Modulation of anti-tumor immunity by the brain's reward system

Welcome to our Monthly Journal Club! Each month I post a paper or two that I have read and find interesting. I use this as a forum for open discussion about the paper in question. Anyone can participate in the journal club, and provide comments/critiques on the paper. This month’s paper is “Modulation of anti-tumor immunity by the brain’s reward system” by Asya Rolls and colleagues at the Technion - Israel Institute of Technology. I will provide a brief overview of the techniques/approaches used to make it more understandable to potential non-expert readers. If I am not familiar with something, I’ll simply say so.

Figure 1: Using DREADDs to selectively manipulate VTA-Dopamine neurons in the context of cancer.

Discussion

In their paper, Rolls and colleagues used viral vectors encoding Cre-dependent Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) to investigate how the brain alters peripheral cancer growth. By injecting these viral constructs into the ventral tegmental area of tyrosine hydroxylase-Cre mice, specific DREADD expression within only VTA-dopamine neurons (TH+) was accomplished. Gq-coupled DREADDs allow for activation of these neurons by systemic injections of the inert* molecule clozapine-N-oxide (CNO).

This is a good approach for several reasons. (1) it allows for cell-type specific control, (2) the kinetics of CNO are well known, allowing for a good amount of temporal precision, (3) it allows for non-invasive (IP injection) control of the brain, without damaging or destroying the cells. Despite this, there are caveats, such as receptor desensitization from repeated administrations of CNO, and the daily stress of IP injections. These findings will have to be replicated using disparate techniques, like optogenetics, or a more ‘natural’ way to activate these cells, via a ‘CNO drinking’ protocol, or even social/sexual interaction which are known to activate VTA-DA neurons.

After achieving cell-type specific DREADD expression, the authors gave mice subcutaneous tumors (LLC or B16 cancer cells), and then gave them daily injections of CNO. Mice that were ‘VTA-activated’ had smaller tumors than control mice that did not express the DREADD in the VTA (as seen in Figure 1). To examine how this signal from the brain might reach the tumor, the authors ablated the sympathetic nervous system using 6-hydroxydopamine (6-OHDA), a neurotoxin which destroys adrenergic nerve terminals (distinguishing feature of the sympathetic nervous system). Mice that were SNS-ablated (or received a beta-adrenergic receptor antagonist) failed to show an effect of VTA-DA activation on tumor growth. They further showed that VTA activation altered norepinephrine concentrations specifically in the bone marrow, an important immune compartment. This strongly supports the hypothesis that VTA-DA neurons alter tumor growth via SNS innervation of the bone marrow.

Narrowing in on the bone marrow, they showed that VTA activation reduces the number of myeloid derived suppressor cells (MDSCs), which can promote tumor growth via immune suppression and the promotion of angiogenesis. MDSCs contained beta-2 adrenergic receptors, which made them sensitive to VTA-DA activation (through the SNS). Finally, adoptively transferring MDSCs from VTA-activated mice to control mice recapitulated the anti-tumor effect of VTA-activation. This suggests that modulation of the immune system via a discrete population of neurons within the brain acts (at least in part) to suppress tumor growth.

Figure 4: ‘VTA-activated’ myeloid derived suppressor cells (MDSCs) are necessary and sufficient to suppress tumor growth. Adoptive transfer of ‘activated’ MDSCs suppressed tumor growth in mice that had not been ‘VTA-activated’.

These findings are in line with early research showing rats with a hyperreactive dopaminergic system have reduced tumor growth, metastasis, and angiogenesis compared to control rats (Teunis et al., 2002). These results need to be confirmed through alternative methodology and cancer models, but this paper represents an exciting new target for peripheral cancer suppression (through modulation of the brain). Indeed, the authors acknowledged this possibility in their earlier paper showing that VTA-DA activation alters both adaptive and innate immunity, suggesting that this may (at least in part) be responsible for the ‘placebo effect’ (Ben-Shaanan et al., 2016).

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*Note: recent research has demonstrated that CNO’s action is likely through it’s metabolism to the bioactive molecule clozapine (Gomez et al., 2017).

Banner Image: VTA-dopamine neurons expressing a sgRNA against BMAL1 (Credit: JCB).