Metabolic reprogramming of skin microenvironment for improved BCG vaccine efficacy
Led by Titular Prof Francisco Javier Sánchez-García (instituto Politécnico Nacional, Mexico), with Asst Prof Steven Smith (Brunel University, UK), Dr Barbara Kronsteiner-Dobramysl (University of Oxford, UK) and Prof Hazel Dockrell (LSHTM, UK)
In theory, every single infectious disease could be eliminated from the earth if good vacccines (plus appropriate social and public health policies) are developed. In this scenario the best example is the vaccine against smallpox that lead to complete eradication of the disease by the year 1980.
Other vaccines, however, have not been so successful, notoriously BCG. This vaccine, first used in humans in 1921, has greatly contributed to reducing TB, but still, TB remains the world's top infectious cause of mortality.
In addition, BCG has proved to be protective in some countries but not in others.
Why is that so? Several hypotheses have been put forward and, for practical purposes, this has conducted researchers to try to develop "better anti-tuberculosis vacicnes".
Here, we propose a different approach: "current BCG is an excellent vaccine, it contains most of the potentially protective antigens. Moreover, it contains its own natural adjuvants; the problem is that we have failed to deliver it into the appropriate metabolic microenvironment".
Recent research shows that the quality of immune cell effector functions are determined by their particular metabolism at the time (i.e. how they produce energy), and that the presence of selected metabolites is capable of modifying cell immune responses locally, such as in solid tumors.
We propose to reprogramme the local cell metabolism at the site where the BCG vaccine will be delivered, to ensure the best immune cell activation and consequently, the highest protection.
This project originated from the need to stop TB and from the fact that the BCG vaccine, used in humans since 1921, has proved to be protective against TB in some countries but not in others. We asked ourselves: why is that so? While BCG fulfils all the criteria of an excellent vaccine, containing most of the potentially protective antigens and its own natural adjuvants, we hypothesized that so far it has not been delivered into the appropriate metabolic microenvironment.
This idea is supported by the recent finding that the quality of immune cell effector functions is largely determined by their particular metabolism at the time, and that the presence of selected metabolites is capable of modifying cell immune responses locally, such as in solid tumours.
We proposed to metabolically pre-condition the local tissue microenvironment to potentiate antigen uptake/processing and trafficking upon the administration of the current BCG vaccine, in order to improve anti-TB protection.
We selected five metabolites to be tested: lactate, succinate, fumarate acetate and butyrate, all of which harbour cell signalling properties. Based on our preliminary results, all were used at a final concentration of 100 µM.
Firstly, we found structural and functional changes of mitochondria from human monocyte-derived macrophages in response to in vitro treatment with the aforementioned metabolites (100 µM). Fumarate induced mitochondrial fusion, polarization, and changes in mitochondrial cristae; in contrast, succinate, lactate and butyrate caused mitochondrial fission and depolarization. All the metabolites tested triggered an increase in cytoplasmic and mitochondrial calcium concentration within a few seconds, and fumarate promoted the highest calcium influx to the cytoplasm whereas the mitochondrial calcium concentrations, following metabolite treatment, were similar for all the metabolites tested.
Furthermore, extracellular flux analysis of human monocytes demonstrated changes in the use of energy pathways upon metabolite treatment. Succinate treatment resulted in an immediate increase of extracellular acidification rate (ECAR), indicative of increased glycolysis, whereas no effect was observed for the other metabolites. Overnight pre- treatment with succinate led to a reduction in mitochondrial ATP production compared to vehicle, whereas fumarate treatment had the opposite effect.
We then exposed human peripheral blood mononuclear cells (PBMC) to the different metabolites for 24 h and cultured the cells in the presence of BCG in order to assess mycobacterial growth inhibition by un-stimulated cells and by metabolite-stimulated cells. Results from six donors showed no statistically significant differences between treatments. Metabolite pre-treatment was able to change the expression of co-stimulatory (CD80, CD86) and co-inhibitory (PD-L1) molecules on the monocytic/macrophage cell line (RAW 264.7) upon stimulation with lipopolysaccharide (LPS), with succinate increasing activation as demonstrated by increased cell size and relative amount of CD80 compared to LPS alone. Preliminary data from in vivo mouse experiments demonstrate that combined injection (i.d.) of BCG and metabolite into the back of mice elicits histopathological changes and differences in immune cell composition as compared to BCG alone.
Overall, our study highlights benefits of metabolite pre-treatment on activation, metabolic properties and function of monocytes/macrophages and suggests that metabolite-induced tissue metabolic reprogramming is feasible.
S Prentice & HM Dockrell 2021. BCG-specific and non-specific effects: different questions, similar challenges. J Infect Dis. DOI: 10.1093/infdis/jiab307
HM Dockrell & E Butkeviciute 2021. Can what we have learnt about BCG vaccination in the last 20 years help us to design a better tuberculosis vaccine? Vaccine DOI: 10.1016/j.vaccine.2021.01.068
C Angélica Pérez-Hernández et al 2020. Mitochondrial signature in human monocytes and resistance to infection in C.elegans during fumarate-induced innate immune training. Frontiers in Immunology 11: 1715
S Prentice & HM Dockrell 2020. Antituberculosis BCG vaccination: more reasons for varying innate and adaptive immune responses. J Clin Invest. 130(10):5121-5123 DOI: 10.1172/JCI141317