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The second locus in the series, RT2, was originally designated AgC, and by linkage similarities involving serum esterase loci appears to be homologous to the mouse Ea-1 locus. RT3 is a blood group locus that was formerly designated AgD. The histocompatibility loci RT4 and RT5 were previously designated H-2 and H-5, respectively and unfortunately most of the common inbred strains have not been characterized for these loci. The RT6 antigens (formerly designated ART-2, AgF, Pta and RT LY-2) are expressed on T lymphocytes in peripheral lymphoid tissues but are absent from thymus and bone marrow. The RT7 antigens (previously designated ART-1 and RT LY-1) are expressed on all thymocytes and peripheral T lymphocytes, while RT9 is a blood group locus linked to RT2, and all strains tested so far except WKA have the a allele. A monoclonal antibody which defines RT10 has been reported.

The study of immunoglobulin allotypes in rats was greatly facilitated by the development of the LOU/C strains, which have a unique susceptibility to the development of immunoglobulin-secreting tumors. Nomenclature conflicts have occurred in this field as well, but a uniform system in agreement with World Health Organization (WHO) guidelines is now in use. Allotypes of many common strains are shown in Table 1.

In addition to the above antigens, a variety of other loci of immunogenetic importance have also been characterized. These include tubular basement membrane antigens, the rat homolog of the CD4 lymphocyte antigen, W3/25 and interleukin 3 (II.-3).

Immunologically unique features of some rat strains are summarized in Table 2. The BB rat is not included in Table 1, as it has not been rigorously inbred. This is a spontaneously diabetic population of rats that was first obtained from a noninbred Wis-tar-derived stock maintained at the Bio Breeding Laboratory of Canada, Ltd, in Ottawa, Ontario. As this model has become increasingly popular, a good deal of inbreeding has undoubtedly occurred, but its genotype has not been reported at many polymorphic markers.

Rat strains have also been characterized for a large number of polymorphic biochemical markers. These include aconitase, aldehyde dehydrogenase, two alkaline phosphatases, two ct-amylases, twelve esterases, fumarate hydratase, a-glycerophosphate dehydrogenase, glyoxalase, hemoglobin, leucine aryl-aminopeptidase, peptidase, pepsinogen, 6-phospho-gluconate dehydrogenase, and seminal vesicle proteins. Many of these genes have been mapped by the use of rat/mouse somatic cell hybrids. Although the genetic map of the rat is not nearly as complete as

Table 2 Rat strains with well-characterized immunological susceptibilities

Strain Susceptibility References

BB Diabetes Nakhooda ef al (1977) Diabetes 26: 100-112

BN Nephritis Mathieson et al (1993) Journal of Experimental Medicine 177: 1309-1316; Hua et al (1993) Toxicology 79: 119-129 BUF Thyroiditis Cohen et al (1988) Cellular Immunology 114: 126-136

DA Arthritis Holmdahl and Kvick (1992) Clinical and Experimental Immunology 88: 96-100

LEC T cell maturation defect Agui et al (1990) Journal of Experimental Medicine 172: 1615-1624; Yamada et al (1991) Immunogenetics 33: 216-219 LEW Arthritis Ulmansky and Naparstek (1995) European Journal of Immunology 25: 952-957

LEW Encephalomyelitis Mor and Cohen (1995) Journal of Immunology 155: 3693-3699

that of the mouse, a great deal of progress has been made. Investigators who wish to map additional genes will benefit from the large numbers of microsatellite polymorphisms that have already been mapped. A growing collection of the oligonucleotides needed for these tests is available from Research Genetics, Inc. (Huntsville, AL, USA).

In some experiments, it is important to know whether sublines of the same strain have diverged from one another. This question has been examined by Matsumoto and colleagues by testing numerous sublines of the SHR and WKY strains for polymorphic variants. The SHR sublines varied at six of the 30 loci that were tested, and the WKY sublines differed at 15 of the 30 loci. Many strains and substrains were also surveyed for their genotypes at 39 loci by Bender and colleagues. These investigators observed variation at only WAG sublines, but there was variation at two of the loci among 13 sublines of BN, at three of the loci among four WF sublines, and at four of the 39 loci among five sublines of BUF. Twelve LEW sublines showed no variation at any of the 39 loci, but LEW/Mol differed from the other LEW strains at eight of the loci. Investigators should therefore be aware that there is a varying degree of subline diversity among inbred strains, and that this may or may not affect any given experiment.

See also: Adjuvant arthritis; Alloantigens; Experimental autoimmune encephalomyelitis (EAE); Inbred strains; Insulin-dependent diabetes mellitus, animal models; MHC, evolution of; Mouse inbred strains; Rheumatoid arthritis, animal models; Thyroid autoimmunity, experimental models.

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