Properties of the promoter region of theT18 d (T13 c )Tla gene

The T18^d (formerlyT13 ^c) gene of BALB/c mice belongs to the category ofTla genes which is expressed by both thymocytes and TL⁺ T-cell leukemias. To elucidate the regulation of T18^d, different restriction fragments of the 5' flanking region between -457 and⁺ 146 were linked to the chloramphenicol acetyltransferase (CAT) gene and transfected into TL⁺ and TL- cells. By comparison of transiently expresssedCAT activity among cells transfected with differentCAT constructs, the results suggest that determination of TL⁺ vs TL phenotypes is located within the region -105 to - 33, and that an element essential to T18 d expression resides within the region - 33 to + 54. Putative DNA-binding factors characterizing particular cell types and displaying selective affinity for particular TI 8^d restriction fragments were identified by electrophoretic mobility shift assays with nuclear extracts (NEs). Two factors (or complexes) which bound specifically to the T18^d fragment-105 to - 33 were expressed preferentially in TL⁺ cells and thus may be involved in determining the tissue-selective expression of TI8^d. The close proximity of negative and positive cis-acting elements within the promoter region is consistent with regulation of T18^d gene expression by a variety of trans-acting factors whose production is attuned to development and differentiation. The data provided may serve as a guide to study the regulation of other categories of Tla genes that are normally silent in thymocytes but may become expressed by leukemia cells.     

Genomic organization of the mouse Tla locus: study of an endogenous retrovirus-like locus reveals polymorphisms related to different Tla haplotypes

A retrovirus element (TLev1) is located within the Thymus leukemia antigen (T7a) locus of the C57BL/10 mouse major histocompatibility complex. Low-copy probes have been isolated from sequences flanking the TLev1 integration site to examine the distribution of TLev1 among inbred mouse strains having genotypically determined variations in TL-antigen expression. It was found that the low-copy probes cross-hybridize to regions within the Tla locus in a genotype-specific manner. Although a strong association was found between TL mouse strains and TLev1, the presence or absence of the TLev1 locus did not exclusively correlate with expression or nonexpression of TL antigens. Analysis of different Mus subspecies indicates that TLev1 integrated into a common ancestor of the species Mus musculus. It is suggested that the loss of the TLev1 locus from certain mouse genomes reflects evolutionary rearrangements in the TL region; the resulting diversity may relate to the differential expression of TL antigens among mouse strains. The probes described here provide a useful tool for examining the genomic expansions and contractions which have occurred during the evolution of the Tla locus     

Biochemical genetics of TL antigens

TL antigens are class I glycoproteins which are expressed on thymocytes and which are coded by the Tla region of the major histocompatibility complex of the mouse. Biochemical analysis of TL molecules from different strains of mice revealed structural variation determined by the Tla region which is detectable by peptide mapping, isoelectric focusing, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, two-dimensional gels, and by differential reactivity of allelic forms of TL molecules with a panel of anti TL reagents. The quantity of TL expressed on thymocytes is also influenced by the Tla region; three quantitative phenotypes were identified: high (Tla ^a, Tla ^d, Tla ^c), intermediate (Tla ^c, Tla ^f), and low (Tla ^b). (Relative amounts: 1000 : 100 : 1.) Some thymic leukemias arising in (Tla ^b, Tla ^c, Tla ^c) mice with genetically determined reduced levels of thymic TL were found to express TL molecules which were structurally indistinguishable from TL isolated from thymocytes but were present in larger amounts. This suggests that TL structural genes are intrinsically capable of full expression in all mice but that the Tla region of mice expressing an intermediate or low quantity of TL is marked by some feature which causes the thymocyte to express less than the full amount of TL possible.     

Genetic control of the target structures recognized in hybrid resistance

Hybrid resistance of lethally irradiated (C57BL/6 × DBA/2)F₁ and (C57BL/10 × C3H)F₁ hybrid mice to the engraftment of parental C57BL/6 or C57BL/10 bone marrow cells is controlled by the H-2-linked Hh-1 locus. This resistance can be specifically blocked or inhibited by the injection of irradiated spleen cells from lethally irradiated, marrow reconstituted donor mice of certain strains. By testing the ability of regenerating spleen cells from various donor strains to block the resistance, we studied the genetic requirements for the expression of putative cell-surface structures recognized in hybrid resistance to H-2^b marrow cells. Strains of mice bearing informative intra-H-2 or H-2/ Qa-Tla recombinant haplotypes provided evidence that the Hh-1 locus is located telomeric to the H-2S region complement loci and centromeric to the H-2D region class I locus in the H-2 ^b chromosome. Two mutations that affect the class I H-2D ^b gene have no effect on Hh-1 ^b gene expression. The H-2D region of the H-2 ^S haplotype contains an allele of the Hh-1 locus indistinguishable from that of the H-2D ^b region, as judged by the phenotypes of relevant strains and F₁ hybrids. Collectively these data indicate that the Hh-1 locus is distinct from the class I H-2D (L) locus in the H-2 ^b or H-2 ^s genome, and favor the view that the expression or recognition of the relevant determinants is not associated with class I gene products.Abbreviations used in this paper BM(C) bone marrow (cells) - CML cell-mediated lympholysis - CTL cytotoxic T lymphocytes - FBS fetal bovine serum - HBSS Hanks' balanced salt solution - SC spleen cells from irradiated, bone marrow-reconstituted mice Address correspondence to: Dr. I. Najamura, Department of Pathology, School of Medicine, State University of New York at Buffalo, Buffalo, NY 14214, USA     

Selective reversal of H-2 linked genetic unresponsiveness to lysozymes

Genes outside of the mouse major histocompatibility complex (H-2) were found to be capable of specifically reversing the previously described nonresponsiveness to hen egg-white lysozyme (HEL) owing to H-2 ^b immune response (Ir) genes. C3H.SW, BALB.B, and C57L, all of the H-2 ^b haplotype, showed responsiveness to HEL, but not to human lysozyme (H UL). Mapping of the reversing gene(s) was attempted by testing H-2 ^b recombinant inbred (RI) strains of mice carrying C3H, BALB, and C57L non-H-2 ^b genes. Analysis of the strain distribution pattern of responsiveness with both CXB and BXH RI strains was consistent with the location of the responsible site within the H-3 region on chromosome 2. The anti-HEL proliferative responsiveness in two H-3 congenic strains of mice, B10.C(28NX) ^SN and B10.C-H-3 ^{cH-3 a} , that have BALB/c genes within the H-3 region confirmed the mapping, as well as localized the reversing gene(s) near the Ir-2 gene. The data are discussed with regard to the site of expression of the reversing gene(s) and its mechanism of action.Abbreviations used in this paper MHC major histocompatibility complex - HEL hen egg-white lysozyme - Ir immune response gene - HUL human lysozyme - SDP strain distribution pattern - PFC plaque-forming cells; ₂ m, ₂-microglobulin - CFA complete Freund's adjuvant - PT-LN parathymic lymph nodes - RI recombinant inbred mice     

Genetic analysis of immune suppression

We have analyzed the genetic control of susceptibility to suppression by 1-J⁺, suppressor-T-cell derived factors (TsF) specific for the synthetic polymer L-glutamic acid⁵⁰-L-tyrosine⁵⁰ (GT). GT-TsF activity was measured as specific inhibition of proliferative responses to GT developed in cultures of lymph-node T cells from mice primed with GT complexed to methylated bovine serum albumin (GT-MBSA). These experiments demonstrated that there is no MHC-encoded genetic restriction between donors and recipients of GT-TsF in suppression of proliferative responses. We have also confirmed the observations that mice of the H-2 ^b, H-2 ^d, and H-2 ^khaplotypes can produce GT-TsF, whereas H-2 ^amice do not, and that H-2 ^b, H-2 ^d, and H-2 ^kmice are sensitive to GT-TsF from all producer strains, whereas H-2 ^bmice are not sensitive to GT-TsF from any strain. Analysis of the effect of GT-TsF on responses by mice bearing recombinant haplotypes suggests that at least two genes are required for susceptibility to GT-TsF and that these genes show coupled complementation.Abbreviations used in this paper GAT random linear terpolymer of L-glutamic acid⁶⁰-L-alanine³⁰-L-tyrosine¹⁰ - GAT-MBSA GAT complexed to methylated bovine serum albumin - GATTsF GAT-specific-T-cell derived suppressor factor - GT random linear copolymer of L-glutamic acid⁵⁰-L-tyrosine⁵⁰ - GT-MBSA GT complexed methylated bovine serum albumin - GT-TsF GT, specific, T-cell derived suppressor factor - ³H-TdR tritiated thymidine - Ir gene immune response gene - MBSA methylated bovine serum albumin - MEM minimal essential media - MHC major histocompatibility complex - PFC plaque-forming cell(s) - PPD purified protein derivative of M. tuberculosis H37Ra     

Genetic control of immune responses of inbred mice: Responses against terpolymers poly(glu57lys38ala5) and poly(glu54lys36ala10)

The immune response patterns of inbred and congenic strains of mice against terpolymers poly(glu⁵⁷lys³⁸ala⁵) and poly(glu⁵⁴lys³⁶ala¹⁰) have been studied. Initial recognition of the polymers is ascribed to ‘GA’ receptors (Ir-GA gene product) on T cells of mice ofH-2 haplotypes,a,b,f,k ands, and ‘GL’ receptors (Ir-GL gene product) of mice ofH-2 ^p,H-2 ^q andH-2 ^j haplotypes, and to GA and/or GL receptors of mice ofH- 2^d andH- 2^r haplotypes. The specificity of the antibody is directed predominantly against GL. The inability to elicit antibody with GA specificity has been ascribed to the lack of significant concentrations of GA sequences in the polymers to interact with appropriate receptors on B cells. The weakest responders were mice of H-2^b haplotype. F¹ hybrids (responders×nonresponders) were all responders demonstrating the dominant character of responsiveness. Wide variations in antibody levels produced among strains of mice of theH-2 ^k andH-2 ^b haplotypes are ascribed to genes not linked toH-2.     

The ability of H-2+ and H-2− cell lines to induce or be lysed by Cytotoxic T cells

We have studied the T cell-mediated lysis of two C58 lymphoma lines: R1(TL⁺), which bears serologically detectable H-2^k and TL1,2,3 antigens, and R1(TL^–), an immunoselected variant which lacks these antigens. Unlike R1(TL⁺) cells, the variant cells are not sensitive to specific lysis by T cells directed against either H-2^k or minor H antigens of C58 mice. An injection of R1(TL⁺) cells into allogeneic H-2-identical or H-2-different mice primes for an excellent secondary cytotoxic response in vitro to R1(TL⁺); but not to R1(TL^–). Immunization in vivo and in vitro with R1(TL)^–) leads to little or no priming or generation of cytotoxic T cells. Both cell lines, however, are sensitive to nonspecific lysis by cytotoxic cells in the presence of PHA or Con A, although even under these conditions, R1(TL⁺) is killed more effectively than is R1(TL^–). We conclude that R1(TL^–) does not express any form of H-2 antigen which can be detected by immunization or by sensitivity to cytotoxic T cells.     

Complementation between a gene of theI-E subregion and a non-H-2 gene in the class-specific suppression of IgG2a antibody to sheep erythrocytes

A low level of IgG2a antibodies is observed in B10 mice after primary immunization with SRBC. Analysis of the response in different H-2^b mice and among B10 animals with differentH-2 haplotypes reveals that this selective isotype deficiency is under the control of at least two genes: a background gene and anH-2-linked gene. Responses ofH-2 recombinant B10 strains map theH-2-linked gene to theI-E subregion. Evidence is presented for complementation betweenH-2 and non-H-2 genes in the determination of the low responder phenotype. Low responsiveness appears to be inherited as a dominant trait. Possible functions of the two series of genes are discussed in relation to suppressor mechanisms.     

H-2 effects on cell-cell interactions in the response to single non-H-2 alloantigens

Immune response (Ir) genes mapping in theI region of the mouseH-2 complex appear to regulate specifically the presentation of a number of antigens by macrophages to proliferating T cells. We have investigated the possibility that similarIr genes mapping in theH-2K andH-2D regions specifically regulate the presentation of target antigens to cytotoxic effector T cells. We report that the susceptibility of targets expressing specific non-H-2 H alloantigens to lysis by H-2-compatible, H-antigen-specific cytotoxic effector T cells is controlled by polymorphicH-2K/D genes. This control of susceptibility to lysis is accomplished through what we have defined operationally as antigen-specific regulation of non-H-2 H antigen immunogenicity. High immunogenicity of the H-4.2 alloantigen is determined by a gene mapping in theH-2K region ofH-2 ^b . However, high immunogenicity of H-7.1 is determined by a gene mapping in theH-2D region ofH-2 ^b . High immunogenicity of the H-3.1 alloantigen is determined by genes mapping in both theH-2K andH-2D regions ofH-2 ^b . Therefore, genes mapping in theH-2K andH-2D regions serve a function in presenting antigen to cytotoxic effector T cells. This function is analogous to that played byI-regionIr genes expressed in macrophages which present antigen to proliferating T cells. We present arguments for classification of theseH-2K/D genes as a second system ofIr genes and discuss the implications of twoH-2-linkedIr-gene systems, their possible functions, and their evolution.     

Analysis of hybrids between an H-2+, TL− lymphoma and an H-2+, TL+ lymphoma and its H-2−, TL− variant subline

Hybrids between an H-2⁺, TL⁺ lymphoma and an H-2⁺, TL^– lymphoma were studied for their expression of H-2 and TL antigens. The H-2 antigens of both parents were expressed, the TL specificity of the TL⁺ parent was retained, and the TL specificity characteristic of the mouse strain from which the TL^– lymphoma was derived was not expressed. There was no evidence that the genome of either parent altered the expression of the TL antigens coded for by the genome of the opposite parent. Hybrids between the H-2⁺, TL^– lymphoma and an H-2^–, TL^– variant line derived from the H-2⁺, TL⁺ cell line expressed both theK- andD-regioncoded H-2 antigens and the TL specificity characteristic of the parental cell line from which the variant cell was derived. This result is consistent with the defect in the variant cell's being the result of a mutation affecting a gene coding for a positive element necessary for expression of both TL and the serologically detectable H-2 antigens on the cell surface.     

H-2 class I antigen expression on mouse teratocarcinoma cell lines

Immunity against PCC3 teratocarcinoma cells (129, H-2 ^b) was induced in allogeneic (C3H, H-2 ^k) mice by preimmunization with L cells (C3H, H-2 ^k) expressing cosmid-introduced K ^b or D ^b genes, but not with nontransfected L cells. In addition, the growth of PCC3 cells in sublethally irradiated (C3H × B6-H-2 ^bm1)F₁ and (C3H × B6-H-2 ^bm13 )F₁ mice bearing the K ^bm1 and D ^bm13 mutations, respectively, was either prevented, stopped, or delayed in comparison with the (C3H × B6)F₁ (k × b) mice, which failed to reject the PCC3 cells. The teratocarcinoma line OC15S was exceptional because it reacted specifically with K^b- and D^b-specific (but not I^b-specific) alloantisera, and because K^b- and D^b-specific antibodies could be absorbed by OC15S cells. The subpopulation of OC15S cells bearing the ECMA-7 antigen characteristic for embryonic carcinoma (EC) cells was isolated by the fluorescence-activated cell sorter and was shown to react specifically with K^b- and D^b-specific antisera. These experiments show that teratocarcinoma cells express antigens similar or identical to the K-and D-region products of differentiated cells. The lack of expression of class I antigens is thus neither a condition nor a consequence of the pluripotentiality of the EC cells. The exact nature of the major histocompatibility complex antigens on EC cells has yet to be established using the methods of molecular biology and biochemistry.     

Structural and genetic properties of the Eb recombinational hotspot in the mouse

The Eb gene of the mouse contains a recombinational hotspot which plays a predominant role in meiotic crossing-over within the I region of the mouse major histocompatibility complex (MHC). The nucleotide sequences of five recombinants derived from H-2 ^k /H-2 ^b heterozygotes at the Eb locus placed the sites of recombination in each recombinant haplotype within a 2.9 kilobase (kb) segment located fully within the second intron of the Eb gene. Further resolution of the crossover sites was not possible since the nucleotide sequences of the parental and recombinant haplotypes are identical within this segment. The molecular characterization of these five recombinants considered in conjunction with three previously reported intra-Eb recombinants indicates that there are at least two distinct sites of recombination within the Eb recombinational hotspot. In a related study, an examination of the nucleotide sequence of the H-2 ^p allele of the Eb gene revealed a major genetic rearrangement in the 5' half of the intron in this haplotype. A 597 base pair (bp) nucleotide sequence found in the H-2 ^p haplotype is replaced by a 1634 bp segment found in the H-2 ^b and H-2 ^k haplotypes. Sequence analysis of this 1634 bp segment shows strong nucleotide sequence similarity to retroposon long terminal repeat (LTR), env, and pol genes indicating that this segment of the second intron has evolved through retroposon insertion. The location of these retroposon sequences within the 2.9 kb recombination segment defined by the five H-2 ^k /H-2 ^b recombinant haplotypes suggests a possible relationship between these retroviral elements and site-specific recombination within the second intron of the Eb gene. Offprint requests to: H. C. Passmore     

Isolation of a retroviruslike sequence from the TL locus of the C57BL/10 murine major histocompatibility complex.

Two retroviruslike sequences have been isolated from the TL locus of the major histocompatibility complex of C57BL/10 mice. One sequence (TLev2) hybridizes only with probes derived from the pol region of the murine leukemia provirus AKR; the other sequence (TLev1) hybridizes with gag, pol, and env AKR region probes. This 9-kilobase endogenous, TL region-associated virus (TLev1) has been further characterized. The TLev1 genome has been shown to contain murine leukemia virus-related sequences bounded by retroviruslike, VL30 long terminal repeats. Hybridization of TLev1-derived probes to mouse genomic digests reveals multiple copies which show distinct patterns compared with those observed with murine leukemia virus probes. The study of TLev1 may prove significant with respect to the interaction of retroviral sequences within the genome, expression of genes within the TL locus, and polymorphisms within the major histocompatibility complex.     

Genetic analysis of the non-H-2-linked Ir genes controlling the cytotoxic T-cell response to H-Y in H-2 d mice

The T-cell mediated immune responses to the male specific minor histocompatibility antigen H-Y in mice have been studied extensively as a model for immune responses to other weak antigens like tumor antigens or autoantigens. In a recent analysis of the strain distribution of the cytotoxic T-cell (Tc-cell) responsiveness to H-Y, it has been found that genes both within and outside the H-2 complex exert an interactive control. Whereas the H-2 ^b strains all are high responders, independent of their non-H-2 background, other H-2 haplotypes (d, k, and s) only allow for a response if they are combined with certain non-H-2 genes. The H-2-linked immune response genes (Ir-genes) have been previously mapped to the I and K or D region of the H-2 complex, but the mapping of the non-H-2 genes has not yet been established. In this study evidence is presented, using recombinant inbred strains and immunoglobulin heavy chain (Igh) congenic strains of mice, to show that there is more than one non-H-2 Ir-gene involved, that the main controlling genes are not linked to the Igh complex, and that at least one non-H-2 Ir-gene is linked to the H-3 region on chromosome 2. This region includes genes for beta-2-microglobulin (2m), the Ly-mllalloantigen a polymorphic cell surface glycoprotein (Pgp-1), a B-cell specific antigen Ly-4, a transplantation antigen H-3, and genes (Ir-2) controlling the immune response to Ea-1 and H-13.     

Non-H-2 and H-2-Linked immune response genes control the cytotoxic T-cell response to H-Y

The immunoregulation of cytotoxic T-cell responses to the male-specific antigen H-Y in mice has been found to be genetically controlled by genes of the major histocompatibility complex (H-2). Responsiveness was mainly confined to H-2 ^b strains, but it has also been found in recombinant strains, F₁ hybrids, and chimeras that carry at least part of the H-2 ^b haplotype. By using a different immunization procedure it has been shown recently that an H-2 ^k mouse strain (CBA) is also able to mount an equivalent H-Y-specific response. We investigate here, by applying this immunization technique, the responsiveness of other H-2 ^k strains and of strains of other independent H-2 haplotypes. Both responders and nonresponders are found in three haplotypes: k, s, and d. The strain distribution pattern of responsiveness shows a combined influence of non-H-2 and H-2 genes. In certain strains there is a high variability in responsiveness between genetically indentical individual animals. We discuss a model of immune response (Ir) gene function which could account for these observations.     

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.     

The influence of Igh-1 genes on the class and subclass distribution of oxazolone-specific antibodies

Previous studies have demonstrated that the level of the oxazolone-specific antibody response induced by contact sensitization is under the control of H-2 and Igh-1-linked genes. The aim of the present study was to clarify the role of H-2 and Igh-1 genes in the regulation of antibody affinity and isotype composition of oxazolone-specific antibodies. Analysis of the antibody response to oxazolone has revealed different ratios of IgG2a and IgG2b antibodies in mice carrying the Igh-1 ^b allele and in strains carrying alleles a, c, and e. The characteristic ratio of IgG2a and IgG2b isotypes persisted during the whole period of the primary and secondary antibody response of CBA and CBA-Ig ^b Igh-C congenic mice. The Igh-1-linked genes influenced the isotype distribution and not the affinity of oxazolone-specific antibodies induced by contact sensitization.Abbreviations used in this paper c.Ig chicken immunoglobulin - Igh-C constant region of immunoglobulin heavy chain - DNCB 2,4-dinitrochlorobenzene - DTH delayed-type hypersensitivity - FITC fluorescein isothiocyanate - Igh-1 cluster of structural heavy chain allotype genes of IgG2a - KLH keyhole limpet hemocyanin - KSCN potassium-sulfocyanid - MHC major histocompatibility complex - Ox oxazolone (4-ethoxymethylene-2-phenyl-5-oxazolone - Ox-BSA oxazolone-bovine serum albumin - Ox-cap oxazolone-capronic acid - Ox-MSA oxazolone-mouse serum albumin - NP 4-hydroxy-3nitrophenyl acetyl - PVP polivinylpirrolidon - RIA radioimmunoassay - SRBC sheep red blood cell - T_H T helper - T_S T suppressor - Igh-V variable region of immunoglobulin heavy chain     

Cytolytic T lymphocyte-defined retroviral antigens on normal cells: Encoding by the Akv-1 proviral locus

We previously described the generation and specificity of H-2-restricted cytolytic lymphocytes (CTL) directed against tumors induced by AKR/Gross murine leukemia viruses (MuLV). Such anti-AKR/Gross virus CTL demonstrated type specificity; only tumors induced by endogenous MuLV that expressed the Gross cell-surface antigen were lysed. These CTL and their precursor also recognized normal spleen cells from AKR-H-2 ^b , but not AKR-H-2 ^b , Fv-1 ^b mice, however, suggesting that N-ecotropic, retrovirus-associated antigens were primarily involved. Here, expression of these CTL-defined retroviral antigens by H-2^b-positive AKR × C57L recombinant inbred strains was examined by using normal spleen cells as stimulators in the generation of specific anti-AKR/Gross virus CTL. Analysis of the strain distribution pattern of stimulation indicated that a single proviral locus, Akv-1, was primarily, if not entirely, responsible for CTL-defined retroviral antigen expression. The lack of correlation with two other well-defined proviral loci was interesting. Whereas Akv-3 is known to encode a defective virus, Akv-4 has been shown to code for an infectious virus thought to be very similar or identical to that of Akv-1. Although quantitative differences cannot be formally excluded, dose response experiments argued against this possibility and suggested that Akv-1 and Akv-4 may exhibit qualitative differences germane to antiviral CTL recognition.     

H-2 effects on cell-cell interactions in the response to single non-H-2 alloantigens

Individual mice were tested for their proliferative T-cell response to H-Y- and H-3-incompatible stimulator cells in secondary mixed lymphocyte culture. Responders expressing the H-2 ^bhaplotype were restricted in their response to stimulators presenting H-Y and H-3 in the context of H-2 ^b. Lymphocytes from individual B10 females proliferated in response to H-Y presented with I-A ^band D ^b. The ratio of I-A ^b/D ^b-restricted responses varied between individual responders, indicating significant qualitative variation between genetically identical responders. The majority of the proliferative response in all tested mice was restricted to the entire H-2 ^bhaplotype suggesting complementation of I-A ^b- and D ^b-region genes in presenting the H-Y antigen. Similar observations were made in the response of individual B10.LP mice to the H-3 antigen. H-3-specific, proliferating T cells were restricted to H-3 antigen presented with K ^bA^band D ^bwith significant variation between individuals in their preference for H-3 plus K ^bA^band D ^b. In contrast to the response to H-Y, the proliferative response to H-3 plus H-2 ^bcould be accounted for by the summation of the proliferative responses to H-3. plus K ^bA^band D ^b. These observations demonstrate that the proliferative response to non-H-2 H antigens in the context of I-region determinants is not a sine qua non for the T-cell response to these antigens. Further, the individual qualitative and quantitative variation observed with individual genetically identical mice has strong implications for our knowledge of intrastrain variation in immune responsiveness and the characterization of inbred strains for immune responsiveness.