The New Kid on the Block: HLA-C, a Key Regulator of Natural Killer Cells in Viral Immunity

Abstract

The human leukocyte antigen system (HLA) is a cluster of highly polymorphic genes essential for the proper function of the immune system, and it has been associated with a wide range of diseases. HLA class I molecules present intracellular host- and pathogen-derived peptides to effector cells of the immune system, inducing immune tolerance in healthy conditions or triggering effective immune responses in pathological situations. HLA-C is the most recently evolved HLA class I molecule, only present in humans and great apes. Differentiating from its older siblings, HLA-A and HLA-B, HLA-C exhibits distinctive features in its expression and interaction partners. HLA-C serves as a natural ligand for multiple members of the killer-cell immunoglobulin-like receptor (KIR) family, which are predominately expressed by natural killer (NK) cells. NK cells are crucial for the early control of viral infections and accumulating evidence indicates that interactions between HLA-C and its respective KIR receptors determine the outcome and progression of viral infections. In this review, we focus on the unique role of HLA-C in regulating NK cell functions and its consequences in the setting of viral infections.

Keywords: HLA-C; NK cells; killercell immunoglobulin-like receptors; viral infection.

Figure 3

KIR family and respective HLA class I ligands. Illustration of the structure and distribution of inhibitory and activating KIRs and their respective ligands. Killer-cell immunoglobulin-like receptors (KIR) (orange), expressed predominantly on NK cells, interact mainly with HLA class I molecules (purple) and its presented peptides (yellow). Each KIR exhibits a specificity for only a selection of HLA class I molecules, including HLA-C allotypes (bold). For example, inhibitory KIR2DL1 and activating KIR2DS1 exclusively interact with HLA-C2, while the inhibitory receptors KIR2DL2/L3 are cross-reactive for the HLA-C1 and -C2 allotypes. Inhibitory KIRs (and the poliovirus receptor (PVR)) carry immunoreceptor tyrosine-based inhibitory motifs (ITIM, red circles), signaling NK cell inhibition upon receptor-ligand engagement, whereas activating KIRs associate with adapter molecules that contain immunoreceptor tyrosine-based activation motifs (ITAMs, green circles) conferring an activating signal. Created with BioRender.com.

Figure 4

Modulation of HLA-C by viral proteins. Viruses utilize multiple mechanisms to evade immune recognition, including downmodulation of HLA class I molecules or selection of specific peptide variants. Viral escape mechanisms can influence HLA-C expression on the transcriptional, translational, post-translational and protein level. HIV-1 is able to decrease HLA-C surface expression by specific variants of the accessory protein Vpu. CMV encodes different US proteins that target HLA-C heavy chains for proteasomal degradation or block the transport of peptides into the ER. Moreover, HIV-1 and HCV are able to select for specific HLA-C-restricted peptides that modulate the activation of NK cells by altering KIR binding. Created with BioRender.com.

Figure 1

Location and structure of the HLA-C gene. HLA class I genes are located on the short arm of chromosome 6. The transcription of HLA-C is regulated by core promoter elements but also by distal regulators. The core promoter consists of the EnhancerA, ISRE and a SXY box. Compared to HLA-A and -B, the EnhancerA of HLA-C has no functional binding site for NFκB. ISRE activation is mediated through IFNγ stimulation which recruits the transcription factor IRF. The SXY box is composed of the W/S, X1, X2 and Y and is important for the binding of NLRC5 and formation of the enhanceosome. Transcription factors for W/S are still unknown, but X1 has binding sites for RFX and ETS, X2 has binding sites for CREB and ATF1 and Y for NFY. Moreover, the non-coding region of HLA-C contains an OCT1 binding site ~800 bp upstream of the core promoter region. HLA-C has 8 exons. Exon 1 encodes the signal peptide. Exon 2 and 3 encode the α1 and α2 domains which build the peptide binding grove. The α3 domain (exon 4) is connected to the transmembrane domain (TM) and cytoplasmic tail (CYT) (exon 5–7), anchoring the molecule to the cell membrane. Created with BioRender.com.

Figure 2

Protein synthesis pathway of HLA-C. Transcription of HLA-C is regulated through various transcription factors in the promoter region. Once the HLA-C mRNA is translated, the generated polypeptide undergoes proper folding, assembly and peptide loading. Translation of the HLA-C mRNA is regulated by micro-RNA-148 (miR148), which binds to the 3’ untranslated region. The assembly of the HLA-C heavy α-chain with β2m and a peptide is facilitated by a multi-subunit complex, composed of Cnx (calnexin), TAP (transporter associated with antigen processing), Tpn (type I transmembrane glycoprotein tapasin), the thiol oxidoreductase ERp57 and Crt (calreticulin). The HLA complex is loaded with high-affinity peptides that are generated by proteasome-mediated protein degradation in the cytosol. Peptides are transported into the ER and loaded onto the peptide binding grove. After peptide loading, the mature HLA-C:peptide complex dissociates from the multiprotein complex and is transported to the Golgi apparatus and then to the cell surface. Only HLA-C molecules with high-affinity peptides are transported to the cell surface to present the peptide to immune cells: Created with BioRender.com.