# The murine MHC-E molecule Qa-1b is surface displayed in a peptide-free conformation in homeostasis

## TL;DR

The study reveals that murine Qa-1b, a nonclassical MHC-E molecule, can be displayed on the cell surface without bound peptides under normal conditions.

## Contribution

The study identifies a novel regulatory mechanism involving peptide-free MHC-E molecules beyond the NKG2x/CD94 immune checkpoint.

## Key findings

- Qdm presentation by Qa-1b requires interferon-γ and the peptide-loading complex.
- 6A8.6F10 antibody binds to peptide-free Qa-1b complexes, independent of PLC components.
- Peptide-free Qa-1b may represent an additional regulatory layer in immune responses.

## Abstract

Qa-1b, the murine ortholog of the nonclassical MHC-E family, contains minimal polymorphism and exhibits reduced surface stability compared with classical MHC class I molecules. To investigate Qa-1b conformations and their immunological relevance, we employed two antibodies: EXX-1, which selectively recognizes Qa-1b bound to the canonical leader peptide Qdm, and 6A8.6F10, a broadly used Qa-1b-reactive antibody. Genome-wide CRISPR screens revealed that Qdm presentation was induced by interferon-γ and required the components of the peptide-loading complex (PLC) and endoplasmic reticulum quality control. EXX-1 binding thus reflected broad cellular integrity and mirrored CD94/NKG2x receptor engagement. In contrast, 6A8.6F10 staining occurred independently of PLC components such as ERAP1 and tapasin, and intriguingly increased in their absence. Accordingly, exogenous pulsing with Qa-1b-binding peptides markedly reduced 6A8.6F10 antibody binding and resonance shift assays revealed that 6A8.6F10 selectively recognizes peptide-deficient Qa-1b complexes. These findings suggest an additional layer of regulation beyond the immune checkpoint NKG2x/CD94, involving peptide-free MHC-E.