Background:  Down-regulation of classical major histocompatibility complex (MHC) class I molecules in HIV infection protects the virus from MHC I-restricted cytotoxic T-lymphocytes, but may expose HIV-infected target cells to attack by natural killer. However, HIV-infected  lymphocytes are resistant to lysis by NK cells despite reduced MHC I expression.  Of note, HIV down-regulates HLA-A and -B, but not HLA-E, an important regulator of NK cell activity. Thus, we studied the role of HLA-E in protection of HIV-infected cells against NK cells.

Methods:  We enrolled in our study 15 HIV-infected patients and 10 healthy HIV-negative subjects. Phytohemagglutinin-treated lymphocytes from HIV-negative donors were infected in vitro with HIV-1. Expression of surface molecules was measured by flow cytometry using the HLA-E-specific monoclonal antibody 3D12 and an HLA-A, -B, -C-specific antibody. HLA-E and TAP mRNA levels were determined by real-time PCR. A chromium-51 release assay was performed following standard protocols.

Results:  We found a significantly increased percentage of HLA-E-positive CD4⁺ (2,92%±1.59 vs 0.59%±0.35; p=0.002) and CD8⁺ cells (1.83%±1.67% vs. 0.43%±0.13%, p<0.002) in HIV-RNA positive patients as compared to uninfected subjects.  Furthermore, in vitro infection of lymphocytes with HIV-1 resulted in marked up-regulation of HLA-E expression leading to reduced susceptibility of HIV-infected cells to NK cell lysis. Up-regulation of HLA-E expression was neither due to HIV-mediated induction of HLA-E mRNA  nor due to up-regulation of TAP1 expression as proven by rt PCR. Using HLA-E-transfected K-562 cells we identified the well-known HIV T-cell epitope p24 aa14-22 as a ligand for HLA-E that stabilizes surface expression of HLA-E and thus inhibits NK cell cytotoxicity. Blocking experiments with monoclonal antibodies specific for NKG2A and HLA-E confirmed that this inhibitory effect of HIV p24 aa14-21 on NK cells was due to interactions of the inhibitory NK cell receptor complex CD94/NKG2A with HLA-E.

Conclusions:  In conclusion, these results propose HIV-mediated up-regulation of HLA-E expression as an efficient evasion strategy targeting the antiviral activities of NK cells and allowing the virus to establish itself as a chronic infection.

Keywords: HLA-E; NK cells; Immune evasion