Page 81 - Haematologica March 2020

Cytokine blockade reduces myeloid expansion in RA
by selective expansion of myeloid progenitors and mature myeloid cells and activation of myeloid lineage genes in HSC. Despite chronic inflammation, HSC from CIA mice retained their long-term potential and maintained a quies- cent cell cycle state associated with the induction of a pro- liferation arrest gene program. Strikingly, we found that cytokine blockade therapy was able to attenuate these effects (Figure 8).
Myeloid expansion, chronic anemia of inflammation and immunosenescence are well-documented hematopoietic phenotypes in human RA patients, although the precise causes remain elusive and may be multifactorial.14,15,33 Strikingly, myeloid expansion in CIA mice closely resembles that in other models of chronic inflammation including those induced by lipopolysaccharide, IL-1, IFN and pathogen infection.8,10,34 Pro-inflammatory signals can direct- ly activate myeloid transcription factors including Spi1/PU.1 and C/EBP family members, which 'override' competing lin- eage programs and drive expansion of myeloid-biased pro- genitors.35-37 In line with this, we observed increased expres- sion of Spi1/PU.1 and its target genes in HSC from CIA mice. In addition, transplantation of purified CIA HSC revealed an increased proportion of donor-derived myeloid cells and phenotypic MPP3, suggesting that BM inflamma- tion primes HSC to differentiate preferentially into myeloid- lineage progenitors. Given that PU.1 activation in HSC and myeloid expansion are features of both our CIA and chronic IL-1 models, they are likely stereotypical responses to ongo- ing inflammation rather than a disease-specific mechanism, wherein PU.1 could serve as a central 'node' that activates a myeloid gene program in HSC following a variety of inflam- matory insults. Newly developed pharmacological
inhibitors of PU.138 could thus provide therapeutic benefit by blocking this ‘node’ independently of cytokines in an inflammatory disease.
In the literature, inflammation is often associated with increased HSC proliferation, typically in response to acute challenges.8,10 On the other hand, we previously found that HSC can maintain quiescence in the context of ongoing chronic type I IFN signaling.16 Likewise, here we showed that HSC quiescence, phenotypic pool size and long-term repopulating capacity are maintained in CIA mice. Quiescence protects HSC by preventing excessive apopto- sis, differentiation, or replicative ‘aging’ associated with pro- liferation, and HSC lacking quiescence maintenance genes, such as Ckdn1a and Cdkn1c, become exhausted in response to stress.25 Notably, we found that HSC maintained a quies- cent state despite ongoing BM remodeling, including decreased phenotypic endosteal MSC. MSC are required for HSC maintenance and quiescence,6 and there is evidence that MSC function may be degraded in the context of RA.13,33 Our data suggest there are mechanism(s) that may limit HSC proliferation in response to ‘emergency’ inflammatory signals and/or BM niche remodeling. Indeed, HSC from CIA mice activated a proliferation arrest gene program character- ized by downregulation of genes including Myc and Ccnd1/2, alongside upregulation of Cdkn1a, Cdkn1b and Cdkn1c. Likewise, MYC and CCND2 are downregulated in CD34+ hematopoietic stem and progenitor cells from human RA patients.39 Such a program could protect HSC pool integrity by increasing the threshold necessary for HSC cell cycle entry, thereby compensating for impaired BM niche function and/or elevated levels of pro-inflammatory cytokines. The functional significance of this gene program
Figure 8. Model of inflammation-driven hematopoietic alterations in mice with collagen-induced arthritis. Under homeostatic conditions, hematopoietic stem cells (HSC) are largely quiescent but occasionally enter the cell cycle and give rise to lineage-biased multipotent progenitor (MPP) subsets, leading to a balanced lineage output and nominal HSC self-renewal capacity. In mice with collagen-induced arthritis (CIA), chronic inflammation leads to expansion of myeloid-biased MPP3 and granulocyte-macrophage progenitors (GMP), resulting in increased myeloid cell production. Concurrently, inflammation induces a myeloid lineage gene program in HSC that may bias HSC toward further overproduction of MPP3. Despite ongoing inflammation, HSC are maintained in a quiescent state characterized by repression of cell cycle and mRNA translation genes alongside induction of cell cycle inhibitor genes. Notably, pro-inflammatory cytokine blockade, here using anakinra, atten- uates myeloid expansion and altered gene expression in HSC. These data indicate that chronic inflammation drives aberrant hematopoiesis in rheumatoid arthritis, and this phenotype can be attenuated by cytokine-blocking therapy.
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