Abstract LB-272: Histidine decarboxylase (Hdc)-expressing myeloid cells support Foxp3+ Treg cells and promote colorectal cancer progression
Authors: Chen Xiaowei , Takemoto Yoshihiro , Nagar Karan K. , Chu Timothy H. , Jiang Zhengyu , Chang Wenju , Friedman Richard A. , Tailor Yagnesh H. , Worthley Daniel L. , Wang Timothy C. 
Published: July 2016

Abstract
The tumor microenvironment contains a diverse population of myeloid cells that are recruited from bone marrow and converted to immunosuppressive cells, thus mediating tumor cell escape from immune checkpoint. We have identified a subset of dynamic bone marrow myeloid cells, which can be identified by histidine decarboxylase (Hdc) mRNA and GFP expression in Hdc-GFP transgenic mice. Gene expression profiling showed that Hdc-GFP+ CD11b+Gr1+ myeloid cells express higher levels of cell cycle promoting genes such as Ccnd2, Ccnd3 and Cdc14a, while cell proliferation repressors including Cdc14b, Cdc16 and Cdk4 were downregulated in Hdc-GFP+ myeloid cells.
To further elucidate the role of Hdc+ myeloid cells in tumor progression, we performed lineage-tracing studies using Hdc-creERT2;R26-LSL-TdTomato reporter mice. Pulsed with tamoxifen, the majority of TdTomato+ cells were localized initially to a group of CD11b+Gr1+ myeloid cells representing the highest Hdc mRNA expression in bone marrow, spleen and blood. Later on, TdTomato+CD11b+Gr1- F4/80+ macrophages can be detected, indicating a hierarchy of Hdc+ myeloid cells in which Hdc+CD11b+Gr1+ myeloid cells reside at the apex. In general, CD11b+ myeloid cells have a relative short lifespan (&lt;2 weeks); however, treatment with dextran sodium sulfate (DSS) in the drinking water increased the longevity and frequency of Hdc+ myeloid cells, also promoted the differentiation of Gr1+ immature myeloid cells into neutrophils and macrophages.
In a mouse model of azoxymethane (AOM)/DSS colorectal carcinogenesis, Hdc-creERT2;R26-LSL-TdTomato;R26-LSL-DTA mice were injected with 10 mg/kg AOM and followed by 3 circles of 10 days 1.5% DSS ad libitum in drinking water. We found that Hdc-TdTomato labeled a proportion of tumor infiltrating CD11b+Gr1+ myeloid cells that expressed higher levels of Arg-1, Cox2, and Pdl1 transcripts. Continuous depletion of Hdc+ myeloid cells by administration of tamoxifen chow to induce DTA (diphtheria toxin subunit A) killing in Hdc-expressing myeloid cells abrogated half of the tumor-infiltrating MDSCs and released tumoricidal CD8+ T cells (&gt; 15-fold), leading to decreased tumor number. This tumor suppression could be rescued by Hdc+ CD11b+Gr1+ cell adoptive transfer. Serum chemokine profiling revealed that Hdc+ DTA mediated myeloid depletion also decreased serum chemokine levels, among which Cxcl13 decreased the most (&gt;30-fold). Cxcl13 protein and Cxcl13 mRNA also decreased in the colonic tumor tissue in the Hdc+ myeloid cell depleted AOM/DSS treatment group. Along with reduction in Cxcl13 levels, we also detected a significant reduction of Foxp3+ Treg cells in the tumor frozen sections stained with antibody against Foxp3 compared to R26-LSL-TdTomato;R26-LSL-DTA controls received the same treatments. Pre-knockdown of Cxcl13 by Dicer-substrate SiRNA (DsiRNAs) in a co-culture of splenic Hdc+ CD11b+Gr1+ myeloid cells from colon tumor-bearing mice with splenic Foxp3-GFP+ Treg cells induced apoptosis and decreased numbers of GFP+ cells compared to the scramble knockdown control group.
Taken together, our data suggest that Hdc marks a distinct subset of myeloid cells with greater potency for promoting tumorigenicity, in part through supporting Tregs and suppressing CD8+ Tcells in the tumor microenvironment.
Citation Format: Xiaowei Chen, Yoshihiro Takemoto, Karan K. Nagar, Timothy H. Chu, Zhengyu Jiang, Wenju Chang, Richard A. Friedman, Yagnesh H. Tailor, Daniel L. Worthley, Timothy C. Wang. Histidine decarboxylase (Hdc)-expressing myeloid cells support Foxp3+ Treg cells and promote colorectal cancer progression. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr LB-272.