HLA-DRB1 amino acid disparity is the major stimulus of interleukin-2 production by alloreactive helper T-lymphocytes

The generation of interleukin-2 (IL-2)-mediated helper activity is a central step in the immune response induced by allogeneic histocompatibility antigens, and IL-2-producing helper T-lymphocyte precursor (HTLp) frequencies have been proposed as a measure of alloreactivity in transplant recipients. We analyzed the influence of HLA-matching on the alloresponse of HTLp in limiting dilution assays derived from healthy individuals. Mean HTLp frequencies were significantly higher in HLA-DR antigen-mismatched than HLA-DR-matched combinations. Significant differences in the effect of one or two mismatched HLA-DR antigens on mean HTLp frequencies were also detected. Mean HLA class I (HLA-A, -B, -Cw) mismatches were not significantly different in each group and had no apparent influence on HTLp frequencies. Analysis of HLA protein sequence disparities revealed no significant difference in the number of mismatched amino acid residues at the HLA-DRB1 locus between one and two HLA-DR antigen-mismatched combinations but correlated strongly with HTLp frequency. The positive correlation was evident with mismatched residues in the beta sheet and alpha helical regions of the HLA-DRB1 molecule, suggesting a predominant influence of bound peptides in the stimulation of alloreactive helper cells. This finding was supported by analysis of the location of individual residue mismatches. Evidence of an effect of polymorphism in the CD4-binding region in the beta-2 domain of HLA-DRB1 molecules was also found. Our results demonstrate the major influence of HLA-DR amino acid sequence mismatching on alloreactive HTLp frequencies but also suggest that additional genetic or environmental influences affect the alloreactive helper T-cell repertoire.