Background. Our recent results indicate that Nlrp3 inflammasome expressed in hematopoietic stem/progenitor cells (HSPCs) and in the bone marrow (BM) microenvironment are required for optimal mobilization of murine cells (Leukemia 2020 Jun;34(6):1512-1523, Stem Cell Rev Rep. 2019 Jun;15(3):391-403). Moreover, Nlrp3 inflammasome expressed in these cellular contexts are also required for normal homing and engraftment after hematopoietic transplantation (Stem Cell Rev and Rep (2020) in press. https://doi.org/10.1007/s12015-020-10005-w). Consistent with these results, Nlrp3-KO mice are poor mobilizers, cells from Nlrp3-KO mice engraft poorly, and, as transplant recipients, Nlrp3-KO mice show a delay in hematopoietic recovery after transplantation of normal BM cells. Activation of Nlrp3 inflammasomes leads to the release in caspase-1-dependent manner from HSPCs and cells in the BM microenvironment of two potent pro-inflammatory cytokines, interleukin 1b (IL-1b) and interleukin 18 (IL-18). Hypothesis. Based on the aforementioned results, we envisioned that the activated Nlrp3 inflammasome caspase-1 autocrine feedback loop involving IL-1bor IL-18 signaling potentiates HSPCs trafficking by amplifying Nlrp3 inflammasome activation in HSPCs and the BM microenvironment.Materials and Methods. We employed normal control and caspase-1-KO mice for mobilization and homing/engraftment experiments. Mice were mobilized with G-CSF or AMD3100 in the absence or presence of administered IL-1b or IL-18, and we measured i) the total number of white blood cells (WBCs) and ii) the number of clonogenic colony-forming unit granulocyte/macrophage (CFU-GM) progenitors and Sca-1+c-kit+lineage- (SKL) cells circulating in PB. Next, caspase-1-KO cells were transplanted into lethally irradiated wild type animals, while caspase-1-KO mice were transplanted with wild type BMMNCs. We then evaluated homing and engraftment by measuring the number of PKH27-labeled cells, the number of clonogenic progenitors 24 hours after transplantation, and the numbers of day-12 CFU-S colonies and day-12 CFU-GM progenitors in the BM of recipient mice. In control experiments, to provide proof of our hypothesis, we also mobilized Nlrp3-KO mice with AMD3100 or G-CSF in the presence of administered IL-1b and IL-18. Results. We found that IL-1b or IL-18 alone mobilizes HSPCs and, significantly, that caspase-1-KO mice were poor mobilizers. Radiation chimera experiments revealed that this pro-mobilizing defect is dependent on the lack of caspase-1 in hematopoietic cells. To our surprise, administration of IL-1b and IL-18 did not improve G-CSF- or AMD3100-induced mobilization in Nlrp3-KO animals. We also found that both of these pro-inflammatory cytokines activate Nlrp3 inflammasomes in an Myd88-dependent manner. Moreover, HSPCs from caspase-1-KO mice show defective migration in response to a BM-released SDF-1 gradient and other supportive homing chemoattractants (sphingosine-1 phosphate; S1P and extracellular adenosine triphosphate; eATP). Finally, caspase-1-KO mice recipients were also engrafted poorly with normal HSPCs. To explain this phenomenon, we observed that the Nlrp3 inflammasome-caspase-1-IL-1b axis is important in upregulating SDF-1 and other key HSPC chemoattractants in the BM microenvironment conditioned for transplantation. Conclusions. We demonstrate for the first time that an Nlrp3 inflammasome-caspase-1-IL-1b/IL18 autocrine feedback mechanism operating in HSPCs and the BM microenvironment is involved in the normal trafficking of HSPCs during mobilization and homing/engraftment. This research demonstrates the unexpected role of caspase-1, which is situated downstream of the Nlrp3 inflammasome and directs the release of the potent pro-inflammatory cytokines IL-1b and IL18 that by feedback mechanism, in turn, activate Nlrp3 inflammasomes in HSPCs and the BM microenvironment, which is required for optimal trafficking of HSPCs. These results are important for understanding the novel casapase-1-mediated autocrine mechanisms involved not only in pharmacological mobilization but also in the inflammatory response of the organism to external challenges, such as infection and tissue/organ damage.

Disclosures

No relevant conflicts of interest to declare.

Author notes

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Asterisk with author names denotes non-ASH members.