21Fibrosis is a condition shared by numerous inflammatory diseases. Our incomplete 22 understanding of the molecular mechanisms underlying fibrosis has severely hampered effective 23 drug development. CXCL4 is associated with the onset and extent of fibrosis development in 24 systemic sclerosis, a prototypic inflammatory and fibrotic disease. Here, we integrated 65 paired 25 longitudinal transcriptional and methylation profiles from monocyte-derived cells with/without 26 2 CXCL4 exposure. Using data-driven gene regulatory network analyses, we demonstrate that 27 CXCL4 dramatically alters the trajectory of monocyte differentiation, inducing a novel pro-28 inflammatory and pro-fibrotic phenotype mediated via key transcriptional regulators including 29 CIITA. CXCL4 exposed monocyte-derived cells directly trigger a fibrotic cascade by producing 30 ECM molecules and inducing myofibroblast differentiation. Inhibition of CIITA mimicked 31 CXCL4 in inducing a pro-inflammatory and pro-fibrotic phenotype, validating the relevance of 32 the gene regulatory network. Our study unveils CXCL4 as a key secreted factor driving innate 33 immune training and forming the long-sought link between inflammation and fibrosis. 34 35 106 antigen processing and presentation (Figure 1-figure supplement 1E). For instance, CXCL4-107 moDCs, in the absence of further stimulation, up-regulate expression of several inflammatory 108 molecules such as CTSL, FLT1, CD86, LAMP1, CHI3L1, and down-regulate signaling receptors 109 such as CLEC10A, IL1R1, IL1R2 compared to moDCs (Figure 1G-I and Figure 1-figure 110 supplement 1A-D). Strikingly, CXCL4 exposure also leads to dramatic changes in expression of 111 genes regulating metabolism and transcription (Figure 1-figure supplement 1E), reminiscent 112 of changes previously observed in myeloid cells undergoing immune training (Saeed et al., 2014). 113 Thus, CXCL4 orchestrates a differentiation process dramatically different than that of the 114 conventional moDCs. 115 116 Mature CXCL4-moDCs are functionally distinct from conventional moDCs 117 5To study the effects of CXCL4 on moDC maturation, we stimulated the cells with polyI:C on 118 day 7. This perturbed the expression of 8,949 and 7,767 genes in CXCL4-moDCs and 119 conventional moDCs, respectively, compared to the day 7 transcriptional profiles of their 120 unstimulated counterparts (Figure 1C and 1E, left and middle panels). 2,397 genes responded 121 differently to polyI:C stimulation in CXCL4-moDCs compared to conventional moDCs ( Figure 122 1C and Figure 1-figure supplement 2). Several pathways involved in inflammatory responses 123 such as TLR signaling, interferon signaling, and cytokine signaling, were significantly 124 upregulated in CXCL4-moDCs compared to conventional moDCs (Figure 1-figure 125 supplement 2E). Confirming our previous findings (Silva-Cardoso et al., 2017), these 126 transcriptional changes were followed by increased production of pro-inflammatory mediators 127 such as IL-1β, IL-6, IL-12, IL-23, IL-27, TNF and...