Cellular expression, trafficking, and function of two isoforms of human ULBP5/RAET1G

Robert A Eagle, Gillian Flack, Anthony Warford, Jesús Martínez-Borra, Insiya Jafferji, James A Traherne, Maki Ohashi, Louise H Boyle, Alexander D Barrow, Sophie Caillat-Zucman, Neil T Young, John Trowsdale

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Abstract

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. METHODOLOGY/PRINCIPAL FINDINGS: 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/SIGNIFICANCE: 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.
Original languageEnglish
Article numbere4503
Number of pages14
JournalPloS ONE
Volume4
Issue number2
DOIs
Publication statusPublished - 18 Feb 2009

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Keywords

  • carrier proteins
  • gene expression
  • histocompatibility antigens class I
  • humans
  • killer cells, natural
  • ligands
  • membrane proteins
  • NK cell lectin-like receptor subfamily k
  • protein isoforms
  • protein transport
  • RNA, messenger

Cite this

Eagle, R. A., Flack, G., Warford, A., Martínez-Borra, J., Jafferji, I., Traherne, J. A., ... Trowsdale, J. (2009). Cellular expression, trafficking, and function of two isoforms of human ULBP5/RAET1G. PloS ONE, 4(2), [e4503]. https://doi.org/10.1371/journal.pone.0004503