MHC class I chain-related protein A and B (MICA and MICB) are predominantly expressed intracellularly in tumour and normal tissue

Background: Major histocompatibility complex (MHC) class I chain-related protein A (MICA) and MHC class I chain-related protein B (MICB) are polymorphic proteins that are induced upon stress, damage or transformation of cells which act as a 'kill me' signal through the natural-killer group 2, member D receptor expressed on cytotoxic lymphocytes. MICA/B are not thought to be constitutively expressed by healthy normal cells but expression has been reported for most tumour types. However, it is not clear how much of this protein is expressed on the cell surface. Methods: Using a novel, well-characterised antibody and both standard and confocal microscopy, we systematically profiled MICA/B expression in multiple human tumour and normal tissue. Results: High expression of MICA/B was detected in the majority of tumour tissues from multiple indications. Importantly, MICA/B proteins were predominantly localised intracellularly with only occasional evidence of cell membrane localisation. MICA/B expression was also demonstrated in most normal tissue epithelia and predominantly localised intracellularly. Crucially, we did not observe qualitative differences in cell surface expression between tumour and MICA/B expressing normal epithelia. Conclusions: This demonstrates for the first time that MICA/B is more broadly expressed in normal tissue and that expression is mainly intracellular with only a small fraction appearing on the cell surface of some epithelia and tumour cells.