Background/Purpose: Monocytes are myeloid cells important for the initiation of inflammation and have been implicated in the pathogenesis of autoimmunity. Monocytes interact with their environment through expression of cell-surface and endosomal pattern recognition receptors, including Toll-like receptors (TLRs), which are activated by endogenous and exogenous ‘danger’ signals. Persistent activation through TLRs is thought to drive chronic inflammation in multiple rheumatic diseases. However, the cellular mechanisms driving feed-forward inflammation downstream of repeated TLR stimulation remain unknown.

Methods: We treat C57BL/6 mice with repeated doses of CpG1826, a TLR9 agonist, to induce a feed-forward inflammatory response. Yet40 interleukin (IL)-12 reporter mice are used to identify the TLR9 responsive, IL-12 producing cells in this model. C-C chemokine receptor type 2 (CCR2)^-/- mice are used to define CCR2- dependent and independent cell trafficking during TLR9-mediated inflammation. Flow cytometric identification of myeloid progenitor cells and in vitromyelopoiesis assays are used to characterize inflammation-induced myelopoiesis.

Results: We demonstrate that repeated stimulation through TLR9 leads to a feed-forward inflammatory response driven by the accumulation of TLR9 responsive inflammatory monocytes. Interestingly, TLR9-activated CCR2^-/- mice are not protected from disease and accumulate similar numbers of peripheral blood and splenic inflammatory monocytes as wildtype mice, despite previous work demonstrating defective inflammatory monocyte egress from the bone marrow in CCR2^-/- mice. Peripheral monocytosis does not coincide with an expansion of bone marrow myeloid progenitor cells, but rather correlates with an accumulation of myeloid progenitor cells in the spleen and liver of TLR9-activated mice. Furthermore, myeloid progenitor cell accumulation at peripheral sites of inflammation is independent of CCR2, implicating in situ generation of inflammatory monocytes as an explanation for the peripheral monocytosis seen in TLR9-activated CCR2^-/- mice. Finally, repeated TLR9 activation in vivoreprograms extramedullary myeloid progenitor cells to have increased inflammatory monocyte production capacity demonstrating another mechanism driving feed-forward myelopoiesis downstream of repeated TLR9 activation. 

Conclusion: Our data indicate that profound changes in extramedullary myelopoiesis occur during TLR9-driven inflammation leading to increased production of inflammatory monocytes. The accumulation of TLR9 responsive monocytes leads to enhanced inflammatory responses downstream of repeated TLR9 activation generating a feed-forward inflammatory loop. As extramedullary myeloid progenitor cells are found in patients with rheumatoid arthritis, systemic lupus erythematous, scleroderma, dermatomyositis, and psoriasis, it is intriguing to speculate that enhanced myelopoiesis downstream of repeated TLR activation is a critical factor driving systemic inflammatory responses in multiple rheumatic diseases.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/enhanced-myelopoiesis-downstream-of-toll-like-receptor-9-activation-drives-a-feed-forward-inflammatory-response/