The HLA‐A10 crossreacting group consists of the A25, A26, A34, A43 and A66 antigens. Here, we report allelic sequences for A43 and for 2 subtypes of both A26 and A34. Combining these results with previously determined sequences for A25, A26 and A66 enables molecular comparison of all the serologically defined A10 antigens. They form a closely related and well‐defined group of alleles which may have originated with A*2601. Patterns of serological crossreactivity are correlated with sequence and a public epitope shared by A33 and members of the A10 family is localized to residues R62 and N63. The A*2501, A*4301 and A*6601 alleles appear to have derived from A*2601 by single gene conversion events with other HLA‐A alleles. In the case of A*4301, the donor allele was probably an A29 allele as A*4301 has a small element of sequence in the α1helix (residues L62 and Q63) uniquely shared with A29. The chimaeric structure of A43 explains the reactivity of A43 molecules with both A10 and A29 alloantisera. The rare Oriental variant of A26 (A26v*) is encoded by an allele (A*2602) that differs from A*2601 by a unique nucleotide substitution which changes aspartate to asparagine at position 116 in the floor of the peptide binding groove. Thus A*2602 is a functionally distinct allele that originated by a point mutation. Alleles encoding A34 and A66 antigens are found to have very similar structures, explaining the difficulty in their serological definition. Further illustration of serological difficulties in discriminating the antigens of the A10 group is the finding that the A26.2 electrophoretic variant of A26 is identical in structure to A*6601.

Alleles encoding five HLA-A and B Ag characteristic of black populations have been isolated and their nucleotide sequences determined. In each case, the "black" allele is similar to a "related" allele found in caucasoid populations. The primary differences between these pairs of alleles are localized clusters of nucleotide substitutions that change two to five residues of the Ag recognition site. The pattern of differences indicates that the pairs of black and caucasoid alleles diverged primarily as a result of interallelic conversion events.

Previous analysis has emphasized the correlation between primary structures of class I HLA molecules and their patterns of serologic cross-reactivity. Here we describe the structures of two serologic groups of HLA-B alleles for which this is not the case. HLA-B45, an allele associated with black populations, is serologically paired with B44 in the B12 group; its structure, however, is divergent from that of B44 but closely related to B50. The BN21 (B*4005) allele is associated with native Americans and is serologically grouped with B50 in the B21 group; its structure, however, is more closely related to alleles of the B40 group. The B44 and B45 serologically cross-reactive molecules differ at seven functional positions of the Ag recognition site; the B50 and BN21 molecules differ at four such residues. These differences are predicted to alter peptide presentation and be capable of eliciting strong alloreactive T cell responses. For these pairs of B12 and B21 Ag, serology appears dominated by epitopes formed by short sequences of the alpha2 helix which have been shuffled by recombination between alleles. The implications of these results for HLA matching in transplantation are discussed.

THE Kaingang and Guarani are culturally and linguistically distinct tribes of southern Brazil 1,2. Like all Amerindian groups 3,4 they show limited HLA polymorphism, which probably reflects the small founder populations that colonized America by overland migration from Asia 11,000-40,000 years ago, 5,6. We find the nucleotide sequences of HLA-B alleles from the Kaingang and Guarani to be distinct from those characterized in caucasian, oriental and other populations 7. By comparison, the HLA-A and C alleles are familiar. These results and those reported in the accompanying paper 8 on the Waorani of Ecuador reveal that a marked evolution of HLA-B has occurred since humans first entered South America. New alleles have been formed through recombination between pre-existing alleles, not by point mutation, giving rise to distinctive diversification of HLA-B in different South American Indian tribes.

In the search for genetic variability in individual susceptibility to mucocutaneous leishmaniasis, a disease caused mainly by Leishmania (Viannia) braziliensis, HLA typing was performed on 43 patients presenting mucosal lesions and 111 matched controls. Antigen specificities of the HLA-A, -B, -C, -DR, and -DQ loci were determined and their frequencies in patients and controls were compared. There was a significant decrease in the frequency of HLA-DR2 [1 out of 38 (2.6%) patients vs. 29 out of 102 (28.4%) controls, corrected p value 0.004, relative risk 0.07, preventive fraction of the total population 0.26] as well as a significant increase of HLA-DQw3 [29 out of 38 (76.3%) patients vs. 43 out of 99 (43.4%) controls, corrected p value 0.006, relative risk 4.2, etiologic fraction of the population 0.58]. These results support participation of HLA class II molecules in individual susceptibility to mucocutaneous leishmaniasis and in the pathogenesis of metastatic, mucosal disease.

A monomoprhic monoclonal antibody (LA45 antibody) reactive with "a new activation-induced surface structure on human T lymphocytes" (LA45 antigen) that resembled free class I heavy chains has recently been described (Schnabl, E., H. Stockinger, O. Majdic, H. Gaugitsch, I.J.D. Lindley, D. Maurer, A. Hajek-Rosenmayr, and W. Knapp. 1990. J. Exp. Med. 171:1431). This antibody was used to clone a class I-like heavy chain (LA45 gene) from the HUT 102 tumor cell, which paradoxically did not give rise to the LA45 antigen on transfection into monkey COS cells. We show here that the LA45 gene is HLA-Aw66.2, a previously uncharacterized allele of the HLA-A locus. The previously determined LA45 sequence differs from that of HLA-Aw66.2, from HUT 102, and the CR-B B cell line derived from the same individual as HUT 102 by substitution of tryptophan for serine at position 4 in the alpha 1 domain. Transfection of HLA-Aw66.2, and of a mutant of this gene with serine 4 substituted for tryptophan, into a human B cell line (C1R) both resulted in expression of the LA45 epitope. Furthermore, we find expression of the LA45 epitope on Epstein Barr virus-transformed B cell lines as well as lectin-activated T cells, but not on long-term T cell lines or unstimulated peripheral blood T cells. The specificity of the LA45 antibody is polymorphic and the presence of the LA45 epitope is precisely correlated with the sequence arginine, asparagine (RN) at residues 62 and 63 of the helix of the alpha 1 domain. The LA45 epitope is broadly distributed, being associated with half the alleles of both HLA-A and -B loci but none of the HLA-C locus. All the results are consistent with the presence of pools of free HLA-A and -B heavy chains at the surfaces of certain cell types but not others. Such molecules are probably responsible for the HLA-associated class I alloantigens of lectin-activated T cells. We hypothesize the free heavy chains result from dissociation of beta 2-microglobulin from subpopulations of empty HLA-A,B molecules, or molecules with weakly bound peptides, that vary in size depending on cellular activation and peptide supply.