Cellular expression, trafficking, and function of two isoforms of human ULBP5/RAET1G.
BACKGROUND
The activating immunoreceptor NKG2D is expressed on Natural Killer (NK) cells and subsets of T cells. NKG2D contributes to anti-tumour and anti-viral immune responses in vitro and in vivo. The ligands for NKG2D in humans are diverse proteins of the MIC and ULBP/RAET families that are upregulated on the surface of virally infected cells and tumours. Two splicing variants of ULBP5/RAET1G have been cloned previously, but not extensively characterised.


RESULTS
We pursue a number of approaches to characterise the expression, trafficking, and function of the two isoforms of ULBP5/RAET1G. We show that both transcripts are frequently expressed in cell lines derived from epithelial cancers, and in primary breast cancers. The full-length transcript, RAET1G1, is predicted to encode a molecule with transmembrane and cytoplasmic domains that are unique amongst NKG2D ligands. Using specific anti-RAET1G1 antiserum to stain tissue microarrays we show that RAET1G1 expression is highly restricted in normal tissues. RAET1G1 was expressed at a low level in normal gastrointestinal epithelial cells in a similar pattern to MICA. Both RAET1G1 and MICA showed increased expression in the gut of patients with celiac disease. In contrast to healthy tissues the RAET1G1 antiserum stained a wide variety or different primary tumour sections. Both endogenously expressed and transfected RAET1G1 was mainly found inside the cell, with a minority of the protein reaching the cell surface. Conversely the truncated splicing variant of RAET1G2 was shown to encode a soluble molecule that could be secreted from cells. Secreted RAET1G2 was shown to downregulate NKG2D receptor expression on NK cells and hence may represent a novel tumour immune evasion strategy.


CONCLUSIONS
We demonstrate that the expression patterns of ULBP5RAET1G are very similar to the well-characterised NKG2D ligand, MICA. However the two isoforms of ULBP5/RAET1G have very different cellular localisations that are likely to reflect unique functionality.
Eagle
Robert A
RA
Cambridge Institute for Medical Research, Wellcome Trust/MRC Building, Addenbrookes Hospital, Cambridge, UK. robeagle@caltech.edu
Flack
Gillian
G
Warford
Anthony
A
Martínez-Borra
Jesús
J
Jafferji
Insiya
I
Traherne
James A
JA
Ohashi
Maki
M
Boyle
Louise H
LH
Barrow
Alexander D
AD
Caillat-Zucman
Sophie
S
Young
Neil T
NT
Trowsdale
John
J
D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism / Q000502:physiology; D015870:Gene Expression; D015395:Histocompatibility Antigens Class I / Q000378:metabolism; D006801:Humans; D007694:Killer Cells, Natural; D008024:Ligands; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism / Q000502:physiology; D055655:NK Cell Lectin-Like Receptor Subfamily K / Q000378:metabolism*; D020033:Protein Isoforms; D021381:Protein Transport; D012333:RNA, Messenger / Q000032:analysis
D002352:Carrier Proteins; D015395:Histocompatibility Antigens Class I; C527085:KLRK1 protein, human; D008024:Ligands; C099187:MHC class I-related chain A; D008565:Membrane Proteins; D055655:NK Cell Lectin-Like Receptor Subfamily K; D020033:Protein Isoforms; C488761:RAET1G protein, human; D012333:RNA, Messenger