The expression level of HLA class-I
proteins is known to influence pathological outcomes: pathogens downregulate HLA to evade host immune responses, host inflammatory reactions upregulate HLA, and differences among people with regard to the steady-state expression levels of HLA associate with
disease susceptibility. Yet precise quantification of relative expression levels of the various HLA loci is difficult because of the tremendous polymorphism of HLA. We report relative expression levels of
HLA-A,
HLA-B,
HLA-C, and HLA-E
proteins for the specific haplotype A*02:01, B*44:02, C*05:01, which were characterized using two independent methods based on flow cytometry and mass spectrometry. PBLs from normal donors showed that
HLA-A and
HLA-B proteins are expressed at similar levels, which are 13-18 times higher than
HLA-C by flow cytometry and 4-5 times higher than
HLA-C by mass spectrometry; these differences may reflect variation in the conformation or location of
proteins detected. HLA-E was detected at a level 25 times lower than that of
HLA-C by mass spectrometry. Primary CD4(+) T cells infected with HIV in vitro were also studied because HIV downregulates selective HLA types.
HLA-A and
HLA-B were reduced on HIV-infected cells by a magnitude that varied between cells in an infected culture. Averaging all infected cells from an individual showed
HLA-A to be 1-3 times higher and
HLA-B to be 2-5 times higher than
HLA-C by flow cytometry. These results quantify substantial differences in expression levels of the
proteins from different HLA loci, which are very likely physiologically significant on both uninfected and HIV-infected cells.