Abstract 394: Murine Cre/flox Studies Indicate Yolk Sac Derived Macrophages Are the Cellular Source of FXIIIA

Fibrin clot cross-linking FXIIIA has a plasma and cellular pool. It is present in platelets, macrophages and connective tissues. However, it is unknown how FXIIIA is secreted and which cell type maintains the plasma pool, although the platelet is considered the main candidate. To address this we have induced lineage specific deletion in mice floxed in coding exon 7 of the FXIIIA gene. Mice homozygous for the floxed gene and transgenic for Pf4cre-recombinase (thrombopoietic deletion), CD11bcre or LysMcre (myeloid deletions) were generated. A FXIIIA KO was obtained using a CMV-driven cre. Plasma and platelet FXIIIA protein & activity and FXIIIA mRNA in brain, heart and aorta were determined and compared to WT, FXIIIA floxed and thrombocytopenic Mpl (thrombopoietin receptor) KO mice. FXIIIA KO and Pf4FXIIIA KO mice lack platelet FXIIIA protein & activity, whereas both are normal in the Mpl KO. pFXIIIA is absent in the FXIIIA KO and decreased in Pf4FXIIIA KO (by 85%), CD11bFXIIIA KO (40%) and LysMFXIIIA KO (60%) whereas in WT, FXIIIA floxed and Mpl KO pFXIIIA was unaltered. Mpl KO platelet size as determined by FACS was also unaltered. These data confirm that platelets are not the source of pFXIIIA. These data also demonstrate that the pFXIIIA secreting cells are Pf4 dependent & Mpl independent: a combination not seen in BM-derived cells. Thus, we are looking for a macrophage-like cell outside of the BM. Resident macrophages in the brain are yolk sac-derived and uniquely, not replaced from BM under normal conditions. In young uninjured brain FXIIIA mRNA levels were knocked down by 96.8% in Pf4FXIIIA KO and by 37.8% in the Mpl KO, confirming, for the first time, the existence of a Pf4 dependent Mpl independent cell from the yolk sac. In heart cell fractions, the smallest Thy1+ fraction (3% of total) contained the largest FXIII-A mRNA pool, which decreases in Pf4cre & CD11bcre-Flox mice, further supporting involvement of yolk sac-derived macrophages. Further study of FXIIIA mRNA expression in Pf4 dependent, Mpl independent cells in the cardiovascular system demonstrated that the pFXIIIA profile was most closely mirrored by the reduction in arterial wall FXIIIA mRNA suggesting that yolk sac derived arterial wall resident macrophages are the cellular source of FXIIIA.