24Ras-Erk MAPK signaling controls many of the principal pathways involved in 25 metazoan cell motility, drives metastasis of multiple cancer types and is targeted in 26chemotherapy. Yet, its putative roles in immune cell functions or in infections have 27 remained elusive. Here, using primary dendritic cells (DCs) in an infection model with 28 the protozoan Toxoplasma gondii, we show that two pathways activated by infection 29 converge on Ras-Erk MAPK signaling to promote migration of parasitized DCs. We 30 identify signaling through the receptor tyrosine kinase Met (also known as HGFR) as a 31 driver of T. gondii-induced DC hypermotility. Further, we show that voltage-gated Ca 2+ 32 channel (VGCC, subtype CaV1.3) signaling impacts the migratory activation of DCs 33 via calmodulin-calmodulin kinase II. We report that VGCC and Met signaling converge 34 on Ras GTPase to drive Erk1/2 phosphorylation and migratory activation of T. gondii-35 infected DCs. The data provide a molecular basis for the hypermigratory mesenchymal-36 to-amoeboid transition (MAT) of parasitized DCs. The emerging concept suggests that 37 parasitized DCs acquire metastasis-like migratory properties to promote infection-38 related dissemination. 39 40 understood (Ebert et al., 2016; Scheele et al., 2007). Further, in primary dendritic cells 65 (DCs), Ras-Erk signaling remains largely unexplored (Riegel et al., 2019). 66Owing to host-pathogen coevolution with reciprocal selection, the study of host-67 pathogen interactions has emerged as a powerful approach to gain insight into basic cell 68 biology. The protozoan Toxoplasma gondii is a model obligate intracellular pathogen 69 due to its wide host range among warm-blooded vertebrates and ability to actively 70 invade nucleated cells (Sibley, 2004). 71

The development of B cells relies on an intricate network of transcription factors critical for developmental progression and lineage commitment. In the B cell developmental trajectory, a temporal switch from predominant Foxo3 to Foxo1 expression occurs at the CLP stage. Utilizing VAV-iCre mediated conditional deletion, we found that the loss of FOXO3 impaired B cell development from LMPP down to B cell precursors, while the loss of FOXO1 impaired B cell commitment and resulted in a complete developmental block at the CD25 negative proB cell stage. Strikingly, the combined loss of FOXO1 and FOXO3 resulted in the failure to restrict the myeloid potential of CLPs and the complete loss of the B cell lineage. This is underpinned by the failure to enforce the early B-lineage gene regulatory circuitry upon a predominantly pre-established open chromatin landscape. Altogether, this demonstrates that FOXO3 and FOXO1 cooperatively govern early lineage restriction and initiation of B-lineage commitment in CLPs.

The development of B cells relies on an intricate network of transcription factors critical for developmental progression and lineage commitment. In the B cell developmental trajectory, a temporal switch from predominant Foxo3 to Foxo1 expression occurs at the CLP stage. Utilizing VAV-iCre mediated conditional deletion, we found that the loss of FOXO3 impaired B cell development from LMPP down to B cell precursors, while the loss of FOXO1 impaired B cell commitment and resulted in a complete developmental block at the CD25 negative proB cell stage. Strikingly, the combined loss of FOXO1 and FOXO3 resulted in the failure to restrict the myeloid potential of CLPs and the complete loss of the B cell lineage. This is underpinned by the failure to enforce the early B-lineage gene regulatory circuitry upon a predominantly pre-established open chromatin landscape. Altogether, this demonstrates that FOXO3 and FOXO1 cooperatively govern early lineage restriction and initiation of B-lineage commitment in CLPs.