Isolation and characterisation of equine dendritic cells.

Despite their important role in initiating T-cell responses in other species, dendritic cells have not been studied in the horse. A method for isolating blood dendritic cells by adherence and metrizamide gradients was adapted to equine cells. A number of monoclonal antibodies (mAbs), including some which label dendritic cells in other species, were tested for immunochemical reactivity with the isolated blood dendritic cells, and sections of lymph node and spleen. 62 +/- 6% of the isolated blood cells were MHC Class II positive and had typical dendritic cell morphology and only 4 +/- 2% contained non-specific esterase, a marker of mature macrophages. These dendritic cells also expressed MHC Class I, LFA-1, EqWC1 and EqWC2. Amongst the potentially cross-reactive antibodies a mAb against bovine CD1b was the most interesting by staining lymph node, but not blood, dendritic cells. Monoclonal antibodies against equine CD5 (T-cells), surface immunoglobulin (B-cells) and macrophages (CZ2.2) were used to enumerate the contaminating cells in preparations from blood by flow cytometry. 39 +/- 7% of the cells did not express T and B cell markers or CZ2.2 but were large and MHC Class II positive. Comparison of immuno-chemistry and flow data, together with examination of alveolar macrophages and adhered blood cells, all support the view that CZ2.2 detects a myeloid marker not seen on mature macrophages and possibly shared with dendritic cell precursors. The functional capacity of the isolates was assessed in terms of their stimulating ability in the mixed leukocyte reaction (MLR). Dendritic cell enriched isolates were more potent stimulators of MLRs than peripheral blood mononuclear cells or adherent cells. Thus equine dendritic cells isolated from blood express high levels of MHC Class I and II and LFA-1 and stimulate a vigorous MLR. They do not express markers characterising T and B cells but, by virtue of expression of the equine macrophage marker CZ2.2, appear closely related to mononuclear phagocytes.