3193 – EXPRESSION OF CD61 MARKS FUNCTIONALLY DISTINCT POPULATIONS OF AGED HEMATOPOIETIC STEM CELLS

Abstract

Hematopoietic stem cells (HSCs) give rise to all hematopoietic cells throughout the lifespan of an organism. Upon ageing, murine HSCs gradually lose their self-renewal and regenerative potential, skew towards the myeloid lineage and expand in numbers. Also, the aged HSC pool becomes increasingly heterogeneous where some cells have remained ‘young-like'. Here we aimed to identify and isolate these ‘young-like' HSCs from aged mice to better understand the mechanisms that contribute to HSC ageing. We focused on CD61 (Integrin beta 3), a membrane-associated gene that we previously identified as one of the most consistently differentially expressed gene between young and old murine HSCs. CD61 plays a role in a variety of cellular processes, including cell adhesion and migration. We hypothesized that the level of CD61 expression on HSCs marks distinct populations of stem cells. We competitively transplanted 500 CD61High and 500 CD61Low young and aged long-term hematopoietic stem cells (LT-HSCs) alongside 2x106 competitor bone marrow cells into lethally irradiated recipients. Chimerism levels were analyzed by FACS every 4 weeks for 20 weeks. LT-HSCs from primary recipients were then competitively transplanted into secondary recipients to further investigate their functionality. In primary transplantation, young CD61High and CD61Low HSCs provided comparable engraftment. However, CD61High HSCs from aged donors resulted in, on average, 80% chimerism whereas the CD61Low HSCs provided only 20% chimerism. In contrast to primary transplantations, secondary transplantations with young CD61Low HSCs resulted in 20% lower chimerism compared to CD61High HSCs. Our data demonstrate that expression of CD61 becomes increasingly important as HSCs age. In addition, our data from secondary transplantation suggest that CD61 might mark intrinsically superior cells in the HSCs pool.