HLA-DPA1 and pulmonary arterial hypertension: The importance of antigen-presenting and recognition in PAH is underlined by the most significant genetic variant discovered in the PAH GWAS in the HLA-DPA1/DPB1 locus encoding class II major histocompatibility complex (MHC) molecules [16], which associated with three HLA-DPB1 alleles, all containing a glutamic acid at amino acid residue 69 (Glu69).