Do macrophages follow the beat of circadian rhythm in TIME (Tumor Immune Microenvironment)?

Abstract

Advances in cancer research have made clear the critical role of the immune response in clearing tumors. This breakthrough in scientific understanding was heralded by the success of immune checkpoint blockade (ICB) therapies such as anti-programmed cell death protein 1 (PD-1)/ programmed death-ligand 1 (PD-L1) and anti-cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), as well as the success of chimeric antigen receptor (CAR) T cells in treating liquid tumors. Thus, much effort has been made to further understand the role of the immune response in tumor progression, and how we may target it to treat cancer. Macrophages are a component of the tumor immune microenvironment (TIME) that can promote tumor growth both indirectly, by suppressing T cell responses necessary for tumor killing, as well as directly, through deposition of extracellular matrix and promotion of angiogenesis. Thus, understanding regulation of macrophages within the tumor microenvironment (TME) is key to targeting them for immunotherapy. However, circadian rhythms (24-hour cycles) are a fundamental aspect of macrophage biology that have yet to be investigated for their role in macrophage-mediated suppression of the anti-tumor immune response Circadian rhythms regulate macrophage-mediated immune responses through time-of-day-dependent regulation of macrophage function. A better understanding of the circadian biology of macrophages in the context of the TME may allow us to exploit synergy between existing and upcoming treatments and circadian regulation of immunity.

Keywords: Circadian; cancer; immunity; tumor immunology.

Figure 1.. Key aspects of macrophage function are under circadian regulation.

While the potential circadian regulation of various aspects of macrophage biology remain unknown, proteins downstream of pathogen sensing confer a circadian signature on the magnitude of cytokine secretion. KLF4, which promotes silent clearance of cells via a pro-resolution profile on macrophage following efferocytosis has also been found to be circadian. RhoA, which is key to phagocytosis, appears to be circadian-regulated, but whether phagocytosis is circadian-regulated remains unclear. Figure created with BioRender.com. KLF4, Krüppel-like factor 4; RhoA, Ras homolog family member A; PD-L1, programmed death-ligand 1; ARG1, Arginase 1; PAMP, pathogen-associated molecular patterns; DAMP, damage-associated molecular patterns; TLR, toll-like receptors; PD-1, programmed cell death protein 1; MHC II, major histocompatibility complex class II.

Figure 2.. Macrophages in tumorigenesis.

While tumor-associated macrophages are heterogeneous, they are overall immunosuppressive due to the tumor microenvironment. Hypoxia, lactate, and acidic pH in the tumor microenvironment promote pro-growth, tissue repair, and immunosuppressive functions of tumor-associated macrophages while suppressing their pro-inflammatory functions. What aspects of macrophage function in the tumor microenvironment remain under circadian regulation is an open question. Figure created with BioRender.com. ARG1, Arginase 1; MHC II, major histocompatibility complex class II; PD-L1, programmed death-ligand 1; ECM, extracellular matrix.

Figure 3.. Circadian expression of the checkpoint protein PD-1 by macrophages within the tumor and PD-1/PDL-1 blockade therapy efficacy.

In B16/BL6 (melanoma) tumor-bearing mice, the administration of PD-1/PD-L1 inhibitor during the dark phase (ZT16) results in higher anti-tumor effects when compared with the same treatment administered during the light phase (ZT8). Figure created with BioRender.com. PD-1, programmed cell death protein 1; PD-L1, programmed death-ligand 1.