Cellular and molecular dynamics of healthy ageing in the human Haematopoietic Stem Cell compartment

Blood comprises more than a dozen distinct cell types which have vital functions and are constantly replaced, at a rate of 30 millions cells per minute in humans. This extreme regenerative capacity can be maintained thanks to haematopoietic stem cells (HSC), rare cells that have the unique ability to produce more of themselves as well as all blood cell types. Interestingly there isn't a single type of HSC but rather HSC come in different flavours (subtypes), which vary in their capacity to divide and have preferences for production of particular differentiated blood cell types. Importantly it is thanks to this heterogeneity in the HSC compartment that blood production can be maintained under normal conditions or after injury or infection.

The unique potential of HSC is exploited in bone marrow transplantation, a procedure routinely used in the clinic to treat leukaemia and other conditions. HSC are harvested from different haematopoietic organs depending on the clinical case. There is evidence that the source of HSC can affect the outcome of the transplantation, but why is not currently understood. In addition, work with animal models has demonstrated that HSC function is compromised with age. Correspondingly, in elderly people there are many changes in the composition of the blood and an increased incidence of infections, anaemia and blood cancers. However despite their importance for human health and for treatment, almost nothing is known to date on the composition of the human HSC compartment in different organs, how it changes with ageing, and what are the consequences for blood production.

This project will use a number of innovative single cell analysis techniques and computational methods to compare the composition of the healthy human HSC compartment in different haematopoietic organs: neonatal cord blood as well as adult bone marrow, peripheral blood and spleen. For each organ we will define how many HSC subsets there are, and identify their gene expression profiles, epigenetic features and functional properties. We will also determine how these change with healthy ageing by analysing samples ranging from infants to 70 years old individuals. Single cell resolution is absolutely necessary to understand how HSC age. The information derived from this comprehensive study will characterise how human HSC, and consequently blood production, change over a lifetime. Our molecular characterisation will also provide a resource to develop new strategies to improve current transplantation protocols and to identify targets that may help to pharmacologically rejuvenate old HSC and correct some of the age-related blood deficiencies.

Haematopoietic stem cells (HSC) are essential to maintain blood supply throughout life and regenerate it in case of injury. The HSC compartment is not homogeneous: rather it is composed of distinct HSC subsets, with specific self-renewal, differentiation and cycling properties, whose concerted action is necessary for efficient and durable blood production. In mice, the balance of these HSC subsets changes from the embryo to adulthood to old age, when HSC function is compromised. In humans, healthy ageing is accompanied by a number of changes in blood cell types, and increased incidence of anaemia and leukaemias, many of which may originate from changes in the HSC compartment. However, despite the routine use of HSC for clinical transplantations, current knowledge of the composition of the human HSC pool is extremely limited, especially in adults.

This project couples state-of-the-art flow cytometry, single cell RNA-seq, epigenomics and stem cell assays to provide a global integrative view of the functional and molecular heterogeneity observed in the human healthy HSC compartment in different organs (cord blood, bone marrow, spleen and peripheral blood) and across different ages. In particular we will use index sorting technology and bioinformatics methods to associate single cell molecular programmes to novel combinations of cell surface markers and specific stem cell properties. This strategy will comprehensively define human HSC subsets, uncover HSC molecular programmes specific to each haematopoietic organ and determine how these programmes are perturbed during healthy ageing. This single cell resolution analysis is a necessary step to understand the dynamics of HSC maintenance and their impact on blood production over a human lifetime. It will inform on putative novel strategies to command mobilisation of human HSC for medical needs and/or possibly reverse some of their age-related deficiencies.

This proposal aims to understand how the composition of the human haematopoietic stem cell (HSC) compartment changes molecularly and functionally in different haematopoietic organs and at different stages of human life, and how this affects blood formation and function during healthy ageing.

The demographic shift to an older population worldwide poses a number of societal and healthcare challenges. One of these is how to help the soaring numbers of individuals suffering from chronic diseases and a generally decreased wellbeing. Elderly people suffer from complex age-related blood deficiencies (increased incidence of anaemia and of myeloid malignancies, changes in the immune system resulting in higher levels of infections and inflammation), many of which originate from impaired HSC activity. However there has been no high-resolution investigation of the changes in function and molecular properties of HSC that occur with age in humans. This information is an important prerequisite to implement novel strategies aimed at attenuating the detrimental effects of ageing on blood formation and function. Our findings will thus provide a very important resource to design new protocols and treatments to improve the quality of life of the elderly.

In addition, HSC transplantation is routinely used in the clinic, with more than 50,000 transplants being carried out each year worldwide, to treat haematopoietic cancers at early and late stages but also non-malignant disorders. HSC are being sourced depending on the clinical circumstances from several haematopoietic organs: adult bone marrow or peripheral blood from the patient, a related donor or a non-related donor, as well as cord blood. Based on the current clinical practice there is evidence that the organ from which HSC are harvested as well as the age of the donor, have an influence on the outcome of the transplantation. But systematic knowledge of these effects is lacking. Moreover, the biological basis for that is unclear and no study has rigorously compared the attributes of human HSC populations in different haematopoietic organs and at different ages. We expect that our findings will provide a basis to further refine current protocols and optimize HSC sources for particular clinical recommendations, providing healthcare benefits in the long-term.

Finally, there is considerable public interest in ageing and stem cell related issues. We therefore aim to disseminate our findings and their implications to a large lay audience, by participating in a number of outreach events with the innovative and interactive activities delineated in the Pathways to Impact section. We will place our own results in a larger context to educate children and adults on the basic principles of stem cell biology and their applications in particular towards countering the deleterious effects of ageing. As a young and dynamic team of researchers, we hope that participating in highly engaging events and encouraging public discourse will also allow us to inspire a new generation of scientists.