Cloning and expression of a cDNA encoding a non-classical MHC class I antigen (HLA-E) in eosinophils from...

A cDNA library, constructed from purified blood eosinophils, was screened with the B cell CD23 cDNA probe. A clone designated EO15 has been isolated and found to encode a non-classical HLA class I gene transcript. EO15 was compared with HLA-E and found to be 99.9% similar at the nucleotide level and to extend further in the 3' untranslated region. The presence of an additional polyadenylation signal in the EO15 3' end suggests that EO15 clone represents a copy of the 3.3-kb mRNA species detected in Northern blot analyses. HLA-E transcripts of 1.9 and 3.3 kb have been described in a variety of cell types. The two EO15 mRNA species, similar in size to the previously defined HLA-E mRNA, were present at high levels in blood leukocyte populations and at variable levels in different cell lines. The EO15 transcripts were found at abundant levels in hypodense and normodense eosinophils from hypereosinophilic patients. In situ hybridization confirmed the expression of EO15 mRNA in eosinophils. Neutrophils and lymphocytes from normal donors of from patients with hypereosinophilia also strongly expressed EO15 mRNA. Among the cell lines studied, the highest levels of EO15 transcripts were detected in B and monocytic cell lines, whereas intermediate and lower levels were found in eosinophilic, NK-like, megakaryocytic, and T cell lines, respectively. Similar to its effect on classical HLA class I transcripts, IFN-gamma increased the levels of EO15 mRNA in eosinophils and neutrophils from hypereosinophilic patients. These results suggest that purified blood eosinophils as well as neutrophils express EO15/HLA-E mRNA; however, further experiments are needed to investigate the localization and the function of EO15 protein products.