Introduction Pig Monocytes
The study of pig monocyte heterogeneity started with the finding that anti-human monoclonal antibodies cross-react with porcine monocytes (Jacobsen, Veterinary Immunol Immunopathol, 39: 461, 1993) and with the discovery that there exists a CD14high and a CD14low population among CD172+ (SWC3+) monocytes (Ziegler-Heitbrock et al, Scand J Immunol 40: 509, 1994). In an in-depth analysis, monocytes were then shown to consist of a CD163- positive CD14low and CD163-negative CD14 high population, with the positives having higher class II, CD16 and VLA4 expression levels (Sanchez et al J Immunol, 162: 5230, 1999). Functionally the CD163+ cells compared to the CD163-ve monocytes showed higher allo-stimulatory capacity and an increased production of IL-1 and TNF cytokines after LPS stimulation (Sanchez et al, J Immunol, 162: 5230, 1999). While the CD163 is reverse in man, i.e. it is low in the CD14 low monocytes, these features suggest that the CD14low CD163+ cells represent non-classical monocytes. This is supported by chemokine studies, which show at the mRNA level high CX3CR1in CD163+ cells and high CCR2 in CD163-ve cells along with selective migration in response to porcine CCL2 by these cells (Moreno et al, Vet. Res. 41:76, 2010). The homology of the human and pig subsets is further supported by the analysis of gene expression patterns for CD14high and CD14low monocytes in both species (Fairbairn et al, J Immunol, 190: 6389, 2013). Of note, the resolution of the monocyte subsets with these markers was shown to depend on the race of the animals in that Large White x Landrace show a good separation while Pietrain does not separate well (Fairbairn et al, J Immunol, 190: 6389, 2013)
The classical and non-classical monocytes in average account for 50% of all monocytes each. Upon infection the percentage of CD163+ cells substantially increases to 80% after inoculation of Actinobacillus pleuropneumoniae (Ondrackova et al, Vet. Res. 41:64, 2010). Also, M-CSF can strongly increase the number of blood monocytes in the pig, with preferential expansion of CD163+ monocytes to about 80% of all monocytes (Sauter et al, Am J Physiol Gastrointest Liver Physiol 311: G533, 2016.)