Phenotypic heterogeneity of peripheral monocytes in healthy dogs

Monocytes are key cells of the innate immune system. Their phenotypic and functional roles have been investigated in humans, mice and other animals, such as the rat, pig and cow. To date, detailed phenotypic analysis of monocytes has not been undertaken in dogs. Two important surface markers in human monocytes are CD14 and MHC class II (MHC II). By flow cytometry, we demonstrated that canine monocytes can be subdivided into three separate populations: CD14^posMHC II^neg, CD14^posMHC II^pos and CD14^negMHC II^pos. Both light and transmission electron microscopy confirmed the monocytic identity of all three populations. The CD14^posMHC II^neg population could be distinguished on an ultrastructural level by their smaller size, the presence of more numerous, larger granules, and more pseudopodia than both of the other populations.