Heinz W. Kunz

The major histocompatibility complex of the rat.

Immunogenetic studies in the rat began with the investigation of blood group antigens (1-7) and evolved into the investigation of histocompatibility antigens and transplantation phenomena (8-17). In the course of this work, several nomenclature systems evolved that were eventually reconciled in a series of comparison studies (18-24). The biennial Workshops on Alloantigenic Systems in the Rat held under the aegis of the Transplantation Society (24) have provided the forum for the continued evolution of this field. In addition, several reviews (24-31) and books (32-35) have provided periodic summaries of different aspects of rat immunogenetics. This review will focus on the structure of the major histocompatibility complex (MHC) of the rat from the serological, biochemical, and molecular points of view.

Genetic studies in inbred rats. I. Two new histocompatibility alleles

Two genetically inbred strains of rats with new histocompatibility alleles—the KGH and WKA strains—were characterized. Neither the KGH nor WKA strain cells reacted with known Ag‐B * specific antisera; specific antisera against these strains did not react with cells from any known Ag‐B group; and there was no cross‐reactivity between the histocompatibility antigens of the two strains. On the basis of these data, the KGH strain was assigned the Ag‐B7 allele and the WKA strain, the Ag‐B8 allele. Both strains were low responders to the test antigen poly(Glu52Lys33Tyr15). In various crosses, the antibody response segregated as expected, i.e. low response was associated with the Ag‐B7 or Ag‐B8 alleles in the F2 and backcross generations.

Instability of neural xenografts placed in neonatal rat brains.

Embryonic mouse retinae placed in neonatal rat brains differentiate normally, form appropriate connections with the host brain, and may survive for longer than 1 year. However, such grafts are susceptible to rejection, either spontaneously or after challenge. Advanced spontaneous rejection of the transplant was identified in about 10% of the animals. In addition, two circumstances have been defined in which mouse retinal grafts can be subsequently induced to undergo rejection. One is following placement of a mouse skin graft on the flank of a rat that has received a retinal transplant in the brain, and the other is following removal of a host eye. After each of these procedures, the neural grafts become infiltrated with lymphocytes and undergo degeneration. It is proposed that this system may provide a useful approach not only for studying the immunology and genetics of neural transplantation, but also for examining the circumstances that precipitate the degenerative events associated with certain autoimmune diseases of the central nervous system.

Immunologic sensitization prior to birth

Abstract A series of clinical and experimental reports have been presented in which there is evidence to indicate that prenatal sensitization of the fetus to specific antigens can occur during apparently normal pregnancies. It appears that the mechanism involved in the alteration of the immune response may be the prenatal passage of antigen and/or maternal cells to the developing fetus and the production of specific immunologic reactivity. If future clinical and experimental investigations define more clearly the mechanism by which this event occurs, methods of specifically altering the immune response of the offspring could be developed. This type of “immunologic engineering” could afford the offspring an enhanced immune response against infectious agents and vertically transmitted oncogenic agents or could induce specific tolerance to selected tissue antigens.

The role of MHC and non-MHC antigens in the rejection of intracerebral allogeneic neural grafts

Embryonic DA retinal allografts that have survived for prolonged periods after having been transplanted into the brains of neonatal BN rats can be induced to reject following peripheral sensitization with a DA skin graft. The results show that histocompatibility antigens play the major role in the rejection of grafts placed in the CNS and that a disparity between the retinal and skin grafts for MHC antigens induces a more severe rejection response than does a non-MHC antigen disparity.

Immunogenetic aspects of transplantation in the rat brain

The nature of the brain as an mmunologically privileged site is controversial. Using congenic rats, we have demonstrated that grafts that differ from the recipient in the A locus of the MHC produce sensitization following intracerebral transplantation, thus indicating that the brain is not immunologically privileged. The IC grafts show histologic signs of rejection, which are delayed compared with the changes found in the subsequent orthotopic grafts, suggesting that the immune response is weaker in the brain.

Localization of the Pa antigen on the placenta of the rat.

A unique class I MHC antigen, the Pa antigen, is the major immunogenic molecule on the placenta of the rat. It carries only a widely shared public antigenic determinant, and it is located on the basal trophoblast.

Elicitation of the maternal antibody response to the fetus by a broadly shared MHC class I antigenic determinant

The antibody response to the semiallogeneic fetus in the rat was examined in eight mating combinations that were chosen specifically to across different major histocompatibility complex (MHC)2 and nonMHC loci. The BN×DA and BN×BN.IA(DA) matings (n anti-a) were the only ones to show an antibody response to the fetus, and the response occurred in 25–64% of the matings. All the other six mating combinations studied were negative by hemagglutination, cytotoxicity, and cellular radioimmunoassay. The response appeared in the postpartum period after the first litter and reached its maximal frequency during the postpartum periods of the second and third matings. There was no correlation between the litter size and the presence or absence of an antibody response to the fetal antigens. The antibodies elicited by the fetus were hemagglutinating but not cytotoxic, and consisted of only one isotype, IgG2a, whereas in the alloantiserum raised by skin grafting and the injection of lymphocytes, they were distributed among the IgG2a > IgG1 > IgM isotypes and were both hemagglutinating and cytotoxic. Using reciprocal congenic strains, the pregnancy-induced antibody response was shown to be against MHC antigens only. This finding was confirmed by testing the sera against rats from a population that was segregating for the haplotypes a, a/n, and n/n. Comparison of the pregnancy-induced antibody with the BN anti-DA alloantibodies by absorption studies showed that the former was less crossreactive, did not react with the RT1.Aa haplotype-specific antigenic determinant, and did react with an antigenic determinant shared among the a, d, o, i, e, f, r10, b, and m haplotypes—but not with the n, c, g, k, l, h, or u haplotypes. Using the DA.II and r10 strains in cellular radioimmunoassay, absorption, and immunoprecipitation experiments, it was shown that the pregnancy-induced antibody was elicited by a class I antigenic determinant, provisionally designated Pa, which is encoded by a locus in the vicinity of RT1.A.

ORIENTATION OF LOCI IN THE MAJOR HISTOCOMPATIBILITY COMPLEX OF THE RAT AND ITS COMPARISON TO MAN AND THE MOUSE

Among 290 F2 progeny of an r10 x ACP cross were two recombinants which allowed the loci for glyoxalase‐1 and neuraminidase‐1 to be mapped relative to the RT1.A and dw‐3 loci in the major histocompatibility complex (MHC) of the rat. In 673 progeny of the same cross there was a recombinant between ft and dw‐3, and in 403 progeny of the backcross BY1 x (BY1 x BDIX)F1 there was another recombinant between ft and dw‐3. These data, combined with those from previous studies, provide the information for constructing a detailed map of the rat major histocompatibility complex: the gene order and size in the rat are very similar to those in the mouse and different from those in man and in the other species that have been studied. Comparison of the structures of the MHC in the various species leads to a hypothesis about the evolution of the MHC which involves sequential duplications of the genes coding for class I and class II loci and an inversion in the prototypic muridae which placed the class II loci between the class I loci.

Pregnancy-induced monoclonal antibody to a unique fetal antigen

An IgM monoclonal antibody was produced by fusing maternal splenic lymphocytes obtained on the day of delivery of the first litter in a WFxDA mating with P3–653 myeloma cells, and at the time when the lymphocytes were isolated there was no circulating antibody in the pregnant animal. The monoclonal antibody had the same specificity as the antibodies in the postpartum pregnancy serum: it reacted with a unique class I MHC antigen, designated the pregnancy-associated (Pa) antigen, which is not one of the currently known, haplotype-specific class I antigens. The use of recombinant strains mapped the locus encoding the Pa antigen to the region of RT1.A and to the left of RT1.B.