Identification of fetal liver tyrosine kinase 3 (Flt3) ligand domain required for receptor binding and function using naturally occurring ligand isoforms

We used a comparative approach to identify the fetal liver tyrosine kinase 3 (flt3) ligand structure required for binding and function. Two conserved bovine flt3 ligand isoforms, which differ in a defined region within the ex tracellular domain, were identified and shown to be uniformly transcribed i n individuals with diverse MHC haplotypes, Notably, at the amino acid level , the extracellular domain of the bovine flt3 ligand isoform 1 is 81 and 72 % identical with the extracellular domains of the human and murine flt3 lig ands, respectively, whereas isoform-2 has a deletion within this domain. Bo vine flt3 ligand isoform 1, but not 2, bound the human flt3 receptor and st imulated murine pro B cells transfected with the murine flt3 receptor, This retention of binding and function allowed definition of key residues by id entifying sequences conserved among species, We have shown that a highly co nserved, 18 aa sequence within the flt3 ligand extracellular domain is requ ired for flt3 receptor binding and function. However, a peptide representin g this sequence is insufficient for receptor binding as demonstrated by its failure to inhibit the bovine flt3 ligand isoform 1 binding to the human f lt3 receptor. The requirement for flanking structure was confirmed by testi ng bovine flt3 ligand isoform 1 constructs truncated at specific residues o utside the 18 aa sequence. Overall, the flt3 ligand structure required for function is markedly similar to that of the related hemopoietic growth fact ors, CSF-1 and steel factor. This definition of the required BU ligand stru cture will facilitate development of agonists to enhance dendritic cell rec ruitment for vaccines and immunotherapy.