combination of therapies, targeting multiple modes of communication and effect, to combat this multi-factorial disease taking into account the patients’ steady-state commensal bacteria complexity and load, and how that is impacted by therapy.
Figure 3
Figure 3. The priming of cancer immunity
The cycle of anti-tumor immunity starts presumably with presentation of cancer antigens liberated in the process of cell turn-over; this same pathway can be followed for vaccination as illustrated herein. Antigens are sensed and captured either by tissue resident DCs or by DCs in draining lymph nodes (LNs). DCs initiate an immune response by presenting captured antigens, in the form of peptide–major histocompatibility complex (MHC) molecule complexes, to naive (that is, antigen inexperienced) T cells in lymphoid tissues. When compared with other APCs, such as macrophages, DCs are extremely efficient and can elicit very low numbers of T cells to respond. Naïve CD8+ T cells differentiate into CTLs in lymphoid organs upon encounter with DCs presenting tumor-derived peptides in the context of co-stimulation through CD8, CD70 and 4-1BB, as well as DC-derived cytokines such as IL-12 and IL-15. Naive CD4+ T cells can give rise to helper cells (e.g., TH) with distinct cytokine profiles, or to regulatory T cells (Treg) whose role is to dampen the immune response. T cells migrate through blood and lymphatics. Upon arrival in tumor beds, CD8+ T cells must confront numerous barriers including: i) intrinsic regulators, for example CD28-CTLA-4, PD1-PDL1, and ILTs, as well as extrinsic regulators cells such as Tregs, Bregs or myeloid cells; ii) a corrupted TME with pro-tumor inflammation; iii) impaired cross presentation due to TME-based DC inhibition; iv) antigen loss and immune evasion of tumor target; and v) tissue-specific alterations such as fatty cells in breast cancer or desmofibrosis in pancreatic cancer stroma. Killing of tumor cells either via T cells or by standard therapy can lead to endogenous antigen release and DC activation so called “endogenous vaccination” thereby closing the cycle. Inevitable to this is the induction of tissue resistance mechanisms, for example, expression of PD-L1 on neoplastic cells, as the result of powerful effector immunity including actions of IFNγ. Thus, future immunotherapy approaches will be based on combinations of different therapeutics targeting distinct components of this cycle, for example, via intratumoral delivery of activating agents able to reprogram the function of infiltrating leukocytes.
Figure 4
Figure 4. Multi-modal biomarker-based approach for optimal immune-mediated tumor control
Cancer medicine is evolving. Going forward, individuals with cancer will be evaluated for biomarkers enabling stratification to determine most optimal combinations for therapy based on tumor-based and systemic biomarkers. Eventually, all patients with cancer will be treated with checkpoint inhibitors, either directly or after interventions targeting inflammation (for example with TH2-blockade therapies, radiation therapy or epigenetic modulation), or vaccination via DCs to boost T cell repertoires, or adoptive T cell transfer. Based on the known tissue-embedded programs empowered to control auto-immunity, it is reasonable to anticipate that a majority of patients will develop acquired resistance followed by immune escape; this will lead to the next cycle of treatments incorporating multi-modal biomarkers (e.g., based on microbiome phenotype, circulating free DNA (cfDNA), circulating cytokine levels) and perhaps NK cells recognizing loss of MHC class I by neoplastic cells, thus rendering them invisible to T cells. Cytotoxic treatments such as with NK cells or standard cytotoxic therapy (CTX or RT), or oncolytic viruses will release neo-antigens that can be used for generation of the next round of effector T cells. Whole exome sequencing (WES) of tumor samples as well as cfDNA will yield information on mutational load that can in turn be used as one class of neoantigens for vaccination and priming of new T cell repertoires. T cell receptors (TCR) can be assessed using genomic approaches enabling sequencing of TCRβ chains to assess repertoire diversity. Given the importance of T cell specificity for relevant antigens, strategies enabling paired sequencing of α and β TCR chains will be invaluable as well as high-throughput tetramer analysis. In addition, RNAseq and epigenetic analysis of tumors and their infiltrates will enable assessment of the type and flavor of inflammation. Future studies will incorporate metabolomics to this biomarker portfolio.
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