Tumors and Ly6Chigh Monocytes

Abstract

Cancer immunotherapy represents a treatment strategy which is being clinically tested to complement surgery, radiotherapy and chemotherapy – the current cornerstones of our fight against cancer. It has become clear now, that tumors not only escape immune recognition but also actively suppress antitumor immune responses. In order to improve cancer immunotherapy, effective manipulation of the immune system to break self-tolerance need to be designed and approaches that counteract immunosuppressive mechanisms need to be developed. The tumor microenvironment encompasses a wide variety of immune cells, which macrophages comprise the most dominant portion of them and thus are the major players in the connection between inflammation and cancer. These tumor-associated macrophages (TAMs) are derived from blood monocyte precursors and subsequently acquire distinct characteristics as a result of tumor micro-environmental cues. Monocytes are a heterogeneous population in the blood with an enormous plasticity whose fate and functions are dictated by the microenvironment. Therefore, the roles of specific Monocytes subsets in tumor progression and the molecular mechanisms for their impacts need to be elucidated. Ly6Chigh and Ly6Clow are two main different types of murine monocytes subsets that have been defined by distinct phenotypes and immunoregulatory functions. Recent data demonstrates that Ly6Chigh monocytes can recruit to inflammation loci while Ly6Clow monocytes are patrolling cells. We have developed a method to produce large number of Ly6Chigh monocytes in vitro. In our study we observed that, injection of these cells affects tumor progression in breast cancer and C26 colon carcinoma. Activation of Ly6Clow monocytes by pro- or anti-inflammatory cytokines, results in a genetic expression profile, corresponding to pro- or anti-inflammatory genes, respectively. Injection of pre- activated Ly6Clow monocytes toward pro- or anti-inflammatory polarization in C26 colon carcinoma showed that anti-inflammatory activated monocytes are more beneficial in delaying cancer cachexia. Increased knowledge of monocytes improves the chances to find therapies against a broad spectrum of diseases including cancer, where monocytes have opposing roles of either being beneficial or detrimental to the host.

Using C26 cancer model we were interested to study the impact of activation of Ly6Chigh monocytes toward pro or anti-inflammatory phenotype on the progression of cancer. We conducted the C26 cancer model injecting Ly6Chigh monocytes treated with IL-4 or INF-. before injection. Among the groups of treatment, Ly6Chigh monocytes activated with IL-4 were the most beneficial on cancer progression, since they had the highest survival rate and the least tumor volume rise among all groups. Since the whole cachectic muscles are inflamed and injured in C26-bearing mice and activated Ly6Chigh monocytes injected in this cancer model apart from recruiting to tumor site have played substantial role by affecting cachectic muscle repair.

In summary, we defined a new regulatory role of recruiting Ly6Chigh monocytes in cancer, which might be clinically relevant in developing novel immunotherapeutic strategies. Although, underlying mechanism by which Ly6Chigh monocytes influences the tumor progression have yet to be established and it requires further studies to characterize the phenotype of these cells after recruitment in cancer. So far, since the inflammatory genes involved in tumor progression were differently regulated in tumors infiltrated with Ly6Chigh monocytes, our hypothesis is that the recruitment of Ly6Chigh monocytes, alter the balance of pro-inflammatory/anti-inflammatory pool of macrophages in the cancer and this is the main reason why modulation impacts occur in this study. While pro-inflammatory macrophages will be able to induce wound hilling and revascularization, the anti-inflammatory macrophages will block the tumor growth through the production of fibrosis.