Disulphide-isomerase-enabled shedding of tumour-associated NKG2D ligands

Abstract

Tumour-associated ligands of the activating NKG2D (natural killer group 2, member D; also called KLRK1) receptor-which are induced by genotoxic or cellular stress-trigger activation of natural killer cells and co-stimulation of effector T cells, and may thus promote resistance to cancer. However, many progressing tumours in humans counter this anti-tumour activity by shedding the soluble major histocompatibility complex class-I-related ligand MICA, which induces internalization and degradation of NKG2D and stimulates population expansions of normally rare NKG2D+CD4+ T cells with negative regulatory functions. Here we show that on the surface of tumour cells, MICA associates with endoplasmic reticulum protein 5 (ERp5; also called PDIA6 or P5), which, similar to protein disulphide isomerase, usually assists in the folding of nascent proteins inside cells. Pharmacological inhibition of thioreductase activity and ERp5 gene silencing revealed that cell-surface ERp5 function is required for MICA shedding. ERp5 and membrane-anchored MICA form transitory mixed disulphide complexes from which soluble MICA is released after proteolytic cleavage near the cell membrane. Reduction of the seemingly inaccessible disulphide bond in the membrane-proximal alpha3 domain of MICA must involve a large conformational change that enables proteolytic cleavage. These results uncover a molecular mechanism whereby domain-specific deconstruction regulates MICA protein shedding, thereby promoting tumour immune evasion, and identify surface ERp5 as a strategic target for therapeutic intervention.

Protein disulfide isomerases are antibody targets during immune-mediated tumor destruction

“…Other surface proteins such as CD44 and PBEF might serve as the relevant targets for the antibody-dependent cytotoxicity of RENCA cells, although this will require additional investigation. Nonetheless, we confirmed the strong surface expression of PDI and ERp5 in multiple murine and human cancer cell lines, 53 including tumor types similar to the patients reported here. Unfortunately, autologous cancer cells were not available for characterization, as these had been used for vaccine manufacture.…”

“…Among the antigens, the human ortholog of PDI manifested immunogenicity in a limited number of cancer patients, whereas the closely related disulfide isomerase ERp5 elicited humoral reactions that were temporally associated with clinically significant anti-tumor effects in 4 different solid and hematologic malignancies. Because ERp5 facilitates immune escape through the enhancement of MICA shedding 53 and may also promote tumor cell invasion through modifying surface thiols, 56,57 our results raise the possibility that this disulfide isomerase may be a useful target for cancer immunotherapy.…”

“…23,54,55 MICA is induced in tumor cells through the DNA damage response, but the surface localization of ERp5 renders the ␣3 domain of MICA susceptible to proteolysis; the release of soluble ligand in turn provokes the down-regulation of NKG2D. 22,53 We previously reported that some advanced melanoma and non-small cell lung carcinoma patients who responded to vaccination with irradiated, autologous, GM-CSF-secreting tumor cells and antibody blockade of CTLA-4 generated high-titer antibodies to MICA. 24 These therapy-induced humoral reactions antagonized the immunosuppressive effects of shed MICA and augmented innate and adaptive anti-tumor cytotoxicity.…”

“…46,47,53 MICA is a ligand for NKG2D, a receptor expressed on NK cells, ␥␦ T cells, and CD8 ϩ T cells that upon engagement triggers perforin dependent cytolysis and provides costimulation. 23,54,55 MICA is induced in tumor cells through the DNA damage response, but the surface localization of ERp5 renders the ␣3 domain of MICA susceptible to proteolysis; the release of soluble ligand in turn provokes the down-regulation of NKG2D.…”

The prognostic significance of soluble NKG2D ligands in B-cell chronic lymphocytic leukemia

“…11 Especially for MICA and ULBP2 molecules it could be shown that considerable amounts are shed from tumor cells by the action of disulfide isomerase, ERp5 and metalloproteases. [39][40][41] Thus, cell turnover and shedding have to be taken into consideration as mechanisms for the release of NKG2DL molecules.…”