Identification of a lineage negative cell population in bovine peripheral blood with the ability to mount a strong type I interferon response

Lineage negative dendritic cells, or natural interferon-producing cells (NIPC), also referred to as plasmacytoid dendritic cells (pDC) constitute a small population of leukocytes secreting high levels of type I interferon (IFNα/β) in response to certain danger signals. Here, we provide initial data towards the identification of so far uncharacterised circulating bovine pDC like cells. A lineage negative cell population (LIN ^- cells) was isolated from PBMC which showed characteristics similar to that of pDC in other species. Isolated LIN ^- cells presented lymphoid morphology with a semi-crescent nucleus, extensive ER and Golgi network; indicative of pDC. In addition phenotypic analysis of LIN ^- cells described them as distinct from other bovine DC subsets; expressing both lymphoid and myeloid surface markers. LIN ^- cells did not express lineage specific markers, but were MHC class II ⁺, CD45RO ⁺, CD80/86 ⁺, CD6 ⁺, WC1 ⁺, CD26 ⁺ and expressed the myeloid markers CD205, CD172a and CD11a. In keeping with pDC, LIN ^- cells express TLR7 mRNA transcripts; however, in a resting state do not express TLR8 or TLR9. Functionally, LIN ^- cells, but not PBMC, monocytes and monocyte derived DC produce large amounts of IFNα/β in response to different CpG oligonucleotides. Taken together, we present data suggesting that an enriched circulating population of bovine LIN ^- cells are uniquely capable of producing IFNα/β in response to CpG oligonucleotides and thus this population likely contain the functional equivalent of bovine pDC.