Biochemical studies on the H-2K mutant B6.C-H-2bm10

Gene conversion can create new MHC alleles

It is possible to measure gene conversion of MHC genes with the help of a semi-nested PCR assay. Several considerations are of utmost importance when such an assay is set up. Using this assay, we have found that gene conversion occurs in MHC class II genes in mouse sperm, but not in somatic cells tested. Although this gene conversion occurs in germline cells, it is already completed in spermatogonia, and consequently is mitotic event unlinked to meiosis. The frequency of gene conversion events in MHC class II genes varies strongly from one allele to another, with the highest detected frequencies as high as 1/40,000 for an individual heterozygous for both donor and acceptor sequences. Deletions or insertions in one gene relative to the other seem to lower the efficiency of gene conversion considerably. Stretches within MHC genes amenable to gene conversion are located in CpG clusters, whereas MHC genes not involved in gene conversion have background CpG levels. DNA damage, either chemical or radiation induced, increases the frequency of gene conversion of MHC class II genes in cultured cells of the fibroblastoid lineage. The effect of chemical DNA damage seems roughly dose dependent, whereas irradiation has a maximal effect at low doses.

Immunoselection of structural H-2Kb variants: use of cloned cytolytic T cells to select for loss of a CTL-defined allodeterminant

Functional studies concerning the unique interaction between class I H-2 allodeterminants and cytolytic T lymphocyte (CTL) antigen receptors have benefitted from the development of H-2Kb mutant mouse strains and somatic H-2 variants selected with monoclonal antibody. Here, we describe the development of a novel approach to immunoselection of somatic H-2Kb variants employing a Kb-specific CTL clone as the negative selective agent. The rationale for this method is that the use of an alloreactive CTL clone as the selective agent should enable us to detect the emergence of structural Kb variants based on their loss of the relevant CTL-defined allodeterminant. Thus, these structural variants are well suited to an in-depth analysis of the functional relationship between H-2 antigens and receptor recognition by CTL. Using this approach, we successfully isolated two types of structural Kb variants, as well as numerous Kb-loss variants. The functional studies described in this paper indicate that these structural variants exhibit alterations in expression of both CTL-defined and serologically defined H-2Kb allodeterminants. The structural characterization of such variants should enable us to identify the precise amino acid residues responsible for the creation of the relevant CTL-defined Kb allodeterminants.

An H-2K gene of the tw32 mutant at the T/t complex is a close parent of an H-2Kq gene

Two recombinant mice have been recovered from the progeny of Ttf/tw32 + animals. They have lost the tw32 lethality factor(s) and gained tufted, presumably from the T chromosome. Southern blot analysis of class I genes of these two new partial tPA027 and tPA286 haplotypes indicates that they have retained at least part of the major histocompatibility complex of the tw32 chromosome (H-2 haplotype H-2w28). We have prepared a phage library of Eco RI-digested DNA from homozygous tPA027 animals. Upon screening the library with a cDNA probe specific for H-2K genes, we isolated a class I gene displaying all of the distinctive features of a genuine H-2K gene, and which could thus be defined as an H-2Kw28 gene. The H-2Kw28 gene is 92-95% homologous to H-2Kb and H-2Kd genes and differs significantly from the other class I genes sequenced so far. Homology with the H-2Kb sequence reaches nearly 100% in the 3' part of the H-2Kw28 gene. Moreover, the homology with an H-2Kq cDNA sequence reaches 99.8%. Several hypotheses can account for the near identity of H-2Kb, H-2Kq, and H-2Kw28 gene sequences: either recombination between H-2w28 and H-2b and H-2q sequences occurred before or at the time the strain was established, or the class I genes of the tw32 chromosome and the H-2b and H-2q genes found in inbred strains of mice have separated from each other rather recently.

Evolution within the multigene family coding for the class I histocompatibility antigens: the case of the mouse t-haplotypes

The t-haplotypes of the house mouse are defined by several genetic events which involve about one-third of chromosome 17 and include the major histocompatibility complex (MHC). One of the primary features of complete t-haplotypes is the nearly complete suppression of recombination between a t-chromosome and a wild-type chromosome 17. If we assume that all genetic exchanges are abolished because of the inhibition of recombination, the class I genes of the MHC can be used as vicinity markers of t-haplotypes. We have studied the H-2K, D and L genes of 10 independant t-haplotypes and have found that their H-2K genes are rather polymorphic, although always very similar or identical to H-2K genes of laboratory mice. By contrast, the DL-Tla region is nearly constant. It is suggested that the t-haplotypes reveal the existence of several mechanisms involved in the evolution of the MHC, one of which is possibly the greater "mobility" of H-2K genes in the genome.

Restriction fine specificity of long-term, hapten-specific cytotoxic T cell clones: analysis with H-2Kbm-mutant mice and H-2Kb-specific monoclonal antibodies

The cell-mediated cytotoxic response against autologous cells modified with the sulfhydryl reagent I-AED [N-iodo-acetyl-N-(5-sulfonic-1-naphthyl) ethylene diamine] has been described by Levy, R. B., Shearer, G. M., Richardson, J. C. and Henkart, P. A., [J. Immunol. 1981. 127: 523.]. We have established two H-2Db- and eight H-2Kb-restricted, AED-specific long-term cytotoxic T lymphocyte (CTL) clones of C57BL/10 origin. The growth of these clones has been dependent upon presence of both antigen and interleukin 2. Cytotoxicity and proliferation analysis of AED-specific Kb-restricted CTL clones with target and stimulator cells from Kbm-mutant mice demonstrated two categories of clones (A and B) based on different reactivity patterns against hapten-modified Kbm-mutant cells. AED-modified target cells of bm5, bm6 and bm9 origin were lysed by type A clones but not by type B clones, in contrast to AED-modified bm3, bm8 and bm11 target cells which were lysed by type B but not by type A clones. None of the clones lysed AED-modified bm1 target cells, but all of them lysed AED-modified bm4 and bm10 target cells. The restriction fine specificities for both cytolytic and proliferative activities of the analyzed clones were identical. A panel of monoclonal antibodies (mAb) directed against the Kb molecule was used to inhibit lysis of AED-modified target cells by CTL clones. mAb which recognize a cluster of allodeterminants located in the second external domain (C1 domain) of the Kb molecule [Hämmerling, G. J., Rüsch, E., Tada, N., Kimura, S. and Hämmerling, U., Proc. Natl. Acad. Sci. USA 1982. 79: 4737] only inhibited type B clones, whereas inhibition of both type of clones was observed with mAb specific for allodeterminants clustered in the N-terminal region of Kb. These findings suggest that different regions on the Kb molecule serve as self determinants for H-2-restricted cytotoxicity. We also demonstrate that allodeterminants recognized by mAb and self determinants involved in H-2 restriction need not be identical. Our data also support the notion that covalent AED modification of H-2 molecules is not necessary to generate the self plus X antigen for CTL recognition.

Biochemical studies of H-2K antigens from a group of related mutants. I. Identification of a shared mutation in B6-H-2bm5 and B6-H-2bm16

Structural studies of the H-2 gene products from a group of five closely related but independent C57BL/6H-2 mutant mice were undertaken. Each of the mutants exhibits reciprocal graft rejection with the parent. The group is remarkable, however, because each member of this group can accept skin grafts from any other member. The results of biochemical analysis of the H-2 glycoproteins from two of these related mutants, bm5 and bm16, are presented in this report. Evidence is given that the H-2K molecules from these two mutants are identical to each other based on comparative tryptic peptide mapping profiles with the parent. From partial amino acid sequence analysis, K products of both mutants have at least one common difference from the parental type located at residue number 116. Definitive studies established that in both bm5 and bm16 a tyrosine found in the parent molecule is substituted with a phenylalanine in the mutant. These results show that a biochemical difference between the K products of the two mutants and of the parent can be detected, that the mutants appear to be identical with one another even though they arose independently, and that they differ from the other H-2Kb mutants analyzed.