Flt3 and its ligand as an ancient regulators of dendritic cells: evidence in the amphibian Xenopus laevis

Abstract

Abstract The evolution of hematopoietic-derived dendritic cells that are so critical for T cell activation is still poorely understood. In mammals, fms related tyrosine kinase 3 (Flt3) and its ligand (Flt3lg) are important regulator of dendritic cell homeostasis. Flt3 and Flt3lg co-evolution was investigated in all jawed vertebrates with special focus in an ancient allotetraploid tetrapod Xenopus laevis. flt3 synteny and predicted amino-acid structure are conserved in all jawed vertebrate including in the duplicated S and L homeologous chromosomes of X. laevis. Flt3lg gene is conserved among X. laevis homeologous chromosomes and most jaw vertebrates with the exception of ray-finned fish. Amino acid comparison, 3D structure modelling and tissue distribution suggest a subfunctionalization of Flt3 and Flt3lg homeologs. Tagged recombinant Flt3lg.S and Flt3lg.L were produced to investigate cell surface expression of Flt3s. Both Flt3lg.S and L specifically stain a splenic population of leukocyte MHC class IIhigh+ and CD8+ phenotypically similar to the recently described X. laevis dual follicular/dendritic-like XL cells. Our results indicate that Flt3lg.S and L are both involved in XL cell homeostasis and that XL cells have hematopoietic origin. In X. laevis, the non-polymorphic MHC class I-like XNC4 restricting innate-like T cells is critical for tadpoles’ resistance against the non-TB mycobacteria M.marinum. Preliminary experiments indicate that XL cells are XNC4+ and are being recruited to the infection site together with XNC4-restricted T cells. This suggests that XL cells are involved in T cell activation, which is consistent with the an ancestral dual follicular/dendritic cell function of XL cells. Supported by NIH (R24-AI059830 – R21-AI139718), NSF (1754274) and 2021 Rochester Vaccine Fellowship