Immunotherapy-activated T cells recruit and skew late-stage activated M1-like macrophages that are critical for therapeutic efficacy.

Total tumor clearance through immunotherapy is associated with a fully coordinated innate and adaptive immune response, but knowledge on the exact contribution of each immune cell subset is limited. We show that therapy-induced intratumoral CD8+ T cells recruited and skewed late-stage activated M1-like macrophages, which were critical for effective tumor control in two different murine models of cancer immunotherapy. The activated CD8+ T cells summon these macrophages into the tumor and their close vicinity via CCR5 signaling. Exposure of non-polarized macrophages to activated T cell supernatant and tumor lysate recapitulates the late-stage activated and tumoricidal phenotype in vitro. The transcriptomic signature of these macrophages is also detected in a similar macrophage population present in human tumors and coincides with clinical response to immune checkpoint inhibitors. The requirement of a functional co-operation between CD8+ T cells and effector macrophages for effective immunotherapy gives warning to combinations with broad macrophage-targeting strategies.

Neutralizing Antibodies Impair the Oncolytic Efficacy of Reovirus but Permit Effective Combination with T cell-Based Immunotherapies.

Reovirus type 3 Dearing (Reo), manufactured for clinical application as pelareorep, is an attractive anticancer agent under evaluation in multiple phase 2 clinical trials for the treatment of solid tumors. It elicits its anticancer efficacy by inducing both oncolysis and intratumoral T-cell influx. Because most people have been preexposed to Reo, neutralizing antibodies (NAb) are prevalent in patients with cancer and might present a barrier to effective Reo therapy. Here, we tested serum of patients with cancer and healthy controls (n = 100) and confirmed that Reo NAbs are present in >80% of individuals. To investigate the effect of NAbs on both the oncolytic and the immunostimulatory efficacy of Reo, we established an experimental mouse model with Reo preexposure. The presence of preexposure-induced NAbs reduced Reo tumor infection and prevented Reo-mediated control of tumor growth after intratumoral Reo administration. In B cell-deficient mice, the lack of NAbs provided enhanced tumor growth control after Reo monotherapy, indicating that NAbs limit the oncolytic capacity of Reo. In immunocompetent mice, intratumoral T-cell influx was not affected by the presence of preexposure-induced NAbs and consequently, combinatorial immunotherapy strategies comprising Reo and T-cell engagers or checkpoint inhibitors remained effective in these settings, also after a clinically applied regimen of multiple intravenous pelareorep administrations. Altogether, our data indicate that NAbs hamper the oncolytic efficacy of Reo, but not its immunotherapeutic capacity. Given the high prevalence of seropositivity for Reo in patients with cancer, our data strongly advocate for the application of Reo as part of T cell-based immunotherapeutic strategies.

Invasive margin tissue-resident macrophages of high CD163 expression impede responses to T cell-based immunotherapy.

BACKGROUND: Primary and secondary resistance is a major hurdle in cancer immunotherapy. Therefore, a better understanding of the underlying mechanisms involved in immunotherapy resistance is of pivotal importance to improve therapy outcome. METHOD: Here, two mouse models with resistance against therapeutic vaccine-induced tumor regression were studied. Exploration of the tumor microenvironment by high dimensional flow cytometry in combination with therapeutic in vivo settings allowed for the identification of immunological factors driving immunotherapy resistance. RESULTS: Comparison of the tumor immune infiltrate during early and late regression revealed a change from tumor-rejecting toward tumor-promoting macrophages. In concert, a rapid exhaustion of tumor-infiltrating T cells was observed. Perturbation studies identified a small but discernible CD163hi macrophage population, with high expression of several tumor-promoting macrophage markers and a functional anti-inflammatory transcriptome profile, but not other macrophages, to be responsible. In-depth analyses revealed that they localize at the tumor invasive margins and are more resistant to Csf1r inhibition when compared with other macrophages. In vivo studies validated the activity of heme oxygenase-1 as an underlying mechanism of immunotherapy resistance. The transcriptomic profile of CD163hi macrophages is highly similar to a human monocyte/macrophage population, indicating that they represent a target to improve immunotherapy efficacy. CONCLUSIONS: In this study, a small population of CD163hi tissue-resident macrophages is identified to be responsible for primary and secondary resistance against T-cell-based immunotherapies. While these CD163hi M2 macrophages are resistant to Csf1r-targeted therapies, in-depth characterization and identification of the underlying mechanisms driving immunotherapy resistance allows the specific targeting of this subset of macrophages, thereby creating new opportunities for therapeutic intervention with the aim to overcome immunotherapy resistance.

Intertumoral Differences Dictate the Outcome of TGF-ß Blockade on the Efficacy of Viro-Immunotherapy.

The absence of T cells in the tumor microenvironment of solid tumors is a major barrier to cancer immunotherapy efficacy. Oncolytic viruses, including reovirus type 3 Dearing (Reo), can recruit CD8+ T cells to the tumor and thereby enhance the efficacy of immunotherapeutic strategies that depend on high T-cell density, such as CD3-bispecific antibody (bsAb) therapy. TGF-ß signaling might represent another barrier to effective Reo&CD3-bsAb therapy due to its immunoinhibitory characteristics. Here, we investigated the effect of TGF-ß blockade on the antitumor efficacy of Reo&CD3-bsAb therapy in the preclinical pancreatic KPC3 and colon MC38 tumor models, where TGF-ß signaling is active. TGF-ß blockade impaired tumor growth in both KPC3 and MC38 tumors. Furthermore, TGF-ß blockade did not affect reovirus replication in both models and significantly enhanced the Reo-induced T-cell influx in MC38 colon tumors. Reo administration decreased TGF-ß signaling in MC38 tumors but instead increased TGF-ß activity in KPC3 tumors, resulting in the accumulation of α-smooth muscle actin (αSMA+) fibroblasts. In KPC3 tumors, TGF-ß blockade antagonized the antitumor effect of Reo&CD3-bsAb therapy, even though T-cell influx and activity were not impaired. Moreover, genetic loss of TGF-ß signaling in CD8+ T cells had no effect on therapeutic responses. In contrast, TGF-ß blockade significantly improved therapeutic efficacy of Reo&CD3-bsAb in mice bearing MC38 colon tumors, resulting in a 100% complete response. Further understanding of the factors that determine this intertumor dichotomy is required before TGF-ß inhibition can be exploited as part of viroimmunotherapeutic combination strategies to improve their clinical benefit. Significance: Blockade of the pleiotropic molecule TGF-ß can both improve and impair the efficacy of viro-immunotherapy, depending on the tumor model. While TGF-ß blockade antagonized Reo&CD3-bsAb combination therapy in the KPC3 model for pancreatic cancer, it resulted in 100% complete responses in the MC38 colon model. Understanding factors underlying this contrast is required to guide therapeutic application.

Preinduced reovirus-specific T-cell immunity enhances the anticancer efficacy of reovirus therapy.

BACKGROUND: Many solid tumors do not respond to immunotherapy due to their immunologically cold tumor microenvironment (TME). We and others found that oncolytic viruses (OVs), including reovirus type 3 Dearing, can enhance the efficacy of immunotherapy by recruiting CD8+ T cells to the TME. A significant part of the incoming CD8+ T cells is directed toward reovirus itself, which may be detrimental to the efficacy of OVs. However, here we aim to exploit these incoming virus-specific T cells as anticancer effector cells. METHODS: We performed an in-depth characterization of the reovirus-induced T-cell response in immune-competent mice bearing pancreatic KPC3 tumors. The immunodominant CD8+ T-cell epitope of reovirus was identified using epitope prediction algorithms and peptide arrays, and the quantity and quality of reovirus-specific T cells after reovirus administration were assessed using high-dimensional flow cytometry. A synthetic long peptide (SLP)-based vaccination strategy was designed to enhance the intratumoral frequency of reovirus-specific CD8+ T cells. RESULTS: Reovirus administration did not induce tumor-specific T cells but rather induced high frequencies of reovirus-specific CD8+ T cells directed to the immunodominant epitope. Priming of reovirus-specific T cells required a low-frequent population of cross-presenting dendritic cells which was absent in Batf3-/- mice. While intratumoral and intravenous reovirus administration induced equal systemic frequencies of reovirus-specific T cells, reovirus-specific T cells were highly enriched in the TME exclusively after intratumoral administration. Here, they displayed characteristics of potent effector cells with high expression of KLRG1, suggesting they may be responsive against local reovirus-infected cells. To exploit these reovirus-specific T cells as anticancer effector cells, we designed an SLP-based vaccination strategy to induce a strong T-cell response before virotherapy. These high frequencies of circulating reovirus-specific T cells were reactivated on intratumoral reovirus administration and significantly delayed tumor growth. CONCLUSIONS: These findings provide proof of concept that OV-specific T cells, despite not being tumor-specific, can be exploited as potent effector cells for anticancer treatment when primed before virotherapy. This is an attractive strategy for low-immunogenic tumors lacking tumor-specific T cells.

IL-6 signaling in macrophages is required for immunotherapy-driven regression of tumors.

BACKGROUND: High serum interleukin (IL-6) levels may cause resistance to immunotherapy by modulation of myeloid cells in the tumor microenvironment. IL-6 signaling blockade is tested in cancer, but as this inflammatory cytokine has pleiotropic effects, this treatment is not always effective. METHODS: IL-6 and IL-6R blockade was applied in an IL-6-mediated immunotherapy-resistant TC-1 tumor model (TC-1.IL-6) and immunotherapy-sensitive TC-1. CONTROL: Effects on therapeutic vaccination-induced tumor regression, recurrence and survival as well on T cells and myeloid cells in the tumor microenvironment were studied. The effects of IL-6 signaling in macrophages under therapy conditions were studied in Il6rafl/fl×LysMcre+ mice. RESULTS: Our therapeutic vaccination protocol elicits a strong tumor-specific CD8+ T-cell response, leading to enhanced intratumoral T-cell infiltration and recruitment of tumoricidal macrophages. Blockade of IL-6 signaling exacerbated tumor outgrowth, reflected by fewer complete regressions and more recurrences after therapeutic vaccination, especially in TC-1.IL-6 tumor-bearing mice. Early IL-6 signaling blockade partly inhibited the development of the vaccine-induced CD8+ T-cell response. However, the main mechanism was the malfunction of macrophages during therapy-induced tumor regression. Therapy efficacy was impaired in Il6rafl/fl×LysMcre+ but not cre-negative control mice, while no differences in the vaccine-induced CD8+ T-cell response were found between these mice. IL-6 signaling blockade resulted in decreased expression of suppressor of cytokine signaling 3, essential for effective M1-type function in macrophages, and increased expression of the phagocytic checkpoint molecule signal-regulatory protein alpha by macrophages. CONCLUSION: IL-6 signaling is critical for macrophage function under circumstances of immunotherapy-induced tumor tissue destruction, in line with the acute inflammatory functions of IL-6 signaling described in infections.

Interleukin-6-mediated resistance to immunotherapy is linked to impaired myeloid cell function.

High serum levels of interleukin-6 (IL-6) correlate with poor prognosis and chemotherapy resistance in several cancers. The underlying mechanisms and its effects on immunotherapy are largely unknown. To address this, we developed a human papillomavirus type 16 (HPV16)-associated tumor model expressing IL-6 to investigate the impact of tumor-expressed IL-6 during cisplatin chemotherapy and HPV16 synthetic long peptide vaccination as immunotherapy. The effects of tumor-produced IL-6 on tumor growth, survival and the tumor microenvironment were analyzed. Our data demonstrated that tumor-produced IL-6 conferred resistance to cisplatin and therapeutic vaccination. This was not caused by a changed in vitro or in vivo growth rate of tumor cells, or a changed sensitivity of tumor cells to chemotherapy or T-cell-mediated killing. Furthermore, no overt differences in the frequencies of tumor-infiltrating subsets of T cells or CD11b+ myeloid cells were observed. IL-6, however, affected the systemic and local function of myeloid cells, reflected by a strong reduction of major histocompatibility complex (MHC) class II expression on all major myeloid cell subtypes. Resistance to both therapies was associated with a changed intratumoral influx of MHC class II+ myeloid cells toward myeloid cells with no or lower MHC class II expression. Importantly, while these IL-6-mediated effects provided resistance to the immunotherapy and chemotherapy as single therapies, their combination still successfully mediated tumor control. In conclusion, IL-6-mediated therapy resistance is caused by an extrinsic mechanism involving an impaired function of intratumoral myeloid cells. The fact that resistance can be overcome by combination therapies provides direction to more effective therapies for cancer.

Lack of myeloid cell infiltration as an acquired resistance strategy to immunotherapy.

BACKGROUND: Immunotherapy of cancer is successful but tumor regression often is incomplete and followed by escape. Understanding the mechanisms underlying this acquired resistance will aid the development of more effective treatments. METHODS: We exploited a mouse model where tumor-specific therapeutic vaccination results in tumor regression, followed by local recurrence and resistance. In depth studies on systemic, local and tumor intrinsic changes were performed with flow and mass cytometry, immunohistochemistry, transcriptomics and several perturbation studies with inhibitors or agonistic antibodies in mice. Main findings were recapitulated in vaccinated patients. RESULTS: Full tumor regression and cure of tumor-bearing mice is dependent on the magnitude of the vaccine-induced T-cell response. Recurrence of tumors did not involve classical immune escape mechanisms, such as antigen-presentation alterations, immune checkpoint expression, resistance to killing or local immune suppression. However, the recurrent tumors displayed a changed transcriptome with alterations in p53, tumor necrosis factor-α and transforming growth factor-ß signaling pathways and they became immunologically cold. Remarkably, ex vivo cell-sorted recurrent tumors, directly reinjected in naïve hosts retained their resistance to vaccination despite a strong infiltration with tumor-specific CD8+ T cells, similar to that of vaccine-responsive tumors. The influx of inflammatory mature myeloid effector cells in the resistant tumors, however, was impaired and this turned out to be the underlying mechanisms as restoration of inflammatory myeloid cell infiltration reinstated the sensitivity of these refractory tumors to vaccination. Notably, impaired myeloid cell infiltration after vaccination was also associated with vaccine resistance in patients. CONCLUSION: An immunotherapy-induced disability of tumor cells to attract innate myeloid effector cells formed a major mechanism underlying immune escape and acquired resistance. These data not only stresses the importance of myeloid effector cells during immunotherapy but also demands for new studies to harness their tumoricidal activities.