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Semaphorin 4A Mediates Inflammation Resistance of Myeloid-Biased HSC
Dorsa Toghani1, Elmir Mahammadov2, Negar Seyehassatehrani3 , Antonio Scialdone2, Joel Spencer3 and Lev Silberstein1
1Fred Hutchinson Cancer Center, Seattle, WA, USA,
2Helmholtz Institute for Epigenetics, Munich, Germany,
3University of
California Merced, CA, USA
Background: Inflammation significantly impairs hematopoietic stem cell (HSC) self-renewal. Despite this, HSC maintain
regenerative capacity throughout life suggesting that they are relatively resistant to inflammation-induced damage.
However, very few HSC “inflammation resistance” signals are currently known. This represents an important hurdle for
understanding not only normal hematopoietic regeneration but also leukemogenesis, since overactivation of normal
inflammation resistance pathways has been suggested as a potential mechanism for pre-malignant clonal expansion and
survival of AML blasts.
Aims: In the current study, we aimed to define a role of a newly-defined niche factor, Semaphorin 4A (Sema4A), in
conferring HSC resistance to acute and chronic inflammation. Further, we sought to understand the molecular events
downstream of Sema4A signaling and explore recombinant Sema4A as an HSC-protective molecule in the setting of
acute inflammatory stress.
Methods: We utilized mice with germline and conditional deletion of Sema4A and recombinant Sema4A-Fc protein, in
conjunction with models of acute inflammatory stress, aging and transplantation. In order to identify underlying cellular
and molecular mechanisms, we performed intravital imaging, as well as bulk and single cell RNA-sequencing studies.
Results: We discovered that Sema4A controls HSC dormancy. Unexpectedly, we found that this property of Sema4A is
restricted to myeloid-biased HSC (myHSC) – an HSC subset which plays a key role in inflammatory response and aging.
We found that following acute lipopolysaccharide (LPS)-induced inflammatory stress, Sema4AKO myHSC failed to
adequately expand and generated fewer numbers of downstream myeloid progeny. Although the impact of Sema4A
absence in this setting was reversible, chronic inflammatory exposure, as exemplified by aging, led to a complete loss of
myHSC self-renewal potential and capacity for reconstitution. Transcriptomic studies in both settings revealed that the
absence of Sema4A was associated with hyperactivated inflammatory signaling and premature myeloid differentiation,
thus identifying this molecule as a key inflammation resistance signal.
Transplantation studies demonstrated that the cellular source of Sema4A is non cell-autonomous, since we observed
that while conditional Sema4A deletion from myHSC was functionally inconsequential, 50% of Sema4AKO animals
transplanted with WT myHSC died between days 11-20 post-transplant. Intravital imaging revealed markedly enhanced
early proliferation of transplanted WT myHSC in Sema4AKO, consistent with the lack of dormancy-inducing effect of
Sema4A causing impaired self-renewal and engraftment failure in a highly. Importantly, systemic administration of
Sema4A-Fc protein protected myHSC from acute inflammatory injury, as evidenced by improved colony-forming
capacity and significantly higher levels of reconstitution post-transplant by myHSC from the mice which we pre-treated
with recombinant Sema4A-Fc protein prior to LPS-induced acute inflammatory exposure.
Conclusion: Our results highlight a crucial role of Sema4A-mediated inflammation resistance for preservation of myHSC
short- and long-term regenerative function. Interestingly, we observed that both Sema4A and its putative receptor,
PlxnD1, are overexpressed in AML blasts as compared to normal bone marrow, and that higher expression of PlxnD1 in
AML blasts is associated with a significantly worse survival. This suggests that hyperactivation of Sema4A-PlxnD1
inflammation resistance pathway may promote leukemogenesis and chemoresistance in myeloid malignancies, which
will be addressed by our future studies.
Speakers
Lev Silberstein