Tumor-associated monocytes and macrophages protect and promote tumor-initiating stem cell proliferation upon being recruited to the niche

A common side-effect of chemoradiotherapy of colorectal cancer (CRC) results in expansion of therapy-resistant cells that are enriched in cancer stem cells (CSCs). Recent reports have highlighted the unique capacity of CSCs in immune resistance. However, immune cells can also support tumor growth, but how this occurs is poorly understood. We conducted single cell RNA sequencing (scRNA-seq) of adenomas derived from APCMin/+ mice to dynamically monitor therapy-induced transcriptomic changes. In the slow-cycling cell population that survived therapy and supported tumor growth, we found the genes Ascl2 and Krt15 to be typically expressed, and thus marked “therapy-resistant tumor-initiating CSCs” (TrCSCs). Furthermore, using serial block electronic microscopy to examine the adenoma microenvironment, we uncovered that innate immune myeloid derived cells (MDCs) were recruited to the TrCSC niche. Within this niche, MDCs were subsequently converted into tumor associated macrophages (TAMs), which promoted TrCSC proliferation through the PGE2-stimulated Akt and b-Catenin signaling. Inhibition of the PGE2 pathway reduced the ability of MDCs to promote adenoma-organoid growth ex vivo. Inhibition or depletion of TAMs resulted in reduced CSCs and attenuated tumorigenesis. In human CRC, the interaction of MDCs with Krt15+ or Ascl2+ CSCs coincided with activation of TrCSCs. Taken together, our work uncovers how dynamic changes in the immune microenvironment actively promotes CSC survival and regeneration, which we also demonstrate a clinically-applicable strategy to prevent.