The biology of cells of the mononuclear phagocyte system has been studied
extensively in the mouse. Studies of the pig as an experimental model have
commonly been consigned to specialist animal science journals. This thesis
considered some of the many ways that pigs may address the shortcomings of mice
as models for the study of macrophage differentiation and activation in vitro, and the
biology of sepsis and other pathologies in the living animal.
Flow cytometry was used initially to phenotype cells from the porcine lung,
peritoneal cavity, blood and bone marrow using the LPS receptor CD14 and the FC
receptor CD16, markers frequently employed to differentiate human monocytes into
subsets. The expression of SIRP-alpha (SWC3a, CD172a), which is present on all
cells of myeloid origin, and the haemoglobin scavenger receptor, CD163 which has
previously been used to study monocyte differentiation in the pig was also studied.
The findings validated previous work where blood monocytes were divided into
subsets on the expression of CD14 and CD163. Furthermore, like human and
mouse, pig monocytes also exhibited variation in CD16 expression, having a subset
which was CD14hiCD16lo and another which was CD14loCD16hi. A whole genome
approach was then used to study the differences between the monocyte subsets in the
pig, using monocytes sorted into two populations based on the expression of CD14
and CD163. The gene expression profiles obtained were then compared to publically
available data from monocyte subsets in human and mouse.
This thesis also investigated the expression of genes that are known to be
differentially expressed between human and mouse. To do this gene expression in
porcine bone marrow derived macrophages was analyzed across an LPS time course.
Like human macrophages, pig macrophages did not induce nitric oxide nor any
arginine metabolizing genes in response to LPS. Instead they responded with robust
induction of indoleamine 2,3-dioxygenase (IDO) and other enzymes of the
tryptophan metabolism pathway such as kynurenine hydroxylase, kynureninase and
tryptophan-tRNA synthetase. The tryptophan metabolism pathway has been
implicated in sepsis in man and the absence of this pathway in the mouse may be one of the reasons why an adequate rodent model of sepsis has not been developed. The
IDO inhibitor 1-methyl-tryptophan (1-MT) has been used to treat mouse
macrophages where it had a protective effect after LPS administration. Similar
experiments on pig macrophages did not show the same protective effect and
induction of key immune genes was increased after treatment with 1-MT suggesting
IDO is involved in feedback control of the immune system.
With the completion of the genome sequence and the characterisation of many key
regulators and markers, the pig has emerged as a tractable model of human innate
immunity and disease that should address the limited predictive value of rodents in
preclinical studies. This project aimed to address the gap in our knowledge of the
control of innate immunity in the pig and provided further evidence that the pig can
function as an ideal model to study innate immunity
