Immune checkpoint blockade in microsatellite instable colorectal cancers: Back to the clinic

Abstract

The active Th1/CTL immune microenvironment of Microsatellite Instable colorectal cancer (CRC) is counterbalanced by up-regulated expression of multiple immune checkpoints, suggesting that defective mismatch repair may be a biomarker to select CRC patients for treatment with checkpoint inhibitors. This hypothesis is currently being tested in two clinical trials.

Inflammation is an essential component of the tumor microenvironment (TME), but its role is highly ambiguous and controversial. However, it has become clear that the nature of cancer-associated inflammation strongly impacts the clinical outcome and tumor-associated immune signatures are critical biomarkers of prognosis. In CRC, expression of a cluster of genes associated with Th17 immune signature (IL17A, RORC) has been associated with a worse prognosis whereas the presence of a Th1 immune signature (TBX21, IRF1, IL12RB2, STAT4) signifies a better outcome.¹ The high genetic instability that characterizes some cancers seemingly results in the appearance of frameshift mutations that may generate neoepitopes recognized by cytotoxic T cells (CTL) in the context of HLA-I restriction. Expansion of tumor infiltrating lymphocytes (TIL) recognizing immunogenic somatic mutations in different cancers has been shown to coincide with clinical responses (CR) to immunotherapy, thereby demonstrating the beneficial role of immune surveillance in fighting cancer.^2,3 However, despite the high immunogenicity of some cancers associated with a high mutational load, the inexorable growth of tumors emphasizes the ability of tumor cells to escape destruction by infiltrating immune cells. The recent breakthrough use of immune checkpoint inhibitors (anti-CTLA-4 and anti-PD-1/PD-L1 antibodies) to treat refractory solid tumors demonstrated that: (1) immunogenic cancers trigger endogenous immune responses independent of immunotherapy; (2) the antitumor immune response induces the expression of inhibitory molecules (immune checkpoints) that regulate T cell function; and (3) blocking these immunoregulatory pathways harnesses the endogenous antitumor immune response and, in some cases, leads to durable CR.⁴ Anti-CTLA-4 and anti-PD-1 antibodies are now approved by the food and drug administration (FDA) for treatment of metastatic melanoma. A variety of additional checkpoint inhibitors such as anti-LAG-3 antibody or small molecule inhibitors of indoleamine 2,3-dioxygenase 1 (IDO1) are in clinical evaluation, opening the perspective of personalized combinatorial immunotherapies.⁴ While the CR of checkpoint blockade therapy is remarkable in some cases, particularly in some difficult to treat cancers such as non-small cell lung cancer (NSCLC), only a fraction of patients respond to such approaches, demonstrating that the mechanisms behind differential sensitivity of cancer are largely unclear.⁵ For example, in three clinical trials only one out of 56 CRC patients elicited an objective response (OR) to inhibitors of the PD-1 / PD-L1 pathway.^6-8 Importantly, this means that adaptive immune-resistance may not be universal and multiple mechanisms of escape will impede immunotherapy efficacy. Therefore, biomarkers identifying these mechanisms are needed to predict cancer populations who would be eligible for personalized approaches.

We recently showed that the genetic background of the CRC (microsatellite instable [MSI] vs. stable [MSS]) dictates the nature of the immune checkpoint expressed in the TME and likely impacts sensitivity to checkpoint blockades.⁹ Whereas MSS are, in general, poorly infiltrated, MSI CRC exhibit a Th1/Tc1-type immune infiltration that is geographically associated with the upregulation of a variety of T cell checkpoints, including CTLA-4, PD-L1, LAG-3, and IDO-1. These results suggest that blocking T cell checkpoints expressed in MSI CRC should unleash endogenous antitumor immune responses (Fig. 1). In fact, the CRC patient who experienced a durable complete CR (> 4 year) in the initial phase I anti-PD1 clinical trial happened to be MSI+.⁸ This CRC patient, like in the case of non-CRC patients (melanoma, renal cell carcinoma and NSCLC) responding to anti-PD-1 or anti-PD-L1, was found to express PD-L1 in pretreatment tumor biopsies.⁶ Altogether, these findings identified two potentially predictive biomarkers that may validate the eligibility of CRC for immunotherapy: (1) the genomic signature, distinguishing MSI from MSS CRC; and (2) the immune signature, as determined by immune checkpoint expression in the TME. These findings initiated two highly anticipated clinical trials. One phase II multi-center clinical trial is using Pembrolizumab (anti-PD-1) in MSI vs. MSS CRC patients (NCT01876511) and the other one compares Nivolumab (anti-PD-1) to Nivolumab+Ipilimumab (anti-CTLA-4) to treat metastatic MSI⁺ CRC patients (NCT02060188).

Figure 1.

Immunologic microenvironment of CRC may dictate the sensitivity to checkpoint blockade. (A) The high density of mutations in MSI CRC increases the number of neoepitopes presented by tumor cells to tumor-infiltrating immune cells. Activation of the immune effector cells (CTL in the figure) is associated with the production of inflammatory mediators, including IFNγ which induces the upregulation of checkpoint ligands in the TME (on tumor cells in the figure). The recognition of neoepitopes by T cells and the nature of the checkpoints expressed in the TME will drive the sensitivity of the tumor to checkpoint blockade and will guide the nature of the inhibitors to be used. (B) Despite displaying a low density of mutations, some MSS CRC, present neoepitopes and are capable of activating specific immune responses. The frequency of such immunogenic MSS is yet unknown. The aforementioned immunogenic MSS CRC should be also eligible for the immune checkpoint blockade. (C) Most of the MSS CRC are poorly immunogenic and do not qualify for immune checkpoint blockade as a therapeutic approach.

Although our immunologic exploration of CRC suggests that MSI CRC have a better chance to exhibit an OR to checkpoint blockade, we anticipate that, ultimately, the immune infiltration (PD-1^hi CTL immune signature) may be a better predictor of a CR. Indeed, we have identified MSS CRC with low mutational load which display MSI-like immune signatures (Fig. 1), suggesting that although the high density of mutation in MSI CRC increases the chance of generating neoantigens recognized by CTL, it is the nature of intratumoral mutations which may drive immunogenicity more than the sheer number of mutations.⁹ On this note, Snyder et al. recently showed that high mutational load is associated with CR to anti-CTLA-4 treatment of metastatic melanomas but remains insufficient to predict clinical benefit. They instead validated neoepitope signatures as predictive biomarkers.¹⁰ Hence, whole cancer exome sequencing and neoepitopes prediction will be instrumental in defining genomic signature across CRC patients (MSI and MSS) eventually responsive to anti-PD-1 or anti-PD-1/CTLA-4. Therefore, we propose that immunogenic genomic signatures (neoepitope identification) associated with immune checkpoints expression will serve to identify a broader CRC population that will benefit from immunotherapy rather than the mere MSI CRC discrimination which represents only a limited fraction of CRC.

Disclosure of Potential Conflicts of Interest

No potential conflicts of interest were disclosed.