Traits That Influence Longevity in Mice: A Second Look

Analysis of genetic interactions in the F2 of an intercross of (C57BL/6 x DBA/2) F1J revealed influences of genetic factors on life span. Females lived longer than males. Dilute brown females died sooner than females of other colors. H-2b/H-2b males died sooner than H-2b/H-2d or H-2d/H-2d males, except that among dilute brown males those of typeH-2b/H-2d died sooner. Cluster analysis suggested that male and female genotypes each fall into two groups, with female dilute brown mice having shorter lives than other females, and male H-2b/H-2b mice except dilute brown and dilute brown H-2b/H-2d mice having shorter lives than other males. The association of heterozygosity with life span was clearer in females than in males, yet the longest-lived female genotype was homozygous H-2d/H-2d, of dominant Black phenotype at the Brown locus of chromosome 4, and homozygous dd at the Dilute locus of chromosome 9. The shortest-lived females were dilute brown H-2b/H-2b. The longest-lived and shortest-lived male genotypes were dilute brown H-2d/H-2d and dilute brown H-2b/H-2d, respectively. Although histological findings at postmortem differed between the sexes, there was no association of particular disorders with other genetic markers. The importance of H-2 in males was confirmed, but the allelic effects were perturbed, possibly by the absence of Sendai infection in this experiment. Overall our studies suggest that genetic influences on life span involve interactions between loci, and allelic interactions may change with viral infections or other environmental factors.     

Traits That Influence Longevity in Mice

Analysis of genetic interactions in the segregating backcross [(C57BL/6 x DBA/2)F₁ x DBA/2] mice revealed influences of genetic and environmental factors on life span. Using determinants of coat color (brown locus of chromosome 4 and dilute locus of chromosome 9), serologically determined H-2 antigens (chromosome 17 ) and sex as genetic markers, we studied the effects of these genes on longevity. The results suggested that genes in the brown locus (b) segment of chromosome 4, genes in a segment of the sex chromosomes and, to a more limited extent, genes in the segment of chromosome 17 which contains the H-2 haplotype all influenced longevity. The coat color (b locus) segment of chromosome 4 was associated with life span predominantly in females, whereas the chromosome 17 (H-2 haplotype) segment was associated with longer life primarily in males. The dilute locus d segment on chromosome 9 did not affect life span. Longevity appears to be influenced by interactions between genes in the chromosomal segment carrying H-2, those in the b segment, gender and the month of birth. Greater heterozygosity at the loci studied was associated with longer life span. Histopathological findings on mice that died at or after 28 months of age were comparable for all genetic combinations except that there was an increased frequency of lymphoma in females and an increased frequency of amyloidosis in males. Our analysis emphasizes the need for comprehensive studies of aging and longevity that would simultaneously determine the effects of several genetic regions and their interactions with the environment with respect to possible causes of death.     

Polymorphic and autoreactive H-2-specific monoclonal antibody isolated after injections of syngeneic sendai virus-coated lymphocytes

An H-2-specific monoclonal antibody (mAb Q-1) was obtained from B10.Q (H-2 ^q) mice injected with syngeneic Sendai virus-coated cells. The IgM monoclonal antibody recognizes the public determinant H-2.25 shared by H-2 ^k (K ^k) and H-2 ^r haplotypes and cross-reacts with H-2^d, H-2^s, H-2^p, and H-2^q cells, the latter being the haplotype of the challenged B-cell donor. The binding of mAb Q-1 to H-2^d, H-2^s, H-2^q, and H-2^p cells was lower than to H-2^k and H-2^r and of decreasing affinity but could be clearly distinguished from the negative reactions with H-2^b and H-2^f cells. MAb Q-1 distinguishes between Sendai virus-coated and uncoated lymphocytes only cells with low-affinity binding. On virus-coated or infected (H-2^p, H-2^q, H-2^d, H-2^s) cells lysis was stronger than on normal lymphocytes. We interpret the enhanced lysis of Sendai virus-positive cells by mAb Q-1 to be due to recognition of a modified exposure of public H-2 determinants induced by Sendai virus.On leave from The Institute of Immunology and Experimental Therapy, Wroclaw, Poland     

Genetic analysis of diabetes in the nonobese diabetic mouse. I. MHC and T cell receptor beta gene expression

Backcross nonobese diabetic (NOD) ((NOD x SWr)F1 x NOD) mice (108 females and 105 males) were typed for MHC, TCR V beta, and monitored for 350 days for the onset of diabetes. The presence of "antipolar" antibodies in the sera and the occurrence of insulitis was examined in a proportion of these backcross mice. There was no difference in the incidence of diabetes in mice heterozygous for TCR V beta b/a vs those homozygous for TCR V beta b/b. Among the 17 diabetics (all female) detected in this backcross, 14/17 were H-2nod/nod but 3/17 were H-2nod/q. This supports a previous observation suggesting that the MHC-linked diabetogenic gene originally thought to be recessive may rather be dominant but have a low penetrance in the heterozygous state. Antipolar autoantibodies were found in both female and male backcross mice, and were similarly distributed in diabetic and nondiabetic mice. There appeared to be no correlation between the level of these auto-antibodies and development of diabetes. The incidence and severity of insulitis was linked to MHC but no influence of TCR genes on insulitis nor an association between insulitis and antipolar antibodies could be demonstrated in this study. Further analyses of H-2nod/nod intercross mice homozygous for TCR V beta a or TCR V beta b are currently underway.     

The effectiveness of a microisolator cage system and sentinel mice for controlling and detecting MHV and Sendai virus infections

Experiments were conducted to determine (a) whether BALB/c mice housed on soiled bedding can be used as sentinels for the detection of Sendai virus and MHV from infected mice housed in microisolators, and (b) whether the microisolator caging system protects mice against Sendai virus and MHV infections. Sentinel mice were housed in microisolator cages, exposed continuously to soiled bedding and bled at 21 and 42 days for serology. All sentinel mice were seropositive for MHV by 42 days; however, sentinel mice exposed to soiled bedding were seronegative for Sendai virus at 21 and 42 days. These results suggest that sentinels housed on soiled bedding may not detect all infectious murine viruses. This study also showed that the microisolator caging system provided an effective barrier against MHV infection at the cage level and suggests that the microisolators should protect mice against other infectious agents.     

H-2 haplotype specificity of molecular requirements for trinitrophenyl recognition by anti-hapten cytotoxic T lymphocytes

The requirement of direct covalent association of trinitrophenyl (Tnp) groups with cell surface components for functional interactions with anti-Tnp cytotoxic T lymphocytes (CTLs) was analyzed. The question was approached by comparing three different methods of modifying target cells with Tnp groups and analyzing the ability of three anti-Tnp effector populations with different H-2 haplotypes (H-2^k, H-2^d, and H-2^b) to lyse the syngeneic Tnp-modified cells. All effector cell populations were able to lyse in an H-2 restricted manner the appropriate target cell modified with 2,4,6-trinitrobenzenesulfonic acid. As previously shown, H-2^k anti-Tnp CTLs exhibited true H-2 restriction while H-2^d anti-Tnp and H-2^b anti-Tnp CTLs lysed the haptenated syngeneic target cell preferentially but not exclusively. Cells modified by either trintrophenylated bovine serum albumin (Tnp₃₅-BSA) or trinitrophenylated Sendai virus (Tnp-SV) were rendered susceptible to lysis depending upon the H-2 haplotypes of the target cells and the anti-Tnp effector cells. H-2^k anti-Tnp CTLs were able to lyse H-2^k target cells modified with either Tnp₃₅-BSA or Tnp-SV; however, H-2^d anti-Tnp or H-2^b anti-Tnp CTLS did not significantly lyse the H-2^d or H-2^b target cells modified by either Tnp₃₅-BSA or Tnp-SV. The results suggest that Tnp groups not covalently linked with cell surface specific components can be recognized by H-2^k anti-Tnp CTLs, but not by H-2^d or H-2^b anti-Tnp CTLs.     

Murine complement factor B (BF) Sexual dimorphism and H-2-Linked polymorphism

Polymorphism of murine BF is described using agarose gel electrophoresis of EDTA-plasma. The proteins were blotted onto cellulose nitrate sheets and BF was detected by incubation of these sheets with anti-BF serum, anti-IgG serum, and ¹²⁵I-labeled protein A successively. After autoradiography, four or five main BF bands were found in plasma of male mice. The strain WLL/BrA (H-2 ^bs carried a more anodal variant than the strains 020/A (H-2 ^pz , B10 (H-2 ^b , B10.A (H-2 ^a , B10.M (H-2 ^f ), and OIR (H-2 ^q ). In backcross and F₂ generations the BF variants always cosegregated with the H-2 haplotypes. In this way linkage to H-2 could be established. When the electrophoretic BF patterns of males and females were compared, a sexual dimorphism was discovered; the females of each strain had only three main BF bands compared with the four or five found in males. However, no differences in level between males and females could be detected, probably because the three BF bands in the females were stronger. These data extend the information on the interspecies homology of the MHC and may open new possibilities for studies of the genetic organization and hormonal regulation of the H-2 complex.     

High-resolution mapping of a minor histocompatibility antigen gene on mouse chromosome 2

Minor histocompatibility (H) loci are significant tissue transplantation barriers but are poorly understood at the genetic and molecular level. We describe the construction of a high-resolution genetic map that positions a class II MHC-restricted minor H antigen locus and orders 12 other genes and genetic markers within the we-un interval of mouse Chromosome (Chr) 2. An intersubspecific backcross between 10.UW/Sn-H-3 ^b and CAST/Ei, an inbred stock of Mus musculus castaneus, was used for this purpose. A total of 1168 backcross mice were generated, and 71 we-un recombinants were identified. Significant compression of the genetic map in males versus females and transmission distortion of CAST-derived we, un, and A ^w genes were observed. Monoclonal T cell lines specific for two minor H alloantigens, Hd-1^a and Hd2^a, encoded by gene(s) that map to the we-un interval were used to antigen type the backcross mice. The results suggest the Hd-1^a and Hd-2^a antigens are most likely encoded by a single gene, now referred to as H-3b. The determined gene order is we-0.09±0.09-Itp-0.62±0.23-D2Mit77-0.26±0.15[Evi-4, Pcna, Prn-p]-0.26±0.15-Scg-1-0.44±0.19-[Bmp2a, D2Mit70]-0.09±0.09-[D2Mit19, D2Mit46]-1.59±0.36-D2Mit28-0.97±0.28-D2Lerl-1.50±0.35-H-3b-0.26±0.15-un (% recombination±1 SE). Because the average resolution of the backcross is 0.09 cM, the backcross panel should facilitate the physical mapping and molecular identification of a number of genes in this chromosome region.     

A genetic analysis of natural resistance to nonsyngeneic cells: The role of H-2

The genetic control of natural resistance in vivo to four natural killer (NK) cell-resistant H-2 homozygous lymphoid tumor cell lines was investigated by following the survival and organ distribution of cells prelabeled with radioactive iododeoxyuridine. Backcross mice derived from DBA/2J and CBA/J parents were injected with H-2 ^dtumor cells and tumor cell elimination was lowest in H-2 ^dhomozygotes. Natural killer cell activity was also reduced in mice with the H-2 ^dhaplotype, but no direct correlation between NK cell levels against YAC-1 or SL2-5 lymphoma cells and natural resistance in vivo was demonstrable. Analysis of 23 BXD recombinant inbred strains indicated that natural resistance to H-2 ^dtumors was restricted to H-2 ^bstrains. There was no direct association of NK cell activity with H-2 type in the BXD strains and NK cell levels did not correlate with tumor survival in vivo. By comparing natural resistance to H-2 ^dand H-2 ^btumors in DBA/2, C57BL/6, B6D2F₁, and B10.D2 mice we found that H-2 nonidentity between the tumor and the host, rather than the host H-2 haplotype, determined whether natural resistance occurred. Again, NK cell activity against YAC-1 cells was not predictive of tumor survival in these strains. These results provide genetic evidence that NK cells alone cannot account for natural resistance to H-2 nonidentical cells of hemopoietic origin.     

Identification of Hepatocarcinogen-Resistance Genes in Dba/2 Mice

Male DBA/2J mice are ~20-fold more susceptible than male C57BL/6J mice to hepatocarcinogenesis induced by perinatal treatment with N,N-diethylnitrosamine (DEN). In order to elucidate the genetic control of hepatocarcinogenesis in DBA/2J mice, male BXD recombinant inbred, D2B6F(1) X B6 backcross, and D2B6F(2) intercross mice were treated at 12 days of age with DEN and liver tumors were enumerated at 32 weeks. Interestingly, the distribution of mean tumor multiplicities among BXD recombinant inbred strains indicated that hepatocarcinogen-sensitive DBA/2 mice carry multiple genes with opposing effects on the susceptibility to liver tumor induction. By analyzing D2B6F(1) X B6 backcross and D2B6F(2) intercross mice for their liver tumor multiplicity phenotypes and for their genotypes at simple sequence repeat marker loci, we mapped two resistance genes carried by DBA/2J mice, designated Hcr1 and -2, to chromosomes 4 and 10, respectively. Hcr1 and Hcr2 resolved the genetic variance in the backcross population well, indicating that these resistance loci are the major determinants of the variance in the backcross population. Although our collection of 100 simple sequence repeat markers allowed linkage analysis for ~95% of the genome, we failed to map any sensitivity alleles for DBA/2J mice. Thus, it is likely that the susceptibility of DBA/2J mice is the consequence of the combined effects of multiple sensitivity loci.     

Loss of reactivity of a myeloma tumor with H-2 compatible thymus-derived lymphocytes.

It was previously shown that MOPC-315-EL, a subline of the BALB/c myeloma tumor MOPC-315, reversibly alters its reactivity with T cells that recognize H-2^d, 2,4,6-trinitrophenyl, and minor histocompatibility antigens. This report demonstrates (a) that CTLs directed against vesicular stomatitis virus and Sendai virus are unable to recognize viral antigens associated with the unreactive tumor cell, and (b) that incorporation of Sendai virus antigens into the same membrane as the H-2 gene products is required for effective recognition by cytotoxic T lymphocytes.     

Effect of mouse thymic medullary epithelial cell line on thymocyte development and functional maturationin vivo

Thymic medullary type epithelial cell line (MTEC1), which expressed H-2D^d and Ia^d, was derived from BALB/c mouse. MTEC1 cells were introduced by intrathymic injection into irradiated H-2^b mice reconstituted with H-2^bxd F1bone marrow cells. Two months later, the injected MTEC1 cells were found to be still present in the recipient thymus. Splenocytes from chimeric mice, inin vitro functional assays, were analyzed to investigate whether the MTEC1 cellsin vivo could induce the production of H-2^d restricted antigen-specific T cells. The H-2^d restricted VSV-antigen specific proliferating and IL-2 producing T cells as well as H-2^d restricted influenza virus specific cytotoxic T cells were found in chimeric mice injected with MTEC1 cells, and these cells were shown to be tolerant to H-2^d selfantigen. On the contrary, H-2^d restricted antigen-specific and H-2^d self-antigen tolerant T cells were not shown in control mice injected with saline. These results suggest that intrathymically injected MTEC1 cells could induce T lineage cell development and functional maturation in the intact thymus. A hypothesis of “second thymic selection” in thymic medulla has been postulated and its implication discussed.     

Genetic control of streptococcus-induced hepatic granulomatous lesions in mice

Hepatic granulomatous lesions were induced in mice by a single intraperitoneal injection of 3 mg of disrupted Streptococcus pyogenes cell-wall material. Mice carrying the H-2 ^b or H-2 ^k haplotypes were highly susceptible to the induction and three weeks after the injection produced numerous granulomas. In contrast, mice of the H-2 ^d haplotype were resistant and produced only a few hepatic granulomas. Resistance was inherited as a dominant trait and in the backcross generation segregated together with the H-2 ^d phenotype. Testing of the H-2-recombinant mice indicated that the putative gene(s) determining resistance/susceptibility is located to the right of the S and to the left of the D region. Thes location corresponds to the recently described gene cluster consisting of tumor necrosis factor (TNF) and lymphotoxin genes and several BAT sequences. The known effect of TNF on granuloma formation in mice is consistent with a possible effect of TNF genes, and their variants, on S. pyogenes-inducibility of hepatic granulomas in mice. Address correspondence and offprint requests to: M. B. Zaleski.     

Effect of mouse thymic medullary epithelial cell line on thymocyte development and functional maturation in vivo

Thymic medullary type epithelial cell line (MTEC1), which expressed H-2D^d and Ia^d, was derived from BALB/c mouse. MTEC1 cells were introduced by intrathymic injection into irradiated H-2^b mice reconstituted with H-2^bxd F1bone marrow cells. Two months later, the injected MTEC1 cells were found to be still present in the recipient thymus. Splenocytes from chimeric mice, inin vitro functional assays, were analyzed to investigate whether the MTEC1 cellsin vivo could induce the production of H-2^d restricted antigen-specific T cells. The H-2^d restricted VSV-antigen specific proliferating and IL-2 producing T cells as well as H-2^d restricted influenza virus specific cytotoxic T cells were found in chimeric mice injected with MTEC1 cells, and these cells were shown to be tolerant to H-2^d selfantigen. On the contrary, H-2^d restricted antigen-specific and H-2^d self-antigen tolerant T cells were not shown in control mice injected with saline. These results suggest that intrathymically injected MTEC1 cells could induce T lineage cell development and functional maturation in the intact thymus. A hypothesis of “second thymic selection” in thymic medulla has been postulated and its implication discussed. Project supported by the National Natural Science Foundation of China (Grant No. 39230320).     

Altered splenic T cell function of BALB/cByJ mice infected with mouse hepatitis virus or Sendai virus

Mouse hepatitis virus and Sendai virus are among the most common viruses naturally infecting laboratory mice. Concanavalin A-stimulated in vitro proliferative responses of splenocytes were examined after infection of BALB/cByJ mice with the JHM strain of mouse hepatitis virus (MHV-JHM) or Sendai virus. Mice were exposed to these viruses by presumed natural routes (per os or intranasally). Immunodepression was marked but transient among BALB/cByJ mice exposed to MHV-JHM. Among mice exposed to Sendai virus and examined over a 21-day period, spleen cells from only one mouse, sacrificed 10 days postinoculation, exhibited a severely impaired ability to respond to concanavalin A. Lymphokine production by spleen cells from control and infected mice was then assessed. IL 2 was either absent or present at very low levels in culture supernates of concanavalin A-unresponsive spleen cells from MHV-JHM-infected mice. Spleen cells from the single Sendai virus-infected mouse also produced very low levels of IL 2. In contrast, IL 1 was detected in supernatants of all spleen cell cultures derived from control, MHV-JHM-infected, or Sendai virus-infected mice. There was not a clear correlation between concanavalin A responsiveness and the ability of spleen cells to produce interferon-gamma. These results stress the importance of using laboratory mice of known microbiological status for immunologic experiments.     

Anti-MHC immunity detected prior to intentional alloimmunization

Cell fusion was performed between spleen cells from young BALB/cBy (H-2 ^d) mice which have never been immunized and SP2/0 mouse plasmacytoma cells. A monoclonal H-2-specific cytotoxic IgM antibody was obtained (By-1) which detected a new public biregional H-2 specificity, H-2.m210. The mcAb By-1 reacted strongly with H-2K^d, D^d, and H-2^s antigens, gave weak cross-reactions with H-2K^k, D^q, H-2^r, and H-2^v antigens and was negative with H-2^b, H-2^f, H-2^p, and H-2L^d antigens. A polymorphic reaction pattern was also observed on a panel of lymphocytes from B 10.W strains. The intriguing finding on this reaction pattern was the reactivity on H-2^d cells, including the syngeneic BALB/cBy and truly autologous cells. As shown by capping and immunoprecipitation experiments on H-2^d cells and by studies on H-2^d-transfected mouse L cells, the target molecules for McAb By-1 were H-2K^d and H-2D^d molecules. The BALB/cBy mouse, from whose spleen cells the McAb By-1 was obtained, survived after the fusion experiment, and serum was examined for the presence of cytotoxic H-2-specific antibodies during the rest of its life. At the time of the fusion, no autoreactive serum antibodies were found, but about 4 months later, we found in the serum of this mouse autoreactive H-2-specific cytotoxic IgM antibodies. The serum antibodies followed the same reaction pattern as that of the McAb By-1. As far as we know, this is the first report of autoreactive H-2-specific antibodies in serum of a mouse which has never been immunized and of the first natural autoreactive H-2-specific monoclonal antibody.Abbreviations McAb monoclonal antibody - MHC major histocompatibility complex - H-2 major histocompatibility complex of mice - CTLs cytotoxic T cells - FMF flow microfluorometry - FITC fluorescein isothiocyanate - LPS lipopolysaccharide W.E. coli 0111:134 - PBS phosphate-buffered saline - Iodogen 1,3,4,6,-tetrachloro-3,6-diphenylglycoluril - GAMIg goat-antimouse immunoglobulin - Staph-A Staphylococcus aureus Cowan I     

Backcross and partial advanced intercross analysis of nonobese diabetic gene-mediated effects on collagen-induced arthritis reveals an interactive effect by two major loci

Genetic segregation analysis between NOD and C57BL strains have been used to identify loci associated with autoimmune disease. Only two loci (Cia2 and Cia9) had earlier been found to control development of arthritis, whereas none of the previously identified diabetes loci was of significance for arthritis. We have now made a high-powered analysis of a backcross of NOD genes on to the B10.Q strain for association with collagen-induced arthritis. We could confirm relevance of both Cia2 and Cia9 as well as the interaction between them, but we did not identify any other significant arthritis loci. Immune cellular subtyping revealed that Cia2 was also associated with the number of blood macrophages. Congenic strains of the Cia2 and Cia9 loci on the B10.Q background were made and used to establish a partial advanced intercross (PAI). Testing the PAI mice for development of collagen-induced arthritis confirmed the loci and the interactions and also indicated that at least two genes contribute to the Cia9 locus. Furthermore, it clearly showed that Cia2 is dominant protective but that the protection is not complete. Because these results may indicate that the Cia2 effect on arthritis is not only due to the deficiency of the complement C5, we analyzed complement functions in the Cia2 congenics as well as the PAI mice. These data show that not only arthritis but also C5-dependent complement activity is dominantly suppressed, confirming that C5 is one of the major genes explaining the Cia2 effect.     

A serological survey on 15 murine pathogens in mice and rats

A serological survey for 15 murine pathogens was performed on 269 mouse sera collected from 21 conventional and 12 barrier colonies, and on 376 rat sera collected from 21 conventional and 23 barrier colonies. Animals having an antibody against at least one of the antigens were contained in 81.0% of conventional and 16.7% of barrier mouse colonies and also in 81.0% of conventional and 43.5% of barrier rat colonies. Main contaminants were mouse hepatitis virus and Sendai virus in mice, and Sendai virus and pneumonia virus of mice in rats. Results also indicated that antibodies to Toolan's H-1, minute virus of mice and PVM were positive in mice from a considerable number of colonies and those to Kilham rat virus, Mycoplasma pulmonis and Toolan's H-1 were sometimes detected in rats, suggesting prevalences of these pathogens in mice and rats in Japan.     

Genetic Linkage between the AH Locus and a Major Gene That Inhibits Susceptibility to Audiogenic Seizures in Mice

Mice of the DBA/2 (D2) strain are highly susceptible to sound-induced seizures at 21 days of age; whereas, mice of the C57BL/6 (B6) strain are resistant to these seizures. Although the difference in susceptibility to audiogenic seizures (ASs) between these two strains is inherited as a multiple-factor trait, an association was observed between susceptibility to ASs and the Ah locus. The Ah locus controls the inducibility of aryl hydrocarbon hydroxylase (AHH) activity by a number of aromatic hydrocarbons. B6 mice carry the Ah^b allele and have inducible AHH activity; whereas, D2 mice carry the Ah^d allele and have noninducible activity. Inducibility is inherited as a Mendelian dominant trait in crosses between these strains. Mice carrying the Ah^b allele are generally less susceptible to ASs at 21 days of age than are mice carrying the Ah^d allele. The combined results from B6 x D2 recombinant inbred strains, congenic strains (where the Ah^b allele was placed into the D2 genome and the Ah^d allele placed into the B6 genome), the B6D2F₁ x D2 backcross generation, and a random survey of various inbred strains, suggest that the association between these two traits is due to genetic linkage, rather than to pleiotrophy or to chance. A major gene that inhibits susceptibility to ASs appears to be closely linked to the Ah locus. This gene has been designated Ias, for inhibition of ASs. A large portion of the genetic variability of AS susceptibility may be due to the segregation of Ias.     

Genetic control of contact sensitivity in mice: Effect ofH-2 and nonH-2 loci

The genetic control of delayed-type hypersensitivity in mice was investigated by contact sensitization with picryl chloride. Distribution patterns of contact sensitivity in 11 inbred strains of mice showed significant differences among strains. Comparison of levels of response between congenic-resistant lines and their inbred partners, at 9 to 11 weeks of age, revealed a clear association betweenH-2 haplotype and the magnitude of response. Testing ofH-2 recombinants further suggested the influence of two genes mapping at either end of theH-2 complex. While theH-2K ^d andH-2D ^k alleles were associated with a high response, theH-2K ^k ,H-2K ^b ,H-2D ^d , andH-2D ^b alleles were associated with a low response. Analysis of the ontogeny of response suggested that theH-2 haplotype manifests its effect through the maturation of contact sensitivity. On both the C57BL/6By and C57BL/10Sn backgrounds, theH-2 ^d haplotype was associated with early maturation of response, while theH-2 ^b haplotype was associated with late maturation. Analysis of the response of congenic lines with different genetic backgrounds and of CXB recombinant-inbred lines further revealed the marked effects of yet other genes on this trait.