Peter K. Gregersen

Strong association of rheumatoid arthritis with the presence of a polymorphic Ia epitope defined by a monoclonal antibody: comparison with the allodeterminant DR4

SummaryAmong individuals with rheumatoid arthritis the presence of the polymorphic Ia antigen epitope detected by the monoclonal antibody 109d6 is more strongly correlated with disease susceptibility than are other specificities, such as HLA DR4, DRw53 (MT3) or the antigenic determinant, defined by the monoclonal antibody 17-3-3S. The cells of 93% of Caucasian and Hispanic patients react with the 109d6 reagent. As was the case in normal individuals, all DR4-positive patients express the 109d6 determinant; however, 26% of those with rheumatoid arthritis have the epitope recognized by antibody 109d6, but lack the specificity DR4. Of these, one-third expresses only HLA DR1 and DQw1 (MT1, MB1) determinants. Studies of family members reveal that here the determinants 109d6, DR1, and DQw1 are encoded by the same unusual haplotype. In certain other individuals with rheumatoid arthritis who express DR4, DRw53, and the 109d6 determinants, family studies show that the 109d6 epitope is encoded not only by the haplotype specifying DR4 but also by the opposite haplotype that does not bear the genes for DR4. This suggests that homozygosity for certain Ia epitopes is relevant to determining the disease-susceptibility state. These studies emphasize the utility of monoclonal antibodies as reagents for the recognition of Ia epitopes that are more closely involved in the determination of disease susceptibility than are allomorphic molecules detected by conventional typing alloantisera.

Evidence for the separate molecular expression of four distinct polymorphic Ia epitopes on cells of DR4 homozygous individuals

The monoclonal antibodies 109d6 and IVD12 reacted with separate polymorphic Ia epitopes in immunofluorescent studies. Using a panel of lymphoblastoid B cell lines, antibody 109d6 reacted with all HLA-DR4 or DR7 positive lines in a pattern resembling the MT3 specificity recognized by human alloantisera. The antibody IVD12 reacted with all HLA-DR4 and two of three DR5 positive B cell lines suggesting that it recognized a specificity similar to MB3. The intensity of fluorescence was greater on DR5(+) cell lines than on DR4(+) cell lines relative to the amount of a nonpolymorphic Ia determinant. Among 45 unrelated control individuals reactivity with antibody 109d6 was correlated most closely with (r = 0.724) but not identical in occurrence to the MT3 specificity. Cocapping experiments demonstrated that the 109d6 epitope and the IVD12 epitope were present on independently redistributed cell surface molecules of DR4 homozygous lymphoblastoid cell lines. Furthermore, the DR4 alloantigens detected by an absorbed polyclonal human alloserum were similarly identified on molecules that were independent from those bearing either the 109d6 epitope or the IVD12 epitope. Taken together, these data indicate the existence of at least four distinct, serologically defined Ia molecular species.

Polymorphism of the DR1 Haplotype: Structural and Functional Analysis

HLA-DR haplotypes vary in the number of Dw subtypes defined by the mixed lymphocyte reaction (MLR). DR2 and DR4 have been shown to be quite complex (1,2) whereas DR1 is relatively homogeneous in populations of Northern European extraction (3). In Caucasians, almost all DR1 individuals express a DR1,DQw5,Dwl haplotype. The DR1 haplotype in American blacks is always associated with DQw5, but approximately 50% of DR1 positive American blacks express a Dw allele undefined by mixed lymphocyte typing (Dw blank or Dw-) (4). Undefined Dw specificities associated with DR1 have been reported in Nigerians (5) and in HLAB14-related haplotypes (6–8). It is not clear whether the DR1,Dw-haplotype found in American blacks, which is not associated with HLA-B14, is the same variant described by others. The structure and function of the American black DR1,DQw5,Dw-haplotype have been studied and compared to the Caucasian DR 1 haplotype using alloproliferative T-cell clones and cDNA sequencing.

DR3 Heterogeneity in American Blacks

The heterogeneity of DR3-associated haplotypes in American Blacks is striking when compared with that in the Caucasoid populations. In American Blacks, only 7.7% of DR3-positive individuals are Dw3. Three DR3-associated haplotypes are generally observed in the American Black population. Two appear to be unique to Blacks and are 1) A30, Bw42, Dw-, DRw18, DRw52a, DQw4, and 2) Bw53 or Bw71, Dw3v, DRw 17, DRw52b, DQw2. HLA-D typing responses for these haplotypes are Dw- (DNV>70) and Dw3v (DNV 35–55), respectively. The third DR3 haplotype is identical to the Caucasian Al, B8, Dw3, DRw17, DRw52a, DQw2.

Evolution of DQw2-Related Haplotypes

The majority of HLA-DR alleles have been grouped into two families on the basis of supertypic serologic specificities, DRw52 and DRw53. Although members of each family share a common evolutionary pathway, there is evidence that the DRw52 and DRw53 pathways have crossed producing haplotypes that express genes found in both families. Haplotypes that share DQw2-like α genes are an example of this convergence. The haplotypes in this group include DRw52-associated DR3 (DQw2 and DQw4), DR5 (DQw3), DRw8 (DQw4), and DRw- (DQw2); and DRw53-associated DR7(DQw2 and DQw3) and DRw9 (DQw2). Some of these haplotypes share the DQw2 serologic specificity whereas others share only a DQw2-like α gene. A unique DQB2 Hinc II DNA restriction fragment is found associated with many of these haplotypes. Reciprocal recombination, mutation, and gene conversion play a major role in the generation of diversity in these haplotypes.