Page 603 - Veterinary Immunology, 10th Edition

innate NK cell system. Type I NKT cells also express receptors for
VetBooks.ir inflammatory cytokines produced by antigen-presenting cells. They

can be activated either in combination with TCR-mediated signals
or even without such signals.

Type II NKT cells, in contrast, use an oligoclonal TCR where both
α and β chains contribute equally to lipid recognition. Their
functions are predominantly antiinflammatory and they recognize
diverse hydrophobic antigens. The type II cells also suppress the

type I cells, suggesting that they have an immunoregulatory role.

Species Differences

Equine NKT cells recognize the cell wall lipids of Rhodococcus equi
when they are bound to CD1. The equine genome contains 13

expressed CD1 genes, which makes it the largest CD1 family yet
recognized in mammals. All but one of these molecules are
expressed on equine antigen-presenting cells. The structural

differences between these proteins are located in their antigen-
binding sites suggesting that horse NKT cells can recognize many
different lipid antigens. It is possible that this unusually large
number of CD1 molecules reflects their key role in protection
against R. equi. Other cells that express CD1 include dendritic cells,

Kupffer cells, endothelial cells, and hepatocytes. Pigs and horses
possess functional CD1 genes that form receptors similar to those
found in humans. In pigs, type I NKT cells recognize α-

galactosylceramide. Their production is enhanced by IL-2, -15 and
-33. The CD1 genes in cattle and other ruminants are pseudogenes,
so functional NKT cells may not be present. Cattle do, however,
possess a novel T cell subset that has features of both T and NK
cells. Thus these cells express both NCR1 and CD3. They can be

activated by either the NKR or TCR pathways and can kill Theileria
parva-infected cells. Presumably these are the functional equivalents
of NKT cells in other species.

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