Abstract
Obesity is associated with poor outcomes in many cancers. However, retrospective studies have reported obesity (Body Mass Index; BMI>30) is associated with improved survival outcomes in patients receiving Immune Checkpoint Inhibitor immunotherapy. These studies raise intriguing questions about how obesity-related factors may be influencing host immunity and response to immunotherapy.
Recent studies have shown primary exogenous stimuli (e.g. fungal component β-glucan) induce metabolic and epigenetic changes in innate immune cells (i.e. monocytes) that reprogramme or ‘prime’ them to react with altered response to subsequent stimuli. This concept of ‘primed innate immunity’ further opens the possibility that cells of the innate immune system may be therapeutically manipulated prior to arrival at the tumour to improve immunotherapy outcome.
Previous studies have demonstrated that monocytes primed with either exogenous or endogenous stimuli exhibit altered cytokine secretion and surface marker expression profiles. However, the functional consequences of human monocyte priming by endogenous stimuli in a tumour context remain to be investigated. In this study, monocytes exposed to primary stimuli β-glucan or oxLDL were found to exhibit altered phenotype and/or functional activity in response to secondary stimulation (LPS, PMA or SK-MEL-28 melanoma cells). Monocytes primed with β-glucan showed upregulated surface marker TLR4 presentation following secondary LPS challenge, and increased CD11b and PD-L1 presentation in response to secondary SK-MEL-28 stimulation. In contrast, monocytes primed with β-glucan showed downregulated IL1β production after secondary stimulation by LPS, and reduced ROS production following secondary stimulation by PMA. Monocytes primed with oxLDL also exhibited an increased rate of superoxide (SO) production and enhanced matrix metalloprotease (MMP)9 activity. These changes in function could enhance immunotherapy outcome through potentially promoting anti-tumour immune responses, but this remains to be validated.
Metabolic and histone epigenetic modifications are well-documented mechanisms underlying innate priming. However, limited studies have investigated priming induced changes in DNA methylation, and/or assessed intracellular pathways that remain activated within monocytes 5 days after primary stimulation. In the current study, bulk RNAseq analysis of non-primed monocytes and monocytes primed with 17β-estradiol or oxLDL showed differential gene expression related to immune function, metabolism and epigenetic modification. NMR analysis corroborated RNAseq data and demonstrated that monocytes primed with oxLDL had altered intracellular metabolic pathways relevant to immune function, energy production and epigenetic regulation. Moreover, mass spectrometry showed that monocytes primed with β-glucan or oxLDL exhibited modified levels of global DNA methylation. Together, these findings emphasise changes in the TCA cycle and cholesterol synthesis pathway, which may modulate the epigenome and cellular function. In addition, this study provides the first direct evidence that DNA methylation contributes towards endogenous innate priming of monocytes by endogenous stimuli.
To explore the relevance of obesity-related endogenous compounds to immunotherapy response, blood was collected from individuals receiving anti-PD-1 immunotherapy (Pembrolizumab) for the treatment of advanced cancers prior to ICI drug infusions 1-3. In the clinic, serum concentrations of carnosine, tyrosine, citrulline, ethanolamine, phenylalanine and oxLDL were present at greater levels prior to first immunotherapy infusion in patients who would go on to discontinue treatment due to progressive disease or adverse events before the third infusion. These results provide preliminary evidence of novel biomarkers that could be used to predict early immunotherapy response. The association observed in this study between high circulating levels of oxLDL and early discontinuation of therapy may be of particular interest due to the identified role of oxLDL in driving innate priming. However, further research will be necessary to understand and validate the relationship between endogenous priming and therapeutic outcome.
In conclusion, this study has shown that the endogenous compounds 17β-estradiol and oxLDL can prime monocytes, modifying transcriptomic, epigenetic and/or metabolic profiles that could influence response to secondary challenge. However, 17β-estradiol priming did not lead to measurable changes in monocyte functional activity, and as oxLDL concentrations were not correlated with BMI, endogenous monocyte priming could not be identified as an influencing factor to the improved ICI response observed in obese patients. Nevertheless, this study provides further evidence that obesity-related endogenous compounds can prime monocytes and establishes a foundation for future research into the role of metabolites in innate priming within the cancer setting.