Young foal and adult horse monocyte-derived dendritic cells differ by their degree of phenotypic maturity

Newborn foals are very susceptible to infections by opportunistic pathogens such as Rhodococcus equi. This susceptibility is thought to be due to the immaturity of their immune system, in particular their inability to produce interferon-gamma. This deficiency may result from an insufficiency in accessory signals. We therefore compared monocyte-derived dendritic cells (MoDC) from foals and from adult horses. CD172, MHC-I and MHC-II were generally expressed on more than 90% MoDC from foals and adults. CD1w2+CD86+ cells tended to be less represented in 2–3-week-old foals than in adults. This difference was significant among CD14− cells. The percentage of CD14−CD1w2+CD86+ cells tended to be increased at 3 months. This suggests that very young foal dendritic cells are quantitatively less mature than their adult counterparts. The expression of IL-1, IL-12, IL-15 and IL-18 mRNA was not different in foal and adult MoDC, but the levels of TNF-α, IL-10, MCP-1 and TGF-β were lower in foal cells. TNF-α and IL-10 expression was increased by LPS; TNF-α even reached the level of adult MoDC. This may mean that the lack of IFN-γ in foals is not due to decreased levels of IL-12, IL-15 or IL-18, but rather to lower constitutive levels of TNF-α.