Early ontogeny of monocytes and macrophages in the pig

Prenatal development of cord blood monocytes and tissue macrophages was stu died in pig foetuses by immunophenotyping and functional assays. The functi on of peripheral blood monocytes was compared in germ-free and conventional piglets. First macrophages were identified by electron microscopy in foeta l liver on the 25th day of gestation. Monoclonal antibodies against porcine CD45 and SWC3 antigens were used for flow cytometric identification of mye lomonocytic cells in cell suspensions prepared from the yolk sac, foetal li ver, spleen and cord blood. Leukocytes expressing the common myelomonocytic antigen SWC3 were found in all organs studied since the earliest stages of development. Opsonized zymosan ingestion assay was used to determine the p hagocytic capacity of foetal mononuclear phagocytes isolated from cord bloo d, liver and spleen. In the foetal liver, avid phagocytosis of apoptic cell s had been found to occur before cells were able to ingest zymosan in vitro . The first cells capable of ingesting zymosan particles were found on the 40th day of gestation in umbilical blood and 17 days later in foetal spleen and liver. Their relative proportion increased with age. Cord blood monocy tes and peripheral blood monocytes in germ-free piglets had low oxidatory b urst activity as shown by iodonitrophenyl tetrazolium reduction assay. A re markable increase of oxidatory burst activity was observed in conventional piglets, probably due to activation of immune mechanisms by the microflora colonizing gastrointestinal tract.