Abstract B035: CXCL8/CXCR1 signaling promotes angiogenesis and hematopoietic stem and progenitor cell function

CXCL8 (IL-8) is a chemokine with pleiotropic roles in host defense, angiogenesis and tumor metastasis. CXCL8 and its specific receptor, CXCR1, are broadly expressed within the hematopoietic and vascular systems. In an effort to identify novel secreted factors with effects on hematopoietic stem and progenitor cell (HSPC) function, we have recently identified CXCL8/CXCR1 signaling as a positive regulator of HSPC colonization of the zebrafish caudal hematopoietic territory (CHT). The CHT is a vascular niche that serves as the primary site of hematopoiesis from 36 hours post fertilization (hpf) to 6 days post fertilization (dpf). This observation raised the question whether CXCR1 signaling might induce dynamic changes in the CHT that favor HSPC colonization. CXCR1 was expressed at high levels in endothelial cells using a kdrl(VEGFR2):CXCR1;kdrl:mCherry double transgenic line. The CHT was imaged by fluorescence confocal microscopy, reconstructed in 3 dimensions and the volume measured using digital image analysis software. Overexpression of CXCR1 within the endothelial cells of these animals increased the volume of the CHT by 28% (p = 0.02). To understand how CXCR1 affects the dynamics of niche development, we globally overexpressed CXCR1 beginning at 36 hpf using a heat shock induction system and performed time lapse confocal microscopy from 52 to 72 hpf. This revealed that overexpression of CXCR1 consistently increased the CHT volume from 53 to 72 hpf compared to control (21% increase at 72 hpf, p = 0.004). These studies did not show whether CXCR1 acted directly on the vascular niche or whether CXCR1 expression in endothelial cells might induce expression of soluble factors or activate circulating cells that then cause expansion of the niche through indirect mechanisms. To address this, we created parabiotic zebrafish by fusing kdrl:mCherry embryos at 4 hpf. One half of each parabiotic animal was modified by DNA microinjection to globally overexpress CXCR1 or GFP as a control via heat shock induction at 36 and 48 hpf. The volume of the CHT was measured in each half of each parabiotic animal at 72 hpf. In control parabiotics overexpressing GFP, there was no difference in CHT volume between modified and unmodified sides of the organism. However, in parabiotics overexpressing CXCR1, the CHT of the modified side was 27% larger compared with the unmodified side (p = 0.012), consistent with our previous results and suggesting that CXCR1 acts directly on the niche in this system. We then asked whether this volume change could affect HSPC engraftment. Parabiotic animals were created using Runx1:mCherry embryos that carry an HSPC-specific reporter transgene as “donors” and WT embryos as “recipients”. The recipient niche was modified as before to overexpress CXCR1 or GFP as a control. At 72 hpf there was no difference in HSPC colonization of donor and recipient niches when the recipient niche expressed GFP. However, when the recipient niche expressed CXCR1, there was a significant increase in HSPC colonization of the recipient niche compared to the donor niche (11.4+/-2.4 vs 19.8+/-3.5 HSPCs per CHT, p = 0.02). Taken together, these results identify a novel role for CXCL8/CXCR1 signaling in angiogenesis and HSPC biology and they provide a new example of how innate immune signaling pathways are important for stem cell function. Administration of CXCL8 to hematopoietic stem cell transplant recipients may therefore improve HSPC engraftment and clinical outcomes in patients who are being treated for hematologic malignancies. Citation Format: Bradley W. Blaser, Jessica L. Moore, Elliott Hagedorn, Brian Li, Vera Binder, Owen Tamplin, Leonard I. Zon. CXCL8/CXCR1 signaling promotes angiogenesis and hematopoietic stem and progenitor cell function [abstract]. In: Proceedings of the Second CRI-CIMT-EATI-AACR International Cancer Immunotherapy Conference: Translating Science into Survival; 2016 Sept 25-28; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(11 Suppl):Abstract nr B035.