For the past several years, we have been studying the Tla region of the mouse, located on chromosome 17 immediately to the right of (distal to) the major histocompatibility complex. We first became interested in this region after our initial discovery that it may code for several cell surface antigens in addition to TL ( 1, 2). Since then we have made a determined effort to define and more precisely locate these newly discovered cell surface loci and determine their characteristics. So far, six cell surface loci have been mapped to this region; two which determine skin and tumor graft rejection -- H-31 and H-32 ( 2) -- and four which determine differentiation antigens on lymphoid cells -- Tla ( 3), Qa-1 ( 4), Qa-2 ( 5), and Qa-3 ( 6). In addition, two genes which determine isozyme variations -- Pgk-2 ( 7) and Ce-2 ( 8, 9) -- have also been located here.
In this report, I will present the general characteristics of the serologically defined cell surface loci with particular emphasis on the newly discovered Qa series of antigens, pointing out the similarities and differences between them. I will also present data on the strain and substrain distributions of some of these antigens.
Table 1 and Figure 1 give a general description and mapping of these loci. Fuller descriptions are given below.
Tla. The Tla locus was first discovered in 1963 by Old and co-workers ( 3). It determines a series of thymus-leukemia specific antigens called TL. They found that there were three alleles determining four TL antigens ( 4). The fourth allele and fifth antigenic specificity were found by Flaherty et al. ( 12) in an analysis of the antibodies contained within an anti-H-2 serum.
These TL antigens have several unusual properties (for review see reference 11). First, they undergo a process known as antigenic modulation in which the presence of TL antibody or its Fab fragment induces the phenotypic loss of TL from the cell surface. Second, they appear on leukemias of mice which do not ordinarily express these antigens on their thymuses. Moreover, this leukemic TL expression is genetically determined since only certain TL phenotypes can appear on a leukemia from a given Tla genotype. And third, TL antigens have a reciprocal quantitative relationship with the antigens determined by a closely linked locus, H-2D, such that a TL+ cell expresses less H-2D antigen than a TL- cell.
Because of these characteristics and others, Boyse and Old ( 13) have suggested that there are actually two components to the Tla locus, one "structural" and one "expressional." The structural component directly specifies the TL protein while the expressional component determines the presence or absence of it. No crossing-over has ever been observed within the Tla locus separating these components. (For a further discussion of these two components and the mapping of them to the Tla locus, see reference 13).
Biochemically, the molecules expressing the TL antigens have a molecular weight of 45,000 with a β2-microglobulin subcomponent making them similar to the H-2D and H-2K molecules ( 14). Qa-1. The Qa-1 locus determines the presence or absence of the Qa-1 antigen ( 4). It appears on thymocytes as well as on a precise subpopulation of peripheral T lymphocytes. This tissue distribution distinguishes it from the known set of TL antigens which are found exclusively in the thymus.
The Qa-1 locus (or loci) may be complex. Stanton and Boyse ( 4) originally observed that different anti-Qa-1 sera gave slightly different strain distributions, consequently they have suggested that at least one of their anti-Qa-1 sera may have more than one antibody against peripheral lymph node cells.
Using the B6.K1 and B6.K2 recombinant strains, they have preliminarily mapped the Qa-1 locus to the chromosome stretch between H2-D and Qa-2. (B6.K1, B6.K2, and B6-Tlaa are all Qa-1+ while B6 and B6- H-2k are Qa-1-; see Figure 2.) However, in a further serological analysis of the Qa-1 antigen and its strain distribution, Stanton has found that a change in the source of rabbit complement or antiserum batch alters the activity of the anti-Qa-1 serum against B6.K1 and B6.K2 ( 15). 2 These results suggest that there is more than one activity in anti-Qa-1 sera and thus makes the mapping of the Qa-1 locus difficult. We have therefore placed a question mark after Qa-1 in our chromosomal map of the Tla region and feel that the main Qa-1 activity may be due to a locus more closely linked to Tla.
The Qa-1 antigen has not been biochemically analyzed.
Qa-2. The Qa-2 locus located between H-2D and Tla determines the presence or absence of the differentiation antigen Qa-2. We originally described this locus as a complex determining at least two antigenic specificities ( 5). By further serological analysis, we have now subdivided this locus into two loci, Qa-2 and Qa-3 ( 6). Qa-3 will be described in the next section.
As with the Qa-1 antigen, Qa-2 is on a restricted population of lymphocytes as seen by its low expression on thymocytes (~20% are Qa-2+) and its high expression on lymph node cells (>65% are Qa-2+). It is also present in the spleen and bone marrow. By cytotoxicity testing, it is predominantly expressed on Thy-1+ lymphocytes although at least some Thy-1- cells are Qa-2+ ( 6). In addition, it is present on some T cell leukemias; both TL+ and TL- leukemias can be Qa-2+.
Qa-2 is also associated with a CML (cell-mediated lympholysis) locus. Forman and Flaherty ( 16) have found that Qa-2b lymphocytes will kill Con A blasts from a Qa-2a mouse in a secondary in vitro CML test. This killing correlates completely with our strain distribution for the serologically detectable Qa-2 antigen.
Qa-3. The Qa-3 locus was separated from Qa-2 on the basis of both the strain and tissue distributions of the Qa-2 and Qa-3 antigens which they determine. Qa-3 is more limited in its tissue distribution than Qa-2 and is only present on lymph node and spleen cells. It is absent from at least two inbred strains of mice which type Qa-2+ (Qa-2a) ( 6). These results indicate that Qa-3 is different from Qa-2 and is determined by a different locus, Qa-3. So far we have not detected any recombinants between Qa-2 and Qa-3, and therefore the precise position of Qa-3 is not known (see Figure 1).
Using immunoprecipitation techniques, Michaelson et al. ( 17) have shown that an antiserum which contains both Qa-2 and Qa-3 activity precipitates a molecule of approximately 45,000 daltons with a β2-microglobulin subcomponent. Subsequent studies have preliminarily shown that the main molecule precipitated by this serum is the Qa-3 molecule (Michaelson et al., personal communication). The molecular weight of the Qa-2 molecule is still not known.
In the course of testing different strains of mice for their Qa phenotype, we have found that certain BALB/c sublines differ at both the Qa-2 and Qa-3 loci ( 6).
The BALB/c strain was derived from a colony of albino mice maintained by Bagg. In the early 1920s, MacDowell obtained this colony from Little and proceeded to inbreed it. In 1932, it was given to Snell in the F26 generation, who subsequently gave it to Andervont, Green, and Scott. It was then widely distributed to many laboratories. Figure 3 gives a partial pedigree of the BALB/c strain with our current knowledge of its distribution ( 18, 19, 20). In our Qa-2 and Qa-3 typings, we have found that the BALB/c sublines have two Qa phenotypes -- all lines derived from Green or Scott are Qa-2+ Qa-3+, while most of the lines derived from Andervont are Qa-2- Qa-3-. One BALB/cAn-derived line, BALB/cBoy (and therefore also all strains derived from it such as BALB/cFla), is Qa-2+ Qa-3+. The simplest explanation for this genetic divergence is that either a mutation or deletion occurred in the BALB/cAn line. It is unlikely that it is due to foreign genes still segregating from the original Bagg albino stock since the BALB/c strain was at least in its F29 generation when given to Andervont. We have also typed these sublines for the neighboring loci, H-2D, D' and Tla, and found them to be identical ( Table 2). If the Qa-2 and Qa-3 phenotypes were maintained from the original heterogeneity in the Bagg albino stock, it would be very unlikely that neighboring genes would not also be affected. Therefore, our hypothesis of a mutation or a deletion seems to be the most tenable one at the present time. The skin grafting results presented in Table 2 between BALB/cBy and BALB/cFla also indicate that there are other genetic differences between these two lines. This is not surprising since these strains are at least 60 generations apart.
If our hypothesis that a mutation or deletion occurred in the BALB/cAn subline is correct, it would indicated that this genetic event (or two consecutive events) modified both Qa-2 and Qa-3.
In our typings of a number of inbred and recombinant strains of mice, we have noticed several correlations between the alleles within the Tla region and between the alleles of the Tla region and the H-2 region ( Table 3).
First, there is a strong correlation between the Qa-1 genotype and the Tla genotype. Most, if not all, tested strains which are Qa-1a are either Tlaa or Tlad, while all tested strains which are Qa-1b are either Tlab or Tlac. This may simply be because Qa-1 and Tla are tightly linked. Since many of the inbred strains have a common ancestry ( 21) it would be likely that the alleles of these two loci would then be preserved together. At present the position of Qa-1 in relationship to Tla is somewhat in doubt, and therefore this hypothesis must be considered a viable one. Alternatively, of course, there may be some functional relationship between these two genes. Currently, there are not enough data to suggest which of these hypotheses is correct.
With few exceptions, there is a strong correlation between the strain distribution of the H-2D.28 specificity and the Qa-2 antigen. All tested inbred strains which have been described as H-2D.28- are also Qa-2- (H-2f,k,p,r). We have investigated this correlation and found that one reason for it is probably due to the contamination of at least one anti-H-2.28 typing serum with anti-Qa-2 activity ( 6). By absorption and direct cytotoxicity testing, we have found that the anti-H-2.28 serum (B10.BR x A.CA) anti-A.SW contains a substantial amount of anti-Qa-2 (and/or Qa-3) activity. Thus a strain would only appear negative with this typing antiserum if it were also negative for Qa-2 and appear positive if it were positive for either H-2.28 or Qa-2.
Structurally the Qa-3 and TL molecules are very similar to the H-2K, H-2D, and D' molecules; they are all approximately 45,000 daltons in size and associated with β2-microglobulin. Immunoprecipitation techniques have so far been unsuccessful in determining the molecular weights of the Qa-1 and Qa-2 molecules. These similar molecular weights might indicate that these genes have a common evolutionary ancestry.
The most interesting relationship to us is that between the expression of Qa-2 and Qa-3. In typing a large number of inbred and recombinant strains plus a few wild haplotypes, we have only observed three Qa phenotypes -- Qa-2+ Qa-3+, Qa-2- Qa-3- and Qa-2+ Qa-3- (and not Qa-2- Qa-3+). This is also true for the tissue and tumor distributions of these antigens. There seems to be an obligatory requirement for Qa-2 in order for Qa-3 to be expressed. This is similar to the relationship between two of the TL antigens, Tl.2 and Tl.3 ( 22). In all known cases, there is no strain or leukemia which expresses TL.3 without expressing TL.2.
I would like to comment on one last point. Even though our information is still limited on the Qa loci, we have observed that in contrast to the neighboring H-2 complex, the genes of the Tla region, Qa-1, Qa-2, Qa-3, and Tla, do not appear to be highly polymorphic and each appears to have an allele which is serologically undetectable. In the cases of Qa-2, Qa-3, and Tla we have made several efforts to induce antibodies against these alternative gene products especially those potentially governed by null alleles but have been unsuccessful. This is also true of the CML locus associated with Qa-2. Forman and Flaherty ( 16) have found that this reaction is unidirectional; Qa-2b Qa-3b lymphocytes will kill Qa-2a Qa-3a cells but the reciprocal is not true. The level of cell-mediated cytotoxicity against Qa-2a Qa-3a cells also does not vary significantly from strain to strain, indicating that, if polymorphism exists, all the differing gene products behave similarly. We realize that congenic immunizations are sometimes difficult ( 23) and that Ir genes are sometimes important in detecting mouse alloantigens ( 24); however, it is still interesting to speculate that the Tla region might have several true null alleles which do not code for allogenic cell surface antigens.
Research on the Tla region and the differentiation antigens which it determines is relatively new. Three of these antigens were only discovered within the last two years and have not been fully characterized. Even with our limited knowledge of this genetic region, it is becoming apparent that this region is complex and might have some unique properties. We hope that it will provide us with information not only on the genetic control of the lymphoid cell surface but also on its differentiation.
The author wishes to thank Ted Hansen for the serum, BALB/c-H-2db anti-BALB/c and Don Bailey for his help in constructing the BALB/c pedigree.
1This work was supported in part by NIH grant AI 12603 awarded by the National Institute of Allergy and Infectious Diseases and NIH Grant CA 23027 awarded by the National Cancer Institute, DHEW.
2Where these strains were originally strongly Qa-1+ with one source of complement and antiserum, they later appeared to be either weakly positive or negative. In our laboratory we have also confirmed these findings. Using a different anti-Qa-1 serum, (A.SW x A.TL) anti-A.TH, we have found that certain sources of rabbit complement will show B6.K1 and B6.K2 as weakly positive where others will show these strains to be negative ( 9).
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See also MGI.
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See also MGI.
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See also MGI.
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See also MGI.
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See also MGI.
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See also MGI.
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See also MGI.
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