Circadian rhythms are evolutionarily conserved mechanisms that exist in almost all organisms. They integrate environmental changes with internal physiology, allowing the synchronization of physiological mechanisms and behavior. In mammals, circadian rhythms are generated in the suprachiasmatic nucleus of the hypothalamus, which in turn distributes synchronization signals to the rest of the body via humoral cues and peripheral innervation. In the context of immunity, the peripheral nervous system, particularly the sympathetic nervous system, has been shown to control the circadian function of leukocytes, making this axis a potential target for the regulation of circadian immunity in a number of pathological conditions. For instance, previous studies from our lab and others showed that diurnal oscillations in leukocyte composition and distribution control the growth profile of tumors. However, whether sympathetic inputs are involved in the diurnal leukocyte composition of the tumor microenvironment and whether the sympathetic nervous system is involved in rhythmic anti-tumor activities remains unknown. In my PhD thesis, I aim to demonstrate the connection between the peripheral nervous system and the immune system, particularly in tumor progression.
In the first part of this thesis, I investigate how chemical and genetic denervation affects the rhythmicity of tumor growth. Interestingly, I demonstrate that genetic deletion of beta-2 adrenergic signaling reverses the time-of-day dependent tumor growth pattern. The abundance of tumor-infiltrating leukocytes correlates with this inverted tumor growth phenotype. Interestingly, by flow cytometry analysis of infiltrating leukocytes in wild-type in comparison with Adrb2 KO, I show that natural killer cells are one of the major leukocyte subsets affected, and their distribution correlate with the inversion pattern of tumor growth at later stages. Furthermore, I show that the inversion of tumor growth after depletion of NK cells in wild-type mice is comparable to the inversions in mice deficient in beta-2 adrenergic signaling. In addition, NK cells show oscillations in clock genes and cytotoxicity after synchronization ex vivo. Together, these data show that adrenergic signaling in NK cells is important for controlling tumor growth in a time-dependent manner and advocate for improved immunotherapies such as combinatorial therapies using adrenergic signaling modulators or CAR-NK cells.
In the second part of the thesis, I focus on imaging tumor-associated neuro-immune interactions using confocal microscopy in different murine tumor models. Specifically, I show the presence of sympathetic nerve fibers in the T-cell zone of lymph nodes in wild-type mice. Additionally, in a mouse model of a bladder tumor, I show a reduction in TReg cells at the primary bladder tumor site, where the tissue is rich in sensory innervation. These studies highlight the importance of understanding neuro-immune interactions both in lymphoid organs and in the tumor microenvironment to provide insight into neuro-immune control of tumor growth ultimately resulting in novel and/or improved therapeutic approaches.
