Tumor-associated calreticulin variants functionally compromise the peptide loading complex and impair its recruitment of MHC-I

J Biol Chem. 2018 Jun 22;293(25):9555-9569. doi: 10.1074/jbc.RA118.002836. Epub 2018 May 16.

Abstract

Major histocompatibility complex-I-β2m dimers (MHC-I) bind peptides derived from intracellular proteins, enabling the immune system to distinguish between normal cells and those expressing pathogen-derived or mutant proteins. The peptides bind to MHC-I in the endoplasmic reticulum (ER), and this binding is facilitated by the peptide loading complex (PLC), which contains calreticulin (CRT). CRT associates with MHC-I via a conserved glycan present on MHC-I and recruits it to the PLC for peptide binding. Somatic frameshift mutations in CRT (CRT-FS) drive the proliferation of a subset of myeloproliferative neoplasms, which are chronic blood tumors. All CRT-FS proteins have a C-terminal sequence lacking the normal ER-retention signal and possessing a net negative charge rather than the normal positive charge. We characterized the effect of CRT-FS on antigen presentation by MHC-I in human cells. Our results indicate that CRT-FS cannot mediate CRT's peptide loading function in the PLC. Cells lacking CRT exhibited reduced surface MHC-I levels, consistent with reduced binding of high-affinity peptides, and this was not reversed by CRT-FS expression. CRT-FS was secreted and not detectably associated with the PLC, leading to poor MHC-I recruitment, although CRT-FS could still associate with MHC-I in a glycan-dependent manner. The addition of an ER-retention sequence to CRT-FS restored its association with the PLC but did not rescue MHC-I recruitment or its surface expression, indicating that the CRT-FS mutants functionally compromise the PLC. MHC-I down-regulation permits tumor cells to evade immune surveillance, and these findings may therefore be relevant for designing effective immunotherapies for managing myeloproliferative neoplasms.

Keywords: antigen presentation; antigen processing; calreticulin; major histocompatibility complex (MHC); myeloproliferative neoplasms; peptide-loading complex; protein export; protein secretion; tumor immunology/immunotherapy.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Antigen Presentation / immunology*
  • Calreticulin / antagonists & inhibitors
  • Calreticulin / genetics*
  • Calreticulin / immunology
  • Calreticulin / metabolism
  • HEK293 Cells
  • Histocompatibility Antigens Class I / immunology*
  • Histocompatibility Antigens Class I / metabolism
  • Humans
  • Mutation*
  • Neoplasms / genetics
  • Neoplasms / immunology*
  • Neoplasms / pathology
  • Peptide Fragments / immunology*
  • Peptide Fragments / metabolism
  • Signal Transduction

Substances

  • CALR protein, human
  • Calreticulin
  • Histocompatibility Antigens Class I
  • Peptide Fragments

Associated data

  • PDB/6ENY