Background & Aims Immunotherapies that induce T-cell responses have shown efficacy against some solid malignancies in patients and mice, but these have little effect on pancreatic ductal adenocarcinoma (PDAC). We investigated whether the ability of PDAC to evade T-cell responses induced by immunotherapies results from the low level of immunogenicity of tumor cells, the tumor's immunosuppressive mechanisms, or both. Methods KrasG12D/+; Trp53R172H/+; Pdx-1-Cre (KPC) mice, which develop spontaneous PDAC, or their littermates (controls) were given subcutaneous injections of a syngeneic KPC-derived PDAC cell line. Mice were then given gemcitabine and an agonist of CD40 to induce tumor-specific immunity mediated by T cells. Some mice were also given clodronate-encapsulated liposomes to deplete macrophages. Tumor growth was monitored. Tumor and spleen tissues were collected and analyzed by histology, flow cytometry, and immunohistochemistry. Results Gemcitabine in combination with a CD40 agonist induced T cell-dependent regression of subcutaneous PDAC in KPC and control mice. In KPC mice given gemcitabine and a CD40 agonist, CD4+ and CD8+ T cells infiltrated subcutaneous tumors, but only CD4+ T cells infiltrated spontaneous pancreatic tumors (not CD8+ T cells). In mice depleted of Ly6Clow F4/80+ extra-tumor macrophages, the combination of gemcitabine and a CD40 agonist stimulated infiltration of spontaneous tumors by CD8+ T cells and induced tumor regression, mediated by CD8+ T cells. Conclusions Ly6Clow F4/80+ macrophages that reside outside of the tumor microenvironment regulate infiltration of T cells into PDAC and establish a site of immune privilege. Strategies to reverse the immune privilege of PDAC, which is regulated by extra-tumor macrophages, might increase the efficacy of T cell immunotherapy for patients with PDAC.

Purpose Local tumor growth is a major cause of morbidity and mortality in nearly 30% of patients with pancreatic ductal adenocarcinoma (PDAC). Radiotherapy (RT) is commonly used for local disease control in PDAC, but efficacy is limited. We studied the impact of selectively intervening on RT-induced inflammation as an approach to overcome resistance to RT in PDAC. Experimental Design PDAC cell lines derived from primary pancreatic tumors arising spontaneously in KrasLSL-G12D/+;Trp53LSL-R172H/+;Pdx-1 Cre (KPC) mice were implanted into syngeneic mice and tumors were focally irradiated using the Small Animal Radiation Research Platform (SARRP). We determined the impact of depleting T cells and Ly6C+ monocytes as well as inhibiting the chemokine CCL2 on RT efficacy. Tumors were analyzed by flow cytometry and immunohistochemistry to detect changes in leukocyte infiltration, tumor viability and vascularity. Assays were performed on tumor tissues to detect cytokines and gene expression. Results Ablative RT alone had minimal impact on PDAC growth but led to a significant increase in CCL2 production by tumor cells and recruitment of Ly6C+CCR2+ monocytes. A neutralizing anti-CCL2 antibody selectively inhibited RT-dependent recruitment of monocytes/macrophages and delayed tumor growth but only in combination with RT (p<0.001). This anti-tumor effect was associated with decreased tumor proliferation and vascularity. Genetic deletion of CCL2 in PDAC cells also improved RT efficacy. Conclusions PDAC responds to RT by producing CCL2, which recruits Ly6C+CCR2+ monocytes to support tumor proliferation and neovascularization after RT. Disrupting the CCL2-CCR2 axis in combination with RT holds promise for improving RT efficacy in PDAC.

Inflammation mediated by activation of Janus kinase/signal transducer and activator of transcription (JAK/STAT) signaling is a major cause of chemotherapy resistance in cancer. We studied the impact of selectively blocking the IL-6 receptor (IL6R) as a strategy to inhibit IL-6-induced STAT activation and to overcome chemoresistance in pancreatic ductal adenocarcinoma (PDAC). To do this, STAT activation was investigated in tumors arising spontaneously in LSL-KrasG12D/+;LSL-Trp53R172H/+;Pdx-1Cre (KPC) mice. Plasma from patients with PDAC was assessed for its ability to activate STAT3/SOCS3 in human monocytes using immunofluorescence microscopy and quantitative gene expression assays. KPC mice and syngeneic mice (wild-type and IL6−/−) implanted with KPC-derived cell lines were treated with an IL6R blocking antibody (anti-IL6R). The impact of treatment on tumor growth in KPC mice and mice with KPC-derived tumor implants was monitored using ultrasonography and calipers, respectively. Tumors were analyzed by immunohistochemistry to detect changes in STAT activation, tumor viability and proliferation. We found that STAT3 was the most activated STAT protein in PDAC tumors from KPC mice. Plasma from patients with advanced PDAC stimulated STAT3/SOCS3 activation in human monocytes. In mice, anti-IL6R antibodies targeted Ly6Chi monocytes, inhibited STAT3 activation in tumor cells and decreased tumor cell proliferation in vivo. IL6R blockade in combination with chemotherapy induced tumor cell apoptosis, tumor regressions and improved overall survival. Overall, we show that IL-6 signaling drives STAT3 activation in tumor cells and mediates chemoresistance in PDAC. Thus, disrupting IL-6 signaling using anti-IL6R antibodies holds promise for improving chemotherapy efficacy in PDAC.

Activation-induced deaminase (AID), a member of the larger AID/ APOBEC family, is the key catalyst in initiating antibody somatic hypermutation and class-switch recombination. The DNA deamination model accounting for AID's functional role posits that AID deaminates genomic deoxycytosine bases within the immunoglobulin locus, activating downstream repair pathways that result in antibody maturation. Although this model is well supported, the molecular basis for AID's selectivity for DNA over RNA remains an open and pressing question, reflecting a broader need to elucidate how AID/APOBEC enzymes engage their substrates. To address these questions, we have synthesized a series of chimeric nucleic acid substrates and characterized their reactivity with AID. These chimeric substrates feature targeted variations at the 2′-position of nucleotide sugars, allowing us to interrogate the steric and conformational basis for nucleic acid selectivity. We demonstrate that modifications to the target nucleotide can significantly alter AID's reactivity. Strikingly, within a substrate that is otherwise DNA, a single RNA-like 2′-hydroxyl substitution at the target cytosine is sufficient to compromise deamination. Alternatively, modifications that favor a DNA-like conformation (or sugar pucker) are compatible with deamination. AID's closely related homolog APOBEC1 is similarly sensitive to RNA-like substitutions at the target cytosine. Inversely, with unreactive 2′-fluoro-RNA substrates, AID's deaminase activity was rescued by introducing a trinucleotide DNA patch spanning the target cytosine and two nucleotides upstream. These data suggest a role for nucleotide sugar pucker in explaining the molecular basis for AID's DNA selectivity and, more generally, suggest how other nucleic acid-modifying enzymes may distinguish DNA from RNA.protein nucleic acid interactions | DNA cytosine deamination

IntroductionMyeloid-derived suppressor cells (MDSCs) are known to support the progression of multiple types of cancer through immunosuppression, angiogenesis, tumor cell survival, and metastasis (1) and by activating fibroblasts (2) in the tumor microenvironment (TME). MDSCs are separated into 2 groups based upon their histological characteristics: polymorphonuclear MDSCs (PMN-MDSCs) are similar to neutrophils, while monocytic MDSCs (M-MDSCs) share phenotypic characteristics of monocytes. Both PMN-and M-MDSCs have been implicated in the pathogenesis of human cancer (3), where they function not only within the primary tumor, but also in peripheral lymphoid organs (4).We have previously reported high expression levels of the ectoenzyme CD38 on the surface of MDSCs (particularly M-MDSCs) in a murine model of esophageal cancer (5), and expression of CD38 on MDSCs has been reported in multiple myeloma (6). It should be noted that CD38 is also expressed on T cells, B cells, and NK cells (7). Since there are several monoclonal antibodies targeting human CD38 available (8-10), with daratumumab already being approved by the FDA for treatment of multiple myeloma (11), we set out to characterize CD38 + MDSCs in colorectal cancer (CRC). BACKGROUND.Myeloid-derived suppressor cells (MDSCs) are a population of immature immune cells with several protumorigenic functions. CD38 is a transmembrane receptor-ectoenzyme expressed by MDSCs in murine models of esophageal cancer. We hypothesized that CD38 could be expressed on MDSCs in human colorectal cancer (CRC), which might allow for a new perspective on therapeutic targeting of human MDSCs with anti-CD38 monoclonal antibodies in this cancer.