Andrew Brooks

The heterodimeric natural killer cells antigen CD94 (CD94)–NKG2‐A/NKG2‐B type II integral membrane protein (NKG2A) receptor family expressed on human and mouse natural killer (NK) cells monitors global major histocompatibility complex (MHC) class I cell surface expression levels through binding to MHC class Ia‐derived leader sequence peptides presented by HLA class I histocompatibility antigen, alpha chain E (HLA‐E; in humans) or H‐2 class I histocompatibility antigen, D‐37 (Qa‐1 b ; in mice). Although the molecular basis underpinning human CD94–NKG2A recognition of HLA‐E is known, the equivalent interaction in the murine setting is not. By determining the high‐resolution crystal structure of murine CD94–NKG2A in complex with Qa‐1 b presenting the Qa‐1 determinant modifier peptide (QDM), we resolved the mode of binding. Compared to the human homologue, the murine CD94–NKG2A–Qa‐1 b –QDM displayed alterations in the distribution of interactions across CD94 and NKG2A subunits that coincide with differences in electrostatic complementarity of the ternary complex and the lack of cross‐species reactivity. Nevertheless, we show that Qa‐1b could be modified through W65R + N73I mutations to mimic HLA‐E, facilitating binding with both human and murine CD94–NKG2A. These data underscore human and murine CD94–NKG2A cross‐species heterogeneity and provide a foundation for humanising Qa‐1b in immune system models.

Ectopic protein overexpression in immortalised cell lines is a commonly used method to screen host factors for their antiviral activity against different viruses. However, the question remains as to what extent such artificial protein overexpression recapitulates endogenous protein function. Previously, we used a doxycycline-inducible overexpression system, in conjunction with approaches to modulate the expression of endogenous protein, to demonstrate the antiviral activity of IFITM1, IFITM2, and IFITM3 against influenza A virus (IAV) but not parainfluenza virus-3 (PIV-3) in A549 cells. We now show that constitutive overexpression of the same IFITM constructs in A549 cells led to a significant restriction of PIV-3 infection by all three IFITM proteins. Variable IFITM mRNA and protein expression levels were detected in A549 cells with constitutive versus inducible overexpression of each IFITM. Our findings show that overexpression approaches can lead to levels of IFITM1, IFITM2, and IFITM3 that significantly exceed those achieved through interferon stimulation of endogenous protein. We propose that exceedingly high levels of overexpressed IFITMs may not accurately reflect the true function of endogenous protein, thus contributing to discrepancies when attributing the antiviral activity of individual IFITM proteins against different viruses. Our findings clearly highlight the caveats associated with overexpression approaches used to screen cellular host proteins for antiviral activity.